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

This biotope occurs in the subtidal and is, therefore, protected from exposure to air so that the thermal regime is more stable and desiccation is not a factor. Examples of distribution and thermal tolerances tested in laboratory experiments are provided as evidence to support the sensitivity assessment. In general, populations can acclimate to prevailing conditions, which can alter tolerance thresholds, and care should, therefore, be used when interpreting reported tolerances.

The encrusting coralline, Lithophyllum incrustans, is close to the northern edge of its reported distribution range in the UK, and found further south than the UK (Kain, 1982; Guiry & Guiry, 2015), and is therefore considered likely to be tolerant of an increase in temperature, particularly in this subtidal biotope, where it is protected from desiccation. Edyvean & Ford (1984b) suggest that populations of Lithophyllum incrustans are affected by temperature changes and salinity, and that temperature and salinity ‘shocks’ induce spawning, but no information on thresholds was provided. Populations of Lithophyllum incrustans were less stable in tide pools with a smaller volume of water, which were more exposed to temperature and salinity changes due to lower buffering capacity. Sexual algae (or their spores) were suggested to be more susceptible than asexual ones to extremes of local environmental variables (temperature, salinity, etc.) as they occurred with greater frequency at sites where temperature and salinity were more stable (Edyvean & Forde, 1984b). In a multidecadal analysis, Fernández (2016) reported that in northern Spain, resurveys of 20 shores previously sampled in 1977 found declines in cold-temperate algae species but increases in warm-temperate taxa, including coralline algae such as Lithophyllum incrustans, suggesting that some crustose corallines may expand under warming conditions.

Qui-Minet et al. (2019) tested physiological responses of Lithophyllum incrustans alongside rhodolith-forming Lithothamnion coralloides and Phymatolithon calcareum under a high-emissions future climate scenario (+3 °C, reduced pH; IPCC RCP 8.5) with and without nutrient enrichment. Responses were species-specific and seasonally dependent. In winter, future conditions impaired light harvesting and photoprotective mechanisms in the rhodolith species, and calcification rates declined to net dissolution under low nutrient concentrations, although nutrient enrichment reduced these losses. In contrast, Lithophyllum incrustans showed enhanced calcification in the dark under nutrient enrichment in summer. While all three species displayed higher maximal calcification rates in winter under future conditions, inhibition at high irradiance occurred in Phymatolithon calcareum, and summer calcification dropped in Lithothamnion coralloides. The study highlights how local nutrient and irradiance regimes can mediate coralline responses to climate stressors. Simon-Nutbrown et al. (2020) modelled future distributions of non-geniculate corallines in the UK and predicted significant range contractions under a range of future climate warming predictions, with declines of 38% following an increase of <2°C (IPCC RCP2.6) and 84% under the “worst-case” climate scenario representing a temperature increase of 3°C (IPCC RCP 8.5).

Although this biotope is characterized by crustose corallines, several of the species that can occur in this biotope, including Lithothamnion crispatum, Lithothamnion glaciale, and Phymatolithon lusitanicum, can form free-living rhodoliths in addition to crusts, and so studies on their rhodolith forms can provide additional insight into potential physiological responses, albeit with caution given morphological and ecological differences. Koerich et al. (2021) showed that Lithothamnion crispatum rhodoliths off Brazil experienced reduced photosynthetic performance when exposed to combined warming (an increase of 6°C), eutrophication and acidification consistent with worst-case future projections (IPCC RCP 8.5) for five days, but recovered to pre-experiment values within five days once conditions returned to ambient. Species distribution models projected a future range expansion under RCP 8.5. Bélanger & Gagnon (2021) reported that Lithothamnion glaciale rhodoliths from Newfoundland maintained normal growth between approx. 1 and 16°C, with resilience to higher temperatures than normally experienced in situ. Legrand et al. (2017) found that net primary production in Lithothamnion coralloides was not altered by a +3°C treatment, although gross primary production declined at higher winter temperatures. Muñoz et al. (2018) reported that Lithothamnion crispatum showed reduced physiological performance under combined future pH and temperature conditions (30 ± 2°C, pH 7.4), but suggested that their overall biological viability indicated capacity to persist under projected climate change scenarios. Schubert et al. (2019) recorded significant declines (40 to 55%) in calcification of Lithothamnion crispatum at 28°C, although photosynthesis and respiration both increased with temperature. Sordo et al. (2019) found that photosynthesis, calcification, and respiration of Phymatolithon lusitanicum rhodoliths increased with temperature in addition to high CO₂.

Spirobranchus triqueter is described as a temperate species by Kupriyanova & Badyaev (1998) and is found in both warmer and colder waters than experienced in the UK (De Kluijver et al., 2016). Spirobranchus triqueter is recorded as abundant in sub-tidal habitats of Trondheimsfjord (63°N) (Kukliński & Barnes, 2008); the most northern records for the species are in northern Norway, ca. 70°N (OBIS, 2026). The most southerly records are from the Iberian Peninsula, Spain (Ramos, 2010), as well as from the Alexandria coast of Egypt, Mediterranean Sea (Dorgham et al., 2013). Across this latitudinal gradient, Spirobranchus triqueter is likely to experience a range of temperatures from approximately 5 to 28°C (Seatemperature, 2015).

Balanus crenatus is described as a boreal species (Newman & Ross, 1976) and is found throughout the northeast Atlantic from the Arctic to the west coast of France as far south as Bordeaux; east and west coasts of North America and Japan (Dauvin et al., 2021; Regel, 2024). In Queens Dock, Swansea, where the water was on average 10°C higher than average due to condenser effluent, Balanus crenatus was replaced by the subtropical barnacle Balanus amphitrite. After the water temperature cooled, Balanus crenatus returned (Naylor, 1965). The increase in water temperature in Queens Dock was greater than an increase at the pressure benchmark (2 to 5°C). Balanus crenatus has a peak rate of cirral beating at 20°C, and all spontaneous activity ceases at about 25°C (Southward, 1955). The tolerance of Balanus crenatus, collected in the summer (and thus acclimated to higher temperatures), to increased temperatures was tested in the laboratory. The median upper lethal temperature tolerance was 25.2°C (Davenport & Davenport, 2005), confirming the observations of Southward (1955).

Merder et al. (2018) conducted a temporal analysis of change in the rocky intertidal community of Ireland based on the raw dataset described in Simkanin et al. (2005), cited in Merder et al. (2018) (all fieldwork was carried out from March to November 2003 by the same two operators). The averaged sea-surface temperature (2000 to 2003) for every site ranged from 9.76 to 13.6°C, and showed a clear differentiation between north-east and south-west, with the south-west consistently approx. 2 to 3°C warmer than the north-east of Ireland (Merder et al., 2018). Balanus crenatus was documented as being restricted to the east (Merder et al., 2018). Balanus crenatus is a low shore specialist and is known to be intolerant of the higher temperatures that could result from heating up of the substratum by insolation at low tide (Southward, 1958, cited in Merder et al., 2018). Merder et al. (2018) noted that the temperature variables presented a largely latitudinal trend, whereas there was a longitudinal (east/west) trend in community composition.

In Sayda Bay, Russian Barents Sea, Balanus crenatus experience temperature ranges of 1°C in March to 10°C in June, and during their observations, Dvoretsky & Dvoretsky (2024) noted that Balanus crenatus infestation levels of red king crab were lower in winter, most likely due to less favourable temperature conditions.

Sensitivity assessment

Typical surface water temperatures around the UK coast vary seasonally from 4 to 19°C (Huthnance, 2010). The biotope would probably tolerate a 2°C increase in temperature for a year. An acute increase at the pressure benchmark may be tolerated in winter, but a sudden return to typical temperatures could lead to mortalities among acclimated animals. The distribution of Lithophyllum incrustans and other crusting corallines in southern Europe and the Mediterranean suggests that these are tolerant of higher temperatures. Field observations in Australia indicate that crustose coralline cover may decline under prolonged seasonal warming above ~29°C, although carbonate production rates remain stable (Dee et al., 2021), and elevated summer mortality has also been recorded following marine heatwaves (Short et al., 2015). However, the temperatures observed in these studies exceed those assessed under the benchmark. Experimental and modelling studies on rhodolith-forming corallines show broad tolerance to moderate warming but potential declines under more extreme scenarios (e.g. Legrand et al., 2017, 2019; Muñoz et al., 2018; Schubert et al., 2019; Simon-Nutbrown et al., 2020), but these results may not be directly transferable to the crustose forms in this biotope. Spirobranchus triqueter records from the Alexandria coast of Egypt, Mediterranean Sea (Dorgham et al., 2013) indicate the species is unlikely to be affected at the benchmark level. An acute increase of 5°C in summer could be close to the lethal thermal temperature for Balanus crenatus, and the loss of this species would alter the character of the biotope, although it would remain recognizable as seasonal scour probably causes seasonal change in the abundance of the component species. Biotope resistance is, therefore, assessed as ‘Medium’, resilience as ‘High’, and biotope sensitivity as ‘Low’.

Temperature decrease (local)

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

Evidence

This biotope occurs in the subtidal and is therefore protected from exposure to air, so that the thermal regime is more stable and desiccation is not a factor. Examples of distribution and thermal tolerances tested in laboratory experiments are provided as evidence to support the sensitivity assessment. In general, populations can acclimate to prevailing conditions, which can alter tolerance thresholds, and care should, therefore, be used when interpreting reported tolerances.

Lithophyllum incrustans are close to the northern edge of their reported distribution range in the UK (Guiry & Guiry, 2015). Edyvean & Forde (1984b) suggest that populations of Lithophyllum incrustans are affected by temperature changes and salinity, and that temperature and salinity ‘shocks’ induce spawning, but no information on thresholds was provided (Edyvean & Ford, 1984b). 

Although this biotope is characterized by crustose corallines, several of the species that can occur in this biotope, including Lithothamnion crispatum, Lithothamnion glaciale, and Phymatolithon lusitanicum, can form free-living rhodoliths in addition to crusts, and so studies on their rhodolith forms can provide additional insight into potential physiological responses, albeit with caution given morphological and ecological differences. Ronowicz et al. (2024) observed high settlement of Lithothamnion sp. during the polar night in Svalbard, indicating that spore release and colonization can occur at temperatures as low as 1.4°C without solar radiation. Arnold et al. (2022) found that Lithothamnion glaciale rhodoliths maintained normal growth rates when exposed to near-zero temperatures (approx. 0.5 to 1 °C) for 85 days, with irradiance a stronger predictor of growth than temperature. Bélanger & Gagnon (2021) reported that Lithothamnion glaciale rhodolith growth ceased at approx. 0.5 °C in field experiments, but was otherwise unaffected between approx. 1 and 16 °C in mesocosms, suggesting resilience to temperature decreases across a broad range.

Spirobranchus triqueter is described as a temperate species by Kupriyanova & Badyaev (1998) and is found in both warmer and colder waters than experienced in the UK (De Kluijver et al., 2016). Spirobranchus triqueter is recorded as abundant in sub-tidal habitats of Trondheimsfjord (63°N) (Kukliński & Barnes, 2008); the most northern records for the species are in northern Norway, ca. 70°N (OBIS, 2026). Averaged across several years, the lowest winter temperature within Trondheimsfjord is 4.9°C (Seatemperature, 2015). Spirobranchus triqueter is unable to build calcareous tubes below 7°C. However, this effect is not considered to lead to mortality in adults at the duration (one month) of the acute pressure benchmark (Thomas, 1940). Intertidal populations of Spirobranchus triqueter were reported to suffer 50% mortality at Mumbles, on the Gower, after the extreme winter of 1962 to 1963 (Crisp, 1964b). However, the decrease in temperature exceeds the pressure benchmark. Mature adults may survive a decrease at the pressure benchmark; however, larvae may not be able to attach to the substratum (Riley & Ballerstedt, 2005) if a temperature decrease co-occurred with cold winter temperatures in the UK. However, settlement is reportedly low during winter (See resilience section), and therefore, the effects on recruitment are likely to be minor.

Balanus crenatus is relatively tolerant of lower temperatures. Balanus crenatus is described as a boreal species (Newman & Ross, 1976) and is found throughout the northeast Atlantic from the Arctic to the west coast of France as far south as Bordeaux; east and west coasts of North America and Japan (Dauvin et al., 2021; Regel, 2024). Balanus crenatus was unaffected during the severe winter of 1962 to 1963, when average temperatures were 5 to 6°C below normal (Crisp, 1964a, b). The tolerance of Balanus crenatus collected from the lower intertidal in the winter (and thus acclimated to lower temperatures) to low temperatures was tested in the laboratory. The median lower lethal temperature tolerance was -1.4°C (Davenport & Davenport, 2005). An acute or chronic decrease in temperature, at the pressure benchmark, is therefore unlikely to negatively affect this species. Meadows (1969) noted that the severe winter of 1962 to 1963 decreased sea temperatures at Newcastle but did not affect fauna, including Balanus crenatus, on settlement panels that were deployed in the area. In Sayda Bay, Russian Barents Sea, Balanus crenatus experience temperature ranges of 1°C in March to 10°C in June, and during their observations, Dvoretsky & Dvoretsky (2024) noted that Balanus crenatus infestation levels of red king crab were lower in winter, most likely due to less favourable temperature conditions.

Sensitivity assessment

Overall, a long-term chronic change in temperature at the pressure benchmark is considered likely to fall within natural variation and to be tolerated by the characterizing and associated species, although Lithothyllum incrustans may experience reduced growth (as it is primarily a southern species). An acute change at the pressure benchmark is considered unlikely to adversely affect the biotope, as the characterizing species can potentially adapt to a wide range of temperatures experienced in both northern and southern waters (Spirobranchus triqueter) or are found primarily in colder, more northern waters (Balanus crenatus). Lithophyllum incrustans may be less tolerant, but reductions in growth, rather than mortalities, may result. Biotope resistance is, therefore, assessed as ‘Medium’ and resilience as ‘High’, so that sensitivity is assessed as ‘Low’.

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

The crustose corallines that occur in this biotope may also be found on rocky shores and in rockpools where salinities may fluctuate markedly during exposure to the air. Edyvean & Ford (1984b) suggest that populations of Lithophyllum incrustans are affected by temperature changes and salinity and that temperature and salinity ‘shocks’ induce spawning, but no information on thresholds was provided (Edyvean & Ford, 1984b). Populations of Lithophyllum incrustans were less stable in rockpools with a smaller volume of water that were more exposed to temperature and salinity changes due to lower buffering capacity. Sexual plants (or the spores that give rise to them) were suggested to be more susceptible than asexual plants to extremes of local environmental variables (temperature, salinity, etc.) as they occur with greater frequency at sites where temperature and salinity were more stable (Edyvean & Ford, 1984b).

Lyster (1965) tested the tolerance of Spirobranchus triqueter larvae to various hyper- and hypo-salinity treatments. Larvae were placed in cultures ranging from 0 to 90‰, and notes were made on the time taken for larvae to die or begin displaying abnormal behaviour. Spirobranchus triqueter larvae were tolerant of salinities ranging from 20 to 50‰. Salinities above 50‰ caused high mortality. Spirobranchus triqueter is, therefore, unlikely to be affected at the pressure benchmark.

Balanus crenatus occurs in both full-salinity environments, such as in Sayda Bay, Russian Barents Sea, where it experiences salinity between 32 psu in summer and 34 psu in winter (Dvoretsky & Dvoretsky, 2024), and estuarine areas, and is therefore adapted to variable salinity (Davenport, 1976). When subjected to sudden changes in salinity, Balanus crenatus closes its opercular valves so that the blood is maintained temporarily at a constant osmotic concentration (Davenport, 1976). Early stages may be more sensitive than adults. Experimental culturing of Balanus crenatus eggs found that viable nauplii larvae were obtained between 25 and 40‰, but eggs did not develop to viable larvae when held at salinities above 40‰, and only a small proportion (7%) of eggs exposed at later stages developed into viable nauplii, and these were not vigorous swimmers (Barnes & Barnes, 1974). When exposed to salinities of 50‰ and 60‰, eggs exposed at an early developmental stage did not produce viable larvae and, again, only a small proportion of eggs (7% and 1%, respectively) exposed at a later developmental stage produced nauplii; these were deformed and probably non-viable. There was no larval development at 70‰ (Barnes & Barnes, 1974).

Sensitivity assessment

This biotope is recorded in full salinity (30 to 35 ppt) habitats (Connor et al., 2004), and the sensitivity assessment considers an increase from full to >40 ppt. Some increases in salinity may be tolerated by the characterizing species. However, the biotope is probably sensitive to an increase in salinity to >40 ppt (based on species distribution in Barnes & Barnes, 1974, and Edyvean & Ford, 1984b). Therefore, resistance is assessed as ‘Low’ and resilience as ‘High’ (following the restoration of prior salinity), so sensitivity is assessed as ‘Low’.

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

Edyvean & Ford (1984b) suggest that populations of the crustose coralline Lithophyllum incrustans are affected by temperature changes and salinity and that temperature and salinity ‘shocks’ induce spawning, but no information on the relevant thresholds was provided (Edyvean & Ford, 1984b). Populations of Lithophyllum incrustans were less stable in tide pools with a smaller volume of water that were more exposed to temperature and salinity changes due to lower buffering capacity. Sexual plants (or the spores that give rise to them) were suggested to be more susceptible than asexual plants to extremes of local environmental variables (temperature, salinity, etc.) as they occur with greater frequency at sites where temperature and salinity were more stable (Edyvean & Ford, 1984b).

Spirobranchus triqueter has not been recorded from brackish or estuarine waters. Therefore, it is likely that the species will be very intolerant of a decrease in salinity. However, Dixon (1985, cited in Riley & Ballerstedt, 2005) views the species as able to withstand significant reductions in salinity. Lyster (1965) tested the tolerance of Spirobranchus triqueter larvae to various hyper- and hypo-salinity treatments. Larvae were placed in cultures ranging from 0 to 90‰, and notes were made on the time taken for larvae to die or begin displaying abnormal behaviour. Spirobranchus triqueter larvae can survive very well in salinities down to 20‰ and can tolerate salinities down to 10‰. Adults are tolerant of salinities as low as 3‰ and can be found in areas where salinity ranges from 18 to 23‰ (Alexander et al., 1935). Spirobranchus triqueter is also present in the biotope CR.MCR.EcCr.UrtScr, which occurs in variable-salinity (as well as full) habitats (Connor et al., 2004).

Balanus crenatus occurs in both full-salinity environments, such as in Sayda Bay, Russian Barents Sea, where it experiences salinity between 32 psu in summer and 34 psu in winter (Dvoretsky & Dvoretsky, 2024), and estuarine areas, and is therefore adapted to variable salinity (Davenport, 1976). When subjected to sudden changes in salinity, Balanus crenatus closes its opercular valves so that the blood is maintained temporarily at a constant osmotic concentration (Davenport, 1976). Acclimation to different salinity regimes alters the point at which opercular closure and resumption of activity occur (Davenport, 1976). Balanus crenatus can tolerate salinities down to 14 psu if given time to acclimate (Foster, 1970). At salinities below 6 psu, motor activity ceases, respiration falls, and the animal falls into a "salt sleep". In this state, the animals may survive (Barnes & Barnes, 1974) in freshwater for three weeks, enabling them to withstand changes in salinity over moderately long periods (Barnes & Powell, 1953). Larvae are more sensitive than adults. In culture experiments, eggs maintained below 10‰ rupture due to osmotic stress (Barnes & Barnes, 1974). At 15 to 17‰, there is either no development of early stages or the nauplii larvae are deformed and “probably not viable”; similarly, at 20‰, development occurs, but about half of the larvae are deformed and not viable. (Barnes & Barnes, 1974). Normal development resulting in viable larvae occurs between salinities of 25 to 40‰ (Barnes & Barnes, 1974).

Sensitivity assessment

This biotope is recorded in full salinity (30 to 35 ppt) (Connor et al., 2004; JNCC, 2022). A change from full to reduced salinity (18 to 30 ppt) is assessed at the pressure benchmark. The characterizing species are found in a similar biotope (CR.MCR.EcCr.UrtScr), which is present in variable salinities (Connor et al., 2004). It is therefore likely that the characterizing species will tolerate a reduction in salinity from full to reduced.

As the characterizing species are found in biotopes in both full and variable salinity habitats, the biotope is considered ‘Not sensitive’ to a decrease in salinity from full to variable. Biotope resistance is therefore assessed as ‘High’, resilience as ‘High’ (by default), and the biotope is assessed as ‘Not sensitive’.

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

This biotope occurs across a range of flow speeds, from very weak (negligible) to moderately strong (0.5 to 1.5 m/s) (Connor et al., 2004; JNCC, 2022). The suspension feeders within the biotope benefit from high water flow, supplying food.

The coralline crusts characterizing this biotope are securely attached, and as these are flat, they are subject to little or no drag compared to upright growth forms of algae. Colonies of Lithophyllum incrustans appear to thrive in conditions exposed to strong water movement (Irvine & Chamberlain, 1994). 

Spirobranchus triqueter has been recorded in areas with very sheltered to exposed water flow rates (Price et al., 1980; Tillin & Tyler-Walters, 2014). Wood (1988) observed Spirobranchus sp. in strong tidal streams, and Hiscock (1983) found that in strong tidal streams or strong wave action where abrasion occurs, fast-growing species such as Spirobranchus triqueter occur.

Balanus crenatus is found in a very wide range of water flows (Tillin & Tyler-Walters, 2014), although it usually occurs in sites sheltered from wave action (Eckman & Duggins, 1993) and can adapt its feeding behaviour according to flow rates. In the absence of any current, the barnacle rhythmically beats its cirri to create a current to collect zooplankton. The growth of Balanus crenatus (measured as an increase in basal area), maintained for 69 days at constant flow speeds in laboratory experiments, was greatest at intermediate flow speeds (0.08 m/s) and decreased at higher speeds (Eckman & Duggins, 1993). Over the entire range of flow speeds measured (0.02 to 0.25 m/s), Balanus crenatus was able to control the cirrus with little or no deformation by flow observed (Eckman & Duggins, 1993).

Sensitivity assessment

Scour is a key factor structuring this biotope (Connor et al., 2004; JNCC, 2022), so changes in flow exceeding the pressure benchmark may increase or decrease sediment transport and associated scour may lead to indirect changes in the character of the biotope. However, the scour and mobility of the substratum are probably dominated by wave action rather than water flow. As the biotope and the associated species can occur in a range of flow speeds, the resistance of the biotope to changes in water flow is assessed as ‘High’ and resilience as ‘High’ (by default), so that the biotope is assessed as ‘Not sensitive’.

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

Not relevant to subtidal biotopes. However, 100% mortality could be expected in adult Spirobranchus (as Pomatoceros) triqueter after 24.1 h and 35.4 h when exposed to air at 7°C and 13°C, respectively (Campbell & Kelly, 2002).

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

This biotope is recorded from locations that range from 'very exposed' to 'exposed' to wave action (Connor et al., 2004; JNCC, 2022).

Colonies of Lithophyllum incrustans appear to thrive in conditions exposed to strong water movement. Irvine & Chamberlain (1994) observe that the species is best developed on wave-exposed shores. Roik et al. (2016) reported that calcification rates of crustose coralline algae were up to 8.8 times higher at offshore exposed sites compared with nearshore sheltered sites in the Red Sea, suggesting positive effects of higher wave exposure. In addition, the crustose corallines associated with this biotope have a flat growth form and are unlikely to be dislodged by increased wave action. As this biotope occurs across a range of exposures, this was therefore considered to indicate, by proxy, that biotopes in the middle of the wave exposure range would tolerate either an increase or decrease in significant wave height at the pressure benchmark.

Balanus crenatus and Spirobranchus triqueter are suspension feeders relying on water currents to supply food. These taxa, therefore, thrive in conditions of vigorous water flow. Balanus crenatus and Spirobranchus triqueter are firmly attached to the substratum and are unlikely to be dislodged by an increase in wave action at the pressure benchmark. These species are found in biotopes from a range of wave exposures, from extremely wave-sheltered to very wave-exposed (Tillin & Tyler-Walters, 2014). 

Sensitivity assessment

Scour is a key factor structuring this biotope (Connor et al., 2004; JNCC, 2022) and the mobility of the substratum, hence, scour is probably dominated by wave action. Therefore, changes in wave action (wave exposure) could result in significant changes to this biotope. Any activity or pressure that redirected, blocked, or mitigated wave energy reaching the biotope could result in a reduction in scour and reclassification of the biotope.

An increase in wave exposure by one category (the benchmark) from very to extremely wave exposed would probably favour the IR.FIR.SG.CC.Mo sub-biotope, which occurs in areas of severe scour (abrasion), and the characteristic Spirobranchus and Balanus species are limited to refugia. However, the IR.FIR.SG.CC biotope would remain identifiable.

A decrease in wave exposure from exposed to moderately exposed would result in reduced scour and allow other opportunistic species to colonize the biotope. The biotope would probably transition to IR.FIR.SG.FoSwCC, which occurs on surge gully entrances, or allow crustose sponge biotopes typical of wave-surged gullies (e.g. IR.FIR.SG.CrSp), which can be found above this IR.FIR.SG.CC biotope. However, the transition would depend on the time taken for other species to colonize the biotope and is unlikely to be complete within one year (the benchmark duration). Therefore, resistance is assessed as ‘Medium’ to represent temporary changes in the community. Resilience is probably ‘High’, so sensitivity is assessed as ‘Low’ at the benchmark level. Prolonged reduction in wave action (over one year) would result in loss of the biotope.

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 any evidence is presented where available.

No information was found concerning the effects of heavy metals on encrusting coralline algae. Bryan (1984) suggested that the general order for heavy metal toxicity in seaweeds is: organic Hg> inorganic Hg > Cu > Ag > Zn> Cd> Pb. Contamination at levels greater than the pressure benchmark may adversely impact the biotope. Cole et al. (1999) reported that Hg was very toxic to macrophytes. The sub-lethal effects of Hg (organic and inorganic) on the sporelings of an intertidal red algae, Plumaria elegans, were reported by Boney (1971). 100% growth inhibition was caused by 1 ppm Hg.

Contamination at levels greater than the pressure benchmark may adversely impact the biotope. Barnacles accumulate heavy metals and store them as insoluble granules (Rainbow, 1987). Pyefinch & Mott (1948) recorded a median lethal concentration of 0.19 mg/l copper and 1.35 mg/l mercury for Balanus crenatus over 24 hours. Barnacles may tolerate fairly high levels of heavy metals in nature, for example, they are found in Dulas Bay, Anglesey, where copper reaches concentrations of 24.5 µg/l due to acid mine waste (Foster et al., 1978).

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 any evidence is presented where available

Where exposed to direct contact with fresh hydrocarbons, encrusting coralline algae appear to have a high intolerance. Crump et al. (1999) described 'dramatic and extensive bleaching' of 'Lithothamnia' following the Sea Empress oil spill. Observations following the Don Marika oil spill (K. Hiscock, pers. comm.) were of rockpools with completely bleached coralline algae. However, Chamberlain (1996) observed that although Lithophyllum incrustans was affected in a short period of time by oil during the Sea Empress spill, recovery occurred within about a year. The oil was found to have destroyed about one-third of the thallus thickness, but regeneration occurred from thallus filaments below the damaged area.

No information is available on the intolerance of Balanus crenatus to hydrocarbons. However, other littoral barnacles generally have a high tolerance to oil (Holt et al., 1995) and were little impacted by the Torrey Canyon oil spill (Smith, 1968), so Balanus crenatus is probably fairly resistant to oil.

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 any evidence is presented where available.

Cole et al. (1999) suggested that herbicides were (not surprisingly) very toxic to algae and macrophytes. Hoare & Hiscock (1974) noted that, with the exception of Phyllophora species, all red algae, including encrusting coralline forms, were excluded from the vicinity of an acidified halogenated effluent discharge in Amlwch Bay, Anglesey and that intertidal populations of Corallina officinalis occurred in significant numbers only 600 m east of the effluent. Chamberlain (1996) observed that although Lithophyllum incrustans was quickly affected by oil during the Sea Empress spill, recovery occurred within about a year. The oil was found to have destroyed about one-third of the thallus thickness, but regeneration occurred from thallus filaments below the damaged area.

Barnacles have a low resilience to chemicals such as dispersants, dependent on the concentration and type of chemical involved (Holt et al., 1995). They are less intolerant than some species (e.g. Patella vulgata) to dispersants (Southward & Southward, 1978) and Balanus crenatus was the dominant species on pier pilings at a site subject to urban sewage pollution (Jakola & Gulliksen, 1987). Hoare & Hiscock (1974) found that Balanus crenatus survived near an acidified halogenated effluent discharge where many other species were killed, suggesting a high tolerance to chemical contamination. Little information is available on the impact of endocrine disrupters on adult barnacles. Holt et al. (1995) concluded that barnacles are fairly sensitive to chemical pollution, therefore, intolerance is reported as high. The species is an important early colonizer of sublittoral rock surfaces (Kitching, 1937), and it heavily recolonized a site that was dredged for gravel within seven months (Kenny & Rees, 1994). Therefore, recovery is predicted to be high.

Most pesticides and herbicides were suggested to be very toxic to invertebrates, especially crustaceans (amphipods, isopods, mysids, shrimp and crabs) and fish (Cole et al., 1999).

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.

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

Specific information concerning oxygen consumption and reduced oxygen tolerances was not found for the key characterizing species within the biotope. It is likely that, as this biotope occurs in areas that are shallow and tidally flushed, re-oxygenation is likely, limiting the effects of any de-oxygenation events. However, this may mean that the species present have little exposure to low oxygen and may be sensitive to this pressure. In general, respiration in most marine invertebrates does not appear to be significantly affected until extremely low concentrations are reached. For many benthic invertebrates, this concentration is about 2 ml/l, or even less (Herreid, 1980; Rosenberg et al., 1991; Diaz & Rosenberg, 1995).

Balanus crenatus, however, respires anaerobically, so it can withstand some decrease in oxygen levels. When placed in wet nitrogen, where oxygen stress is maximal and desiccation stress is minimal, Balanus crenatus has a mean survival time of 3.2 days (Barnes et al., 1963), and this species is probably ‘Not sensitive’ to this pressure. 

Sensitivity assessment

Based on Balanus crenatus and the mitigation of de-oxygenation by strong water movements, this biotope is considered to have 'High' resistance and 'High' resilience (by default) and is, therefore, 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

Nutrient enrichment at the pressure benchmark is unlikely to affect the fauna within this biotope.

Over geological timescales, periods of increased nutrient availability have experienced increases in the distribution of crustose coralline species at the expense of corals (Littler & Littler, 2013), suggesting that this group have some tolerance for enhanced nutrient levels. For example, the encrusting coralline Lithophyllum incrustans was present at sites dominated by Ulva spp. in the Mediterranean exposed to high levels of nutrient enrichment from domestic sewage (Arévalo et al., 2007). However, Bélanger & Gagnon (2020) found that Lithothamnion glaciale rhodoliths grew two times slower under elevated nutrient concentrations in mesocosms (NO₃⁻: 10.2 to 29.5 µmol/l; NH₃: 14.4 to 33.9 µmol/l; PO₄³⁻: 2.9 to 5 µmol/l compared with ambient NO₃⁻: 3.2 µmol/l, NH₃: 3.5 µmol/l, PO₄³⁻: 0.6 µmol/l). In field experiments, growth was also reduced under elevated phosphate (1.2 µmol/l vs. ambient 0.4 µmol/l), although some recovery was observed after the first six weeks.

Koerich et al. (2021) reported that Lithothamnion crispatum showed reduced photosynthetic performance when exposed to combined warming (+6°C), acidification (pH 7.7), and nutrient enrichment (70 µM ammonium, 14 µM nitrate, 10 µM phosphate) for five days, but recovered after returning to ambient conditions. Qui-Minet et al. (2019) showed that nutrient enrichment (PO₄³⁻: 0.8 to 1.2 µmol/l in winter, 0.9 to 1.7 µmol/l in summer; NO₃⁻: 30 to 50 µmol/l in winter, 25 to 55 µmol/l in summer) reduced dissolution of Lithothamnion corallioides and Phymatolithon calcareum in winter under future climate scenarios, and enhanced dark calcification in Lithophyllum incrustans in summer. Schubert et al. (2019) found that nutrient concentration (ambient: NO₃⁻ 2.0 µmol/l, NH₄⁺ 1.1 µmol/l, PO₄³⁻ 1.6 µM; enriched: NO₃⁻ 21 µmol/l, NH₄⁺ 70 µmol/l, PO₄³⁻ 4 µmol/l) had no significant effect on photosynthesis or respiration in Lithothamnion crispatum, but when combined with high temperature (28°C vs. 23°C) the negative effects were additive, reducing daily carbonate production. Overall, Littler & Littler (2013) suggest that corallines as a group can tolerate both low and elevated levels of nutrients.

Balanus crenatus and Spirobranchus triqueter are suspension feeders relying on water currents to supply food. Nutrient enrichment of coastal waters that enhances the population of phytoplankton may be beneficial to both species in terms of an increased food supply, but the effects are uncertain (Hartnoll, 1998). However, high primary productivity in the water column combined with high summer temperature and the development of thermal stratification (which prevents mixing of the water column) can lead to hypoxia of the bottom waters. Balanus crenatus was the dominant species on pier pilings, which were subject to urban pollution (Jakola & Gulliksen, 1987).

Sensitivity assessment

Limited evidence on the effects of nutrient enrichment on the characteristic species was found. The evidence suggests that the characteristic species vary in their response to nutrients. However, the evidence is ‘Insufficient’ to form the basis of an assessment.

Organic enrichment

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

Evidence

As the biotope occurs in tide-swept or wave-exposed areas (Connor et al., 2004), water movements will disperse organic matter and reduce the level of exposure to this pressure. For example, the crusting coralline Lithophyllum incrustans was present at sites dominated by Ulva spp. in the Mediterranean exposed to high levels of organic pollution from domestic sewage (Arévalo et al., 2007), suggesting the encrusting corallines are not sensitive to this pressure.

The animals found within the biotope may be able to utilise the input of organic matter as food, or are likely to be tolerant of inputs at the benchmark level. In a recent review, assigning species to ecological groups based on tolerances to organic pollution, characterizing animal species, Balanus crenatus and Spirobranchus triqueter were assigned to AMBI Group II, described as 'species indifferent to enrichment, always present in low densities with non-significant variations with time, from initial state to slight unbalance' (Gittenberger & Van Loon, 2011).

Sensitivity assessment

It is not clear whether the pressure benchmark would lead to enrichment effects in this dynamic, scoured habitat. High water movements would disperse organic matter particles, mitigating the effect of this pressure. This biotope is probably ‘Not sensitive’ to this pressure, based on the AMBI categorisation (Borja et al., 2000; Gittenberger & Van Loon, 2011). Although species within the biotope may be sensitive to gross organic pollution resulting from sewage disposal and aquaculture, they are considered to have ‘High’ resistance to the pressure benchmark, which represents organic enrichment and therefore ‘High’ resilience, so that the biotope is assessed as ‘Not Sensitive’.

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

In the absence of a hard substratum, crustose corallines can propagate as free-living rhodolith nodules and can form extensive subtidal habitats (Littler & Little, 2013). However, these biogenic reefs are not analogous to this habitat type. The loss of hard substrata would remove the habitat, and sediments would be unsuitable for the crustose corallines and other attached algae that characterize this biotope.

Balanus crenatus and Spirobranchus triqueter. These species were reported from a variety of substratum types, including fine (muddy sand, sandy mud, and fine sands) and coarse sediments, where some hard surfaces (such as pebbles or shells) are present for the attached species (Tillin & Tyler-Walters, 2014). Spirobranchus triqueter is also known to colonize floating marine debris, mostly favouring plastics (Gündogdu, Çevik & Karaca, 2017; Abelouah et al., 2024).

Balanus crenatus and Spirobranchus triqueter are fouling organisms and occur on a wide variety of substrata (Harms & Anger, 1983; Andersson et al., 2009). As well as artificial and natural hard substrata, Balanus crenatus and Spirobranchus triqueter also encrust a range of invertebrates. For example, Spirobranchus triqueter has been recorded on the hermit crab, Pagurus bernhardus (Fernandez-Leborans & Gabilondo, 2006). In Northern Norway, Spirobranchus triqueter had a coverage range of 20 to 73% on Buccinum sp. and Littorina sp. shells utilized by Pagurus pubescens (Balazy & Kuklinski, 2018). Similarly, Balanus crenatus has been reported to encrust empty shells of the invasive non-indigenous species Ensis americanus (Donovan, 2011) and Carcinus maenas (Heath, 1976; Campos et al., 2022), and the red king crab Paralithodes camtschaticus (Dvoretsky & Dvoretsky, 2024). Settling on mobile species or floating marine debris may aid Balanus crenatus and Spirobranchus triqueter dispersal and recovery.

Sensitivity assessment 

A permanent change in substratum type to artificial or sedimentary would lead to the reclassification of the biotope. Biotope resistance to this pressure is therefore assessed as ‘None’ (loss of >75% of extent), as the change at the benchmark is permanent, and resilience is assessed as ‘Very low’. Sensitivity, based on combined resistance and resilience, is therefore 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 to biotopes occurring on bedrock.

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

The species characterizing this biotope are epifauna occurring on rock and would be sensitive to the removal of the habitat. However, extraction of rock substratum is considered unlikely, and this pressure is considered to be ‘Not relevant’ to hard substratum 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

The species characterizing this biotope occur on the rock and therefore have no protection from surface abrasion. High levels of abrasion from scouring by mobile sands and gravels are an important structuring factor in this biotope (Connor et al., 2004; JNCC, 2022) and prevent replacement by less scour-tolerant species, such as red algae.

Mechanical abrasion from scuba divers was reported to impact encrusting corallines, with cover of Lithophyllum stictaeforme greater in areas where diving was forbidden than in visited areas (abundance, 6.36 vs 1.4; it is presumed this refers to proportion of cover, although this is not clear from the text, Guarinieri et al., 2012).

Dethier (1994) manipulated surface abrasion on a range of encrusting algae experimentally, including Lithophyllum impressum. Crusts were brushed with either a nylon or steel brush for one minute each month for 24 months. Unbrushed controls grew by approximately 50%, whereas the cover of nylon-brushed crusts and steel-brushed crusts decreased by approximately 25% and 40%, respectively (interpreted from figures in Dethier, 1994). In laboratory tests on chips of Lithophyllum impressum, brushing with a steel brush for one minute once a week for three weeks resulted in no cover loss in two samples, while a third ‘thinned and declined’ (Dethier, 1994).

Hiscock (1983) noted that a community, under conditions of scour and abrasion from stones and boulders moved by storms, developed into a community consisting of fast-growing species such as Spirobranchus (formerly Pomatoceros) triqueter. Off Chesil Bank, UK, the epifaunal community dominated by Spirobranchus (as Pomatoceros) triqueter and Balanus crenatus decreased in cover in October as it was scoured away in winter storms, but recolonized in May to June (Gorzula, 1977). Warner (1985) reported that the community did not contain any persistent individuals, but that recruitment was sufficiently predictable to result in a dynamic stability and a similar community, dominated by Spirobranchus (as Pomatoceros triqueter), Balanus crenatus, and Electra pilosa (an encrusting bryozoan), as was present in 1979, 1980, and 1983 (Warner,1985).

Fishing disturbance is unlikely to affect species on cave and gully walls. However, it can be looked at as an example of how epifaunal communities react to scour. For example, Picton & Goodwin (2007) noted that an area of boulders with a rich fauna of sponges and hydroids on the east coast of Rathlin Island, Northern Ireland, was significantly altered since the 1980s. Scallop dredging had begun in 1989, and boulders were observed to have been turned and the gravel harrowed. In addition, many of the boulders had disappeared, and rare hydroid communities were greatly reduced (Picton & Goodwin, 2007). Furthermore, re-sampling of fishing grounds that were historically studied (from the 1930s) indicated that some encrusting species, including serpulid worms and several species of barnacles, had decreased in abundance in gravel substrata subject to long-term scallop fishing (Bradshaw et al., 2002). These may have been adversely affected by the disturbance of the stones and dead shells onto which they attach (Bradshaw et al., 2002). Where individuals are attached to mobile pebbles, cobbles, and boulders rather than bedrock, surfaces can be displaced and turned over, preventing feeding and leading to smothering. This observation is supported by experimental trawling, carried out in shallow, wave-disturbed areas using a toothed clam dredge, which found that Spirobranchus sp. decreased in intensively dredged areas over the monitoring period (Constantino et al., 2009).

In contrast, a study of Spirobranchus spp. aggregations found that the tube heads were not significantly affected by biannual beam trawling in the eastern Irish Sea (Kaiser et al., 1999). No changes in the number or size of serpulid tube heads were apparent throughout the study, and no significant changes were detectable in the composition of the tube head fauna that could be attributed to fishing disturbance (Kaiser et al., 1999). Subsequent laboratory experiments on collected tube heads found that these were unlikely to resettle on the seabed in an orientation similar to that prior to disturbance (Kaiser et al., 1999). This may lead to the death of resident serpulids and sessile associated fauna.

Sensitivity assessment

The impact of surface abrasion will depend on the footprint, duration, and magnitude of the pressure. High levels of abrasion from scouring by mobile cobbles and pebbles are an important structuring factor in this biotope (Connor et al., 2004; JNCC, 2022), and the persistence of the assemblage probably depends on rapid recovery rather than high resistance (Gorzula, 1977). The evidence for the effects of severe scour and trawling on Balanus crenatus and Spirobranchus triqueter suggests that resistance to a single abrasion event is ‘Low’, but resilience is ‘High’, so that sensitivity is assessed as ‘Low’, based on the information for these species and the characterizing Coralline spp.

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

The species characterizing this biotope group are epifauna or epiflora occurring on rock, which is resistant to subsurface penetration. The assessment for abrasion at the surface only is, therefore, considered to equally represent sensitivity to this pressure.

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

This biotope occurs in scoured habitats, and it is likely, depending on local sediment supply, that the biotope is exposed to chronic or intermittent episodes of high levels of suspended solids as local sediments are re-mobilised and transported. A significant increase in suspended solids may result in smothering (see siltation pressures) where these are deposited. Based on Cole et al. (1999) and Devlin et al. (2008), this biotope is considered to experience intermediate turbidity (10 to 100 mg/l) based on UK TAG (2014). An increase at the pressure benchmark refers to a change to medium turbidity (100-300 mg/l), and a decrease is assessed as a change to clear (<10 mg/l) based on UK TAG (2014).

The biotope occurs in shallow waters where light attenuation due to increases in turbidity is probably low, and the characterizing animals are unlikely to be affected by increased or decreased clarity. Encrusting coralline algae, especially, are known to be shade-tolerant and are common components of the understorey on seaweed-dominated shores. Therefore, an increase or decrease in light intensity is unlikely to adversely affect the crustose corallines, as plants can acclimate to different light levels.

Legrand et al. (2021) reported that the deposition of salmon faeces at a depositional flux of 15 g/m² per day, equivalent to locations between 100 and 200 m from a high-capacity salmon farm, reduced light by 60% in mesocosms, resulting in a 94% decline in calcification rates of Lithophyllum soriferum rhodoliths over six weeks. An accumulation of sediment within the turf may attract more sediment-dwelling interstitial invertebrates such as nematodes, harpacticoids, and polychaetes, although in wave-exposed locations, accumulation of sediment is likely to be minimal.

Increased suspended sediment may also result in increased scour, which may adversely interfere with settling spores and recruitment if the factor is coincident with their major reproductive period. However, coralline algae, especially the crustose forms, are thought to be resistant to sediment scour (Littler & Kauker, 1984) and will probably not be adversely affected at the benchmark level. Recoverability is likely to be very high as species are likely to remain in situ, from which recruitment can occur.

An increase in turbidity could be beneficial if the suspended particles are composed of organic matter. However, high levels of suspended solids with increased inorganic particles may reduce filter-feeding efficiencies. A reduction in suspended solids will reduce food availability for filter-feeding species in the biotope (where the solids are organic), although effects are not likely to be lethal over the course of a year. A reduction in light penetration could also reduce the growth rate of phytoplankton and so limit zooplankton levels. However, light penetration itself is unlikely to be an important factor as both Balanus crenatus and Spirobranchus triqueter are recorded from the lower eulittoral or the lower circalittoral. 

Available evidence indicates that Spirobranchus triqueter is tolerant of a wide range of suspended sediment concentrations (Riley & Ballerstedt, 2005). Stubbings and Houghton (1964) recorded Spirobranchus (as Pomatoceros) triqueter in Chichester Harbour, UK, which is a muddy environment. However, Spirobranchus (as Pomatoceros) triqueter has also been noted to occur in areas where there is little or no silt present (Price et al., 1980). According to Bacescu (1972), sabellids are accustomed to turbidity and silt. Spirobranchus triqueter has also recently been recorded by De Kluijver (1993) from Scotland in the aphotic zone, indicating that the species would not be sensitive to an increase in turbidity.

Barnes & Bagenal (1951) found that the growth rate of Balanus crenatus epizoic on Nephrops norvegicus was considerably slower than that of animals on raft-exposed panels. This was attributed to reduced currents and increased silt loading of water in the immediate vicinity of Nephrops norvegicus. In dredge disposal areas in the Weser estuary, Germany, where turbidity is 35% above the natural rate of 10 to 100 mg/l, the abundance of Balanus crenatus was lower than in reference areas (Witt et al., 2004). Separating the effect of increased suspended solids from increased sedimentation and changes in sediment from sediment dumping is problematic (Witt et al., 2004). Balanids may stop filtration after silt layers of a few millimetres have been discharged (Witt et al., 2004), as the feeding apparatus is very close to the sediment surface.

This community is unlikely to be dependent on suspended sediment. A reduction in suspended sediment will probably reduce the risk of scour and reduce food availability for the suspension-feeding species in the biotope (e.g. barnacles and sabellids present), although effects are not likely to be lethal.

Sensitivity assessment

Overall biotope resistance is assessed as ‘High’ to an increase in suspended solids. Resilience is assessed as ‘High’ (by default), as adults are likely to remain in situ, from which recruitment can occur. Hence, the biotope is assessed as ’Not sensitive’ to decreased suspended solids.

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

The characterizing species occur in biotopes subject to sedimentation and scour (such as CR.MCR.EcCr.UrtScr) and are therefore likely to tolerate intermittent episodes of sediment movement and deposition. At the pressure benchmark, ‘light deposition’ represents a low thickness of deposit (5 cm) but could still completely bury the characterizing species. Removal of ‘light’ sediments by wave action and tidal currents would be quick but could result in scour. The effect of this pressure will be mediated by the length of exposure to the deposit and the nature of the deposit. 

In a review of the effects of sedimentation on rocky coast assemblages, Airoldi (2003) outlined the evidence for the sensitivity of encrusting coralline algae to sedimentation. The reported results are contradictory, with some authors suggesting that coralline algae are negatively affected by sediments while others report that encrusting corallines are often abundant or even dominant in a variety of sediment-impacted habitats (Airoldi, 2003). Encrusting coralline algae, especially, are known to be shade-tolerant and are common components of the understorey on seaweed-dominated shores. Therefore, an increase or decrease in light intensity is unlikely to adversely affect the crustose corallines, as plants can acclimate to different light levels. While studying coralligenous assemblages of the Apulian continental shelf in the Mediterranean Sea, Piazzi et al. (2019) found that sedimentation was higher on deep outcrops and suggested that it was the main driver of differences between shallow and deep assemblages. Furthermore, in these areas of high sedimentation, an abundance of stress-tolerant organisms was observed, such as the anemone Parazoanthus axinellae (Piazzi et al., 2019).

Legrand et al. (2021) reported that the deposition of salmon faeces at a depositional flux of 15 g/m² per day, equivalent to locations between 100 and 200 m from a high-capacity salmon farm, reduced light by 60% in mesocosms, resulting in a 94% decline in calcification rates of Lithophyllum soriferum rhodoliths over six weeks. An accumulation of sediment within the turf may attract more sediment-dwelling interstitial invertebrates such as nematodes, harpacticoids, and polychaetes, although in more wave-exposed locations, accumulation of sediment is likely to be minimal.

Increased suspended sediment may also result in increased scour, which may adversely interfere with settling spores and recruitment if the factor is coincident with their major reproductive period. However, coralline algae, especially the crustose forms, are thought to be resistant to sediment scour (Littler & Kauker, 1984) and will probably not be adversely affected at the benchmark level. Recoverability is likely to be very high as species are likely to remain in situ, from which recruitment can occur.

Crustose corallines have been reported to survive under a turf of filamentous algae and sediment for 58 days (the duration of the experiment) in the Galapagos (species not identified, Kendrick, 1991). The crustose coralline Hydrolithon reinboldii has also been reported to survive deposition of silty sediments on subtidal reefs off Hawaii (Littler, 1973). In an experimental study, Balata et al. (2007) enhanced sedimentation on experimental plots in the Mediterranean (close to Tuscany) by adding 400 g of fine sediment every 45 days on plots of 400 cm² for one year. Nearby sites with higher and lower levels of sedimentation were assessed as control plots. Some clear trends were observed. Crustose corallines declined at medium and high levels of sedimentation (Balata et al., 2007). The experiment relates to chronic low levels of sedimentation rather than a single acute event, as in the pressure benchmark, however, the trends observed are considered to have some relevance to the pressure assessment. 

As small, sessile species attached to the substratum, siltation at the pressure benchmark would bury Balanus crenatus and Spirobranchus triqueter. Spirobranchus triqueter is sessile and thus would be unable to avoid the deposition of a smothering layer of sediment. Furthermore, Spirobranchus triqueter are an encrusting species, and depending on retention time, sediment could block larval settlement.

Available evidence indicates that Spirobranchus triqueter is tolerant of a wide range of suspended sediment concentrations (Riley & Ballerstedt, 2005). Stubbings and Houghton (1964) recorded Spirobranchus (as Pomatoceros) triqueter in Chichester Harbour, UK, which is a muddy environment. However, Spirobranchus (as Pomatoceros) triqueter has also been noted to occur in areas where there is little or no silt present (Price et al., 1980). According to Bacescu (1972), sabellids are accustomed to turbidity and silt. Spirobranchus triqueter has also recently been recorded by De Kluijver (1993) from Scotland in the aphotic zone, indicating that the species would not be sensitive to an increase in turbidity.

Holme & Wilson (1985) described a Pomatoceros-Balanus assemblage on ‘hard surfaces subjected to periodic severe scour and ‘deep submergence by sand or gravel’ in the English Channel. They inferred that the Pomatoceros-Balanus assemblage was restricted to fast-growing settlers able to establish themselves in short periods of stability during summer months (Holme & Wilson, 1985), as all fauna were removed in the winter months. Barnacles may stop filtration after silt layers of a few millimetres have been discharged, as the feeding apparatus is very close to the sediment surface (Witt et al., 2004). In dredge disposal areas in the Weser estuary, Germany, where the modelled exposure to sedimentation was 10 mm for 25 days, with the centre of the disposal ground exposed to 65 mm for several hours before dispersal, Balanus crenatus declined in abundance compared to reference areas (Witt et al., 2004). However, separating the effect of sedimentation from increased suspended solids and changes in sediment from sediment dumping was problematic (Witt et al., 2004).

Sensitivity assessment

This biotope is IR.FIR.SG.CC and its sub-biotopes (CC.Mo & CC.BalSpi) are described as subject to significant scour (Connor et al., 2004; JNCC, 2022). The biotope occurs on the walls of caves and gullies, above the floor of coarse sediments (cobbles and stones) (JNCC, 2022), where the step or vertical surfaces would be immune to smothering and the high-energy water flow would remove a deposit of fine sediment within one tidal cycle. Therefore, resistance has been assessed as ‘High’, and resilience as ‘High’. Sensitivity has been assessed as ‘Not Sensitive’. 

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

The characterizing species occur in biotopes subject to sedimentation and scour (such as CR.MCR.EcCr.UrtScr) and are therefore likely to tolerate intermittent episodes of sediment movement and deposition. At the pressure benchmark, ‘light deposition’ represents a low thickness of deposit (30 cm) but could still completely bury the characterizing species. Removal of ‘light’ sediments by wave action and tidal currents would be quick but could result in scour. The effect of this pressure will be mediated by the length of exposure to the deposit and the nature of the deposit. 

In a review of the effects of sedimentation on rocky coast assemblages, Airoldi (2003) outlined the evidence for the sensitivity of encrusting coralline algae to sedimentation. The reported results are contradictory, with some authors suggesting that coralline algae are negatively affected by sediments, while others report that encrusting corallines are often abundant or even dominant in a variety of sediment-impacted habitats (Airoldi, 2003). Encrusting coralline algae, especially, are known to be shade-tolerant and are common components of the understorey on seaweed-dominated shores. Therefore, an increase or decrease in light intensity is unlikely to adversely affect the crustose corallines, as plants can acclimate to different light levels. While studying coralligenous assemblages of the Apulian continental shelf in the Mediterranean Sea, Piazzi et al. (2019) found that sedimentation was higher on deep outcrops and suggested that it was the main driver of differences between shallow and deep assemblages. Furthermore, in these areas of high sedimentation, an abundance of stress-tolerant organisms was observed, such as the anemone Parazoanthus axinellae (Piazzi et al., 2019).

Legrand et al. (2021) reported that the deposition of salmon faeces at a depositional flux of 15 g/m² per day, equivalent to locations between 100 and 200 m from a high-capacity salmon farm, reduced light by 60% in mesocosms, resulting in a 94% decline in calcification rates of Lithophyllum soriferum rhodoliths over six weeks. An accumulation of sediment within the turf may attract more sediment-dwelling interstitial invertebrates such as nematodes, harpacticoids, and polychaetes, although in more wave-exposed locations, accumulation of sediment is likely to be minimal.

Increased suspended sediment may also result in increased scour, which may adversely interfere with settling spores and recruitment if the factor is coincident with their major reproductive period. However, coralline algae, especially the crustose forms, are thought to be resistant to sediment scour (Littler & Kauker, 1984) and will probably not be adversely affected at the benchmark level. Recoverability is likely to be very high as species are likely to remain in situ, from which recruitment can occur.

Crustose corallines have been reported to survive under a turf of filamentous algae and sediment for 58 days (the duration of the experiment) in the Galapagos (species not identified, Kendrick, 1991). The crustose coralline Hydrolithon reinboldii has also been reported to survive deposition of silty sediments on subtidal reefs off Hawaii (Littler, 1973). In an experimental study, Balata et al. (2007) enhanced sedimentation on experimental plots in the Mediterranean (close to Tuscany) by adding 400 g of fine sediment every 45 days on plots of 400 cm² for one year. Nearby sites with higher and lower levels of sedimentation were assessed as control plots. Some clear trends were observed. Crustose corallines declined at medium and high levels of sedimentation (Balata et al., 2007). The experiment relates to chronic low levels of sedimentation rather than a single acute event, as in the pressure benchmark, however, the trends observed are considered to have some relevance to the pressure assessment. 

As small, sessile species attached to the substratum, siltation at the pressure benchmark would bury Balanus crenatus and Spirobranchus triqueter. Spirobranchus triqueter is sessile and thus would be unable to avoid the deposition of a smothering layer of sediment. Furthermore, Spirobranchus triqueter are an encrusting species, and depending on retention time, sediment could block larval settlement.

Available evidence indicates that Spirobranchus triqueter is tolerant of a wide range of suspended sediment concentrations (Riley & Ballerstedt, 2005). Stubbings and Houghton (1964) recorded Spirobranchus (as Pomatoceros) triqueter in Chichester Harbour, UK, which is a muddy environment. However, Spirobranchus (as Pomatoceros) triqueter has also been noted to occur in areas where there is little or no silt present (Price et al., 1980). According to Bacescu (1972), sabellids are accustomed to turbidity and silt. Spirobranchus triqueter has also recently been recorded by De Kluijver (1993) from Scotland in the aphotic zone, indicating that the species would not be sensitive to an increase in turbidity.

Holme & Wilson (1985) described a Pomatoceros-Balanus assemblage on ‘hard surfaces subjected to periodic severe scour and ‘deep submergence by sand or gravel’ in the English Channel. They inferred that the Pomatoceros-Balanus assemblage was restricted to fast-growing settlers able to establish themselves in short periods of stability during summer months (Holme & Wilson, 1985), as all fauna were removed in the winter months. Barnacles may stop filtration after silt layers of a few millimetres have been discharged, as the feeding apparatus is very close to the sediment surface (Witt et al., 2004). In dredge disposal areas in the Weser estuary, Germany, where the modelled exposure to sedimentation was 10 mm for 25 days, with the centre of the disposal ground exposed to 65 mm for several hours before dispersal, Balanus crenatus declined in abundance compared to reference areas (Witt et al., 2004). However, separating the effect of sedimentation from increased suspended solids and changes in sediment from sediment dumping was problematic (Witt et al., 2004).

Sensitivity assessment

This biotope is IR.FIR.SG.CC and its sub-biotopes (CC.Mo & CC.BalSpi) are described as subject to significant scour (Connor et al., 2004; JNCC, 2022). The biotope occurs on the walls of caves and gullies, above the floor of coarse sediments (cobbles and stones) (JNCC, 2022), where the step or vertical surfaces would be immune to smothering and the high-energy water flow would remove a deposit of fine sediment within one tidal cycle. Therefore, resistance has been assessed as ‘High’, and resilience as ‘High’. Sensitivity has been assessed as ‘Not Sensitive’. 

Litter

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

Evidence

All characterizing species for this biotope are sessile encrusting epifauna. Fishing disturbance is unlikely to affect species on cave and gully walls. However, ghost fishing from discarded or lost gear could be responsible for damage to epifaunal communities (Sheehan et al., 2017; Giusti et al., 2019).

In addition, Spirobranchus triqueter is also known to colonize floating marine debris, mostly favouring plastics (Gündogdu, Çevik & Karaca, 2017; Abelouah et al., 2024). Settling on floating marine debris may aid Spirobranchus dispersal and recovery.

There are no records of ghost fishing affecting the characterizing species for this biotope. However, epifaunal communities are vulnerable to damage from fishing gear, and are likely vulnerable to being dislodged or damaged through lost fishing gear, and possibly certain types of marine litter. At present, there is 'Insufficient evidence' to complete a sensitivity assessment on the effect of litter on this biotope.

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 is still severely lacking. 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. There have been no studies investigating the effect of EMFs at the population or community level for benthic organisms. 

No studies have examined the effect of EMFs on the characterizing species. However, one study was performed on the reef-forming annelid, Ficopomatus enigmaticus (Oliva et al., 2023). Sperm cells from this species were exposed to 0.5 and 1.0 mT of a static magnetic field. After only three hours of exposure, sperm fertilization rate was reduced, and significant increases in DNA damage and mitochondrial activity, indicative of a stress response, were reported. However, there is ‘Insufficient evidence’ on which to base an assessment of the likely sensitivity of this biotope to EMFs.

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

Encrusting corallines can occur in deeper water than other algae, where light penetration is limited. Spiecker & Menge (2024) reported that shading increased crustose algal cover by between 22 and 32% in a New Zealand field experiment, depending on grazer presence. Arnold et al. (2022) found that Lithothamnion glaciale rhodoliths in Newfoundland grew steadily across irradiances from 0.02 to 0.78 mol photons/m²/day, indicating broad light tolerance. Bélanger & Gagnon (2021) reported that Lithothamnion glaciale rhodoliths maintained similar growth rates at 15 m and 25 m depth, despite approx. 60% lower irradiance, showing high light-use efficiency. Sowa et al. (2023) found that crustose corallines dominated experimental settlement plates during the winter in Svalbard. Similarly, Ronowicz et al. (2024) observed high settlement of crustose Lithothamnion sp. during the polar night in Svalbard, indicating that reproduction and recruitment can occur without solar radiation and confirming adaptation to very low light conditions. However, Tâmega & Figueiredo (2019) found that low light inhibited the growth of Porolithon onkodes and Lithophyllum stictaeforme, while Porolithon onkodes dominated high-light environments and grew faster, indicating species-specific responses. Legrand et al. (2021) found that salmon faeces deposition at 15 g/m²/day reduced light by approx. 60% in mesocosms and led to a 94% decline in calcification and 41% lower net primary production of Lithothamnion soriferum rhodoliths after six weeks.

Samples of Lithophyllum impressum suspended from a raft and shaded (50 to 75% light reduction) continued to grow over two years (Dethier, 1994). Similarly, Plocamium cartilagineum grows in shaded conditions beneath laminarian canopies, where irradiance is greater, growth is lower, and it appears that light levels of 0.5 mmol/m²/s are inhibitory (Kain, 1987). In areas of higher light levels, the fronds and bases may be lighter in colour due to bleaching (Colhart & Johansen, 1973). Other red algae in the biotope are flexible regarding light levels and can also acclimate to different light levels. 

Spirobranchus triqueter is found in a variety of light environments, from shallow sublittoral biotopes where light levels are relatively high, to deeper sites that are aphotic (De Kluijver, 1993). 

Balanus crenatus possesses a rudimentary eye and can detect and respond to sudden shading, which may be an anti-predator defence (Forbes et al., 1971). Balanus crenatus tend to orient themselves when settling, with the least light-sensitive area directed towards the light (Forbes et al., 1971), so that the more sensitive area can detect shading from predator movements in the area where light availability is lower (Forbes et al., 1971).

It should be noted that, since 2016, research on artificial light at night (ALAN) has expanded rapidly. ALAN alters benthic communities, algal cover, and ecological interactions in coastal systems (reviewed in Marangoni et al., 2022; Ferretti et al., 2025), with experimental studies demonstrating effects on algal assemblages under both increased and reduced light conditions (Schaefer et al., 2025; Trethewy et al., 2023). Although no studies have tested direct effects on any of the characterizing species, the assumption that light is an insignificant pressure is no longer supported by the wider evidence base.

Sensitivity assessment

Although Spirobranchus triqueter and Balanus crenatus are demonstrably tolerant of variation in natural light and shade (Forbes et al., 1971; De Kluijver, 1993), and crustose and rhodolith-forming corallines also show broad tolerance of natural light variation, with growth and recruitment maintained under shading and very low light (e.g. Bélanger & Gagnon 2021; Arnold et al., 2022; Sowa et al., 2023; Ronowicz et al., 2024), there is now substantial evidence that artificial light at night (ALAN) alters coastal communities and trophic interactions, including algal assemblages (e.g. Marangoni et al., 2022; Trethewy et al., 2023; Ferretti et al., 2025; Schaefer et al., 2025). As no studies have tested direct effects on any of the characterizing species, the sensitivity of this biotope to the introduction of light via anthropogenic means is assessed as ‘Insufficient evidence’.

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

Barriers that reduce the degree of tidal excursion may alter larval supply to suitable habitats from source populations. Conversely, the presence of barriers may enhance local population supply by preventing the loss of larvae from enclosed habitats.  Barriers and changes in the tidal excursion are not considered relevant to the characterizing crusting corallines, as species dispersal is limited by the rapid rate of settlement and vegetative growth from bases rather than reliance on recruitment from outside populations. Resistance to this pressure is assessed as 'High' and resilience as 'High' by default. This biotope is therefore considered to be 'Not sensitive'.

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' to seabed habitats.  NB. Collision by grounding vessels is addressed under surface abrasion

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

Many invertebrate species within the biotope probably respond to light levels, detecting shade and shadow to avoid predators and day length in their behavioural or reproductive strategies. However, their visual acuity is probably very limited, and they are unlikely to respond to a visual disturbance at the benchmark level. This pressure is, therefore, assessed as ‘Not relevant’.

Balanus crenatus possesses a rudimentary eye and can detect and respond to sudden shading, which may be an anti-predator defence (Forbes et al., 1971). Balanus crenatus tend to orient themselves when settling, with the least light-sensitive area directed towards the light (Forbes et al., 1971), so that the more sensitive area can detect shading from predator movements in the area where light availability is lower (Forbes et al., 1971).

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

Key characterizing species within this biotope are not cultivated or translocated. This pressure is therefore considered ‘Not relevant’ to this biotope group.

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

No evidence was found that microbial pathogens cause high levels of disease or mortality in this biotope.

Diseased encrusting corallines were first observed in the tropics in the early 1990s when the bacterial pathogen Coralline Lethal Orange Disease (CLOD) was discovered (Littler & Littler, 1995). All species of articulated and crustose species tested to date are easily infected by CLOD, and it has been increasing in occurrence at sites where it was first observed and spreading through the tropics. Another bacterial pathogen causing a similar CLOD disease has been observed with a greater distribution, and a black fungal pathogen first discovered in American Samoa has been dispersing (Littler & Littler, 1998). An unknown pathogen has also been reported to lead to white ‘target-shaped’ marks on corallines, again in the tropics (Littler et al., 2007). No evidence was found that these pathogens are impacting temperate coralline habitats.

Sensitivity assessment. Based on the lack of reported mortalities of the characterizing species and the available evidence for the characterizing coralline crust, the biotope is judged to have ‘High’ resistance to this pressure. By default, resilience is assessed as ‘High’, and the biotope is classed as ‘Not sensitive’ at the pressure benchmark. 

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

Direct, physical impacts from harvesting are assessed through the abrasion and penetration of seabed pressures. The sensitivity assessment for this pressure considers any biological/ecological effects resulting from the removal of target species from this biotope. No commercial application or harvesting of characterizing or associated species are described in the literature. Hence, this pressure is considered to be 'Not relevant'.

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 removal of the key characterizing species would alter the character of the biotope, resulting in reclassification and the loss of species richness. The ecological services such as primary and secondary production, provided by characterizing and associated species, would also be lost. As most species present in this biotope are relatively large, conspicuous, and either sedentary or attached to rock surfaces, they have little protection against removal.

Sensitivity assessment

Removal of a large percentage of the characterizing species resulting in bare rock would alter the character of the biotope, species richness and ecosystem function. Therefore, resistance is assessed as ‘Low’ and resilience as ‘Medium’ (based on the removal of coralline crusts), so that biotope sensitivity is assessed as 'Medium’.

The American slipper limpet, Crepidula fornicata

Evidence

Crepidula fornicata larvae require hard substrata for settlement. It prefers muddy, gravelly, shell-rich substrata that include gravel, the 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. But it 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; Tillin et al., 2020). Close examination of the literature (2023) shows that evidence of its colonization and density on bedrock in the infralittoral or circalittoral was lacking. Tillin et al. (2020) suggested that Crepidula could colonize circalittoral rock due to its presence on tide-swept rough grounds at 60 metres 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 a gravel substratum with boulders. Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas dominated by boulders. 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. In addition, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats. It was only recorded at low density (0.1-0.9/m²) in one faunal turf biotope (CR.MCR.CFaVS.CuSpH.As) (JNCC, 2015). Faunal turfs are dominated by suspension feeders, so larval predation is probably high, which may prevent colonization by Crepidula. Also, faunal turf species actively compete for space, and many are fast-growing and opportunistic, so they may out-compete Crepidula for space even if it gained a foothold in the community. 

Sensitivity assessment

The wave-exposed infralittoral rock and high levels of scour characterizing this biotope is likely to be unsuitable for the colonization by Crepidula fornicata. Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011) and has been recorded from the lower intertidal to ca 160 m in depth, but it is most common in the shallow subtidal above 50 m (Blanchard, 1997; Thieltges et al., 2003; Bohn et al., 2012, 2015; Hinz et al., 2011; OBIS, 2023; Tillin et al., 2020). However, no evidence was found of the effect of Crepidula populations on coralline crust-dominated habitats. As scouring of this biotope by mobile sediments limits the establishment of all but robust species, resistance to Crepidula fornicata is assessed as ‘High’ and resilience as ‘High’ (by default), so that the biotope is assessed as ‘Not sensitive’.

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). Didemnum vexillum has colonized and established populations in the northeast 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).

Although a widespread invader, Didemnum vexillum has a limited ability for natural dispersal since the pelagic larvae remain in the water column for a short time (up to 36 hours). Therefore, it has a short dispersal phase that can allow the species to build localized populations (Herborg et al., 2009; Vercaemer et al., 2015; Holt, 2024). However, Bullard et al. (2007) suggested that Didemnum vexillum can form new colonies asexually by fragmentation. Colonies can produce long tendrils from an encrusting colony, which can fragment, disperse and settle, attaching to suitable hard substrata elsewhere (Bullard et al., 2007; Lambert, 2009; Stefaniak & Whitlatch, 2014). A fragmented colony can spread naturally for up to three weeks, transported by ocean currents, attached to floating seaweed, seagrass or other floating biota, or as free-floating spherical colonies (Bullard et al., 2007; Lengyel et al., 2009; Stefaniak & Whitlatch, 2014; Holt, 2024). Fragments can reattach to suitable substrata within six hours of contact. Fragments have the potential to disperse around 20 km before reattachment (Lengyel et al., 2009). Valentine et al. (2007a) reported that colonies of Didemnum vexillum enlarged by 6 to 11 times by asexual budding after 15 days and enlarged 11 to 19 times after 30 days. Valentine et al. (2007a) concluded fragments could successfully grow, survive, and help to spread Didemnum vexillum.

While natural fragmentation of tendrils is thought to allow Didemnum vexillum to invade longer distances and increase its dispersal potential, Stefaniak & Whitlatch (2014) found that only one tendril out of 80 reattached to the flat, bare substrata used in their study, because tendrils required an extensive (at least eight-hour) period of contact to reattach. Stefaniak & Whitlatch (2014) suggested that once fragmented from a colony, the success of tendril reattachment was limited, and reattachment was not a major contributor to the invasive success of Didemnum vexillum. However, Stefaniak & Whitlatch (2014) also found that larvae-packed tendril fragments may increase natural dispersal distance, reproduction, and invasive success of Didemnum vexillum, and increase the distance larvae can travel. Not all colonies produce tendrils at all locations.

Human-mediated transport via aquaculture facilities, boat hulls, commercial fishing vessels, and ballast water is probably the most important vector that has aided the long-distance dispersal of Didemnum vexillum and explains its prevalence in harbours and marinas (Bullard et al., 2007; Dijkstra et al., 2007; Griffith et al., 2009; Herborg et al., 2009). Fragmentation of colonies during transport or human disturbance (such as trawling or dredging) could indirectly disperse the species and enable it to find suitable conditions for establishment (Herborg et al., 2009). For example, in oyster farms in British Columbia, large fragments of Didemnum sp. come off oyster strings when they are pulled out of water, and other fragments can be pulled off oysters and mussels and thrown back into the water, which is likely to aid dispersal of the invasive species (Bullard et al., 2007). Dijkstra et al. (2007) hypothesised that Didemnum sp. was introduced to the Gulf of Maine with oyster aquaculture in the Damariscotta River and transported via Pacific oysters.

Didemnum vexillum was likely introduced into the UK from northern Europe or Ireland via poorly maintained or not antifouled vessels, movement of contaminated shellfish stock and aquaculture equipment, or via marine industries such as oil, gas, renewables, and dredging (Holt, 2024). Recent evidence from genetic material suggests that human-mediated dispersal between marinas and shellfish culture sites is the most likely pathway for connectivity of Didemnum vexillum populations throughout Ireland and Britain (Prentice et al., 2021; Holt, 2024). Didemnum vexillum can disperse away from artificial substrata, invading and colonizing natural substrata in surrounding areas (Tillin et al., 2020). Holt (2024) noted that Didemnum vexillum had not spread as far as feared in the UK since it was first recorded. The current evidence of Didemnum vexillum’s ability to spread on natural habitats in this area is sparse and often conflicting, complicated by genetics, its apparent variable habitat preferences and tolerances and its variable ability to adapt to ‘new’ conditions (Holt 2024).

Didemnum vexillum has a seasonal growth cycle that is influenced by temperature (Valentine et al., 2007a). In warmer months (June and July), colonies may be large and well-developed encrusting mats. Populations experience more rapid growth from July to September, sometimes continuing into December. Colonies begin to decline in health and ‘die-off’ when temperatures drop below 5°C during winter months from around October to April (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). Cold-water months cause colonies to regress and reduce in size, yet they often regenerate as temperatures warm (Griffith et al., 2009; Kleeman, 2009; Mercer et al., 2009), although some populations may not survive winter at all (Dijkstra et al., 2007). The early growth phase, from May to July, is initiated by smaller colonies developing from remnants of colonies that survived the cold water (Valentine et al., 2007a). The seasonal growth cycle is also likely influenced by location. For example, the Didemnum sp. growth cycle for colonies in Sandwich tide pool (temperature range from -1 °C to 24 °C, with daily fluctuations), probably does not occur in deep offshore subtidal habitats in Georges Bank (annual temperature range from 4 °C to 15°C, and daily fluctuations are minimal) (Valentine et al., 2007a). Larval release and recruitment typically occur between 14 and 20°C and slow or cease below 9 to 11°C as summer ends (Griffith et al., 2009; McKenzie et al., 2017). In New Zealand, recruitment occurs from November to July, where the highest average temperatures were recorded in February (18 to 22°C), and the lowest average temperatures were recorded in July (9 to 10°C) (Fletcher et al., 2013a). In this New Zealand study, higher water temperatures were associated with a higher level of recruitment (Fletcher et al., 2013a).

Didemnum vexillum requires suitable hard substrata for successful settlement and the establishment of colonies. It can grow quickly and 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). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems because of its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock, gravel, pebble, cobble, or boulders or artificial substrata such as a variety of maritime structures, such as pontoons, docks, wood and metal pilings, chains, ropes and moorings, plastic and ship hulls and at aquaculture facilities (Valentine et al., 2007a&b; Bullard et al., 2007; Griffith et al., 2009; Lambert, 2009; Tagliapietra et al., 2012; 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).

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).

In contrast, Didemnum vexillum’s preference for sheltered conditions, established colonies observed in Georges Bank and Long Island Sound were exposed to moderately strong tidal currents (1 to 2 knots; ca 0.5 to 1 m/s recorded at both sites) that may mobilise sediment (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020). However, Valentine et al. (2007b) describe the substratum as immobile, presumably consolidated, gravel, cobbles, and pebbles. Kleeman (2009) stated that the presence of a consistent mild wave action or ‘swash zone’ appears to favour Didemnum sp. establishment in the intertidal. Although some evidence suggests that waves and currents can facilitate the fragmentation and spread of Didemnum vexillum (McKenzie et al., 2017), the tidal current velocities at some sites where Didemnum vexillum has been reported (for example, New England, where current velocities reach up to around 3 m/s) is lower than the current velocity required for the dislodgement of Didemnum vexillum fragments (around 7.6 m/s) (Reinhardt et al., 2012). This suggests that not all tidal currents are likely to dislodge Didemnum vexillum fragments. When on boat hulls, the species can experience higher current velocities, which are enough to cause dislodgement (Reinhardt et al., 2012).  

Sensitivity assessment

Didemnum vexillum has been recorded in the sublittoral to depths of 81 m in Georges Bank and 30 m in Long Island, USA (Bullard et al., 2007; Valentine et al., 2007b; Mercer et al., 2009). This biotope, and related biotopes, occurs on bedrock, boulders, or cobbles, which could provide a suitable hard substratum for colonization by Didemnum sp. Didemnum vexillum is reported to prefer sheltered conditions but has also been recorded in moderately strong currents (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020) and is predicted to survive stronger currents, as the current velocity which will dislodge Didemnum vexillum is around 7.6 m/s (Reinhardt et al., 2012).

This biotope, and related biotopes, experiences very weak to moderately strong water flow (>0.5 to 1.5 m/s) and moderately exposed to very exposed wave exposure. The biotope is structured by severe wave-driven scour, especially in the winter months, which removes all but the most resistant coralline species. The CC.BalSpi sub-biotope occurs where scour is reduced sufficiently to allow Balanus crenatus and Spirobranchus triqueter to colonize the rock in spring, only to be reduced or removed again in winter. However, other species can occur in crevices and cracks that provide refugia from scour, e.g. Halichrondria sp., Urticina felina, and Cylista elegans. If Didemnum sp. could gain a 'foothold' in such refugia, or occupy other biotopes in the vicinity (e.g. crustose sponge and ascidian-dominated biotopes), then it might be able to colonize the bare rock in spring in the CC.BalSpi biotope, smother, and compete with other epifauna, but it is unlikely to dominate the biotope. Therefore, a resistance of 'Medium' (some, <25% mortality) is suggested as a precaution in case Didemnum vexillum could colonize the biotope, but with 'Low' confidence due to the lack of direct evidence. Resilience is assessed as 'High', as recovery would require the physical removal of Didemnum sp., which is likely due to the scouring nature of this biotope, so sensitivity is assessed as 'Low'. 

The Pacific oyster, Magallana gigas

Evidence

The majority of the evidence indicates that infralittoral rock and other habitats that occur at depths more than 10 m are unlikely to be suitable for Magallana gigas because it is considered an intertidal and shallow subtidal species rarely recorded below extreme low water (Herbert et al., 2012, 2016; Tillin et al., 2020). Therefore, this biotope is probably 'Not sensitive to this INIS.

Wireweed, Sargassum muticum

Evidence

The wave surge and scour, combined with the low levels of light within this biotope, particularly the low walls of gullies, are considered to inhibit colonization of invasive algal species. Hence, it is unlikely to be colonized by Sargassum. Therefore, this biotope is probably 'Not sensitive to this INIS.

Wakame, Undaria pinnatifida

Evidence

The wave surge and scour, combined with the low levels of light within this biotope, particularly the low walls of gullies, are considered to inhibit colonization of invasive algal species. Hence, it is unlikely to be colonized by Undaria. Therefore, this biotope is probably 'Not sensitive’ to this INIS.

Other INIS

Evidence

Increased warming has allowed the Australian barnacle Austrominius (formerly Elminius) modestus to dominate sites previously occupied by Semibalanus balanoides and Balanus crenatus (Witte, 2010). However, on settlement panels deployed in SW Ireland, Austrominius modestus initially dominated panels in the lower subtidal, but post-recruitment mortality over a year allowed Balanus crenatus to become the dominant barnacle (Watson et al., 2005). Balanus crenatus and Austrominius modestus have shown recruitment differences, which may alter the seasonal dominance patterns (Witte, 2010). Free-living aggregations (Balanuliths) of Balanus crenatus have been observed growing on shell fragments of the INIS, Ensis directus (Cadée, 2007).

Sensitivity assessment

As scouring of this biotope by mobile sediments limits the establishment of all but robust species, resistance to INIS is assessed as ‘High’ and resilience as ‘High’ (by default), so that the biotope is assessed as ‘Not sensitive’.