The Marine Life Information Network

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year (Temperature change pressure definition).

Evidence

Kain (1964) stated that Laminaria hyperborea sporophyte growth and reproduction could occur within a temperature range of 0 to 20°C. Upper and lower lethal temperatures were estimated at between 1 to 2°C above or below the extremes of this range (Birkett et al., 1988). Gamete survival is reduced above 17°C (Kain, 1964 and 1971) and gametogenesis is inhibited at 21°C (Dieck, 1992). It is, therefore, likely that Laminaria hyperborea recruitment would be impaired at a sustained temperature increase of above 17°C. Sporophytes, however, can tolerate slightly higher temperatures of 20°C. Temperature tolerances for Laminaria hyperborea are seasonally variable, with more sensitivity to temperature change in winter months than summer months (Birkett et al., 1998b).

Laminaria hyperborea is a boreal northern species with a geographic range from mid-Portugal to northern Norway (Birkett et al., 1998b), and a mid-range within southern Norway (60° to 65° North) (Kain, 1971). The average seawater temperature for southern Norway in October is 12 to 13°C (Miller et al., 2009), and average annual sea temperature, from 1970 to 2014, is 8°C (Beszczynska-Möller & Dye, 2013). In Portugal and the south-west UK, Laminaria hyperborea is near the southern limit of its range where sea surface temperatures are closer to the upper thermal limit for this species. These populations are known as ‘trailing edge’ populations, where the species’ geographic range is contracting due to ocean warming. Trailing edge populations are known to be more sensitive to temperature increases than populations in the centre of their geographic range because they are already living close to, or at the limit of, their thermal tolerance (Smale, 2020; Hereward et al., 2020; Leathers et al., 2024). 

Trailing edge Laminaria hyperborea populations assimilate less carbon than populations in colder waters (Pessarrodona et al., 2018) and therefore accumulate less biomass (Smale et al., 2016). In colder parts of the UK, lamina extension, regrowth, and carbon standing stock were 1.5, 2 and 3 times higher, respectively, than in warmer areas (Smale et al., 2020). Wernberg et al. (2025) observed differences in morphological features between populations at opposing ends of the species’ range, such as stipe height, lamina width, stipe diameter, lamina thickness, and the number of digits. They also found that stipe epiphyte load was far greater in the colder region than in the warmer region (Wernberg et al., 2025), most likely due to greater stipe surface area (Teagle & Smale, 2018).

Temperature increases beyond the thermal optimum for kelp can negatively affect photosynthesis in kelps. Photosynthetic efficiency (measured as Fv/Fm) is widely used for measuring physiological stress in photosynthetic organisms (Trautmann et al., 2024). Burdett et al. (2019) found that simulated heat spikes (+2°C and +4°C) for three days had no overall effect on Laminaria hyperborea oxygen flux or photosynthetic efficiency, with the latter remaining above 0.72 for all treatments (with 0.7 being the widely accepted value which indicates physiological stress – Bass et al., 2023). However, photosynthetic efficiency responses to heat spikes can vary by season, light availability, and by the degree of warming. Bass et al. (2023) observed a decline in average photosynthetic efficiency of 0.33 in high light conditions and 0.11 in low light conditions, with both values falling below 0.7. The biggest decline was observed in the 22°C treatment, while the control (18°C) and 20°C treatments showed no significant change in photosynthetic efficiency. This interactive effect was also observed by Diehl et al. (2024), where photosynthetic efficiency was reduced significantly only in the coldest (0°C) treatment combined with a long photoperiod (24:0 hours light:dark) treatment. Cold and long light conditions significantly decreased chlorophyll a, accessory and xanthophyll pigments, which indicates a photoprotective stress response. In the 10°C treatment, these pigments either decreased or showed no change, suggesting that the relatively higher temperature mitigated light stress. Dry weight increased significantly, despite no measurable change in surface area, when the highest temperature (10°C) treatment was combined with moderate (16:8 h) and long (24:0 h) photoperiods. This increase in dry weight was not detrimental to the kelp and was likely due to the accumulation of storage carbohydrates rather than growth. No significant responses were observed in phlorotannin (compounds that protect against light stress) levels. Mannitol (a storage carbohydrate) decreased under the long night treatment, but this effect is expected and not detrimental to the kelp. Laminarin (the other storage carbohydrate that was measured) increased significantly under both light treatments and the two warmer treatments (5°C and 10°C), which is a positive metabolic response.

The loss of Laminaria hyperborea in some parts of the UK has been attributed to increasing sea surface temperatures in the last several decades (Yesson et al., 2015b). A reduction in abundance was observed in 187 out of 496 sites between 1974 and 2010. Declines were recorded in the English Channel and West Channel and Celtic Sea, the southernmost regions in the study. The English Channel decline was strongly correlated to the sea surface temperature increase of 1 to 2°C in this period. Northernmost sites (around Scotland) were overall unchanged, while populations on the west coast of Ireland increased in abundance (Yesson et al., 2015b).

Between 1977 and 2007, many cool-water macroalgae including Laminaria hyperborea and many other species found along its distribution have almost disappeared and been replaced by warm-water macroalgae on the west coast of Asturias, northern Spain (Fernández, 2016). Moreover, only 21 out of 50 (42%) locations that were surveyed between 1997 and 2023 on the northwest coast of Spain still had dense kelp forests, of which nine were completely dominated by Laminaria hyperborea (Barrientos et al., 2025). Sea surface temperatures overall increased by around 0.01°C to 0.02°C per year in this region and across this period. In 2023, only eight of the 21 remaining dense forests still had the same canopy-forming species as they did in 1997. Laminaria hyperborea was no longer the dominant kelp in any of these sites, and it only persisted in two sites, which it shared with Laminaria ochroleuca. The persistence of these forests was strongly correlated with winter and summer sea surface temperatures as well as higher wave action (Barrientos et al., 2025). Globally, Laminaria hyperborea has experienced a range contraction of 14% between the 1980s and 2010s (Casado-Amezúa et al., 2019). This range loss is estimated to continue to up to 39.34% under the most extreme greenhouse gas emissions projections (Assis et al., 2016).

Another feature of this biotope, subtidal red algae, are less tolerant of temperature extremes than intertidal red algae, surviving between -2°C and 18 to 23°C (Lüning 1990; Kain & Norton, 1990). Temperature increase may affect growth, recruitment or interfere with reproduction processes. For example, there is some evidence to suggest that blade growth in Delesseria sanguinea is delayed until ambient sea temperatures fall below 13°C. Blade growth is also likely to be intrinsically linked to gametangia development (Kain, 1987), and maintenance of sea temperatures above 13°C may affect recruitment success.

Warming can indirectly affect this biotope through cascading effects in the food chain. For example, it is suggested that the decline of cod (Gadus morhua) in the northeast Atlantic due to fishing, combined with increased sea surface temperatures in the past 50 years, has led to the spread of crabs (Cancer pagurus and Carcinus maenas) into this region. In addition, king crabs have spread into the northeast Atlantic since their introduction to Russia from the Pacific in the 1960s. These species are known predators of urchins, and this is believed to be a contributing factor in the recovery of kelp forests in this region (Christie et al., 2019).

Against the pressure benchmark, the available information suggests that Laminaria hyperborea recruitment processes may be affected and associated red algae communities may decline.

No empirical evidence was found for the temperature tolerance of Sabellaria spinulosa. Nevertheless, its widespread distribution suggests that it is tolerant of temperature variation (Gibb et al., 2014). Sabellaria spinulosa has the greatest geographical range of all the sabellariids, according to current records, encompassing Iceland, the Skagerrak and the Kattegat, the North Sea, the English Channel, the northeast Atlantic, the Mediterranean, the Wadden Sea and the Indian Ocean, (Achari, 1974; Riesen & Reise, 1982; Reise & Schubert,1987; Hayward & Ryland, 1998; Foster-Smith, 2000; Collins, 2005). The associated epifauna are highly variable and reflect the assemblages found in adjacent biotopes. 

Weinert et al. (2021) modelled habitat distribution of Sabellaria spinulosa within marine protected areas (MPAs) off the east coast of the UK, based on projected salinity and bottom water temperature. They reported that, from 2001 to 2099, with an increase in bottom water temperature of 5.4°C and a reduction in salinity of 0.6 psu (taken from Mathis & Pohlmann, 2014), Sabellaria spinulosa distribution will decline in two MPAs (from 39 to 11% and 64 to 8%) and will disappear from another entirely.

Sensitivity assessment. In the UK, northern to southern Sea Surface Temperature (SST) ranges from 8 to 16°C in summer and 6 to 13°C in winter (Beszczynska-Möller & Dye, 2013). Overall, a chronic change (2°C for a year) outside the normal range for a year may reduce recruitment and growth, resulting in a minor loss in the population of kelp, especially in winter months or in southern examples of the biotope. One model predicted that a long-term increase in temperature will reduce the abundance of Sabellaria spinulosa (Weinert et al., 2021), however, given the widespread distribution of this species, it is unlikely to be sensitive to temperature increases at the pressure benchmark. However, an acute change (5°C for a month; e.g. from thermal effluent) may result in loss of abundance of kelp or extent of the bed, especially in winter. Therefore, resistance to the pressure is assessed as 'Medium' and resilience as 'Medium' so that the sensitivity of this biotope to increases in temperature is assessed as 'Medium'.

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year (Temperature change pressure definition).

Evidence

Kain (1964) reported that Laminaria hyperborea sporophyte growth and reproduction could occur within a temperature range of 0°C and 20°C. Upper and lower lethal temperatures have been estimated at between 1-2°C above or below the extremes of this range (Birkett et al., 1998b). Subtidal red algae can survive at temperatures between -2°C and 18-23°C (Lüning, 1990; Kain & Norton, 1990). Laminaria hyperborea is a boreal northern species with a geographic range from mid-Portugal to Northern Norway (Birket et al., 1998b), and a mid-range within southern Norway (60°-65° North)(Kain, 1971). The average seawater temperature for southern Norway in October is 12-13°C (Miller et al., 2009).  Also, the average annual sea temperature, from 1970 to 2014, was 8°C (Beszczynska-Möller & Dye, 2013). The available information suggests that Laminaria hyperborea forest and canopy would not be affected by a decrease in sea temperature at the benchmark level.

Sabellaria spinulosa occurs north to the Arctic and is, therefore, probably tolerant of a decrease in temperature at the pressure benchmark. This conclusion is supported by observations that Sabellaria spinulosa appeared unaffected by the cold, on oyster grounds in the River Crouch, throughout the severe winter of 1962–1963 The mean daily temperature was recorded at a depth of one fathom (1.8 m) below low water (equinoctial spring tide) and the lowest temperature recorded was -1.8°C (Crisp, 1964). At Penmon in Bangor, Sabellaria spinulosa also did not to suffer from the low temperatures and live individuals were found readily (Crisp, 1964). 

Sensitivity assessment. Therefore, the evidence suggests that neither the Laminaria hyperborea canopy nor the Sabellaria crusts would be adversely affected by a decrease in temperature at the benchmark level. Therefore, resistance to the pressure is assessed as ‘High’ and resilience as ‘High’ so that the sensitivity of this biotope to a decrease in temperature is assessed as ‘Not Sensitive’ at the benchmark level. 

Salinity increase (local)

Benchmark. An increase in one MNCR salinity category above the usual range of the biotope or habitat (Salinity regime change pressure definition).

Evidence

Lüning (1990) suggested that 'kelps' were stenohaline and that their general tolerance to salinity as a phenotypic group covered16 to 50 psu over a 24-hour period. Optimal growth probably occurs between 30 and 35 psu ('Full' salinity) and growth rates are likely to be affected by periodic salinity stress. Birkett et al. (1998b) suggested that long-term increases in salinity may affect Laminaria hyperborea growth and may result in loss of affected kelp and, therefore, loss of the biotope. 

No evidence for the physiological tolerance of Sabellaria spinulosa to salinity change was found by Gibb et al. (2014). Quintino et al. (2008) examined through laboratory experiments the sub-lethal endpoints of brine exposure on the congener Sabellaria alveolata larvae. Natural seawater where salinities had been increased using commercial salts used to prepare artificial seawater were used as the control. At a salinity of 36 (natural seawater artificially concentrated), 20% of Sabellaria alveolata developed abnormally. At a salinity of 40, about 70% of the larvae developed abnormally, clearly indicating the effect of increasing salinity on larvae. Although not directly relevant to the pressure benchmark, the experiments do suggest that increasing salinity could lead to lethal effects on larvae. It is not clear how these supply effects would ramify at the population level.

Sensitivity assessment. There is no direct evidence on the effect of increased salinity on Sabellaria spinulosa, however, the available evidence for Laminaria hyperborea suggests that the canopy might be affected by hypersaline conditions. Therefore, a precautionary resistance assessment of 'Medium' is suggested but with ‘Low’ confidence. Resilience is probably 'Medium' so that sensitivity is assessed as 'Medium'.  

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat (Salinity regime change pressure definition detail).

Evidence

Lüning (1990) suggested that 'kelps' were stenohaline and that their general tolerance to salinity as a phenotypic group covered 16 to 50 psu over a 24-hour period. Optimal growth probably occurs between 30 and 35 psu ('Full' salinity) and growth rates are likely to be affected by periodic salinity stress. Birkett et al. (1998b) suggest that long-term changes in salinity may result in loss of affected kelp and, therefore, loss of this biotope.

Hopkin & Kain (1978) tested Laminaria hyperborea sporophyte growth at various low salinity treatments. The results showed that Laminaria hyperborea sporophytes could grow 'normally' at 19 psu, but that growth was reduced at 16 psu and sporophytes did not grow at 7 psu. A decrease in one MNCR salinity scale from 'Full' salinity (30 to 40 psu) to 'Reduced' salinity (18 to 30 psu) could result in a decrease of Laminaria hyperborea sporophyte growth. Laminaria hyperborea may also be out-competed by low salinity tolerant species e.g. Saccharina latissima (Karsten, 2007), or the Invasive Non-Indigenous Species Undaria pinnatifida (Burrows et al., 2014). If salinity was returned to 'Full' salinity (30 to 40 psu) Laminaria hyperborea could out-compete Saccharina latissima and re-establish community dominance in 2 to 4 years (Kain, 1975; Leinaas & Christie, 1996), however, full habitat structure may take over 10 years to recover (Birkett et al., 1998b; Christie et al., 1998). The ability of Laminaria hyperborea to out-compete Undaria pinnatifida within the UK is, however, unknown (Heiser et al., 2014), and any interspecific interaction between Laminaria hyperborea and Undaria pinnatifida is not included within this sensitivity assessment.

No evidence for physiological tolerances of Sabellaria spinulosa to decreases in salinity was found by Gibb et al. (2014). Sabellaria spinulosa does not tend to occur in very low salinity areas (Holt et al.,1998) but has been recorded from estuaries including the Crouch, Mersey (Killeen & Light, 2000) and the Thames (Limpenny, 2010). Buhs & Reise (1997) surveyed 12 channel systems in the Wadden Sea and found that Sabellaria spinulosa reefs occurred in the northern tidal inlets, which experienced salinity levels ranging from 28 to 30 psu. There is some speculation (Foster-Smith & Hendrick, 2003) that Mcintosh (1922) misidentified samples of Sabellaria spinulosa as the congener Sabellaria alveolata from the Humber estuarine population (Holt et al., 1998). The majority (> 15,000) of the Sabellaria spinulosa records in OBIS (2025) were reported from 30 to 35 psu, although a small number were report at 25 to 30 or 5 to 10 psu. Weinert et al. (2021) modelled habitat distribution of Sabellaria spinulosa within marine protected areas (MPAs) off the east coast of the UK, based on projected salinity and bottom water temperature. They reported that, from 2001 to 2099, with an increase in bottom water temperature of 5.4°C and a reduction in salinity of 0.6 psu (taken from Mathis & Pohlmann, 2014), Sabellaria spinulosa abundance will decline in two MPAs (from 39 to 11% and 64 to 8%) and will disappear from another entirely.

These records indicate that reduced and variable salinities can be tolerated to some extent, but the paucity of records suggests that areas of reduced salinity do not provide optimal habitat.

Sensitivity assessment. A reduction in salinity from 'Full' (30 to 35) to 'Reduced' (18 to 30) for a year, may result in loss of a proportion of the kelp bed, as sporophyte growth is reduced and loss by wave action and herbivory increases, or via competition with opportunistic, low salinity tolerant, species such as Saccharina latissima. Similarly, Sabellaria may not be able to tolerate low salinities, although the evidence is unclear. Therefore, resistance to the pressure is assessed as ‘Low’, and resilience ‘Medium’ so that the sensitivity of this biotope to decreases in salinity is assessed as ‘Medium’.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s and 0.2 m/s for more than one year (Water flow pressure definition). 

Evidence

Kregting et al. (2013) measured Laminaria hyperborea blade growth and stipe elongation from an exposed and a sheltered site in Strangford Lough, Ireland, from March 2009 to April 2010. Maximal significant wave height (H_m0) was 3.67 and 2 m, and maximal water velocity (Vel_rms) was 0.6 and 0.3 m/s at the exposed and sheltered sites, respectively. Despite the differences in wave exposure and water velocity, there was no significant difference in Laminaria hyperborea growth between the exposed and sheltered sites. Therefore, water flow was found to have no significant effect on Laminaria hyperborea growth at the observed range of water velocities. Biotope structure is, however, different between wave exposed and sheltered sites. 

The morphology of the stipe and blade of kelps varies with water flow. In wave exposed areas, for example, Laminaria hyperborea develops a long and flexible stipe and this is probably a functional adaptation to strong water movement (Sjøtun et al., 1998). In addition, the lamina becomes narrower and thinner in strong currents (Sjøtun & Fredriksen, 1995). However, the stipe of Laminaria hyperborea is relatively stiff and can snap in strong currents. Laminaria hyperborea is usually absent from areas of high wave action or strong currents, although it is found in the Menai Strait, Wales, where tidal velocities can exceed 4 m/s (NBN, 2015) and in tidal rapids in Norway (J. Jones, pers. comm.). Laminaria hyperborea growth can persist in very strong tidal streams (>3 m/s).). Strong currents can, however, cause detachment and reduce the settlement of kelp, therefore reducing their abundance (Bekkby et al., 2019).

Norderhaug et al. (2014) investigated the effects of current speed on species richness and diversity on Laminaria hyperborea holdfasts. Species richness was significantly linked to current speed, with the highest richness observed at intermediate current speeds between 0.12 and 0.18 m/s, (defined as “intermediate” based on the range of values observed in their study). It has been suggested that higher current speeds increase nutrient flow in the area, thereby promoting the growth of epiphytic algae on kelp stipes (Bekkby et al., 2015). Moreover, current speed has an interactive effect with wave exposure, where areas with high tidal flow and high wave action have a higher stipe epiphyte density. In contrast, in areas with high tidal flow and low wave activity, the bidirectional flow of water from tidal forces may increase canopy shading due to drag, while orbital and stochastic wave action can allow more light penetration through the canopy and facilitate epiphyte growth (Bekkby et al., 2015).

Increased water flow rate may also remove or inhibit grazers including Patella pellucida and Echinus esculentus and remove epiphytic algae growth (Pedersen et al., 2012). The associated algal flora and suspension-feeding faunal populations change significantly with different water flow regimes. Increased water flow rates may reduce the understorey epiflora, to be replaced by an epifauna dominated community (e.g. sponges, anemones and polyclinid ascidians) as in the biotope IR.HIR.KFaR.LhypFa. The composition of the holdfast fauna may also change, e.g. energetic or sheltered water movements favour different species of amphipods (Moore, 1985).

Sabellaria spinulosa tend to occur in areas of high water movement where larvae, tube building materials and food particles are suspended and transported (Jones et al., 2000). The relative importance of tidal versus wave-induced movements to support reefs is, however, unclear (Holt et al., 1998). There is currently limited in-situ data on the specific water flow tolerances of Sabellaria spinulosa, although colonies have been found in areas with sedimentary bed forms that suggest current velocities in the range of 0.5 m/s to 1.0 m/s (Mistakidis, 1956; Jones et al., 2000; Davies et al., 2009). Sabellaria spinulosa reefs were recorded in areas exposed to peak spring tidal flows of 1.0 m/s (Pearce et al., 2014) in the southern North Sea close to the coast of England. On rocky boulders sunken to form an artificial reef, one individual worm was observed surviving in flow rates between 0.7 and 1.5 m/s in the Wadden Sea in (Baptist et al., 2025). Davies et al. (2009) also found, through laboratory experiments with Sabellaria spinulosa in tanks, that increasing the water flow to an average of 0.03 m/s was adequate to begin distribution of the sediment rain from the airlift throughout the tank, and that doubling the water flow to almost 0.07 m/s further improved particle distribution throughout the tank. Therefore, it is likely that Sabellaria spinulosa requires habitats with a water flow above 0.07 m/s so that particles are suspended and distributed for the use of tube building and feeding. 

Jackson-Bue et al. (2022) demonstrated that mean bed shear stress was the most important environmental variable predicting Sabellaria spinulosa distribution off the west coast of Anglesey, Wales, where tidal flow can reach 3.7 m/s. They showed that reefs were unlikely to occur where bed shear stress exceeded 2.52 N/m² and suggested that higher flow rates may act as barriers to larval settlement or may increase sediment suspension, thus impact tube building and feeding. Dubois et al. (2009) reported that Sabellaria alveolata filter feeding efficiency declined and remained lower at around 45 mg/l of suspended particulate matter.

Tillin (2010) used logistic regression to develop statistical models that indicate how the probability of occurrence of the congener Sabellaria alveolata changes over environmental gradients within the Severn Estuary. The modelled ranges should be interpreted with caution and apply to the Severn Estuary alone (which experiences large tidal ranges, high currents and extremely high suspended sediment loads and is therefore distinct from many other estuarine systems). However, these ranges do provide some useful information on environmental tolerances. The models indicate that for subtidal Sabellaria alveolata the maximum optimal current speed (the range in which it is most likely to occur) ranges from 1.26 to 2.46 m/s and the optimal mean current speed ranges from 0.5 to 1.22 m/s. Although not directly applicable to Sabellaria spinulosa, this data suggests that tube-building sabellariids are able to occur within a broad range of current speeds.

In cases of reduced water flow, Sabellaria spinulosa is likely to suffer a reduction in the supply of suspended food and particles that are integral for growth and repair. A long-term decrease in water flow may reduce the viability of populations by limiting growth and tube building. No evidence was found for threshold levels relating to impact.

Sensitivity assessment. The IR.MIR.KR.Lhyp and CR.MCR.CSab biotopes and sub-biotopes are characteristic of moderate energy environments due to moderate or greater wave action or strong to weak tidal flow, depending on which source of water movement is dominant in any particular site. This biotope IR.MIR.KR.Lhyp.Sab is found in weak tidal flow but moderate wave exposure, which keeps the sand in suspension. A significant decrease in water flow might reduce the suspension of sand to the detriment of the Sabellaria crusts but benefit red algae, whereas a significant increase in water flow may increase scour to the detriment of the red algal understorey and possibly kelp recruitment but benefit the Sabellaria spinulosa. Nevertheless, wave action is the dominant source of water movement in this biotope and a change in peak mean spring bed flow velocity of between 0.1 to 0.2 m/s for more than one year is not likely to affect the structure of the biotope, especially as both kelp and red algal dominated biotopes and Sabellaria crusts occur in moderately strong (0.5 to 1.5 m/s) and strong water flows (1.5 to 3 m/s). Therefore, resistance to the pressure is assessed as ‘High’, and resilience ‘High’ so that the sensitivity of this biotope to changes in water flow is assessed as ‘Not Sensitive’ at the benchmark level.   

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year, or 2) an increase in relative sea level or decrease in high water level for ≥1 year. (Emergence regime change pressure definition).

Evidence

The upper limit of the Laminaria hyperborea bed is determined by wave action, water flow, desiccation, and competition from the more emergence resistant Laminaria digitata. Laminaria hyperborea exposed at extreme low water are very intolerant of desiccation, the most noticeable effect being bleaching of the frond and subsequent death of the meristem and loss of the plant. An increase in wave exposure (see below), as a result of increased emergence, has been found to exclude Laminaria hyperborea from shallow waters due to dislodgement of the sporophyte or snapping of the stipe (Birket et al., 1998b). Hence, an increase in emergence could lead to mortality of exposed Laminaria hyperborea and the associated habitat. However, a decrease in emergence (at the benchmark level) may increase the upper depth restriction of Laminaria hyperborea forest biotope variants. However, limited light availability at depth will decrease the lower extent of Laminaria hyperborea, and may, therefore, result in a shift from forest to park biotope variants at depth. Further increases in depth will cause a community shift to that characterized by circalittoral faunal species, however, this is beyond the scope of the benchmark. Sabellaria spinulosa crusts are most abundant in the subtidal (e.g. CR.MCR.CSab) but crusts can occur in the lower intertidal. 

Sensitivity assessment. Shallow examples of the biotope may lose a proportion of the Laminaria canopy due to an increase in emergence whereas an increase in emergence due to increase water depth might also limit the deeper extents of the Laminaria canopy. Therefore, resistance to the pressure is assessed as ‘Low’, and resilience ‘Medium’ so that the sensitivity of this biotope to changes in emergence is assessed as ‘Medium’.

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year (Wave action pressure definition). 

Evidence

Laminaria hyperborea biotopes are found across a range of wave exposures, from extremely exposed to sheltered sites, and experience elevated tidal streams. Changes in local wave height outside the typical range for these habitats may affect the dominance of Laminaria hyperborea. If the suitable range of wave exposure levels is exceeded, then kelp recruitment may not be possible. Changes in wave exposure where Laminaria hyperborea forests already occur, such as storms that strongly increase wave activity, could be detrimental to mature kelps by breaking stipes and reducing the canopy cover. If wave exposure is too low, nutrient and light availability would be limited, and would affect the kelp itself and its associated communities. Bekkby et al. (2019) observed a strong interactive effect of wave action, tidal flow and depth on Laminaria hyperborea abundance. They found that the probability of Laminaria hyperborea occurrence was 0.2 in areas with low wave exposure (0.002 km²/s), while it was almost 1.0 in areas with high wave exposure (up to 1.95 km²/s). These values were calculated using a simplified wave model which uses fetch, wind speed and frequency as input variables, is widely used in Scandinavian coastal studies and aligns with the EUNIS wave exposure classification system (described by Rinde et al., 2005).

Laminaria hyperborea responses to extreme levels of wave exposure vary in the literature. In the winter of 2013 – 14, the south of the UK experienced 6 of the 12 most intense storms that had been recorded in the prior five years (Smale & Vance, 2015). Despite wave heights exceeding 7 m and periods exceeding 13 s, Laminaria hyperborea showed much more resistance to the storm than the negatively impacted Laminaria ochroleuca, which had three times more density of severed stipes than Laminaria hyperborea. Earp et al. (2024) observed significant changes in canopy cover after Storm Arwen in the UK in November 2021. In the most wave exposed site (most northerly facing), canopy cover was reduced from above 75% to less than 10%. In the second most exposed site, cover was reduced from 100% to less than 60%. In addition to canopy loss, the understorey algae communities had also almost completely changed in the four most exposed sites. In northwest Portugal, Laminaria hyperborea abundance was reduced from 80 individuals per square metre (m²) to 60/m² in the month following a storm (Pereira et al., 2017). The abundance was recorded at 10/m² 10 months later, and the remaining individuals were almost entirely recruits.

The morphology of Laminaria hyperborea also varies with water flow, with local adaptations likely aimed at reducing drag in highly wave exposed sites. In wave exposed areas, for example, Laminaria hyperborea develops a long and flexible stipe, and this is probably a functional adaptation to strong water movement (Sjøtun, 1998). In addition, the lamina becomes narrower and thinner in strong currents (Sjøtun & Fredriksen, 1995). Kregting et al. (2013) measured Laminaria hyperborea blade growth and stipe elongation from an exposed and a sheltered site in Strangford Lough, Northern Ireland, from March 2009-April 2010. Maximal significant wave height (H_m0) was 3.67 and 2 m at the exposed and sheltered sites, and maximal water velocity (Vel_rms) was 0.6 and 0.3 m/s at the exposed and sheltered sites, respectively. Despite the differences in wave exposure and water velocity, there was no significant difference in Laminaria hyperborea growth between the exposed and sheltered sites. In contrast, other studies have found that populations in areas with higher wave exposure tend to have greater levels of growth, biomass and density than their counterparts in low wave exposed sites (Pedersen et al., 2012; Smale et al., 2016; Gundersen et al., 2021). At low wave exposure, Laminaria hyperborea canopy forming plants were smaller, had lower densities and had higher mortality rates than at exposed sites. This may be due to high epiphytic loading in low-exposure sites, which could reduce light and nutrient uptake, and increase the drag of the host Laminaria hyperborea during extreme storm events (Pedersen et al. (2012).

Smaller changes in local wave height have the potential to cause changes to the understorey community. For example, species richness and diversity were found to be greater in areas with intermediate levels of wave exposure (Norderhaug et al., 2014). The prominent understorey filter feeding community within IR.MIR.KR.LhypT/TX is reliant on high water movement. A decrease in wave surge may result in a decline of filter-feeding fauna and an increase in red seaweeds within the understorey community or vice versa. A decrease in local wave height may also decrease the chance of urchins being dislodged (removed) from biotopes found at sites with traditionally high wave exposure and may, therefore, increase urchin grazing. An increase in urchin grazing may cause a decline in the understorey community abundance and diversity (as in IR.MIR.KR.Lhyp.GzFt/Pk and IR.MIR.KR.LhypPar).

No empirical evidence was found to assess this pressure on Sabellaria spinulosa. However, increases in wave action from storms has been suggested as the main driver of Sabellaria spinulosa reef destruction in winter (Lisco et al., 2017). Intertidal Sabellaria spinulosa is directly exposed to waves but wave exposure can also potentially affect subtidal Sabellaria spinulosa reefs. At depth, the motion from surface waves becomes oscillatory and produces back-and-forth water movement at the seabed (Dubois et al., 2006). In sublittoral habitats, water movements are likely to provide sand and food particles that are necessary for Sabellaria spinulosa to build tubes, feed, and subsequently grow and develop.

Weinert et al. (2021) identified that, other than grain size, wave stress was the most important environmental variable driving distribution models of Sabellaria spinulosa in the North Sea. Models of habitat suitability also identified fetch and wave height as some of the most influential variables in determining suitable habitat for the congener Sabellaria alveolata (Firth et al., 2021, Curd et al., 2023). Optimum wave exposure for these reefs was identified as moderate, such that wave height falls between 1.3 and 1.8 m (Firth et al., 2021). Their models predicted that Sabellaria alveolata was unlikely to occur in areas where wave height exceeded 1.8 m (Firth et al., 2021).

Jackson-Bue et al. (2022) demonstrated Sabellaria spinulosa reefs were unlikely to occur off the west coast of Anglesey, Wales in areas where bed shear stress exceeded 2.52 N/m². They suggested that higher flow rates may act as barriers to larval settlement or may increase sediment suspension, thus impact tube building and feeding. Therefore, should increases in wave action result in higher bed shear stress, this could lead to loss of reefs and suitable habitat.

Tests on the mechanical strength and properties of Sabellaria alveolata tubes were performed by Le Cam et al. (2011). These found that the biomineralized cement the worms produce to bind sand grains into tubes confer wave resistance. Although thresholds of resistance are not known, the visco-elastic behaviour of the cement enables tubes to dissipate the mechanical energy of breaking waves (Le Cam et al., 2011). Therefore, similarly constructed tubes by Sabellaria spinulosa may also confer wave resistance.

Sensitivity assessment. The IR.MIR.KR.Lhyp and CR.MCR.CSab biotopes and sub-biotopes are characteristic of moderate energy environments due to moderate or greater wave action or strong to weak tidal flow, depending on which source of water movement is dominant in any particular site. This biotope IR.MIR.KR.Lhyp.Sab is found in weak tidal flow but moderate wave exposure, which keeps the sand in suspension. Wave action is the dominant source of water movement in this biotope, and a significant reduction in wave action would likely reduce the availability of suspended sediment and reduce scour allowing more macroalgae to colonize the biotope. Conversely, an increase in wave action may increase scour and reduce the habitats suitability for kelps. Nevertheless, a 3 to 5% change is significant wave height is unlikely to affect this biotope, which is typical of moderately wave exposed conditions. Therefore, resistance to the pressure is assessed as ‘High’, and resilience ‘High’ so that the sensitivity of this biotope to changes in water flow is assessed as ‘Not Sensitive’ at the benchmark level.  

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant Transitional metal or organometal (e.g. TBT) contaminants via uncontrolled releases or incidental spills (Transitional metals and organometals pressure definition). 

Evidence

This pressure is Not assessed but evidence is presented where available.

Bryan (1984) suggested that the general order for heavy metal toxicity in seaweeds is: Organic Hg > inorganic Hg > Cu > Ag > Zn > Cd > Pb. Cole et al. (1999) reported that Hg was very toxic to macrophytes. Similarly, Hopkin & Kain (1978) demonstrated sub-lethal effects of heavy metals on Laminaria hyperborea gametophytes and sporophytes, including reduced growth and respiration. Sheppard et al., (1980) noted that increasing levels of heavy metal contamination along the west coast of Britain reduced species richness in holdfast fauna except for suspension feeders which became increasingly dominant. Gastropods may be relatively tolerant of heavy metal pollution (Bryan, 1984). Echinus esculentus recruitment is likely to be impaired by heavy metal contamination due to the intolerance of its larvae. Echinus esculentus is long-lived and poor recruitment may not reduce grazing pressure in the short-term. Although macroalgae species may not be killed, except by high levels of contamination, reduced growth rates may impair the ability of the biotope to recover from other environmental disturbances.

While the effects of heavy metals on Laminaria hyperborea have not been extensively studied in recent literature, closely related species such as Laminaria digitata have received greater attention. Given the close taxonomic relationship between these species, it is possible that physiological responses to metal contamination could be similar. Maulvault et al. (2015) investigated toxic elements in seafood samples around Europe, including Laminaria digitata, bivalves, and fish. The highest levels of arsenic in the study (41 mg per kg) were detected in Laminaria digitata. Cadmium and copper have been shown to significantly decrease photosynthetic efficiency in Laminaria digitata to 0.65 (Anacleto et al., 2017), which is below the commonly accepted indicative value for stress, 0.7 (Bass et al., 2023).

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant hydrocarbon or polyaromatic hydrocarbon (PAH) contaminants via uncontrolled releases or incidental spills (Hydrocarbon & PAH pressure definition).

Evidence

This pressure is Not assessed but evidence is presented where available.

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more synthetic compound contaminants via uncontrolled releases or incidental spills (Synthetic compound contamination pressure definition).

Evidence

This pressure is Not assessed but evidence is presented where available.

O'Brian & Dixon (1976) suggested that red algae are the most sensitive group of macrophytes to oil and dispersant contamination (see Smith, 1968). Although Laminaria hyperborea sporelings and gametophytes are intolerant of atrazine (and probably other herbicides), they may be relatively tolerant of synthetic chemicals overall (Holt et al., 1995). Laminaria hyperborea was found surviving within >55 m from the discharge point of acidified halogenated effluent polluting Amlwch Bay, Anglesey, albeit at low density. These individuals were older than 5 years of age, suggesting that spores and early stages are more sensitive to chemical contamination (Hoare & Hiscock, 1974). Patella pellucida was excluded from Amlwch Bay by the pollution, and the species richness of the holdfast fauna decreased with proximity to the effluent discharge. Amphipods were particularly intolerant, while polychaetes were the least affected (Hoare & Hiscock, 1974). The richness of epifauna/flora also decreased near the source of the effluent and epiphytes were absent from Laminaria hyperborea stipes within Amlwch Bay. The red alga Phyllophora membranifolia appeared to be tolerant of the effluent in Amlwch Bay.

Smith (1968) also noted that epiphytic and benthic red algae were highly sensitive to dispersant or oil contamination following the Torrey Canyon oil spill. Only the epiphytes Crytopleura ramosa and Spermothamnion repens and some tufts of Jania rubens, and sublittoral fringe species such as Osmundea pinnatifida, Gigartina pistillata and Phyllophora crispa. Delesseria sanguinea was probably the most sensitive, showing damage at depths of 6 m (Smith, 1968). Holt et al. (1995) similarly suggested that Delesseria sanguinea is generally sensitive to chemical contamination.

Although Laminaria hyperborea may be relatively tolerant of synthetic chemical pollution, evidence suggests that grazing gastropods, amphipods and red algae are more sensitive. Loss of red algae is likely to reduce the species richness and diversity of the biotope, potentially resulting in the understorey being dominated by encrusting corallines. However, red algae are likely to recover relatively quickly.

There is also currently insufficient evidence for harmful effects of synthetic compound contamination in the closely related kelp Laminaria digitata. Studies which investigate the presence of these compounds in seafood, including kelps, focus on implications for human health (see Alvarez-Munoz et al., 2015) rather than their effects on the species or ecosystems themselves. However, Anacleto et al. (2017) did investigate the effects of a range of pollutants on Laminaria digitata health but found no significant reduction in photosynthetic activity in response to pesticides (diflubenzuron and lindane).

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels (Radionuclides contamination pressure definition).

Evidence

No evidence

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant "other" substances (solid, liquid or gas) contaminants via uncontrolled releases or incidental spills (Introduction of other substances pressure definition). 

Evidence

This pressure is Not assessed but evidence is presented where available.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status) (deoxygenation pressure definition).

Evidence

Reduced oxygen concentrations have been shown to inhibiting both photosynthesis and respiration in macroalgae (Kinne, 1977). Despite this, macroalgae are thought to buffer the environmental conditions of low oxygen, thereby acting as a refuge for organisms in oxygen-depleted regions especially if the oxygen depletion is short-term (Frieder et al., 2012).  Rapid recovery from a state of low oxygen is expected if the environmental conditions are transient. If levels do drop below 4 mg/l negative effects on these organisms can be expected with adverse effects occurring below 2 mg/l (Cole et al., 1999). No information was found regarding Sabellaria spinulosa tolerance to changes in oxygenation. 

Sensitivity Assessment. Reduced oxygen levels are likely to inhibit photosynthesis and respiration but not cause a loss of the macroalgae population directly.  However, small invertebrate epifauna may be lost, causing a reduction in species richness. Therefore, a resistance of ‘High’ is recorded.  Resilience is assessed as ‘High’, and the biotope is assessed as ‘Not sensitive’ at the benchmark level.

Nutrient enrichment

Benchmark. Increased levels of the elements nitrogen, phosphorus, silicon, and iron in the marine environment compared to background concentrations (Nutrient enrichment pressure definition).

Evidence

Holt et al. (1995) suggest that Laminaria hyperborea may be tolerant of nutrient enrichment since healthy populations are found at ends of sublittoral untreated sewage outfalls in the Isle of Man. Increased nutrient levels, e.g. from sewage outfalls, have been associated with increases in abundance, primary biomass and Laminaria hyperborea stipe production, but with simultaneous decrease in species numbers and diversity (Fletcher, 1996). 

Laminaria hyperborea forests near high-effluent salmon farms show different stipe-associated community compositions to those near low-effluent farms and reference sites with no nearby aquaculture activity (Haugland et al., 2021). Bryozoan biomass was significantly higher at the high-effluent sites, whereas stipes at the low-effluent and reference sites were predominantly colonized by epiphytic macroalgae. At the high-effluent sites, the reduced epiphytic community was dominated by Ectocarpus spp., leading to lower heterogeneity within the stipe assemblage and reduced habitat heterogeneity. This suggests that changes in dissolved inorganic nitrogen could potentially shift this biotope from being fauna-dominated to an algae-dominated one (e.g. IR.HIR.KFaR.LhypR). Increased nutrients may result in phytoplankton blooms that increase turbidity (see above), and may favour sea urchins, e.g. Echinus esculentus, due their ability to absorb dissolved organics, potentially increasing grazing pressure leading to loss of understorey epiflora/fauna, reduced kelp recruitment, and possibly to the formation of urchin barrens. Therefore, although nutrients may not affect kelps directly, indirect effects such as turbidity, siltation and competition may significantly affect the structure of the biotope. 

No direct evidence was found to assess this pressure on Sabellaria spinulosa. Bertocci et al. (2017) assessed the effects of nutrient enrichment on the congener Sabellaria alveolata in northern Portugal. Enrichment was performed by deploying 200 g of slow-release fertilizer pellets containing 15 mg of nitrogen (nitrate and ammoniacal nitrogen), 3.9 mg of phosphorous pentoxide, 9.1 mg of potassium oxide, 1.5 mg of magnesium oxide (plus microelements) every two months into rockpools containing Sabellaria alveolata. The temporal variability in nutrient enrichment had no effect on the abundance on Sabellaria alveolata during the 20-month study period. Enhanced phytoplankton production may increase food supply and increased siltation and deoxygenation from algal blooms is likely to be mitigated by water movements in the areas most suitable for Sabellaria spinulosa reefs. 

Sensitivity assessment. While the evidence of nutrient enrichment of Sabellaria spinulosa is unclear, the above evidence suggests that increased nutrients may benefit Laminaria hyperborea kelp beds but alter the associated community, possibly resulting in changes in biotope classification. In extreme cases, turbidity and suspended sediment (see changes in suspended sediment) may attenuate light and be detrimental. Hence, resistance is assessed as ‘Medium’ to represent the potential loss of the associated community. Therefore, resilience is assessed as ‘Medium’ and sensitivity as ‘Medium’.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr (Organic enrichment pressure definition).

Evidence

Holt et al. (1995) suggest that Laminaria hyperborea may be tolerant of organic enrichment since healthy populations are found at ends of sublittoral untreated sewage outfalls in the Isle of Man. Increased nutrient levels e.g. from sewage outfalls have been associated with increases in abundance, primary biomass and Laminaria hyperborea stipe production but with concomitant decreases in species numbers and diversity (Fletcher, 1996). Increase in ephemeral and opportunistic algae is associated with reduced numbers of perennial macrophytes (Fletcher, 1996). Increased nutrients may also result in phytoplankton blooms that increase turbidity. Therefore, although organic enrichment may not affect kelps directly, indirect effects such as turbidity may significantly affect the structure of Laminaria hyperborea biotopes.

Sabellaria spinulosa was reported to show enhanced growth adjacent to a sludge dumping area in Dublin Bay (Walker & Rees 1980).  Hence, Sabellaria spinulosa reef biotopes are probably resistant to a high level of organic enrichment.  Information on the levels of organic matter in Dublin Bay was not provided and so it is unclear how the levels experienced relate to the pressure benchmark. Sabellaria spinulosa reefs are found in areas of high water movement of up to 1 m/s (Pearce et al., 2014) that would naturally disperse some organic matter preventing accumulation and siltation.  In larger, dense colonies of Sabellaria spinulosa, sand, detritus, and finer faecal materials collect in between worm tubes.  These detritus layers do not interrupt the normal growth of the individuals or the colony as a whole (Schafer, 1972).  Hence, it seems likely that Sabellaria spinulosa crusts and reefs are resistant to the deposition of a fine layer of organic materials.

Indirect effects arising from inputs of organic matter are possible where habitat quality and species interactions are altered.  In the Wadden Sea, large subtidal areas of Sabellaria spinulosa reefs have been completely lost since the 1920s.  This decline was partly attributed to an increase in coastal eutrophication that favoured blue mussel beds (Dörjes, 1992; Hayward & Ryland, 1998; Benson et al., 2013).  However, a direct causal link has not been established and it is possible that the decline of Sabellaria spinulosa reefs was due to physical damage from fishing activities rather than competitive interactions (Jones et al., 2000).

Sensitivity assessment. Little evidence was found to support this sensitivity assessment. Sabellaria spinulosa and the associated species assemblage (which typically includes attached filter feeders from several phyla) is likely to be able to consume extra organic matter.  This conclusion is supported by the enhanced growth rates recorded in the vicinity of sewage disposal areas (Walker & Rees, 1980). However, the Laminaria canopy and especially epiphytes and red algal epiflora may experience a reduction in abundance due to increased turbidity and competition with opportunistic green algae. Therefore, resistance is assessed as 'Medium' as a precaution, albeit with 'Low' confidence. Hence, resilience is assessed as 'High' and the sensitivity of this biotope as 'Low'.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site (Physical loss pressure definition). 

Evidence

All marine habitats and benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very Low’).  Sensitivity within the direct spatial footprint of this pressure is, therefore ‘High’.  Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata, or vice versa (Physical change in subtratum type pressure definition).

Evidence

Sabellaria spinulosa can colonize artificial structures and have been found on a gas pipeline off the coast of Aberdeen (Mistakidis, 1956) and on old oil and gas infrastructure in the North Sea (Coleen et al., 2020). An increase in the availability of artificial substratum may support reef development in areas where sedimentary habitats were previously unsuitable for colonisation. Within the Wadden Sea, boulder revetments that have been placed into the sea in an effort to introduce natural reefs, were suggested as suitable habitat for Sabellaria spinulosa (Baptist et al., 2025). However, this assumption was based only on the occurrence of one individual (Baptist et al., 2025).

Sensitivity assessment. It should be noted that the basis of the sensitivity assessment for this pressure is the sensitivity of the biotope to changes in substratum type, rather than the mechanism by which this occurs (e.g. complete smothering of the biotope, then any recovery, if possible). If rock substrata were replaced with sedimentary substrata this would represent a fundamental change in habitat type, which Laminaria hyperborea would not be able to tolerate (Birkett et al., 1998b). Sabellaria spinulosa also develops crusts and reefs on coarse sediment and could probably recover.  However, the biotope would no longer be a rock habitat, would lose its macroalgal component and be lost. Therefore, resistance to the pressure is assessed 'None' and resilience 'Very Low' so that sensitivity is assessed as 'High'.

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification) (Physical change in sediment type pressure definition). 

Evidence

Not relevant on hard rock habitats

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock) (Removal of substratum pressure definition). 

Evidence

Not relevant on hard rock habitats

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat) (Surface abrasion/disturbance pressure definition).

Evidence

Kelp harvesting can cause significant changes to this biotope through the removal of Laminaria hyperborea and the habitat space that it provides for its associated communities. Removing 26% of the canopy led to a 67% reduction in epiphytes and an 89% reduction of invertebrates (Norderhaug et al., 2020).

Christie et al. (1998) observed Laminaria hyperborea habitat regeneration following commercial Laminaria hyperborea trawling in south Norway. Trawling removed all large canopy-forming adult Laminaria hyperborea, however, sub-canopy recruits were largely unaffected. Within two to six years of harvesting, a new canopy had formed 1 m off the seabed. The associated holdfast communities recovered in six years. However, the epiphytic stipe community did not fully recover within the same period. In Nord-Trøndelag, Norway, Laminaria hyperborea was harvested in 2010 (Steen et al., 2016). Video surveys and plant sampling conducted two days prior to the trawling and in each year for the following four years, showed that Laminaria hyperborea coverage had returned to pre-harvest levels (around 94%). However, the new canopy, was significantly lower in density, average plant age, length, weight, and epiphyte biomass. In addition, the density of understorey recruits had only recovered by one-third by the end of the study period. It was suggested that 80% of the new canopy consisted of understorey plants that had survived the harvesting, and that the resilience of this biotope was dependent on the frequency of harvesting (Steen et al., 2016). Christie et al. (1998) suggested that kelp habitats were relatively resistant to direct disturbance/removal of Laminaria hyperborea canopy.

Recurrent disturbance on a timescale shorter than the two to six year recovery period could prolong recovery. Kain (1975) cleared sublittoral blocks of Laminaria hyperborea at different times of the year for several years. The first colonizers and succession communities differed between blocks and the time of year they were cleared. However, within two years of clearance, the blocks were dominated by Laminaria hyperborea. Leinaas & Christie (1996) also observed Laminaria hyperborea recolonizing urchin barrens following urchin removal. The substratum was initially colonized by filamentous macroalgae and Saccharina latissima. However, Laminaria hyperborea dominated the community after two to four years.

Laminaria hyperborea forests subjected to regular harvesting support different associated communities compared to unharvested, preserved forests (Leclerc et al., 2015). Macroalgal species richness was consistently higher at the harvested site across all parts of the kelp and on the surrounding rock. Sessile fauna richness was slightly higher on the stipes and surrounding rock at the harvested site, but lower on the holdfast compared to the preserved site. In contrast, mobile fauna richness and density were generally greater on all parts of the kelp in the preserved site, although both were higher on the surrounding rock at the harvested site. Following disturbance, or in areas experiencing frequent disturbance, Laminaria hyperborea recruitment may be affected by interspecific competition with Non-Indigenous Invasive Species (INIS) or ephemeral algae (Brodie et al., 2014; Smale et al., 2013) (see INIS below).

Gibb et al. (2014) found no direct evidence for impacts of the surface only on Sabellaria spinulosa. Studies of intertidal reefs of the congener Sabellaria alveolata (Cunningham et al.,1984) found that the reef recovered within 23 days from the effects of trampling (i.e. treading, walking or stamping on the reef structures) by repairing minor damage to the worm tube porches. Severe damage caused by kicking and jumping on the reef structure, resulted in large cracks between the tubes, and removal of sections (ca 15 x15 x10 cm) of the structure (Cunningham et al., 1984). Subsequent wave action enlarged the holes or cracks. However, after 23 days, at one site, one side of the hole had begun to repair, and tubes had begun to extend into the eroded area. At another site, a smaller section (10x10x10 cm) was lost but after 23 days the space was already smaller due to rapid growth (Cunningham et al., 1984). Plicanti et al. (2016) demonstrated that even low intensity trampling (i.e. being walked over once) caused significant damage to the reef by reducing the amount of intact tubes and that, even after two months, the percentage cover of intact tubes had not recovered to control levels, remaining significantly reduced at sites exposed to medium and high intensity trampling.

Vorberg (2000) used video cameras to study the effect of shrimp fisheries on Sabellaria alveolata reefs in the Wadden Sea. The imagery showed that a 3 m beam trawl easily ran over a reef that rose to 30 to 40 cm, although the beam was occasionally caught and misshaped on the higher sections of the reef. At low tide, there were no signs of the reef being destroyed and, although the trawl had left impressions, all traces had disappeared four to five days later due to the rapid rebuilding of tubes by the worms. The daily growth rate of the worms during the restoration phase was significantly higher (4.4 mm after removal of 2 cm of surface) than undisturbed growth (0.7 mm) and indicated that, as long as the reef is not completely destroyed, recovery could occur rapidly. 

Patches of Sabellaria spinulosa reef of up to 50 cm in height were discovered in the Brown Bank area in the Dutch Continental Shelf (Van der Reijden et al., 2019), an area that is heavily fished by beam trawlers (up to 12 times per year). While Sabellaria spinulosa may be able to withstand some pressure, there is evidence that abrasion from bottom trawling negatively affects the size of reefs, given that reefs of the greatest extent were found in areas of lowest fishing pressures (1 to 2 times per year), but under higher fishing pressures (up to 12 trawls per year) were restricted to refuge areas within valleys of the sand megaripples (Van der Reijden et al., 2019).

Sabellaria spinulosa reefs are thought to be more fragile than Sabellaria alveolata (B. Pearce, pers comm, cited from Gibb et al., 2014) and, therefore, surface abrasion may lead to greater damage and lower recovery rates than observed for Sabellaria alveolata. Sabellaria spinulosa reefs vary in thickness, having been reported between 2.4 to 75 cm in height (Holt et al., 1998, Lisco et al., 2021), however, thinner reef crusts may be more severely damaged or removed by surface abrasion (see also evidence from penetration and disturbance of the substratum, below). No direct observations of reef recovery, through repair, from abrasion were found for Sabellaria spinulosa.

Sensitivity assessment. Based on the evidence discussed above, abrasion at the surface of Sabellaria spinulosa crusts is likely to damage the tubes and result in sub-lethal and lethal damage to the worms. It is also likely to remove a proportion of the Laminaria canopy, attached epiphytes, Laminaria holdfasts and understorey macroalgae (where present). Therefore, resistance is assessed as ‘Low’ (loss of 25-75% of the extent/abundance of component species within the impact footprint). Hence, resilience is assessed as ‘Medium’ (within 2-10 years) and sensitivity as ‘Medium’.  

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat) (Sub-surface penetration pressure definition).

Evidence

Not relevant on hard rock habitats, please refer to pressure 'Abrasion/disturbance of the substratum on the surface of the seabed' above.

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale, e.g. from clear to intermediate for one year (Suspended sediment pressure definition).

Evidence

Suspended Particle Matter (SPM) concentration has a linear relationship with subsurface light attenuation (Kd) (Devlin et al., 2008). An increase in SPM results in a decrease in sub-surface light attenuation. Light availability and water turbidity are principal factors that determine the depth range of Laminaria hyperborea (0-47 m Below Sea Level) (Birkett et al., 1998b). Light penetration influences the maximum depth at which kelp species can grow and it has been reported that laminarians grow at depths at which the light levels are reduced to one per cent of incident light at the surface. Maximal depth distribution of laminarians, therefore, varies from 100 m in the Mediterranean to only 6 to 7 m in the silt-laden German Bight. In Atlantic European waters, the depth limit is typically 35 m. In very turbid waters, the depth at which Laminaria hyperborea is found may be reduced to 2.5 m (Birkett et al. 1998b), or in some cases excluded completely (e.g. Severn Estuary), because of the alteration in light attenuation by suspended sediment (Birkett et al. 1998b; Lüning, 1990).

Laminaria spp. show a decrease of 50% photosynthetic activity when turbidity increases by 0.1/m (light attenuation coefficient =0.1-0.2 /m; Staehr & Wernberg, 2009).  An increase in water turbidity will likely affect the photosynthetic ability of Laminaria hyperborea and decrease its abundance and density (see IR.HIR.KFaR.LhypR.Pk). Kain (1964) suggested that early Laminaria hyperborea gametophyte development could occur in the absence of light. Furthermore, observations from south Norway found that a pool of Laminaria hyperborea recruits could persist growing beneath Laminaria hyperborea canopies for several years, indicating that sporophytes growth can occur in light-limited environments (Christie et al., 1998). However, in habitats exposed to high levels of suspended silts, Laminaria hyperborea is out-competed by Saccharina latissima, a silt tolerant species. Thus, a decrease in water clarity is likely to decrease the abundance of Laminaria hyperborea in the affected area (Norton, 1978). The biotope is expected to be excluded from silt rich environments.

Sabellaria spinulosa do not rely on light penetration for photosynthesis, and it is believed that visual perception is limited such that this species does not rely on sight to locate food or other resources. In a recent review of the sensitivity of Sabellaira spinulosa reefs to anthropogenic disturbance, Fariñas-Franco et al. (2014) concluded that impacts on Sabellaria spinulosa due to a decrease in water clarity resulting from an increase in suspended solids (inorganic or organic) are unlikely, although no thresholds regarding tolerance or intolerance were found. Decreases in suspended particles that reduce the supply of food or tube-building materials may, however, negatively impact this species (Davies et al., 2009; Last et al., 2011).

Sabellaria spinulosa relies on a supply of suspended solids and organic matter to filter feed and build protective tubes and so they are often found in areas with high levels of turbidity. Davies et al. (2009) and Last et al. (2011) developed Vortex Resuspension Tanks (VoRT) to test the effects of a change in the composition of suspended sediment on benthic species. This laboratory experiment manipulated turbidity and current flow and demonstrated the susceptibility of Sabellaria spinulosa to a decrease in suspended particulate matter (SPM). A clear erosion of tubes was observed in the absence of SPM and subsequent starvation of tube building materials. At high and intermediate sediment regimes (high SPM ~71 mg/l) conditions were comparable to what might be expected within only a few hundred meters distance of a primary aggregate extraction site and Sabellaria spinulosa maintained a cumulative growth rate at these rates of SPM. This supports the view that the availability of suspended particles is necessary for Sabellaria spinulosa development and that tolerance of elevated levels is likely (Davies et al., 2009). Indirect evidence for the tolerance of Sabellaria spinulosa for changes in turbidity is provided by the persistence of reefs on the outskirts of aggregate dredging areas (Pearce et al., 2007, 2011a), which appear unaffected by dredging that is likely to have led to sediment plumes.  Such plumes, however, are short-lived (Tillin et al., 2011) and, therefore, the long-term effect depends on the duration of dredging activities.

Lisco et al. (2023) investigated the tube building rate of Sabellaria spinulosa in the presence of different sediment grain sizes. The worms exhibited tube growth rates of 1 to 4 cm per month, and growth was faster when finer sediments were available (125 to 350 um) compared to coarser grains (350 to 500 um), where a maximum growth on  2 mm per month was recorded (Lisco et al., 2023). Therefore, any changes occur to suspended sediment, which decreases the availability of finer grains, could lead to lower growth rates, potentially hampering recovery of reefs after an impact.

Tillin (2010) used logistic regression to develop statistical models that indicate how the probability of occurrence of the congener Sabellaria alveolata changes over environmental gradients within the Severn Estuary. The modelled ranges should be interpreted with caution and apply to the Severn Estuary alone (which experiences large tidal ranges, high currents and extremely high suspended sediment loads and is therefore distinct from many other estuarine systems). The models indicated that the optimal mean neap sediment concentrations for subtidal Sabellaria alveolata ranged from 515.7 to 906 mg/l and optimal mean spring sediment concentrations ranged from 855.3 to 1,631 mg/l. Although not directly applicable to Sabellaria spinulosa this data suggests that tube-building sabellariids are tolerant to very high levels of suspended sediment. Fine sediments such as mud may clog the gills and feeding tentacles of some polychaetes and, therefore, the potential impact will be mediated by the character of the sediment in suspension.

Sensitivity assessment. The benchmark for this pressure refers to a change in turbidity of one rank, e.g. from clear (<10 mg/l) to intermediate (10 to 100 mg/l) or intermediate to medium (100 to 300 mg/l). Sabellaria spinulosa do not photosynthesise and do not rely on sight to locate resources and therefore no effects are predicted for reef biotopes from an increase or decrease in clarity resulting from a change in one rank on the water framework directive scale. Experiments (Davies et al., 2009) and predictive modelling (Tillin, 2010) indicate that tube building sabellariids can tolerate a broad range of suspended solids so the Sabellaria crusts are unlikely to be affected by an increase in suspended sediment. However, if the supply of sand to the habitat was reduced, then the Sabellaria crusts would probably erode and be replaced with a diverse understorey of red algae, similar to IR.MIR.KR.Lhyp. Similarly, an increase in turbidity will reduce the abundance of the Laminaria hyperborea canopy, especially in deeper examples of the biotope, and the Laminaria may be replaced by Saccharina latissima or by an abundant Sabellaria crust similar to CR.MCR.CSab.Sspi. Overall, the biotope is likely to change in character, be reclassified, and, effectively, lost. Therefore, resistance is assessed as ‘Low’, resilience as ‘Medium’, and sensitivity is assessed as 'Medium'.

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event (Smothering pressure definition).

Evidence

Suspended Particle Matter (SPM) concentration has a linear relationship with subsurface light attenuation (Kd) (Devlin et al., 2008). An increase in SPM results in a decrease in sub-surface light attenuation. Light availability and water turbidity are principal factors that determine the depth range of Laminaria hyperborea (0-47 m Below Sea Level) (Birkett et al., 1998b). Light penetration influences the maximum depth at which kelp species can grow and it has been reported that laminarians grow at depths at which the light levels are reduced to one per cent of incident light at the surface. Maximal depth distribution of laminarians, therefore, varies from 100 m in the Mediterranean to only 6 to 7 m in the silt-laden German Bight. In Atlantic European waters, the depth limit is typically 35 m. In very turbid waters, the depth at which Laminaria hyperborea is found may be reduced to 2.5 m (Birkett et al. 1998b), or in some cases excluded completely (e.g. Severn Estuary), because of the alteration in light attenuation by suspended sediment (Birkett et al. 1998b; Lüning, 1990).

Laminaria spp. show a decrease of 50% photosynthetic activity when turbidity increases by 0.1/m (light attenuation coefficient =0.1-0.2 /m; Staehr & Wernberg, 2009).  An increase in water turbidity will likely affect the photosynthetic ability of Laminaria hyperborea and decrease its abundance and density (see IR.HIR.KFaR.LhypR.Pk). Kain (1964) suggested that early Laminaria hyperborea gametophyte development could occur in the absence of light. Furthermore, observations from south Norway found that a pool of Laminaria hyperborea recruits could persist growing beneath Laminaria hyperborea canopies for several years, indicating that sporophytes growth can occur in light-limited environments (Christie et al., 1998). However, in habitats exposed to high levels of suspended silts, Laminaria hyperborea is out-competed by Saccharina latissima, a silt tolerant species. Thus, a decrease in water clarity is likely to decrease the abundance of Laminaria hyperborea in the affected area (Norton, 1978). The biotope is expected to be excluded from silt rich environments.

Sabellaria spinulosa do not rely on light penetration for photosynthesis, and it is believed that visual perception is limited such that this species does not rely on sight to locate food or other resources. In a recent review of the sensitivity of Sabellaira spinulosa reefs to anthropogenic disturbance, Fariñas-Franco et al. (2014) concluded that impacts on Sabellaria spinulosa due to a decrease in water clarity resulting from an increase in suspended solids (inorganic or organic) are unlikely, although no thresholds regarding tolerance or intolerance were found. Decreases in suspended particles that reduce the supply of food or tube-building materials may, however, negatively impact this species (Davies et al., 2009; Last et al., 2011).

Sabellaria spinulosa relies on a supply of suspended solids and organic matter to filter feed and build protective tubes and so they are often found in areas with high levels of turbidity. Davies et al. (2009) and Last et al. (2011) developed Vortex Resuspension Tanks (VoRT) to test the effects of a change in the composition of suspended sediment on benthic species. This laboratory experiment manipulated turbidity and current flow and demonstrated the susceptibility of Sabellaria spinulosa to a decrease in suspended particulate matter (SPM). A clear erosion of tubes was observed in the absence of SPM and subsequent starvation of tube building materials. At high and intermediate sediment regimes (high SPM ~71 mg/l) conditions were comparable to what might be expected within only a few hundred meters distance of a primary aggregate extraction site and Sabellaria spinulosa maintained a cumulative growth rate at these rates of SPM. This supports the view that the availability of suspended particles is necessary for Sabellaria spinulosa development and that tolerance of elevated levels is likely (Davies et al., 2009). Indirect evidence for the tolerance of Sabellaria spinulosa for changes in turbidity is provided by the persistence of reefs on the outskirts of aggregate dredging areas (Pearce et al., 2007, 2011a), which appear unaffected by dredging that is likely to have led to sediment plumes.  Such plumes, however, are short-lived (Tillin et al., 2011) and, therefore, the long-term effect depends on the duration of dredging activities.

Lisco et al. (2023) investigated the tube building rate of Sabellaria spinulosa in the presence of different sediment grain sizes. The worms exhibited tube growth rates of 1 to 4 cm per month, and growth was faster when finer sediments were available (125 to 350 um) compared to coarser grains (350 to 500 um), where a maximum growth on  2 mm per month was recorded (Lisco et al., 2023). Therefore, any changes occur to suspended sediment, which decreases the availability of finer grains, could lead to lower growth rates, potentially hampering recovery of reefs after an impact.

Tillin (2010) used logistic regression to develop statistical models that indicate how the probability of occurrence of the congener Sabellaria alveolata changes over environmental gradients within the Severn Estuary. The modelled ranges should be interpreted with caution and apply to the Severn Estuary alone (which experiences large tidal ranges, high currents and extremely high suspended sediment loads and is therefore distinct from many other estuarine systems). The models indicated that the optimal mean neap sediment concentrations for subtidal Sabellaria alveolata ranged from 515.7 to 906 mg/l and optimal mean spring sediment concentrations ranged from 855.3 to 1,631 mg/l. Although not directly applicable to Sabellaria spinulosa this data suggests that tube-building sabellariids are tolerant to very high levels of suspended sediment. Fine sediments such as mud may clog the gills and feeding tentacles of some polychaetes and, therefore, the potential impact will be mediated by the character of the sediment in suspension.

Sensitivity assessment. The benchmark for this pressure refers to a change in turbidity of one rank, e.g. from clear (<10 mg/l) to intermediate (10 to 100 mg/l) or intermediate to medium (100 to 300 mg/l). Sabellaria spinulosa do not photosynthesise and do not rely on sight to locate resources and therefore no effects are predicted for reef biotopes from an increase or decrease in clarity resulting from a change in one rank on the water framework directive scale. Experiments (Davies et al., 2009) and predictive modelling (Tillin, 2010) indicate that tube building sabellariids can tolerate a broad range of suspended solids so the Sabellaria crusts are unlikely to be affected by an increase in suspended sediment. However, if the supply of sand to the habitat was reduced, then the Sabellaria crusts would probably erode and be replaced with a diverse understorey of red algae, similar to IR.MIR.KR.Lhyp. Similarly, an increase in turbidity will reduce the abundance of the Laminaria hyperborea canopy, especially in deeper examples of the biotope, and the Laminaria may be replaced by Saccharina latissima or by an abundant Sabellaria crust similar to CR.MCR.CSab.Sspi. Overall, the biotope is likely to change in character, be reclassified, and, effectively, lost. Therefore, resistance is assessed as ‘Low’, resilience as ‘Medium’, and sensitivity is assessed as 'Medium'.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event (Smothering pressure definition).

Evidence

Smothering by sediment (e.g. 30 cm of fine material) in a discrete event is unlikely to damage Laminaria hyperborea plants but is likely to affect gametophyte survival, holdfast communities, the epiphytic community at the base of the stipe, and interfere with zoospore settlement. Given the microscopic size of the gametophyte, 30 cm of sediment could be expected to significantly inhibit growth. However, laboratory studies showed that gametophytes can survive in darkness for between 6 and 16 months at 8°C and would probably survive smothering within a discrete event. Once returned to normal conditions the gametophytes resumed growth or maturation within 1 month (Dieck, 1993). Intolerance to this factor is likely to be higher during the peak periods of sporulation and/or spore settlement.  If clearance of deposited sediment occurs rapidly then understorey communities are expected to recover quickly. If inundation is long-lasting then the understorey epifauna/flora may be adversely affected, e.g. suspension or filter-feeding fauna and/or algal species.  However, this biotope is found in sand-laden waters (JNCC, 2015) so that the understorey of red algae and epifauna is limited in abundance.

Sabellaria spinulosa is often found in areas of high water movement with some degree of sediment transport essential for tube-building and feeding. Sabellaria spinulosa reefs adjacent to aggregate dredging areas appear unimpacted by dredging operations (Pearce et al., 2007; Pearce et al., 2011a).  Evidence suggests that the dynamic sedimentary environments in which sabellariids live, their populations can persevere in turbid conditions in spite of ‘typical’ natural levels of burial (Last et al., 2011) and that recovery from burial events is high.  The congener Sabellaria alveolata was reported to survive short-term burial for days and even weeks in the south-west as a result of storms that altered sand levels up to two meters, although they were, killed by longer-term burial (Earll & Erwin 1983).

Last et al. (2011) buried Sabellaria spinulosa worms (isolated into artificial tubes), under three different depths of sediment – shallow (2 cm), medium (5 cm) and deep (7 cm).  The results indicate that Sabellaria spinulosa can survive short-term (32 days), periodic burial by sand of up to 7 cm.  Last et al. (2011) suggested that the formation of ‘emergence tubes’ (newly created tubes extending to the surface) under sediment burial allowed Sabellaria spinulosa to tolerate gradual burial and that perhaps this mechanism allows for continued adult dispersal.  This mechanism occurred most rapidly throughout the 8-day burial at ~1 mm per day (Last et al., 2011) but even though tube-growth still seems possible under burial, it is likely that the dumping of fine and coarse material will block feeding apparatus and, therefore, worm development will be curtailed. 

A Sabellaria spinulosa reef off the coast of Dorset showed periodic burial from large sand waves (Collins, 2003).  The displacement of some colonies that had established themselves on a gas pipeline 1 km off the coast of Aberdeen was also associated with burial (Mistakidis, 1956; cited by Holt et al., 1998).  Furthermore the loss of a 2 km² area of Ross worm reef in Jade Bay, North Sea was attributed to burial as a consequence of mud deposition, although fishing activity may have contributed to the decline (Dörjes, 1992, cited from Hendrick et al., 2011). The evidence above suggests that Sabellaria spinulosa reefs are sensitive to damage from siltation events (Hendrick et al., 2011).  However, recovery is likely to be rapid given that larval dispersal is not interrupted and new reefs are likely to be able to establish themselves over old buried ones as postulated by (Fariñas-Franco et al., 2014).

Sensitivity assessment. No direct evidence was found for the length of time that Sabellaria spinulosa can survive beneath 30 cm of sediment.  Although this biotope occurs in moderate energy habitats (due to wave action), deposition of 30 cm of sediment represents a large volume of material that would probably remain for a number of tidal cycles and is expected to damage understorey flora/fauna as well as juvenile Laminaria hyperborea.   Similarly, burial to Sabellaria crusts to a depth 30 cm of fine sediment may also result in loss the abundance or extent of the crusts, based on the observations in Hendrick et al. (2011).  Therefore, as a precautionary assessment, resistance is assessed as ‘Low’ due to the depth of overburden. Hence, resilience is assessed as ‘Medium’ (2-10 years) and sensitivity as ‘Medium’.  

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline) (Litter pressure definition). 

Evidence

Not assessed.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT (Electromagnetic pressure definition).

Evidence

Evidence on the effect of electromagnetic fields (EMFs) on benthic organisms was severely lacking. No studies examining the effect of EMFs on macroalgae were found. Some studies have investigated the effect of anthropogenically induced EMFs on benthic invertebrates at intensities ranging between 2 nT and 40 mT, which is often much higher than in-situ measurements from subsea cables. While some report changes to behaviour, physiology, reproduction, development, immunology, cytotoxicity and orientation, others demonstrate no effect from exposure to the EMF (Albert et al., 2020; Hutchison et al., 2020), depending on the study species and duration and intensity of exposure. No studies investigating the effect of EMFs at the population or community level for benthic organisms were found.

Sensitivity assessment. Given the lack of data at the level of individual biotopes, resistance and resilience to EMFs cannot be robustly assessed. Sensitivity is therefore recorded as 'Insufficient evidence'.

Underwater noise changes

Benchmark. MSFD indicator levels (SEL or peak SPL) exceeded for 20% of days in a calendar year. Further detail

Evidence

Not relevant

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means (Introduced light or shade pressure definition).

Evidence

Light availability is a key environmental factor influencing the distribution, morphology, and productivity of Laminaria hyperborea. Several studies have demonstrated that biomass accumulation, canopy density, and morphological traits are positively correlated with light levels. For example, Smale et al. (2016) found that summer daytime light had a strong positive effect on canopy biomass and standing stock of carbon. Similarly, Smith et al. (2022) reported that percentage surface irradiance significantly predicted total and canopy density, canopy standing biomass, total fresh weight, blade fresh weight, and blade length in Laminaria hyperborea populations across the UK. In southern regions, blade width, total length, and age were also positively affected.

Stahl et al. (2024) conducted a study on the potential for Laminaria hyperborea afforestation in the German Bight, with a specific focus on light requirements and habitat suitability. Their study identified a minimum compensation irradiance of approximately 30 µmol photons/m²/sec under summer conditions. Monteiro et al. (2015) supported these findings, showing that over 75% of observed kelp species, including Laminaria hyperborea, occurred in areas where more than 3.65% of surface light reached the seafloor. These studies show that Laminaria hyperborea occurs in areas with moderate to high levels of light.

The effects of light on Laminaria hyperborea physiology can vary depending on temperature. For instance, at 10ºC, three different photoperiods: polar day (24:0 light:dark), long day (16:8 light:dark) and polar night (0:24 light:dark) had very little effect on photosynthetic efficiency, while 5ºC and 0ºC treatments had varied photosynthetic responses (Diehl et al., 2024). Photosynthetic efficiency declined significantly over 12 weeks in the polar day and long day photoperiods at 0ºC, but increased slightly with the polar night treatment. In the same time frame, photosynthetic efficiency declined significantly at 5ºC with the polar day treatment, but was relatively unchanged with the other two photoperiods. Other measured responses – such as dry weight, pigments, phlorotannins, and storage carbohydrates – all varied by light and temperature treatments. Cold and long light conditions significantly decreased chlorophyll a, accessory and xanthophyll pigments, which indicates a photoprotective stress response. In the 10°C treatment, these pigments either decreased or showed no change, suggesting that the relatively higher temperature mitigated light stress. Dry weight increased significantly, despite no measurable change in surface area, when the highest temperature (10°C) treatment was combined with moderate (16:8 h) and long (24:0 h) photoperiods. This increase in dry weight was not detrimental to the kelp and was likely due to the accumulation of storage carbohydrates rather than growth. No significant responses were observed in phlorotannin (compounds that protect against light stress) levels. Mannitol (a storage carbohydrate) decreased under the long night treatment, but this effect is expected and not detrimental to the kelp. Laminarin (the other storage carbohydrate that was measured) increased significantly under both light treatments and the two warmer treatments (5°C and 10°C), which is a positive metabolic response.

Shading of the biotope (e.g. by coastal development) could adversely affect the biotope in areas already low in water clarity. This may shift the balance toward shade-tolerant species, leading to loss of the biotope within shaded zones or a reduction in Laminaria hyperborea abundance, shifting from forest to park-type biotopes. Ecklonia radiata show a decrease of 50% photosynthetic activity when turbidity increases by 0.1/m (light attenuation coefficient = 0.1 to 0.2/m; Staehr & Wernberg, 2009). Therefore, any activity that decreases incident light (e.g. shading) may be detrimental.

While incident light has an overall positive effect on Laminaria hyperborea at optimal temperatures, the effects of artificial light on kelp are not yet fully understood. There is now a growing body of evidence to show that artificial light at night (ALAN) is widespread in the marine environment, with biologically relevant levels of light penetrating to depths of up to 50m (Davies et al., 2020; Smyth et al., 2021). ALAN has been shown to change the timing of Ascophyllum nodosum and Fucus serratus reproduction, with receptacles (the reproductive tissues of fucoid macroalgae) continuing to ripen into the winter months instead of peaking in the summer (Moyse et al., 2025). This change in the timing of reproduction could result in gametes being released during suboptimal conditions, such as winter storms, and therefore reduce fertilisation success. Reduced recruitment may lead to shifts in macroalgal assemblages in favour of species which are less sensitive to ALAN, such as Fucus vesiculosus, which seems to be unaffected (Moyse et al., 2025). ALAN can also vary significantly on small spatial scales and therefore affect some macroalgal forests more than others even if they are close to one another. It is therefore possible that ALAN could cause changes in macroalgal assemblages over time.

No studies have examined the effect of light or shading on Sabellaria spinulosa. As filter feeders, any shading of the biotope that reduces primary productivity and negatively impacts trophic cascades may result in a lack of food availability.

Sensitivity assessment. An increase in incident light is likely to increase plant productivity and increase the density of the kelps. Constant artificial light may affect the reproductive cues and recruitment in macroalgae, but no evidence was found specifically for Laminaria hyperborea. However, shading, especially from permanent structures (e.g. pontoons, jetties) are likely to reduce incident light and will probably result in the reduction in kelp density, or even its exclusion from the affected area. The effects of changing light levels on Sabellaria spinulosa is not known. Therefore, a resistance of 'Low' is suggested. Resilience is probably 'High' if the shading is temporary but 'Very low' if permanent. Therefore, a precautionary sensitivity of 'High' is suggested.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion (Barrier to species movement pressure definition).

Evidence

Not relevant. This pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit the dispersal of spores.  But spore dispersal is not considered under the pressure definition and benchmark.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure (Death for collision pressure definition).

Evidence

Not relevant. Collision from grounding vessels is addressed under abrasion above.

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature (Visual disturbance pressure definition). 

Evidence

Not relevant

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species may result in changes in the genetic structure of local populations, hybridization, or a change in community structure (Translocation pressure definition).

Evidence

None of the important structuring characteristic species (e.g. Laminaria hyperborea or Sabellaria spinulosa) are subject to genetic modification or translocation, at present. Therefore, this pressure is considered 'Not relevant'. 

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus) (pathogen or disease pressure definition).

Evidence

Galls on the blade of Laminaria hyperborea and spot disease are associated with the endophyte Streblonema sp. although the causal agent is unknown (bacteria, virus or endophyte). The resultant damage to the blade and stipe may increase losses in storms. The endophyte inhibits spore production and, therefore, recruitment and recoverability (Lein et al., 1991). However, no evidence was found for adverse impacts of microbial pathogens on Sabellaria spinulosa.   Therefore, resistance to the pressure is assessed as ‘Medium’, based on the possible effects on Laminaria hyperborea. However, resilience is probably ‘High’ so that sensitivity is assessed as ‘Low’.

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale (targeted removal pressure definition).

Evidence

Kelp harvesting can cause significant changes to this biotope through the removal of Laminaria hyperborea and the habitat space that it provides for its associated communities. Removing 26% of the canopy led to a 67% reduction in epiphytes and an 89% reduction of invertebrates (Norderhaug et al., 2020).

Christie et al. (1998) observed Laminaria hyperborea habitat regeneration following commercial Laminaria hyperborea trawling in south Norway. Trawling removed all large canopy-forming adult Laminaria hyperborea, however, sub-canopy recruits were largely unaffected. Within two to six years of harvesting, a new canopy had formed 1 m off the seabed. The associated holdfast communities recovered in six years. However, the epiphytic stipe community did not fully recover within the same period. In Nord-Trøndelag, Norway, Laminaria hyperborea was harvested in 2010 (Steen et al., 2016). Video surveys and plant sampling conducted two days prior to the trawling and in each year for the following four years, showed that Laminaria hyperborea coverage had returned to pre-harvest levels (around 94%). However, the new canopy, was significantly lower in density, average plant age, length, weight, and epiphyte biomass. In addition, the density of understorey recruits had only recovered by one-third by the end of the study period. It was suggested that 80% of the new canopy consisted of understorey plants that had survived the harvesting, and that the resilience of this biotope was dependent on the frequency of harvesting (Steen et al., 2016). Christie et al. (1998) suggested that kelp habitats were relatively resistant to direct disturbance/removal of Laminaria hyperborea canopy.

Recurrent disturbance on a timescale shorter than the two to six-year recovery period could prolong recovery. Kain (1975) cleared sublittoral blocks of Laminaria hyperborea at different times of the year for several years. The first colonizers and succession communities differed between blocks and the time of year they were cleared. However, within two years of clearance, the blocks were dominated by Laminaria hyperborea. Leinaas & Christie (1996) also observed Laminaria hyperborea recolonizing urchin barrens following urchin removal. The substratum was initially colonized by filamentous macroalgae and Saccharina latissima. However, Laminaria hyperborea dominated the community after two to four years.

Laminaria hyperborea forests subjected to regular harvesting support different associated communities compared to unharvested, preserved forests (Leclerc et al., 2015). Macroalgal species richness was consistently higher at the harvested site across all parts of the kelp and on the surrounding rock. Sessile fauna richness was slightly higher on the stipes and surrounding rock at the harvested site, but lower on the holdfast compared to the preserved site. In contrast, mobile fauna richness and density were generally greater on all parts of the kelp in the preserved site, although both were higher on the surrounding rock at the harvested site. Following disturbance, or in areas experiencing frequent disturbance, Laminaria hyperborea recruitment may be affected by interspecific competition with Non-Indigenous Invasive Species (INIS) or ephemeral algae (Brodie et al., 2014; Smale et al., 2013) (see INIS below).

Sabellaria spinulosa has no economic value and is not commercially harvested, therefore, not directly impacted by this pressure. 

Sensitivity assessment. Therefore, resistance is assessed as ‘None’, and resilience ‘Medium’ so that the sensitivity of this biotope to the removal of target species is assessed as ‘Medium’.

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale (non-targeted removed pressure definition).

Evidence

Incidental/accidental removal of Laminaria hyperborea from extraction of other marine resources, e.g. fisheries or aggregates, is likely to cause similar effects to that of direct harvesting of Laminaria hyperborea. Hence, the same evidence has been used for both pressure assessments. Kelp harvesting can cause significant changes to this biotope through the removal of Laminaria hyperborea and the habitat space that it provides for its associated communities. Removing 26% of the canopy led to a 67% reduction in epiphytes and an 89% reduction of invertebrates (Norderhaug et al., 2020).

Christie et al. (1998) observed Laminaria hyperborea habitat regeneration following commercial Laminaria hyperborea trawling in south Norway. Trawling removed all large canopy-forming adult Laminaria hyperborea, however, sub-canopy recruits were largely unaffected. Within two to six years of harvesting, a new canopy had formed 1 m off the seabed. The associated holdfast communities recovered in six years. However, the epiphytic stipe community did not fully recover within the same period. In Nord-Trøndelag, Norway, Laminaria hyperborea was harvested in 2010 (Steen et al., 2016). Video surveys and plant sampling conducted two days prior to the trawling and in each year for the following four years, showed that Laminaria hyperborea coverage had returned to pre-harvest levels (around 94%). However, the new canopy, was significantly lower in density, average plant age, length, weight, and epiphyte biomass. In addition, the density of understorey recruits had only recovered by one-third by the end of the study period. It was suggested that 80% of the new canopy consisted of understorey plants that had survived the harvesting, and that the resilience of this biotope was dependent on the frequency of harvesting (Steen et al., 2016). Christie et al. (1998) suggested that kelp habitats were relatively resistant to direct disturbance/removal of Laminaria hyperborea canopy.

Recurrent disturbance on a timescale shorter than the two to six-year recovery period could prolong recovery. Kain (1975) cleared sublittoral blocks of Laminaria hyperborea at different times of the year for several years. The first colonizers and succession communities differed between blocks and the time of year they were cleared. However, within two years of clearance, the blocks were dominated by Laminaria hyperborea. Leinaas & Christie (1996) also observed Laminaria hyperborea recolonizing urchin barrens following urchin removal. The substratum was initially colonized by filamentous macroalgae and Saccharina latissima. However, Laminaria hyperborea dominated the community after two to four years.

Laminaria hyperborea forests subjected to regular harvesting support different associated communities compared to unharvested, preserved forests (Leclerc et al., 2015). Macroalgal species richness was consistently higher at the harvested site across all parts of the kelp and on the surrounding rock. Sessile fauna richness was slightly higher on the stipes and surrounding rock at the harvested site, but lower on the holdfast compared to the preserved site. In contrast, mobile fauna richness and density were generally greater on all parts of the kelp in the preserved site, although both were higher on the surrounding rock at the harvested site. Following disturbance, or in areas experiencing frequent disturbance, Laminaria hyperborea recruitment may be affected by interspecific competition with Non-Indigenous Invasive Species (INIS) or ephemeral algae (Brodie et al., 2014; Smale et al., 2013) (see INIS below).

Sabellaria spinulosa biotopes may be removed or damaged by static or mobile gears that target other species. These direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures. Evidence for ecological interactions between Sabellaria spinulosa and other species is limited. The removal of Sabellaria spinulosa predators as by-catch may be beneficial. Sabellaria spinulosa reefs appear to be important nursery areas for commercially targeted flatfish including Dover sole (Bryony Pearce, pers comm). Assessment of this indirect effect is limited by the lack of empirical evidence for predator-prey relationships. Stomach analysis of fish by Pearce (2001) found that juvenile flatfish captured in reef areas including Dover sole, dab and plaice fed preferentially on Sabellaria spinulosa. Pearce et al. (2011b) found that butterfish Pholis gunnellus and dragonet Callionymus lyra also prey on Sabellaria spinulosa. Previous studies have also shown that Carcinus maenas feeds on Sabellaria spinulosa (Taylor, 1962; Bamber & Irving, 1997). Other invertebrates such as Pandalus montagui and Asterias rubens found in association with Sabellaria spinulosa reefs may also be feeding on the worms or other species associated with the reefs rather than Sabellaria spinulosa.

Due to the limited information available on predator-prey relationships, the impact of predator removal on Sabellaria spinulosa reef biotopes cannot be assessed. Dense aggregations of the brittle star, Ophiothrix fragilis, have been suggested to compete with Sabellaria spinulosa for space and food and potentially to consume the gametes inhibiting recruitment (George & Warwick 1985). Removal of this species as by-catch could potentially be beneficial to the reef biotopes.

Sensitivity assessment. Although the removal of predatory species by commercial fisheries may be beneficial to Sabellaria spinulosa, the accidental removal (e.g. as by-catch) of a proportion of the kelp bed and possibly the Sabellaria spinulosa crusts could impact the biotope. Therefore, resistance is assessed as 'Low' and resilience as ‘Medium’ so that sensitivity is assessed as 'Medium’.

The American slipper limpet, Crepidula fornicata

Evidence

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and the Essex coast in 1887-1890, and has spread into waters around mainland Europe (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 1999, 2018; Hinz et al., 2011; Helmer et al., 2019; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015). It ranges from the Baltic Sea, the Kattegat and Skagerrak, the North Sea coasts of the UK, Germany, and Belgium, through the English Channels and into the Irish sea coasts of Ireland and south Wales with records in east and west Scotland, Northern Ireland, northwest France, Spain and south into the Mediterranean (NBN, 2023; OBIS, 2025).

Abundances at its northern and southern extremes may be low but densities in UK and France are often over 1000 /m² and it may carpet the seafloor in the Solent and Essex. In the UK, it was reported to reach abundances of >1000 /m² (max. 2,748 /m²) in the Milford Harbour Waterway (Bohn et al., 2012), 84 /m² in Portsmouth, 174 /m² in Langstone and 306 /m² in Chichester harbours in 2017 (Helmer et al., 2019). In France, it has been reported to reach >4,700 /m² in the Bay of Marennes-Oleron, France, 11.6 tonnes/ha in Bay of Mont-Saint-Michel, 8.2 tonnes/ha in the Bay of Brest and 2.8 tonnes/ha in the Bay of Saint-Brieuc (Blanchard, 2009; Bohn et al., 2012, 2015; Powell-Jennings & Calloway, 2018).

Its density and ability to spread within and between sites (e.g., bays) depend on the availability of suitable habitat, competition with other species, larval retention within the site, human activities (e.g., dredging), and seasonal temperatures, particularly in the intertidal zone. For example, the Crepidula fornicata population in the Bay of Mont-Saint-Michel grew by 50% between 1996 and 2004, covering 25% of the area at high density (51–100% cover), aided by local oyster farming and shellfish dredging (Blanchard, 2009). However, in Arcachon Bay, France, Crepidula fornicata was limited to only 155 tonnes in 1999 and 312 tonnes in 2011 (De Montaudouin et al., 2001, 2018). It was confined to muddy sediments, which accounted for only approximately 8% of the bay and were colonized by Zostera beds. These areas represented just 0.4% of the suspension feeder biomass compared to the oysters Magallana gigas in the bay, and there was no indication of increasing biomass over a 12-year period. In addition, benthic trawling was prohibited in the bay (De Montaudouin et al., 2001, 2018). As a result, De Montaudouin et al. (2018) concluded that Crepidula fornicata was not invasive in the Bay of Arcachon.

Crepidula fornicata is recorded from shallow, sheltered bays, lagoons and estuaries or the sheltered sides of islands, in variable salinity (from 18 to 40 ppt) although it prefers around 30 ppt (Tillin et al., 2020). Larvae require hard substrata for settlement. It prefers muddy gravelly, shell-rich, substrata that include gravel, or shells of other Crepidula, or other species e.g., oysters, and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. It has also recorded from rock, artificial substrata, and Sabellaria alveolata reefs (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Helmer et al., 2019; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015; Tillin et al., 2020).

In the eastern Solent harbours of Portsmouth, Langstone, and Chichester, 75% to 98% of Crepidula larvae settled on dead Crepidula shells, while ~4% settled on stone, 2.5% on live Crepidula, 0.3% oyster shell, 0.6% cockle shell, 0.3% winkle shell and 0.1% perwinkle shell (Preston et al., 2020). In the Milford Harbour Waterway, the highest densities of Crepidula were found in areas of sediment with hard substrata, e.g., mixed fine sediment with shell, or gravel or both but, while Crepidula density increased as gravel cover increased in the subtidal, the reverse was found in the intertidal (Bohn et al., 2015). However, gravel formed the base of most stacks of Crepidula in the intertidal, which suggested that initial colonization occurred on available hard substrata (i.e., gravel) in the absence of adult shells of Crepidula. The availability of hard substrata (e.g., gravel) may only restrict initial colonization as higher densities of Crepidula functions as substrata for subsequent colonization (Thieltges et al., 2004; Blanchard, 2009). Bohn et al. (2015) also noted that Crepidula density was low in areas of homogenous fine sediment and absent in areas dominated by boulders.

Bohn et al. (2015) suggested that wave action (exposure) probably prevented the establishment of large numbers of Crepidula in high-energy areas. However, Hinz et al. (2011) recorded Crepidula off the Isle of Wight in the English Channel, at ~60 m on rough ground in areas of high tidal flow. Tillin et al. (2020) suggested that the effect of oscillatory wave meditated flow might have a greater effect on Crepidula than tidal flow, presumably due to mobilization of the substratum. Similarly, Crepidula was absent from sandy substrata in Swansea Bay but was most abundant in the shelter of the breakwater at Swansea east site (Powell-Jennings & Calloway, 2018).

Crepidula fornicata has been recorded from the lower intertidal to approx. 160 m in depth but it most common in the shallow subtidal and low water springs (Blanchard, 1997; Thieltges et al., 2003; Bohn et al., 2012, 2015; Hinz et al., 2011; OBIS, 2025; Tillin et al., 2020). Bohn et al. (2012, 2013a, 2013b, 2015) suggested that extreme conditions in the intertidal limited its upward distribution due to early post-settlement mortality. It reached its highest densities in the lower shore (below ~0.7 m) and was absent from high tidal level (~1.8 m) in the Milford Harbour Waterway (Bohn et al., 2015).

When the slipper limpet Crepidula fornicata settles in an area it can increase the amount of pseudofaeces and subsequently, the substratum may be altered from hard substratum to soft sediment which will reduce the substratum available for settlement by other species. This was observed when a 28 to 30% mortality of Mytilus edulis occurred after the introduction of Crepidula fornicata to mussel beds (Thieltges, 2005). Crepidula fornicata has been recorded in association with Sabellaria spinulosa reefs at Hastings Shingle Bank (up to 66 individuals per grab, Pearce, 2007) and in lower numbers in the East Coast REC area (maximum 4 per grab (Pearce et al., 2011a)). The relationship between Crepidula fornicata and Sabellaria spinulosa has not been investigated. However, potential impacts on Sabellaria spinulosa reefs could occur through changes to substratum suitability or other interactions.

Sensitivity assessment. There is currently a lack of evidence of Crepidula fornicata colonization on bedrock in the infralittoral or circalittoral. Tillin et al. (2020) suggested that Crepidula could colonize circalittoral rock due to its presence on tide-swept rough grounds in the English Channel (Hinz et al., 2011). However, Hinz et al. (2011) reported that Crepidula fornicata only dominated one assemblage (with an average of 181 individuals per trawl) on gravel substratum with boulders. Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas dominated by boulders, and Bohn et al. (2013a, 2013b, 2015) and Preston et al. (2020) showed that while Crepidula could settle on slate panels or ‘stone’ it preferred shell, especially that of conspecifics. No evidence was found for Crepidula fornicata currently impacting Sabellaria spinulosa reef biotopes, and the moderately wave exposed conditions characterizing this biotope, may mitigate or prevent the colonization by Crepidula at high densities, although Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011). At present, there is ‘Insufficient evidence’ to suggest that Laminaria hyperborea biotopes are sensitive to colonization by Crepidula fornicata.

The carpet sea squirt, Didemnum vexillum

Evidence

The carpet sea squirt Didemnum vexillum (syn. Didemnum vestitum; Didemnum vestum) is a colonial ascidian with rapidly expanding populations that have invaded most temperate coastal regions around the world (Kleeman, 2009; Stefaniak et al., 2012; Tillin et al., 2020). It is an ‘ecosystem engineer’ that can change or modify invaded habitats and alter biodiversity (Griffith et al., 2009; Mercer et al., 2009).

A lack of published descriptions and an incomplete historical record has led to the widespread misidentification of Didemnum vexillum, and it is often recorded as Didemnum spp. Hence, the native range of the species is not known conclusively (Lambert, 2009; Stefaniak et al., 2012; Mckenzie et al., 2017; Holt, 2024). However, molecular data and limited historical evidence have suggested that the species may be native to Japan, with its native range possibly extending into continental Asia and north-western Pacific (Stefaniak et al., 2012; Tillin et al., 2020; Holt, 2024). Previously unrecorded populations of a colonial ascidian have been recently identified as Didemnum vexillum (Tillin et al., 2020).

Didemnum vexillum has colonized and established populations in the north-east Pacific, Canadian and USA coast; New Zealand; France, Spain, and the Wadden Sea, Netherlands; the Mediterranean Sea and Adriatic Sea (Bullard et al., 2007; Coutts & Forrest, 2007; Dijkstra et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Lambert, 2009; Hitchin, 2012; Tagliapietra et al., 2012; Gittenberger et al., 2015; Vercaemer et al., 2015; Mckenzie et al., 2017; Cinar & Ozgul, 2023; Holt, 2024).

In the UK, Didemnum vexillum has colonized Holyhead marina and Milford Haven, Wales; the west coast of Scotland (marinas around Largs, Clyde, Loch Creran and Loch Fyne), South Devon (Plymouth, Yealm, and Dartmouth estuaries), the Solent, northern Kent, Essex, and Suffolk coasts (Griffith et al., 2009; Lambert, 2009; Hitchin, 2012; Minchin & Nunn, 2013; Bishop et al., 2015; Mckenzie et al., 2017; Tillin et al., 2020, Holt, 2024; NBN, 2024).

Didemnum vexillum has the ability to rapidly overgrow and displace on other sessile organisms such as other colonial ascidians (Ciona intestinalis, Styela clava, Ascidiella aspera, Botrylloides violaceus, Botryllus schlosseri, Diplosoma listerianium and Aplidium spp.), bryozoan, hydroids, sponges (Clione celata and Halichrondria sp.), anemone (Diadumene cincta), calcareous tube worms, eelgrass (Zostera marina), kelp (Laminaria spp. and Agarum sp.), green algae (Codium fragile subsp. fragile), red algae (Plocamium, Chondrus crispus and bush weed Agardhiella subulata), brown algae (Ascophyllum nodosum, Sargassum, Halidrys, Fucus evanescens and Fucus serratus), calcareous algae (Corallina officinalis), mussels (Mytilus galloprovincialis, Perna canaliculus and Mytilus edulis), barnacles, oysters (Magallana gigas, Ostrea edulis and Crassostrea virginica), sea scallops (Placopecten magellanicus), or dead shells (Dijkstra et al., 2007; Gittenberger, 2007; Valentine et al., 2007a; Valentine et al., 2007b; Griffith et al., 2009; Carman & Grunden, 2010; Dijkstra & Nolan, 2011; Groner et al., 2011; Hitchin, 2012; Tagliapietra et al., 2012; Minchin & Nunn, 2013; Gittenberger et al., 2015; Long & Groholz, 2015; Vercaemer et al., 2015).

Didemnum vexillum has been found colonizing the stipes of Laminaria spp. in the Gulf of Maine (Dijkstra et al., 2007) and in Norway (Legrand et al., 2025). However, it has not been recorded in sites exposed to wave action, that is, 'very wave exposed', 'wave exposed' and 'moderately wave exposed' (sensu MNCR, Hiscock, 1996), especially in the intertidal, where wave action is not ameliorated by depth (see Hiscock, 1983).

This species requires suitable hard substrata for successful settlement and the establishment of colonies. It can grow quickly and can establish large colonies of dense encrusting mats on a variety of hard substrata (Valentine et al., 2007a; Griffith et al., 2009; Lambert, 2009; Groner et al., 2011; Cinar & Ozgul, 2023). Mats can be up to several meters in area, covering large portions of the seafloor (Mercer et al., 2009). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems by its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock gravel, pebble, cobble, or boulders (Tillin et al., 2020). Didemnum vexillum has been reported colonizing these types of hard substrata in the USA, Canada, northern Kent and the Solent (Bullard et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Hitchin, 2012; Vercaemer et al., 2015; Tillin et al., 2020). It is therefore likely that the substrate in this biotope is suitable for Didemnum vexillum colonisation. In addition, the depth range at which Laminaria hyperborea biotopes are found (0 to 30 m) overlaps with the depth range that is suitable for suitable for Didemnum vexillum colonization. Didemnum vexillum has been recorded from less than 1 m to at least 81 m deep (Bullard et al., 2007; Tagliapietra et al., 2012; Tillin et al., 2020).

Reinhart et al. (2012) examined the effects of water flow and hydrodynamics on the encrusting and tendril forms of Didemnum vexillum. They reported that a current speed of approx. 7.6 m/s was required to induce fragmentation of tendrils, but that natural tidal flow alone was insufficient to cause fragmentation of tendrils. They suggested that rare instances of wave action such as storms that resulted in wave orbital velocities of ca 8 m/s or (more likely) human activity, could cause fragmentation of tendrils.

Reinhart et al. (2012) noted that the tensile strength of Didemnum vexillum was an order of magnitude higher than Botrylloides sp. and was similar to that of Alyconium digitatum. Alyconium digitatum is reported from sheltered to very wave exposed conditions, but in the sublittoral. Reinhart et al. (2012) also suggested that seasonal changes in the condition of Didemnum vexillum reduced the tensile strength of colonies and was associated with the period of greater larval production, and implied that fragmentation aided dispersal.

The oscillatory nature of wave-mediated water flow (wave orbital velocities) combined with wave pressure in the lacerating zone, where breaking wave causes multidirectional strong water movement (Hiscock, 1983), would probably dislodge and break up Didemnum vexillum colonies, prevent them from forming suffocating mats, and restrict the colonies to crevices and overhangs. However, it is unclear if moderately wave exposed conditions would be adequate to prevent Didemnum vexillum from developing extensive mats in the summer months when wave action is typically reduced. Hitchin (2012) suggested that the presence of Didemnum vexillum in Whitstable, Kent, was contrary to its then known habitat preferences.

Sensitivity assessment. There is no evidence of Didemnum vexillum colonizing this biotope in the UK. However, it has been recorded in similar kelp habitats in Norway (Järnegren et al., 2023). Didemnum vexillum requires hard substrata for successful colonization, therefore, it could colonize the bedrock and boulders that characterize this biotope. Didemnum vexillum can overgrow sessile organisms, including kelp Laminaria sp. However, no direct evidence was found on how Didemnum vexillum affects kelp or if it contributes to Laminaria sp. mortality (Järnegren et al., 2023), although epifaunal growth by Membranacea membrancea was reported to reduce the physical strength of kelp fronds (inc. Laminaria digitata) and make them susceptible to removal by wave action (Krumhansl et al., 2011). In addition, overgrowth by epiphytes contributed to the decline of Saccharina latissima in Norway (Andersen et al., 2011). However, Didemnum vexillum may compete for light and space with kelp and epifauna and could interfere with recruitment, which could lead to the mortality of some epifauna, the loss of kelp, and a reduction in biodiversity. If Didemnum sp. could gain a 'foothold' it might overgrow, smother or cause mortality on the Sabellaria spinulosa encrustations. Didemnum prefer sheltered conditions so the wave exposed conditions that characterize this biotope may mitigate its abundance. Therefore, a resistance of 'Medium' (some mortality, <25%) is suggested as a precaution. Resilience is likely to be 'Very low' as Didemnum vexillum would need to be physically removed to allow recovery. Hence, sensitivity to invasion by Didemnum is assessed as 'Medium'. However, confidence in the assessment is ‘Low’ due to the lack of direct evidence of damage to kelp beds.

The Pacific oyster, Magallana gigas

Evidence

The Pacific oyster, Magallana (syn. Crassostrea) gigas, is native to warm temperate regions from the northwest Pacific to Japan and northeast Asia, including Cape Mariya (Russia) to Hong Kong (China) (Carrasco & Baron, 2010; GBNNSIP, 2011, 2012). It is a fast-growing and tolerant species that has become a successful invader in the coastal waters of all continents, aside from Antarctica (Wrange et al., 2010; Carrasco & Baron, 2010; Padilla, 2010). 

It was initially introduced for aquaculture in Europe and the UK in the 1960s due to a decline in the Portuguese oyster (Crassostrea angulata) and the European flat oyster (Ostrea edulis) (Spencer et al., 1994; GBNNSIP, 2011b, 2012a; Humphreys et al., 2014, cited in Alves et al., 2021; Hansen et al., 2023). It was also introduced to the northeast Adriatic Sea (Ezgeta-Balic et al., 2019) and southwest England from France, possibly via fouling on ships (GBNNSIP, 2011b, 2012a; Padilla, 2010; Ezgeta-Balic et al., 2019).

Magallana gigas has a high fecundity, a long-lived pelagic larval phase (2 to 4 weeks) and can produce up to 200 million eggs during spawning (Herbert et al., 2012, 2016; Alves et al., 2021; Wood et al., 2021; Hansen et al., 2023). Hence, as a broadcast spawner, it has a high dispersal potential of more than 1000 km (Padilla, 2010; Wood et al., 2021). Although larval mortality can be as large as 99% due to sensitivity to environmental conditions (Alves et al., 2021), adults are long-lived so that populations can survive with infrequent recruitment (Padilla, 2010).

Larval dispersal has facilitated the establishment of populations in various regions, such as the Oosterschelde estuary in the Netherlands and the Scandinavian coastlines, where northward drift on tidal and wind-driven currents has been suggested (Hansen et al., 2023). Offshore structures and aquaculture operations can enhance spread (Wood et al., 2021).

Magallana gigas is an ecosystem engineer and can dramatically change habitat structure when it invades. Once successfully settled, groups of Pacific oysters may form dense aggregations, potentially forming a reef, which in some regions can reach densities of 700 individuals/m² (Herbert et al., 2012, 2016). Once, the density of live or dead Pacific oysters reaches or exceeds 200 ind./m², little of the underlying substratum remains visible (Herbert et al., 2016). However, Hansen et al. (2023) suggested that no immediate ecosystem risk is observed where the Pacific oyster occurs sporadically.

Settlement requires hard substrata, including rock, bedrock, chalk, bare boulders, cobbles and pebbles and shells (Kochmann et al., 2012, 2013; Mckinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020). Magallana gigas also attaches to available hard materials in mixed sediment environments such as shingle and sand within otherwise unsuitable mudflats (Spencer et al., 1994; Mckinstry & Jensen, 2013; Tillin et al., 2020).

Populations of Magallana gigas have been found on wave-exposed rocky shores to wave-sheltered soft sediment environments and it has been described as a habitat generalist (Troost, 2010; Kochmann et al., 2012, 2013). For example, in Scotland, wild Magallana gigas are mainly located in the lower intertidal on bedrock, bedrock encrusted with barnacles, within bedrock crevices, and large and small boulders (Cook et al., 2014). Patches of Pacific oyster reefs have been recorded on littoral rock in Kent, southern England and on littoral sediments in southern England, the North Sea, and the English Channel (Herbert et al., 2012, 2016; Morgan et al., 2021).

Magallana gigas has been reported from estuaries growing on intertidal mudflats and sandflats, and other soft sediments (Padilla, 2010; Herbert et al., 2016; Cabral et al., 2020). The settlement of spat on hard substrata within sediments has been observed in the estuaries of the River Dart, Exe, Fal, Fowey, Tamar, Teign, and Yealm in Devon and Cornwall, the Menai Straits, Wales and large estuaries of Lough Swilly, Lough Foyle and the Shannon in Ireland, and the Tagus Estuary in Portugal (Spencer et al., 1994; Kochmann et al., 2012, 2013; Cabral et al., 2020). In Lough Swilly, Lough Foyle and the Shannon, the Pacific oyster was often associated with intertidal mud or sandflats (Kochmann et al., 2013). In contrast, the Pacific oysters were absent from sandflat areas in Poole Harbour (Mckinstry & Jensens, 2013).

While often described as an intertidal and shallow subtidal species, Magallana gigas has been observed across a broader depth range. Although rocky habitats deeper than 10 m are generally considered unsuitable, it has been recorded down to 42 m in the Oosterschelde, Netherlands (Herbert et al., 2012, 2016; Tillin et al., 2020; Smaal et al., 2009).

It frequently occurs between Mean High Water and Mean Low Water in intertidal zones but has also been recorded at 1 to 10 m depth in regions like Sweden, Ireland, and the UK (Kochmann et al., 2013; Herbert et al., 2016; Bergstrom et al., 2021). In Lough Swilly and Lough Foyle, Ireland, oysters were found on shallow subtidal mussel beds and mixed mud and sand habitats (Kochmann, 2012). In the Thames Estuary and parts of Essex and Kent, oysters have also been found subtidally, 2–3 m below chart datum (Tillin et al., 2020).

Bergstrom et al. (2021) suggested the optimal depth in the Skagerrak is around 0.5 m, although presence is documented down to 5 m. In Lim Bay (Adriatic Sea), M. gigas occurs in the intertidal and shallow subtidal (down to 1 m), but not beyond 3 m depth (Stagličić et al., 2020). The species has not been recorded below extreme low water on rocky habitats, although it has been found subtidally on soft sediments in some areas (Herbert et al., 2012).

The Pacific oyster prefers wide intertidal areas with shallow gradients; it is generally absent from steep shores (McKinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020). In Ireland and the Solway Firth, it is more commonly found on intertidal shores over 40–50 m wide (Kochmann et al., 2013; Cook et al., 2014).

It has been suggested that recruitment is enhanced, and abundances are higher in wave-sheltered conditions (Robinson et al., 2005; Ruesink, 2007 cited in Teschke et al., 2020; Tillin et al., 2020). Teschke et al. (2020) found the abundance of Magallana gigas was significantly higher at wave-protected sites within the artificial harbours of Helgoland, North Sea, compared to wave exposed sites outside the harbours. The authors suggested that the successful colonization in wave-protected sites could be due to the relative retention of water masses in the harbours that reduces larval drift and whiplash effect on newly settled larvae. In addition, better growth and higher survival rates were observed at wave-protected sites, whereas mortality rates increased at wave exposed sites, due to the wave exposure causing dislodgement or detachment from the settlement substratum (Teschke et al., 2020; Tillin et al., 2020). Similarly, Bergstrom et al. (2021) noted that the occurrence of high densities of both Ostrea edulis and Magallana gigas decreased with increasing wave exposure.

Carrasco & Baron (2010) suggested that Magallana gigas has successfully adapted to colonize a range of thermal niches. Temperature is important for the life cycle of the Pacific oyster and influences the establishment of feral and wild populations (Alves et al., 2021). Within its native range, Magallana gigas occurs in areas where the sea surface temperatures range from 14.0°C to 28.6°C in the warmest month of the year, and between -1.9°C and 19.8°C in the coldest month (Carrasco & Baron, 2010).

Magallana gigas has a seasonal reproductive cycle (Alves et al., 2021). Spawning occurs in the summer months, when temperatures are 16 to 34°C and larvae require a water temperature of 18°C or above for successful development (Mann 1979; Troost, 2010; Kochmann, 2012; Ezgeta-Balic et al., 2020; Alves & Tidbury, 2022). In Poole, UK, spawning temperatures were estimated at 19.7°C (Alves & Tidbury, 2022). Ezgeta-Balic et al.‘s (2020) study indicated that temperatures in the Mediterranean and the Adriatic were favourable for Pacific oyster larval development, with gametogenesis initiated at temperatures from around 10 to 15°C and spawning initiated at around 24°C. However, the lower thermal limit for spawning was recognized as 16°C (Carrasco & Baron, 2010) and once settled, larvae are unable to survive in temperatures below 3°C (Alves & Tidbury, 2022).

Dense macroalgal cover is unsuitable for the Magallana gigas (Herbert et al., 2012, 2016; Tillin et al., 2020), being rarely found under macroalgal cover in Northern Ireland, absent from exposed bedrock or large boulders with macroalgae cover in the Solway Firth, Scotland, and absent in Poole Harbour, where there was competition with macroalgae (Kochmann et al., 2012, 2013; Mckinstry & Jensen, 2013; Cook et al., 2014; Tillin et al., 2020). Fucus cover significantly reduced larval recruitment of the Pacific oyster in the Wadden Sea (Diederich, 2005). Hence, the Pacific oyster is more likely to colonize bare rock, boulders, or mussel beds without macroalgae (Diederich, 2005; Cook et al., 2014). Kochmann et al. (2013) suggested that macrophyte canopies prevent larvae from settling on the rock underneath, and macroalgae fronds inhibit settlement and recruitment by exuding metabolites.

While no evidence of Magallana gigas interacting with Sabellaria spinulosa has been reported, this species has been observed colonizing littoral intertidal biogenic reefs formed by Mytilus edulis and Sabellaria alveolata (GBNNSS, 2011, 2012; Kochmann, 2012; Kochmann et al., 2013; Herbert et al., 2016; Tillin et al., 2020). The colonization and overgrowth of Magallana gigas may have impacts on Sabellaria alveolata and its habitat formation (Herbert et al., 2012, 2016). Pacific oysters may smother Sabellaria alveolata because it grows over tube ends and could outcompete it for space (Dubois et al., 2006; Desroy et al., 2011). Colonization by the Pacific oyster has been linked to the degradation and deterioration of Sabellaria reef health (Desroy et al., 2011). Desroy et al. (2011) reported several contributing factors, including an increase in silt deposits and fine particles in the sediment from pseudo-faeces produced by the oysters, which can cause increased sedimentation and nutrient enrichment (Green & Crowe, 2013). It has been suggested the increased sediment from oysters might explain why some species normally found in muddy-sand environments were present, further creating new species associations (Dubois et al., 2006).

Dubois et al. (2006) found that Magallana gigas had invaded some Sabellaria alveolata reefs in the Bay of Mont-Saint-Michel, France, resulting in densities of more than 100 oysters /m² on some of them. In this area, Sabellaria alveolata reefs were the only available hard substratum for settlement by the Pacific oyster. The study found that an intermediate covering of the Pacific oyster introduced greater species richness and heterogeneity of diversity on the Sabellaria reefs by creating hard substrata habitats and refuges for sessile or mobile species not usually present (Dubois et al., 2006). Green & Crowe (2013) found a lower percentage cover of Sabellaria alveolata on boulders colonized by Magallana gigas in Ireland. 

Pacific oysters might improve the recruitment of Sabellaria alveolata by increasing the probability of Sabellaria larvae swimming or sinking down the water column (Tillin et al., 2020). However, Dubois et al. (2006) reported that the abundance of smaller class sizes of Sabellaria alveolata was reduced in Sabellaria reefs with epibionts (Pacific oyster or Ulva spp.) indicating negative impacts of Pacific oysters on recruitment, although not as marked as in the presence of algae (Padilla, 2010; Tillin et al., 2020). Secondary impacts have also been reported, including increased recreational harvesting of the oysters on Sabellaria reefs, which led to reef trampling, physical damage and fragmentation (Dubois et al., 2006; Desroy et al., 2011). In the northern part of Bourgneuf Bay, France Magallana gigas was observed in rocky areas usually occupied by Sabellaria alveolata (Cognie et al., 2006; Herbert et al., 2012, 2016). Cognie et al. (2006) suggested that Magallana gigas could compete with Sabellaria alveolata for food and space, leading Herbert et al. (2012) to suggest that the Pacific oyster may prevent new colonization by Sabellaria alveolata.

Sensitivity assessment. While most of the evidence suggests the environmental conditions within this biotope are suitable for Magallana gigas, and the Sabellaria spinulosa encrustations may provide attachment substrata, it is unlikely that they would be able to colonize this biotope without the removal of the kelp canopy. In addition, populations may be limited to low densities due to moderately wave exposed, soured conditions. Therefore, resistance is assessed as ‘High’, resilience as 'High', and this biotope is assessed as ‘Not Sensitive’ to this pressure.

Wireweed, Sargassum muticum

Evidence

Sargassum muticum has been shown to competitively replace Laminaria spp. in Denmark (Staehr et al., 2000). In Nova Scotia, Codium fragile competes successfully with native kelps for space including Laminaria digitata, exploiting gaps within the kelp beds. Once established the algal mat created by Codium fragile prevents re-colonization by other macro-algae (Scheibling et al., 2006).

Sargassum muticum is a circumglobal invasive species (Engelen et al., 2015). It is recorded (2015) from Norway to Morocco and into the Mediterranean in the eastern Atlantic, from Alaska to Baja California in the eastern Pacific, and from southern Russia to southern China in the western Pacific (Engelen et al., 2015). It colonizes a variety of habitats and can tolerate -1°C to 30°C and survive salinities below 10 ppt. Although fertilization does not occur below 15 ppt and growth of germlings is limited below 10°C, it can complete its life cycle as long as temperatures are over 8°C for at least four months of the year (Engelen et al., 2015). However, its distribution is limited by the availability of hard substratum (e.g., stones >10 cm) and light (Staeher et al., 2000; Strong & Dring 2011; Engelen et al., 2015). It is most abundant between 1 and 3 m below mean water, but it has been recorded at 18 m or 30 m in the clear waters of California. However, it is a poor competitor under low light and only develops dense canopies in shallow areas (Engelen et al., 2015).

Sargassum muticum was shown to replace and out-compete leathery, canopy-forming macroalgae such as Saccharina latissima, Halidrys siliquosa, and Fucus spp. and, to a lesser degree, understorey species such as Codium fragile, Chondrus crispus and Dictyota dichotoma in Limfjorden, Denmark between 1984 and 1997 (Staehr et al., 2000; Engelen et al., 2015; de Bettignies et al., 2021). The invasion in Limfjorden had stabilized by 2005 although many of the native macroalgal species continued to decline (Engelen et al., 2015). In Limfjorden, the distribution of Sargassum muticum was limited to areas with hard substratum, in particular stones >10 cm in diameter, while smaller stones, gravel and sand were unsuitable. It was most abundant between 1 and 4 m in depth but had low cover at 0 to 0.5 m or 4 to 6 m, in the turbid waters of the Limfjorden. Limfjorden is wave sheltered although wave exposure has been reported to restrict the growth and survival of Sargassum muticum (Staehr et al., 2000). Viejo et al. (1995) reported that Sargassum muticum transplanted to wave exposed shores in Spain experienced > 80% breakages within a month, and that the growth of undamaged plants was significantly lower than that of plants on sheltered shores. Similarly, Andrew & Viejo (1998) noted that Sargassum muticum was restricted to intertidal rockpools in wave exposed sites in the Bay of Biscay.

Strong & Dring (2011) used canopy removal experiments to investigate inter- and intra-species competition between Sargassum muticum and Saccharina latissima in the Dorn, Strangford Lough, N. Ireland. The Dorn consists of tidal pools, very sheltered from wave action but with moderately strong tidal streams (1 to 2 knots). Sargassum muticum grew better in mixed stands with Saccharina latissima than in the highest density monospecific stands examined. However, the growth of Saccharina was not affected by the proportion of Sargassum in mixed stands. They concluded that Saccharina was not impacted significantly by the alien species while Sargassum benefited from growth in mixed stands. Experimental manipulation of subtidal algal canopies in the San Juan Islands, Washington State, USA, showed that Sargassum muticum reduced the abundance of native macroalgae, including the kelp Laminaria bongardiana due to shading. However, the experimental removal of Sargassum resulted in the recovery of native species within about 1 year (Britton-Simmons, 2004; Engelen et al., 2015). The negative effects of Sargassum muticum on native macroalgae are mainly due to competition for light, rather than changes in nutrient availability, sedimentation, or water flow (Britton-Simmons, 2004; Engelen et al., 2015). Cosson (1999) reported a significant decline in Laminaria digitata at two sites between 1983 and 1997 on the coast of Normandy, France, due to an increase in Sargassum muticum abundance in the same areas. For example, on the Grandcamp rocks, Laminaria digitata has almost disappeared while Sargassum muticum had covered 80% of the lower intertidal and subtidal zone in summer.

This biotope (IR.MIR.KR.Lhyp.Sab) is found within the infralittoral zone with moderate exposure to wave action and weak tidal streams. The evidence above suggests that Sargassum muticum prefers wave sheltered, shallow sites in the sublittoral fringe. It was reported to out-compete and replace Saccharina latissima in the Limfjorden and achieve maximum abundance between 1 and 4 m (Staehr et al., 2000; Engelen et al., 2015) but no evidence of the effects of Sargassum on Laminaria hyperborea beds was found. However, Sargassum is unlikely to survive in this biotope due to the wave exposed conditions within this biotope, and due to the scour, it will be unable to gain a foothold.

Sensitivity assessment. The evidence above suggests that Sargassum muticum prefers wave sheltered, shallow sites in the sublittoral fringe. No evidence of the effects of Sargassum on Laminaria hyperborea beds was found. Therefore, competition with Sargassum is probably site-specific and dependent on local conditions, so it is unlikely that Sargassum will be able to colonize and survive in this biotope due to the moderate wave exposure and scour that characterizes this biotope. Therefore, resistance is assessed as ‘High’, resilience as 'High', and sensitivity is assessed as ‘Not Sensitive’. Overall, confidence is assessed as ‘Low’ due to evidence of variation and the site-specific nature of competition between native kelps and Sargassum muticum. 

Wakame, Undaria pinnatifida

Evidence

Undaria pinnatifida (Wakame or Asian kelp) is a large brown seaweed and an Invasive Non-Indigenous Species (INIS) that could out-compete native UK kelp species (see Farrell & Fletcher, 2006; Thompson & Schiel, 2012; Brodie et al., 2014; Heiser et al., 2014; Arnold et al., 2016; Epstein & Smale, 2017; Epstein & Smale, 2018; Kraan, 2017; Epstein et al., 2019a,b; Tidbury, 2020). Undaria pinnatifida originates from Japan but is established currently on the coastlines of New Zealand, Australia, Northern France, Spain, Italy, the UK, Portugal, Belgium, Holland, Argentina, Mexico, and the USA (De Leij et al., 2017). Undaria pinnatifida was first recorded in the UK in the Hamble Estuary in 1994 (Macleod et al., 2016). It has since proliferated along UK coastlines. One year after its discovery at the Queen Anne Battery marina, Plymouth, it became a major fouling plant on pontoons (Minchin & Nunn, 2014). Although initially restricted to artificial habitats, such as marinas and ports, it is now widespread in natural habitats in several areas, including Plymouth Sound.

Undaria pinnatifida seems to settle better on artificial substrata (e.g., floats, marinas, or piers) than on natural rocky shores among local kelps (Vaz-Pinto et al., 2014). It is found predominantly in low intertidal to shallow subtidal habitats (Epstein et al., 2019b) and is significantly more abundant on artificial substrata compared to natural rocky substrata (Heiser et al., 2014; Epstein & Smale, 2018). James (2017) suggested that Undaria pinnatifida could out-compete native species on artificial substrata (such as marinas and wharf structures). In Plymouth, UK, De Leij et al. (2017) found that natural habitats with dense native macroalgal canopies, such as Laminaria hyperborea, Laminaria ochroleuca, Laminaria digitata and Saccharina latissima had more resistance to Undaria pinnatifida invasion than disturbed or sparse canopies, due to limited space and light availability for Undaria pinnatifida recruits. However, the dense canopies did not always prevent invasion of Undaria pinnatifida as sporophytes were still recorded within dense Laminaria canopies, so canopy disturbance was not always required (De Leij et al., 2017; Epstein & Smale, 2018).

Undaria pinnatifida species behaves as a winter annual and recruitment occurs in winter followed by rapid growth through spring, maturity and then senescence through summer, with only the microscopic life stages persisting through autumn. It exhibits multiple dispersal strategies, such as short-range spore dispersal, and long-range dispersal as whole drift plants or fragments. Undaria pinnatifida has spread rapidly across the UK and Europe, resulting in community-wide responses and impacts (Vaz-Pinto et al., 2014; Epstein & Smale, 2017). Its impacts are complex and context-specific, depending on space, time, and taxa present in the introduced location (Epstein & Smale, 2017; Teagle et al., 2017; Tidbury, 2020).

Undaria pinnatifida has a wide physiological niche meaning it can occur in both coastal and estuarine environments showing tolerance for varying salinities, turbidity, and siltation (Heiser et al., 2014; Epstein & Smale, 2018). Undaria pinnatifida prefers sheltered sites with low wave exposure and weak tidal streams (Heiser et al., 2014; Epstein & Smale, 2018). In natural habitats, Undaria pinnatifida was not recorded if the wave fetch was greater than 642 km but increased in abundance and cover in very sheltered sites (Epstein & Smale, 2018).

In St Malo, France, there was evidence that Undaria pinnatifida could co-exist with Laminaria digitata under certain conditions (Castric-Fey et al., 1993). Epstein et al. (2019b) observed that, in Plymouth Sound, UK, Undaria pinnatifida co-existed with seven species of canopy-forming brown macroalgae within its depth range (+1 to –4 m), including Laminaria digitata; which suggested that they could occupy an overlapping niche. Epstein & Smale (2018) also observed that Undaria pinnatifida was relatively common (abundance of > 70 individuals per 25 m transect) at three sites in Devon, UK (Jennycliff, Bovisand and Beacon Cove) where Laminaria spp. were abundant (40 to 79%) or superabundant (> 80%), which suggested that Undaria pinnatifida could co-exist within refugia amongst areas with dense Laminaria spp..

In many cases, Undaria pinnatifida seems to have minimal impacts on native communities (e.g., Forrest & Taylor, 2002; Valentine & Johnson, 2003; South et al., 2016; Epstein & Smale, 2017; Epstein & Smale 2018). Laminaria digitata forms dense monospecific canopies and its thick, extensive laminae are likely to restrict light penetration to the underlying substratum, including Undaria pinnatifida (De Leij et al., 2017). Disturbance to the native kelp canopy can facilitate the spread of INIS by increasing the availability of resources such as light and space. Experimental full removal of the existing kelp canopy in Plymouth Sound allowed the mean cover of Undaria pinnatifida to increase from ca 10% to ca 50% within three months (De Leij et al., 2017). Their experiment showed that the density of Laminaria digitata was important to Undaria pinnatifida invasion (De Leij et al., 2017).

Similarly, a primary succession experiment by Epstein et al. (2019b) in Plymouth Sound (UK) showed that clearance of Laminaria digitata in 2016 increased Undaria pinnatifida abundance. However, this was quickly followed by the recovery of Laminaria digitata in 2017 and the concurrent decline in Undaria pinnatifida, which suggested that Laminaria digitata had a higher fitness (Epstein et al., 2019b). Within the same study, Epstein et al. (2019b) observed that Undaria pinnatifida exhibited a significant negative relationship with Laminaria digitata on intertidal rocky reef substrata, which suggested that Laminaria digitata negatively affected Undaria pinnatifida abundance. It was also suggested that Undaria pinnatifida has a lower resistance to desiccation than Laminaria digitata. As a result, Epstein & Smale (2019b) concluded that due to its lower fitness, Undaria found within natural habitats in the northeast Atlantic has low ecological and community level impacts and was competitively inferior to Laminaria spp. However, Heiser et al. (2014) found that in Plymouth, UK, Laminaria digitata was significantly less abundant at sites with the presence of Undaria pinnatifida, with only ca 1.5 Laminaria digitata individuals per m² with Undaria pinnatifida, compared to ca 7 individuals per m² at sites without Undaria pinnatifida. Undaria pinnatifida was successfully eradicated on a sunken ship in Chatham Islands, New Zealand, by applying a heat treatment of 70 °C (Wotton et al., 2004). However, numerous other eradication attempts have failed and as noted by Fletcher & Farrell (1998), once established Undaria pinnatifida resists most attempts at long-term removal.

Sensitivity Assessment. Undaria pinnatifida has the potential to colonize and co-exist in refugia within Laminaria hyperborea habitats, especially in shallow examples of their biotopes that are within its depth range (1 to 4 m) and sheltered from wave action. A dense kelp canopy may restrict or slow the proliferation of Undaria pinnatifida, however, there is mixed evidence of its colonization with Laminaria hyperborea beds and in some areas, a lower abundance of Laminaria hyperborea may result in increased Undaria pinnatifida growth. It is unlikely that Undaria pinnatifida will be able to colonize or survive the degree of wave exposure and sand scour that characterize this biotope. Therefore, resistance is assessed as ‘High’, resilience as 'High', and sensitivity is assessed as ‘Not Sensitive’. Overall, confidence is assessed as ‘Low’ due to evidence of variation and the site-specific nature of competition between native kelps and Undaria pinnatifida.

Other INIS

Evidence

The golden kelp Laminaria ochroleuca. This species is a warm-temperate Lusitanian kelp with a distribution ranging from Morocco to the south of the UK. It was first recorded in the southwest UK in 1946 (Parke, 1948) and is projected to expand further northwards under future climate change scenarios (Franco et al., 2018). A small population was recorded in northwest Ireland in 2018 (Schoenrock et al., 2019), further suggesting ongoing poleward expansion. While not considered a traditional invasive species, its northward expansion into the UK has led to competition with Laminaria hyperborea. In Plymouth Sound, southwest UK, estimates of Laminaria ochroleuca standing stock are now comparable to those of Laminaria hyperborea (Taylor-Robinson et al., 2024; also see Smale et al., 2016 for standing stock of Laminaria hyperborea).

It is suggested that Laminaria ochroleuca may have a competitive advantage over Laminaria hyperborea due to its tolerance of warmer waters. Barrientos et al. (2025) investigated changes in kelp forests in northwest Spain between 1997 and 2023. They found that kelp forests had disappeared or severely declined in density at 29 of 50 sites, and the canopy was now dominated by Laminaria ochroleuca at the surviving sites, while Laminaria hyperborea is almost entirely absent, occurring at only two sites. These changes were linked to sea surface temperature (an average increase of 0.01 to 0.02°C per year over the 26-year study period), which suggested that Laminaria ochroleuca was more resistant to warming and could, therefore, outcompete Laminaria hyperborea under global warming scenarios.

There is contrasting evidence on the relative resilience of Laminaria ochroleuca and Laminaria hyperborea to storm damage. Pereira et al. (2017) reported no recovery of Laminaria hyperborea populations in the two years following a storm in northern Portugal, whereas Laminaria ochroleuca showed partial recovery. In contrast, Smale & Vance (2015) found that Laminaria hyperborea was highly resistant to severe storms in the UK during the 2013 to 2014 winter season. The breakage of mature Laminaria hyperborea stipes ranged between 2.3 and 6.9%, while broken Laminaria ochroleuca stipes were on average 8.7 times more prevalent. Given this conflicting evidence, it remains unclear whether Laminaria ochroleuca biotopes could displace Laminaria hyperborea biotopes following storm events.

Another potential advantage of Laminaria ochroleuca is its greater average stipe length compared to Laminaria hyperborea, potentially reducing light availability for Laminaria hyperborea recruits in mixed-population forests (Smale et al., 2014). This shading effect may exaggerate the impacts of marine heatwaves on Laminaria hyperborea, as elevated temperatures increase metabolic demands that cannot be met under light-limited conditions (Bass et al., 2023).

The introduction of Laminaria ochroleuca into Laminaria hyperborea forests can have negative impacts on biodiversity. Kelp stipe assemblages differ significantly between the two species due to the texture of the stipe. Laminaria hyperborea stipes are rough and pitted and, therefore, have a larger surface area, while Laminaria ochroleuca stipes are uniformly smooth. Teagle & Smale (2018) found species from up to 15 different taxonomic groups on Laminaria hyperborea stipes in spring, compared to 2 taxa at most on Laminaria ochroleuca stipes all year round. In addition, the biomass of Laminaria hyperborea stipe assemblages was >3600 more than Laminaria ochroleuca stipe assemblages. Therefore, the proliferation of Laminaria ochroleuca could reduce available habitat space for epibionts that are associated with Laminaria hyperborea biotopes.

Red ripple bryozoan, Watersipora subatra. This species colonizes a variety of substrata and is often found in the intertidal and shallow subtidal, though has been recorded deeper than 10 m, and in salinities between 18 and 49 psu as well as a wide range of wave exposures from sheltered to exposed (Tillin et al., 2020). The availability of suitable hard attachment substrata, depth and salinity range, and the moderately exposed nature in which this biotope can be found makes it suitable habitat for Watersipora subatra (Tillin et al., 2020). 

Bonnemaison’s hook weed, Bonnemaisonia hamifera. This species has been found in salinities between 14.26 and 37.5 psu, settled in lower littoral and sublittoral habitats up to 8 m depth, and while it prefers very sheltered conditions, it has been seen in biotopes of moderate energy (Tillin et al., 2020). This biotope may provide potentially suitable habitat for Bonnemaisonia hamifera based on suitable attachment substrata, depth and salinities (Tillin et al., 2020). 

Japanese skeleton shrimp, Caprella mutica. This species has been found on a range of different substrata between 18 to 35 ppt salinity, intertidally and subtidally up to 20 m deep and in sheltered areas to those with high tidal flow regimes (Tillin et al., 2020). While this species does not usually associate directly with hard surfaces, it favours filamentous structures like hydroids and turf algae that it can hold onto, which can be found growing hard substrata and Sabellaria spinulosa crusts. This biotope has been considered as potentially suitable habitat for Caprella mutica due to its associated epifaunal community and suitable salinity and hydrodynamic regimes (Tillin et al., 2020).

Orange striped anemone, Diadumene lineata. This species tends to be found in brackish waters, particularly in bays, estuaries, and marinas where its only requirement is hard substrata on which to attach. As such, it is often associated with biogenic reefs (Tillin et al., 2020). It can tolerate a large salinity range, from 0.5 to 35 ppt, and is found in shallow waters to depths of a few hundred meters (Cohen, 2011), preferring sheltered areas with low wave exposure (Fofonoff et al., 2003). This species has not been shown to cause negative impacts on the habitats that it colonizes (Fofonoff et al., 2003). This biotope can be found within suitable depths and salinities, and contains suitable hard attachment substrata, making it potentially suitable habitat for Diadumene lineata (Tillin et al., 2020). 

Sensitivity Assessment. Watersipora subatra, Bonnemaisonia hamifera, Caprella mutica and Diadumene lineata may be able to colonize this biotope. However, no evidence of detrimental effects in kelp beds or Sabellaria reefs was found. There is ‘Insufficient evidence’ to assess the sensitivity of this biotope to these species. Therefore, sensitivity to 'Other INIS' will be based on the known impacts of Laminaria ochroleuca. The evidence for Laminaria hyperborea poleward range contraction (Assis et al., 2016; Casado-Amezúa et al., 2019), alongside the expansion of Laminaria ochroleuca into higher latitudes (Franco et al., 2018), suggests that Laminaria ochroleuca could displace existing kelp biotopes in the southern UK. In Plymouth Sound, Laminaria ochroleuca is already rivalling Laminaria hyperborea, which used to be the dominant kelp in the area (Smale et al., 2014; Taylor-Robinson et al., 2024). Its greater stipe length could reduce light availability for smaller kelps, and when combined with elevated temperatures, could create unfavourable conditions for the persistence and recovery of native species. Laminaria ochroleuca, however, does form mixed forests with Laminaria hyperborea in moderately sheltered to exposed shores, and has physiological and morphological advantages that could allow it to proliferate if Laminaria hyperborea density was reduced. Therefore, resistance to Laminaria ochroleuca is assessed as ‘Low’ based on the evidence of Laminaria ochroleuca rivalling Laminaria hyperborea in Plymouth Sound, southwest UK. Hence, resilience is assessed as ‘Very Low’, and sensitivity as ‘High’. While the quality and applicability of the evidence are high, there is contrasting evidence regarding both species’ resistance and resilience to storm damage. Therefore, confidence in this sensitivity assessment is ‘Medium’.