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

Temperature decrease (local)

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

Evidence

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

Kelps are tolerant to short-term daily fluctuations in salinity and are recorded as tolerating 5 to 60 PSU. However, they are much less tolerant to long-term changes with growth and photosynthetic rates declining either side of 20 to 45 PSU (Gordillo et al., 2002; Karsten, 2007). Despite this tolerance, Laminaria digitata is considered to be a stenohaline species, and this biotope is only found in conditions of full salinity (Connor et al., 2004). Other species probably outcompete Laminaria digitata at the limits of its salinity tolerance, so this biotope is unlikely to occur in salinities above 40 PSU. Axelsson & Axelsson (1987) indicated that damage of the plants’ plasma membranes occurs when salinity is below 20 or above 50 PSU. Localized, long-term reductions in salinity to below 20 PSU may result in the loss of kelp beds in affected areas (Birkett et al., 1998b).

Pholas dactylus has been recorded in salinities of 30 to 35 PSU, with a small number of records from 35 to 40 PSU (OBIS, 2025). Barnea candida is reported to extend into estuarine environments in salinities down to 20 PSU (Fish & Fish, 1996). Petricolaria pholadiformis is particularly common off the Essex and Thames estuary, e.g. the River Medway (Bamber, 1985) suggesting tolerance of brackish waters. Zenetos et al. (2009) suggest that at all sites where Petricolaria pholadiformis has been found has some freshwater inflow into the sea. According to the literature, the species in its native range inhabits environments with salinities between 29 and 35 PSU, while in the Baltic Sea it is reported from salinities 10 to 30 PSU (Gollasch & Mecke, 1996, cited from Zenetos et al. 2009). According to Castagna & Chanley (1973, cited from Zenetos et al. 2009) the lower salinity tolerance of Petricolaria pholadiformis is 7.5 to 10 PSU. It thus appears that reduced salinity facilitates its establishment (Zenetos et al., 2009). Filipov et al., (2003, abstract only) tested the salinity tolerances of Hiatella arctica obtained from the White Sea. The salinity tolerance of individuals kept at 25 PSU was 17 to 36 PSU. Acclimation of Hiatella arctica allowed them to adapt to higher or lower salinities with the potential tolerance range of acclimated individuals assessed as 13 to 42 PSU.

Sensitivity assessment. Although some increases in salinity may be tolerated by the species present these are generally short-term and mitigated during tidal inundation. This biotope is considered, based on distribution of Laminaria digitata, piddocks and Hiatella arctica on the mid to lower shore, to be sensitive to a persistent increase in salinity to >40 PSU (although Hiatella arctica is considered to have greater tolerance). Resistance is therefore assessed as ‘Low’ and resilience as ‘Medium’ (based on recovery of piddocks). Sensitivity is therefore assessed as ‘Medium'.

Salinity decrease (local)

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

Evidence

Birkett et al. (1998b) suggested that kelps are stenohaline, in that they do not tolerate wide fluctuations in salinity and require regular salinities of 30 to 35 PSU to maintain optimum growth rates. Growth rate may be adversely affected if the kelp plant is subjected to periodic salinity stress. Laminaria digitata tolerates a large salinity range within a 24-hour period (5 to 60 PSU; Karsten, 2007). In the study by Karsten (2007), kelp thalli were kept at constant salinities for five days, with their photosynthetic rates measured after two and five days. At the extremes of this range, decreases in photosynthetic rates are evident, particularly at low salinities (Gordillo et al., 2002). The lower salinity limit for Laminaria digitata lies between 10 and 15 PSU. On the Norwegian coast, Sundene (1964) found healthy Laminaria digitata plants growing between 15 and 25 PSU. Axelsson & Axelsson (1987) indicated that damage of the plants’ plasma membranes occurs when salinity is below 20 or above 50 PSU. Localized, long-term reductions in salinity, to below 20 PSU, may result in the loss of kelp beds in affected areas (Birkett et al., 1998b). Lebrun et al. (2024) found that reduced salinity (20 to 25 PSU) combined with elevated temperatures (+4°C higher than ambient) and reduced light availability (approx. 20% of incident PAR, equivalent to strong turbidity or shading) did not affect any photosynthetic or biochemical metrics.

Mortensen (2017) measured diurnal carbon dioxide exchange rates (CER) in Laminaria digitata and Saccharina latissima at salinities of 34, 24, 16 and 10 PSU to assess how low‑salinity conditions typical of Norwegian fjords affect performance. Laminaria digitata showed reduced CO₂ uptake at lower salinities, with daily CER falling by about 20%, 30% and 40% at 24, 16 and 10 PSU, respectively. However, unlike Saccharina latissima, the reduction at 10 PSU was fully reversible, indicating that Laminaria digitata can recover after exposure to very low salinity.

Pholas dactylus has been recorded in salinities ranging from 30 to 35 PSU, with a small number of records from 35 to 40 PSU (OBIS, 2025). No information was found for the salinity tolerance of Pholas dactylus. Barnea candida is reported to extend into estuarine environments in salinities down to 20 PSU (Fish & Fish, 1996). Barnea candida is reported to extend into estuarine environments in salinities down to 20 PSU (Fish & Fish, 1996). Petricolaria pholadiformis is particularly common off the Essex and Thames estuary, e.g. the River Medway (Bamber, 1985) suggesting tolerance of brackish waters. Zenetos et al. (2009) suggest that at all sites where Petricolaria pholadiformis has been found has some freshwater inflow into the sea. According to the literature, the species in its native range inhabits environments with salinities between 29 and 35 PSU, while in the Baltic Sea it is reported from salinities 10 to 30 PSU (Gollasch & Mecke, 1996, cited from Zenetos et al. 2009). According to Castagna & Chanley (1973, cited from Zenetos et al. 2009) the lower salinity tolerance of Petricolaria pholadiformis is 7.5 to 10 PSU. It thus appears that reduced salinity facilitates its establishment (Zenetos et al., 2009). Filipov et al., (2003, abstract only) tested the salinity tolerances of Hiatella arctica obtained from the White Sea. The salinity tolerance of individuals kept at 25 PSU was 17 to 36 PSU. Acclimation of Hiatella arctica allowed them to adapt to higher or lower salinities with the potential tolerance range of acclimated individuals assessed as 13 to 42 PSU. Gordillo & Aitken (2000) in a review of environmental factors relevant to re-interpreting Late Quaternary environments from fossil collections suggest that the normal minimum salinity tolerance of Hiatella arctica is 20 PSU, based on Aitken (1990) and Peacock (1993).

Sensitivity assessment. The evidence suggests that Laminaria digitata and Hiatella arctica would be resistant to a reduction in salinity at the benchmark level. However, Pholas dactylus has not been found in salinities below 30 PSU. Therefore, a precautionary resistance of ‘Low’ is given to this biotope in case Pholas dactylus is lost from the biotope. Resilience (following return to full salinity) is assessed as ‘Medium’ (2 to 10 years), and the sensitivity of the biotope is assessed as ‘Medium’.

Water flow (tidal current) changes (local)

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

Evidence

This biotope occurs in a range of water flow conditions from moderately strong (0.5-1.5 meters /second) to weak negligible (Connor et al., 2004, JNCC 2022), indicating that the characterizing species can tolerate a range of flow speeds. In Lough Ine in Ireland, Laminaria digitata forms dense forests in the fast-flowing water of the Rapids where water speeds vary from 4 to 6 knots (ca 2 to 3 m/s) (Ebling et al., 1948). Laminaria digitata is also found in very strong flows (>3.87 m/s) although it is often outcompeted by Alaria esculenta. The biotope is not found in areas where sand scour occurs (associated with high water flow rates). The structure of the substratum of this biotope is also likely to reduce water flow by increasing frictional drag, providing some inherent resistance within the biotope. Therefore, Laminaria digitata and associated community will probably not be affected by a change of 0.1-0.2 m/s in peak mean spring bed flow velocity. 

Laminaria digitata partially achieves survival in a range of water flow conditions due to its blade morphology, which varies with flow, becoming narrower and more digitate as water flow rate increases (Sundene, 1964). In a laboratory study, this morphological adaptation was attributed to longitudinal stress with exposure to this stress over six weeks resulting in narrower blades and a significantly higher rate of cell elongation, compared to plants that had not experienced the same stress. This study also suggested that plasticity would serve to decrease the risk of thallus damage in areas of greater exposure or in stormier conditions (Gerard, 1987).

Biogenic habitat structures reduce the effects of water flows on individuals by slowing and disrupting flow. The fronds of Laminaria digitata and the red algal turf will reduce the flow experienced by the turf. Boller & Carrington (2006), for example, found that the canopy created by the taller turf of Chondrus cripsus reduced drag forces on individual plants by 15 to 65%. The crustose holdfasts of Osmundea pinnatifida, Corallina officinalis and the coralline crusts are securely attached and as these are relatively flat, are subject less drag than upright fronds and are likely to tolerate changes in water flows at the pressure benchmark. Moderate water movement is beneficial to seaweeds as it carries a supply of nutrients and gases to the plants and removes waste products. However, if flow becomes too strong, plants may become dislodged.

Millar et al. (2020) examined how different hydrodynamic conditions specifically waves versus currents affect the growth of Laminaria digitata in the low‑intertidal zone. By comparing three contrasting flow environments (low current/low wave, high current/low wave, and high wave/low current), they found that variation in growth was driven entirely by hydrodynamic forces. Growth in high‑current environments was substantially enhanced: meristematic growth increased by 45%, and whole‑blade growth by 25%, relative to high‑wave sites. The authors suggest that currents promote more sustained, unidirectional water movement, whereas waves exert large, intermittent forces that impose drag and bending stress, reducing growth efficiency.

Kregting et al. (2015) found that in winter, the velocity of unidirectional (tidal current) and oscillatory water movement (waves) had no influence on the growth rate of Laminaria digitata. However, in summer, when nitrate concentrations are low, increased water motion can enhance the supply of nitrates to the kelp and increase growth. Nitrate uptake rates were lowest at velocities <5 cm/s and increased with velocity, regardless of the type of water motion. However, oscillatory motion increased nitrate uptake rates by 20 to 50% compared to unidirectional flow at the same speed. These findings suggest that it is the velocity of water speed and the supply of nutrients, rather than the type of water flow, that are important for macroalgal growth. They also show seasonal variation in the effects of this pressure on this biotope.

Spore dispersal is in part governed by the local hydrodynamic regime; increased turbulence is associated with an increase in biotope connectivity and therefore a loss of spores from the local system. A decrease in wave and current mediated water flow is identified by lower connectivity with other sites and a higher settlement rate within the local biotope (Robins et al., 2013). Therefore, an increase in water flow could result in loss of spores from the local biotope, which if not balanced by a spore influx from another geographically different population, could result in the demise of the biotope’s health, with a shift in the age structure of the population and a death of young algae. A decrease in the level of water flow is unlikely to have a detrimental effect because the species often grow in areas of low water movement where it may form extensive loose-lying populations (Burrows, 1958; cited in White & Marshall, 2007).

Established adult piddocks and Hiatella arctica are, to a large extent, protected from direct effects of increased water flow, owing to their environmental position within the substratum. Increases or decreases in flow rates may affect suspension feeding by altering the delivery of suspended particles or the efficiency of filter feeding. The most damaging effect of increased flow rate would be the erosion of the clay or soft chalk substratum as this could eventually lead to loss of the habitat. No evidence was found to assess the water velocities at which erosion of clay or chalk occurs. Some erosion will occur naturally, and storm events and wave action may be more significant in loss and damage of substratum than surface water flow.

Sensitivity assessment. Based on the exposure of the characterising species to a range of water flows in this and other biotopes (Connor et al., 2004, JNCC 2022), they are considered to be unimpacted by changes within this range as long as these do not lead to increased erosion of the substratum. Resistance is therefore assessed as 'High' and resilience as 'High' (based on no impact to recover from), so the biotope is considered to be ‘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

This biotope occurs in the shallow sublittoral and is therefore exposed to changes in emergence. Increased emergence will expose the biotope to air for longer periods, leading to drying. Laminaria digitata is relatively resistant to desiccation, surviving up to 40% water loss (Dring & Brown, 1982). The desiccation tolerance of Laminaria digitata allows beds to extend another 2 cm into the eulittoral zone, where grazing pressure is removed (Southward & Southward, 1978). When exposed to the air, macroalgal canopies can buffer the effects of high temperatures and water loss on organisms below their fronds, with substratum temperatures on average 8 to 10°C lower under the canopy than bare rock, additionally decreasing water loss by >45% (Bertness et al., 1999). Harris et al. (2025) also reported this buffering effect in Fucus serratus canopies during low tide emersion. They recorded differences of up to 13.8°C between below- and above-canopy temperatures. Below-canopy individuals showed virtually no reduction in photosynthetic efficiency or water loss while their above-canopy counterparts experienced significant, cumulative thermal stress. It is therefore possible that smaller, below-canopy Laminaria digitata may have some level of protection from desiccation stress.

Migné et al. (2015) investigated the photosynthetic activity and productivity of intertidal macroalgae, including Laminaria digitata, under natural conditions of emersion (air exposure) and immersion (submersion). They found that Laminaria digitata exhibited very low photosynthetic activity and productivity during emersion, with electron transport rates (ETR) up to 16 times lower than upper shore species such as Pelvetia canaliculata. Under air exposure, effective quantum yield (a measure of photosynthetic efficiency) dropped to critical levels (<0.1), and respiration often exceeded photosynthesis, resulting in net carbon loss. In contrast, Laminaria digitata performed much better when submerged, showing higher ETR and positive carbon flux. These results indicate that Laminaria digitata is poorly adapted to aerial exposure and physiologically optimized for underwater photosynthesis, explaining its dominance in lower-shore habitats where emersion is brief.

An increase in the benchmark level for air exposure may result in the depression of the biotope’s upper limit, as this species’ lower limit is set by competition with Laminaria hyperborea (Hawkins & Harkin, 1985). The upper, landward limits of Laminaria digitata biotopes are generally set by competition with the brown algae Fucus serratus (Hawkins & Harkin, 1985); therefore, a decrease in the benchmark level for air exposure may result in the extension of the biotope’s upper limit. The main driver of competition between Fucus serratus and Laminaria digitata is based on the ability of Fucus serratus to control its respiration rates based on its desiccation rates, which Laminaria digitata is unable to do. Therefore, longer periods of emergence may result in a compression of Laminaria digitata’s extent as it is outcompeted by Fucus serratus at its upper limit. The kelp is able to resist both an increase and a decrease in emergence; however, this resistance is based on the free movement of this species within its environmental optima, shifting up or down the shore. Therefore, if an obstacle to movement perpendicular to the shoreline (e.g. sea defence) is then combined with a change in the emergence regime, this biotope could undergo compression of its range and potentially local loss.

The characteristic species of this biotope have no mobility and cannot therefore migrate up or down shore to adapt to changes in emergence. Within the substratum, bivalves will be afforded some protection from desiccation following increased emergence by their burrows, which will retain some moisture. However, the shells of piddocks do not completely enclose the animals and therefore cannot be closed to prevent water loss. The tolerance of piddocks to increased and decreased emergence varies between species. Pholas dactylus inhabits the shallow sub-tidal and lower shore, while Barnea candida and Petricolaria pholadiformis live slightly higher up the shore (Duval, 1977). During extended periods of exposure, Pholas dactylus squirts some water from its inhalant siphon and extends its gaping siphons into the air (Knight, 1984). This may result in increased detection and predation by birds. Hiatella arctica occurs within the intertidal and subtidal zones, but the presence of suitable substratum is a more significant factor determining the distribution. Red algal turfs, piddocks and other boring infauna are found higher on the shore in the Fucus serratus biotope LR.MLR.BF.Fser.Pid. Therefore, an increase in emergence could possibly lead to biotope reclassification (where Fucus serratus replaces Laminaria digitata).

Sensitivity assessment. This pressure is a key driver of biotope extent because the upper and lower limits of this species are set by interspecific competition. In the direct footprint of the impact resistance is probably ‘Low’ based on the loss of Laminaria digitata (loss of 25 to 75%). Resilience is suggested as ‘Medium’ (2 to 10 years) following restoration of the emergence regime. This biotope is, therefore, considered to have ‘Medium’ sensitivity to the pressure.

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

The structure of kelps enables them to survive a range of wave conditions from wave exposed to sheltered conditions (Connor et al., 2004; Harder et al., 2006). Physiological differences between kelps are evident between low wave exposure and medium-high wave exposure. The greatest wet weight of Laminaria digitata occurs at low wave exposure (mean significant wave height <0.4 m) decreasing by a mean of 83% in medium to high wave exposures (mean significant wave height >0.4m; Gorman et al., 2013). At medium to high levels of wave exposure, Laminaria digitata biomass has been shown to decrease by 83% in the field (Wernberg & Thomsen, 2005). In areas of high wave exposure, Laminaria digitata may extend its upper limits into the lower eulittoral zone.

Laminaria digitata erosion rates are generally higher on more exposed shores (Gilson et al., 2023). However, this biotope can be found on sheltered, moderately exposed, and exposed shores (JNCC 2022), and erosion rates on exposed shores do not typically result in biotope loss.

The morphology of Laminaria digitata differs between sites with differing levels of wave exposure. Experiments showed that Laminaria digitata individuals translocated from exposed to sheltered sites resulted in frond widening, while individuals translocated from sheltered to exposed sites became thinner and more streamlined (Sundene, 1964; Gerard, 1987). This morphological plasticity is evident during the spore stage; because of this, it is suggested that if wave height is increased or decreased the kelp will adapt morphologically over time to optimise its survival in the new environment.

Millar et al. (2021) compared the mechanical properties, cellular composition, and tissue architecture in Laminaria digitata across sites dominated by waves, currents, or relatively benign flow conditions. They found that kelps from more energetic environments (high waves or strong currents) generally possessed stronger yet more extensible blades than those from sheltered sites. Extensibility was greatest in the meristematic region (the main body of the kelp from which the blades protrude), where enhanced proportions of medulla cells (cylindrical-shaped cells) increased tissue flexibility, allowing blades to stretch and deform safely under high forces. By contrast, the distal blade tips were structurally stronger. Although cellular proportions did not differ significantly between kelps from different hydrodynamic regimes, kelps from high-energy sites showed significantly thicker tissues, suggesting that tissue thickening, rather than major changes in cellular composition, is the key strategy for resisting drag, preventing breakage, and maintaining attachment under extreme hydrodynamic loads. Similarly, Savard-Drouin et al. (2024) found that Laminaria digitata from sites with higher wave exposure had longer and thicker stipes and thicker blades with increased branching.

Kregting et al. (2015) found that in winter, the velocity of unidirectional (tidal current) and oscillatory water movement (waves) had no influence on the growth rate of Laminaria digitata. However, in summer, when nitrate concentrations are low, increased water motion can enhance the supply of nitrates to the kelp and increase growth. Nitrate uptake rates were lowest at velocities <5 cm/s and increased with velocity, regardless of the type of water motion. Oscillatory motion did however increase nitrate uptake rates by 20 to 50% compared to unidirectional flow at the same speed. These findings suggest that it is the velocity of water speed and the supply of nutrients, rather than the type of water flow, that are important for macroalgal growth. They also show seasonal variation in the effects of this pressure on this biotope.

In contrast, Millar et al. (2020) showed that wave movement reduces the growth efficiency in Laminaria digitata. By comparing three contrasting flow environments (low current/low wave, high current/low wave, and high wave/low current), they found that growth in high‑current environments was substantially enhanced: meristematic growth increased by 45%, and whole‑blade growth by 25%, relative to high‑wave sites. The authors suggest that currents promote more sustained, unidirectional water movement, whereas waves exert large, intermittent forces that impose drag and bending stress, reducing growth efficiency.

Storm-induced increases in wave action can be detrimental to Laminaria digitata biotopes. On the Atlantic coast of Nova Scotia, Hurricane Earl generated extreme wave heights of up to 25 m and strong bottom currents which caused a large-scale defoliation of kelp beds in shallow subtidal zones (Filbee-Dexter & Scheibling, 2012). Saccharina latissima and Laminaria digitata were stripped of blades, leaving only stipes and fragments, resulting in a 46% average loss of kelp canopy cover across the surveyed sites. The strong bottom currents also caused the displacement of urchins (Strongylocentrotus droebachiensis). In addition, coralline and filamentous red algae cover increased after the storm due to the loss of kelp.

The associated assemblage of the biotope also influences Laminaria digitata’s ability to withstand increases in wave action. The encrusting epiphyte Membranipora membranacea reduces the ability of individual kelps to withstand wave action, increasing frond breakages and reducing the maximum toughness and extensibility of the kelp blade materials (Krumhansl et al., 2011).

The piddocks and Hiatella arctica are unlikely to be directly affected by changes in wave exposure, owing to their environmental position within the substratum, which protects them. Trudgill & Crabtree (1987) found Hiatella arctica at both sheltered and wave exposed sites, suggesting that substratum, rather than wave action is a more significant factor determining distribution.

Potentially the most damaging effect of increased wave heights would be the erosion of the substratum as this could eventually lead to loss of the habitat. Increased erosion would lead to the loss of habitat and removal of piddocks. No evidence was found to link significant wave height to erosion. Some erosion will occur naturally, and storm events may be more significant in loss and damage of clays than changes in wave height at the pressure benchmark. For example, Micu (2007) observed numerous Pholas dactylus that had been washed out of the clay substratum or exposed due to storm damage to the clay in the Romanian Black Sea. Erosion rates at the Cretaceous chalk cliffs in East Sussex on the south coast of the UK has accelerated by 22 to 32 cm/year due to natural and anthropogenic modification of the coast (Hurst et al., 2016).

Sensitivity assessment. The structure of Laminaria digitata makes it resistant to changes in wave action, although large sudden increases in wave action through events such as storms may result in the removal of individuals from the habitat. Also, an increase in wave exposure to very or extremely exposed conditions could result in a change in kelp species present and the character of the biotope. However, a 3 to 5% change in significant wave height is unlikely to be significant and this biotope is therefore considered as having ‘High’ resistance to changes in wave height at the benchmark level. Resilience is also considered as ‘High’ at the benchmark, as there is no impact to recover from. Therefore, this biotope is ‘Not sensitive’ at the benchmark level.

Transition elements & organo-metal contamination

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

Evidence

Hydrocarbon & PAH contamination

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

Evidence

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

However, exposure to contaminants at levels greater than the benchmark may lead to impacts, although no evidence was found for sensitivity of piddocks. O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil or dispersant contamination, possibly due to the susceptibility of phycoerythrins to destruction, but that the filamentous forms were the most sensitive. Laboratory studies of the effects of oil and dispersants on several red algae species, including Palmaria palmata (Grandy, 1984 cited in Holt et al. 1995) concluded that they were all sensitive to oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages.

Laminaria digitata is less susceptible to coating than some other seaweeds because of its preference for exposed locations where wave action will rapidly dissipate oil. The effects of oil accumulation on the thalli are mitigated by the perennial growth of kelps. No significant effects of the Amoco Cadiz spill were observed for Laminaria populations and the World Prodigy spill of 922 tons of oil in Narragansett Bay had no discernible effects on Laminaria digitata (Peckol et al., 1990). Mesocosm studies in Norwegian waters showed that chronic low level oil pollution (25 µg/L) reduced growth rates in Laminaria digitata but only in the second and third years of growth (Bokn, 1985). Where exposed to direct contact with fresh hydrocarbons, encrusting calcareous algae have a high intolerance. The sensitivities of the faunal components of the kelp bed are not known although amphipods normally suffer high mortality in oil affected areas. Analysis of kelp holdfast fauna after the Sea Empress oil spill in Milford Haven illustrated decreases in number of species, diversity and abundance at sites nearest the spill (SEEEC, 1998).

Synthetic compound contamination

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

Evidence

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

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

O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil contamination, although the filamentous forms were the most sensitive. Laboratory studies of the effects of oil and dispersants on several red algae species (Grandy, 1984 cited in Holt et al., 1995) concluded that they were all intolerant of oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages. Cole et al. (1999) suggested that herbicides, such as simazina and atrazine were very toxic to macrophytes. Hoare & Hiscock (1974) noted that all red algae was excluded from Amlwch Bay, Anglesey by acidified halogenated effluent discharge. No evidence was found for the effects of synthetic chemicals on any of the bivalve species which characterise this biotope.

Radionuclide contamination

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

Evidence

No evidence was found to assess this pressure at the benchmark. Algae bioaccumulate radionuclides (with extent depending on the radionuclide and the algae species). Adverse effects have not been reported at low levels.

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 were not found for important characterizing species within the biotope. Cole et al. (1999) suggested possible adverse effects on marine species below 4 mg O₂/l and probable adverse effects below 2mg O₂/l .

This biotope would only be exposed to low oxygen in the water column intermittently during periods of tidal immersion. In addition, in areas of wave exposure and/or moderately strong current flow, low oxygen levels in the water are unlikely to persist for very long as oxygen levels will be recharged by the incorporation of oxygen in the air into the water column or flushing with oxygenated waters.  

Duval (1963a) observed that conditions within the borings of Petricolaria pholadiformis were anaerobic and lined with a loose blue/black sludge, suggesting that the species may be relatively tolerant to conditions of reduced oxygen.

Reduced oxygen concentrations have been shown to inhibit both photosynthesis and respiration in macroalgae (Kinne, 1977). Despite this, macroalgae are thought to buffer the environmental conditions of low oxygen, thereby acting as a refuge for organisms in oxygen depleted regions especially if the oxygen depletion is short-term (Frieder et al., 2012).

Sensitivity Assessment.  Reduced oxygen levels are likely to inhibit photosynthesis and respiration but not cause a loss of the macroalgae population directly. In wave exposed and tidally flushed habitats, oxygen levels are likely to be recharged by water mixing and the effects of deoxygenation are likely to be mitigated. Therefore, resistance is assessed as ‘High’ and resilience is considered to be ‘High’ (based on no impact), and the biotope is considered ‘Not sensitive’ at the pressure benchmark.

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

This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations. The benchmark is set at compliance with WFD criteria for good status, based on nitrogen concentration (UKTAG, 2014). No evidence was found to assess the sensitivity of piddocks to this pressure, and it is unlikely that they and other animal species present in the biotope, would be directly affected by this pressure. 

High ambient levels of phosphate and nitrogen enhance spore formation in a number of Laminaria species (Nimura et al., 2002), but will eventually inhibit spore production, particularly at the limits of temperature tolerances as seen in Saccharina latissima (studied as Laminaria saccharina; Yarish et al., 1990). Laminaria digitata seems to follow this trend with a growth peak occurring in conjunction with nutrient upwelling from deeper waters in Norway (Gévaert et al., 2001). Macroalgal growth is generally nitrogen-limited in the summer, as illustrated by the growth rates of Laminaria digitata between an oligotrophic and a eutrophic site in Arbroath, Scotland (Davison et al., 1984). Laminaria digitata does not accumulate the significant internal nutrient reserves seen in some other kelp. Higher growth rates have been associated with algae situated close to sewage outfalls. However, after removal of sewage pollution in the Firth of Forth, Laminaria digitata became abundant on rocky shores from which they had previously been absent (Read et al., 1983). Enhancement of coastal nutrients is likely to favour those species with more rapid growth rates including turf-forming algae (Gorgula & Connell, 2004) which could explain Laminaria digitata absence from the Firth of Forth. In addition, epiphytic abundance and biomass on Laminaria longicruris increase under a eutrophic regime decreasing the ability of individual algae to photosynthesise and withstand pressure from water movement (Scheibling et al., 1999).

Hiatella arctica is a fouling species present at fish farms suggesting that it is tolerant of the increased nutrient levels associated with fish aquaculture. However, no direct evidence was found for the effects of nutrient enrichment on these or for the piddocks which characterise this biotope. Nutrient enrichment that enhances phytoplankton productivity may indirectly benefit piddocks and Hiatella arctica by increasing food supply.

Sensitivity assessment. The pressure benchmark is relatively protective and may represent a reduced level of nutrient enrichment in previously polluted areas. Resistance to this pressure is therefore assessed as ‘High’ and resilience as ‘High’ (based on no impact to recover from), so the biotope is assessed as ‘Not sensitive’.

Organic enrichment

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

Evidence

Organic deposition may result in siltation (see smothering and siltation change pressure) and subsequent re-suspension of organic particles reducing water clarity (see change in suspended solids pressure). The deposition of sewage effluent into coastal environments resulted in the absence of Laminaria digitata and many other species from the coastline of the Firth of Forth (Read et al., 1983). Addition of organic matter may  decrease water clarity and increase particulate matter in the water column the effects of these changes are assessed through the pressure ‘Changes in suspended solids’.

Sensitivity assessment. The algae within the biotope are not considered likely to be directly affected by an increase in organic matter. The fronds of algae may intercept particles and may remove these from the chalk surfaces limiting deposition, although red algal turfs may trap silts and organic matter where turf dwelling fauna may consume organic fractions. Suspension feeders inccluding barnacles and piddocks and Polydora spp. may also utilise organic particles as food. Piddocks and the algal turf are likely to be able to withstand a low level of deposition of organic matter (at the pressure benchmark). Resistance is assessed as 'High' and resilience as 'High' so that the biotope is considered to be '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

This biotope is characterized by the soft rock substratum which supports populations of burrowing piddocks. The substratum was formed in prehistoric periods and is therefore unlike sedimentary habitats which may be renewed by water transport of sediment particles. Clay and chalk habitats are restricted in distribution and have been identified as irreplaceable habitats (Tillin et al., 2022). When removed, there is no mechanism by which the substratum can be replaced. Therefore, resistance to this pressure is assessed as ‘None’, resilience as ‘Very low’ because recovery is not possible, and sensitivity is ‘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

This biotope is characterized by the soft rock substratum which supports populations of burrowing piddocks.

This biotope is characterized by the soft rock substratum which supports populations of burrowing piddocks. The substratum was formed in prehistoric periods and is therefore unlike sedimentary habitats which may be renewed by water transport of sediment particles. Clay and chalk habitats are restricted in distribution and have been identified as irreplaceable habitats (Tillin et al., 2022). When removed, there is no mechanism by which the substratum can be replaced. Therefore, resistance to this pressure is assessed as ‘None’, resilience as ‘Very low’ because recovery is not possible, and sensitivity is ‘High’.

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

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

Low-level disturbance (e.g. solitary anchors) is suggested as being unlikely to cause harm to the biotope as a whole due to their small impact footprint. In a review of the effects of trampling on intertidal habitats, Tyler-Walters & Arnold (2008) found no information on the effects of trampling on Laminaria species (Laminaria digitata or Saccharina latissima). The authors reported that laminarians are robust species but that trampling on blades at low tide could potentially damage the blade or growing meristem.

Traditionally Laminaria digitata was used on agricultural lands as fertilizers; now Laminaria spp. are used in a range of different products, with their alginates used in the cosmetic, pharmaceutical and agri-food industries (Kervarec et al.,1999; McHugh, 2003). Collection of Laminaria digitata by mechanical harvesting (trawling) is not done in the UK, (Netalgae, 2012). Trawling, used to harvest Laminaria hyperborea in Norway results in whole alga being removed from the substratum, and substantial scouring of the substratum, indicating that the use of trawls in a Laminaria digitata biotope is likely to detrimentally affect the biotope, regardless of the target species. In France, Laminaria digitata is harvested with a ‘Scoubidou’, a curved iron hook which is mechanically operated. This device is considered to be selective; only harvesting individuals older than 2 years (Arzel, 2002). France reportedly harvests 75,000t kelp, mainly consisting of Laminaria digitata annually (FAO, 2007). Davoult et al. (2011) suggested that the maintenance of a sustainable crop of Laminaria digitata was possible if the industry continues employing small vessels evenly dispersed along the coastline. This could protect against habitat fragmentation and buffer over exploitation (Davoult et al., 2011). A fallow period of 18 to 24 months has been suggested for Laminaria digitata in France, where competition between the juvenile sporophytes of Laminaria digitata and Saccorhiza polyschides was indicated as a threat to the continued harvesting effort of Laminaria digitata (Engelen et al., 2011).

Canopy removal of Laminaria digitata has been shown to reduce shading, resulting in the bleaching of sub-canopy algae (Hawkins & Harkin, 1985). Harvesting may also result in habitat fragmentation, a major threat to this biotope’s ecosystem functioning (Valero et al., 2011).

Harvesting and fishing trawls scour and abrade the seabed, dislodging macroalgae and their associated assemblages from the substratum. The impact footprint and recovery period to artificial abrasion by bottom trawling are dependent on the trawl’s characteristics including duration, type and size. There is little evidence in the literature concerning natural or low-level bedrock abrasion.

Wiktor Jr et al. (2022) surveyed Laminaria digitata forests around Spitsbergen. The west and east sides of the island had contrasting thermal regimes: the warmer, Atlantic (west) side had temperatures ranging from 2°C in the winter to 7°C in the summer, while temperatures on the Arctic (east) side of the island ranged from -1.8°C in the winter to 2°C in the summer. At depths down to 5 m, kelp canopies (including Laminaria digitata) on the warm, west side were dense, intact, and covered approx. 30.5% of the surveyed area. However, on the colder eastern side of the island which experiences heavy ice cover and scouring, canopy cover in this depth range was significantly lower (approx. 20.3%), and kelp thalli were uniformly trimmed or damaged. Within 5 to 10 m depth, no significant differences were detected in kelp coverage or damage between the two sites. The authors therefore concluded that the reduced and damaged canopies of the east side of the island were caused by ice scouring.

At Hansneset, Kongsfjorden, reduced ice scouring from increasing summer sea surface temperatures (approx. 2°C between 1996 and 2014) was suggested to have caused a significant increase in kelp biomass (Bartsch et al., 2016). Between 1996 to 1998 and 2012 to 2014 survey periods, kelp biomass at 2.5 m depth had increased dramatically to 14 kg fresh weight/m², an 8.2-fold rise compared to the 1996 to 1998 survey results. This increase was driven primarily by Laminaria digitata, which formed dense stands with up to 70 individuals/m², mostly aged between 2 and 8 years, and exhibited a leaf area index of nearly 10 at this depth. The depth zone of maximum kelp biomass also shifted upwards from around 5 m in the late 1990s to approximately 2.5 m in the 2012 to 2014 surveys. These changes were attributed to reduced ice scouring and a longer open-water period, which have increased kelp survival and growth in shallow zones. This indicates that kelps, especially Laminaria digitata, are able to recover in instances where abrasion pressure is stopped.

The substratum may be impacted by activities that damaged the surface layers, resulting in removal or increased erosion. Natural erosion processes are likely to be ongoing within this habitat type. Where abundant, the boring activities of piddocks also contribute significantly to bioerosion, which can make the substratum habitat more unstable and can result in increased rates of coastal erosion (Evans 1968a, Trudgill 1983, Trudgill & Crabtree, 1987). Pinn et al. (2005) estimated that over the lifespan of a piddock (12 years), up to 41% of the shore could be eroded to a depth of 8.5 mm. Surface erosion is therefore a natural part of the environmental processes the biotope experiences although rates could be enhanced by surface abrasion and disturbance. Erosion rates at the Cretaceous chalk cliffs in East Sussex on the south coast of the UK has accelerated by 22 to 32 cm y^-1 due to natural and anthropogenic modification of the coast (Hurst et al., 2016).

The substratum may be impacted by activities that damaged the surface layers, resulting in removal or increased erosion. Natural erosion processes are likely to be ongoing within this habitat type. Where abundant, the boring activities of piddocks also contribute significantly to bioerosion, which can make the substratum habitat more unstable and can result in increased rates of coastal erosion (Evans 1968a, Trudgill 1983, Trudgill & Crabtree, 1987). Pinn et al. (2005) estimated that over the lifespan of a piddock (12 years), up to 41% of the shore could be eroded to a depth of 8.5 mm. Surface erosion is therefore a natural part of the environmental processes the biotope experiences although rates could be enhanced by surface abrasion and disturbance. Erosion rates at the Cretaceous chalk cliffs in East Sussex on the south coast of the UK has accelerated by 22 to 32 cm y^-1 due to natural and anthropogenic modification of the coast (Hurst et al., 2016). Micu (2007) observed that after storms in the Romanian Black Sea, the round goby, Neogobius melanostomus, removed clay from damaged or exposed Pholas dactylus burrows to be able to remove and eat the piddocks.

Sensitivity assessment. Surface abrasion may remove the algae and epifauna and result in the loss of some piddocks and damage to habitat. Resistance is therefore assessed as ‘Low’ for the algae and ‘Medium’ for piddocks and substratum. The algal canopy is expected to recover within 2 years, so that resilience is considered to be ‘High’ and sensitivity is ‘Low’. However, in a worst-case scenario where a significant amount of the substratum is removed, resilience would be ‘Very Low’ since the substratum cannot recover. and the sensitivity of the overall biotope would be ‘High’.

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

Penetration and disturbance below the surface of the substratum will damage surface fauna and could damage or expose piddocks. Individuals in damaged burrows, or those that are removed from the substratum, are unlikely to be able to rebury and will be predated by fish and other mobile species (Micu, 2007). 

The most significant impact arising from this pressure may be the damage and removal of the chalk and clay substratum. Where abundant, the boring activities of bivalves can make the substratum habitat more unstable and can exacerbate erosion (Evans 1968a, Trudgill 1983, Trudgill & Crabtree, 1987). Pinn et al. (2005) estimated that over the lifespan of a piddock (12 years), up to 41% of the shore could be eroded to a depth of 8.5 mm. Piddock burrowing can therefore make the substratum more vulnerable to damage and removal.

Sensitivity assessment. Sub-surface penetration and disturbance will remove and damage the characterizing algae and surface dwelling fauna and result in the loss of piddocks and damage to the habitat. Resistance is therefore assessed as ‘Low’. All characterising species are expected to recover 2 to 10 years, so their resilience is ‘Medium’. However, as the substratum cannot recover, resilience is assessed as ‘Very Low’ and sensitivity of the overall biotope is considered to be ‘High’. 

Changes in suspended solids (water clarity)

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

Evidence

Suspended Particle Matter (SPM) concentration has a linear relationship with sub-surface light attenuation (K_D) (Devlin et al., 2008). Light availability and water turbidity are principal factors in determining depth range at which kelp can be found (Birkett et al., 1998b). Light penetration influences the maximum depth at which kelp species can grow and it has been reported that laminarians grow at depths at which the light levels are reduced to 1 percent of incident light at the surface. Maximal depth distribution of laminarians therefore varies from 100 m in the Mediterranean to only 6 to 7 m in the silt laden German Bight. In Atlantic European waters, the depth limit is typically 35 m. In very turbid waters the depth at which kelp is found may be reduced, or in some cases excluded completely (e.g. Severn Estuary), because of the alteration in light attenuation by suspended sediment (Lüning, 1990; Birkett et al. 1998b).

The absence of Laminaria digitata in the Firth of Forth was suggested to be caused by the outflow from a sewage treatment plant that increased the turbidity of the water and thus decreased photosynthetic activity, although the effect of turbidity was probably coupled with increased nutrient levels (Read et al., 1983). Blue light is crucial for the gametophytic stages of Laminaria digitata, and several other congenic species (Lüning, 1980). Dissolved organic materials (yellow substance or gelbstoff) absorb blue light (Kirk, 1976), therefore changes in riverine input or other land-based runoff are likely to influence kelp density and distribution. In the silt-laden waters around Helgoland, Germany the depth limit for Laminaria digitata growth may be reduced to between 0 m and 1.5 m (Birkett et al. 1998b). In locations where water clarity is severely decreased, Laminaria species experience a significant decrease in growth from the shading of suspended matter and/or phytoplankton (Lyngby & Mortensen 1996, Spilmont et al., 2009).

Increased suspended particles may enhance food supply (where these are organic in origin) or decrease feeding efficiency (where the particles are inorganic and require greater filtration efforts). Very high levels of silt may clog respiratory and feeding organs of some suspension feeders. Increased levels of particles may increase scour and deposition in the biotope depending on local hydrodynamic conditions. However, the bivalves which characterise this biotope are protected from scour within their burrows. Piddocks occur in habitats such as soft chalks where turbidity may be high and is therefore unlikely to be affected by an increase in suspended sediments at the pressure benchmark. Suspension feeding bivalves have efficient mechanisms to remove inorganic particles via pseudofaeces. Experimental work on Pholas dactylus showed that large particles can either be rejected immediately in the pseudofaeces or passed very quickly through the gut (Knight, 1984). Similarly, Petricolaria pholadiformis are able to tolerate high-levels of suspended solids through the production of pseudofaeces (Purchon, 1955). Increased suspended sediments may impose sub-lethal energetic costs on piddocks by reducing feeding efficiency and requiring the production of pseudofaeces with impacts on growth and reproduction. A significant decrease in suspended organic particles may reduce food input to the biotope resulting in reduced growth and fecundity of piddocks. However, local primary productivity may be enhanced where suspended sediments decrease, increasing food supply. Decreased suspended sediment may increase macroalgal competition enhancing diversity but is considered unlikely to significantly change the character of the biotope.

Regression models developed by Bourget et al. (2003) found that temperature and water transparency (measured in metres and indicating the level of inorganic suspended solids) explained only 40% of the variation in biomass of Hiatella arctica fouling navigation buoys in the Gulf of St Lawrence system (Canada). These findings suggest that other variables play a more significant role in determining settlement, survival and growth over a year in this system. However, the models did indicate that biomass is higher where water transparency was greater (around 15 m) and declined at higher levels of suspended solids (transparency 5 m) although a causal link was not identified (Bourget et al., 2003).

Sensitivity Assessment. A decrease in inorganic suspended solids is likely to support enhanced growth (and possible habitat expansion) of algae. However, an increase in turbidity, is likely to result in a reduction in growth of kelp and potential loss, particularly where this biotope occurs towards the depth limit. Changes in suspended solids is unlikely to affect the characteristic bivalve species, it may affect Laminaria digitata. Resistance to this pressure is assessed as ‘Low’ based on reduction and loss of kelp canopy, especially at the lower limit of this biotope’s depth range. Resilience to this pressure is assessed as ‘High’, and the biotope is regarded as having a ‘Low’ sensitivity.

Smothering and siltation rate changes (light)

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

Evidence

Suspended Particle Matter (SPM) concentration has a linear relationship with sub-surface light attenuation (K_D) (Devlin et al., 2008). Light availability and water turbidity are principal factors in determining depth range at which kelp can be found (Birkett et al., 1998b). Light penetration influences the maximum depth at which kelp species can grow and it has been reported that laminarians grow at depths at which the light levels are reduced to 1 percent of incident light at the surface. Maximal depth distribution of laminarians therefore varies from 100 m in the Mediterranean to only 6 to 7 m in the silt laden German Bight. In Atlantic European waters, the depth limit is typically 35 m. In very turbid waters the depth at which kelp is found may be reduced, or in some cases excluded completely (e.g. Severn Estuary), because of the alteration in light attenuation by suspended sediment (Lüning, 1990; Birkett et al. 1998b).

The absence of Laminaria digitata in the Firth of Forth was suggested to be caused by the outflow from a sewage treatment plant that increased the turbidity of the water and thus decreased photosynthetic activity, although the effect of turbidity was probably coupled with increased nutrient levels (Read et al., 1983). Blue light is crucial for the gametophytic stages of Laminaria digitata, and several other congenic species (Lüning, 1980). Dissolved organic materials (yellow substance or gelbstoff) absorb blue light (Kirk, 1976), therefore changes in riverine input or other land-based runoff are likely to influence kelp density and distribution. In the silt-laden waters around Helgoland, Germany the depth limit for Laminaria digitata growth may be reduced to between 0 m and 1.5 m (Birkett et al. 1998b). In locations where water clarity is severely decreased, Laminaria species experience a significant decrease in growth from the shading of suspended matter and/or phytoplankton (Lyngby & Mortensen 1996, Spilmont et al., 2009).

Increased suspended particles may enhance food supply (where these are organic in origin) or decrease feeding efficiency (where the particles are inorganic and require greater filtration efforts). Very high levels of silt may clog respiratory and feeding organs of some suspension feeders. Increased levels of particles may increase scour and deposition in the biotope depending on local hydrodynamic conditions. However, the bivalves which characterise this biotope are protected from scour within their burrows. Piddocks occur in habitats such as soft chalks where turbidity may be high and is therefore unlikely to be affected by an increase in suspended sediments at the pressure benchmark. Suspension feeding bivalves have efficient mechanisms to remove inorganic particles via pseudofaeces. Experimental work on Pholas dactylus showed that large particles can either be rejected immediately in the pseudofaeces or passed very quickly through the gut (Knight, 1984). Similarly, Petricolaria pholadiformis are able to tolerate high-levels of suspended solids through the production of pseudofaeces (Purchon, 1955). Increased suspended sediments may impose sub-lethal energetic costs on piddocks by reducing feeding efficiency and requiring the production of pseudofaeces with impacts on growth and reproduction. A significant decrease in suspended organic particles may reduce food input to the biotope resulting in reduced growth and fecundity of piddocks. However, local primary productivity may be enhanced where suspended sediments decrease, increasing food supply. Decreased suspended sediment may increase macroalgal competition enhancing diversity but is considered unlikely to significantly change the character of the biotope.

Regression models developed by Bourget et al. (2003) found that temperature and water transparency (measured in metres and indicating the level of inorganic suspended solids) explained only 40% of the variation in biomass of Hiatella arctica fouling navigation buoys in the Gulf of St Lawrence system (Canada). These findings suggest that other variables play a more significant role in determining settlement, survival and growth over a year in this system. However, the models did indicate that biomass is higher where water transparency was greater (around 15 m) and declined at higher levels of suspended solids (transparency 5 m) although a causal link was not identified (Bourget et al., 2003).

Sensitivity Assessment. A decrease in inorganic suspended solids is likely to support enhanced growth (and possible habitat expansion) of algae. However, an increase in turbidity, is likely to result in a reduction in growth of kelp and potential loss, particularly where this biotope occurs towards the depth limit. Changes in suspended solids is unlikely to affect the characteristic bivalve species, it may affect Laminaria digitata. Resistance to this pressure is assessed as ‘Low’ based on reduction and loss of kelp canopy, especially at the lower limit of this biotope’s depth range. Resilience to this pressure is assessed as ‘High’, and the biotope is regarded as having a ‘Low’ sensitivity.

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

Melting glaciers in the Arctic due to global warming releases turbid, fresh-melt waters and inorganic material into the water column. In a study by Ronowicz et al. (2018), this had no effect on arctic kelps (including Laminaria digitata), but it did significantly affect the composition of faunal communities associated with the holdfast of the kelp. Species richness remained similar across sites and levels of disturbance, but the dominant taxa were different. In the lowest impacted sites, kelp holdfast communities were dominated by Hydrozoa, while the highest impacted sites were dominated by Bryozoa.

An experiment by Roleda et al. (2008) illustrated potential benefits to low levels of siltation including UV protection for Saccharina latissima for short periods. When burial under a variety of sediment types was extended beyond 7 days, symptoms of bleaching, tissue loss and diminished photosynthetic function were exhibited (Roleda & Dethleff, 2011). A layer of fine-grained sediment (0.1 to 0.2 cm thick) caused rotting of Saccharina latissima and 25% mortality after 4 weeks of coverage in a laboratory experiment. Saccharina latissima is considered to be more silt tolerant than Laminaria digitata, suggesting that in locations of low wave and current mediated water flow; sedimentation is a threat to this biotope (Lyngby & Mortensen 1996). However, this study was carried out on disc samples from the thalli placed in petri dishes with no current flow. It is unlikely that this biotope would be found in such conditions of low flow, therefore the relevance of this study is questionable, and it is unclear how a whole plant would respond to siltation.

Sedimentation has additional negative effects on the zoospores of brown algae, with spores attaching to the only substratum available. Hence, fine sediment could interfere with recruitment, by preventing and deterring spore attachment to a hard substratum; resulting in their subsequent loss due to waves and currents (Devinny & Volse, 1978, Norton, 1978; Bartsch et al., 2008). Field observations reveal that kelp is associated with accelerating sediment deposition and additionally prevent sediments being washed away because of their influence on local water current by increasing drag and thus particulate fall out (Airoldi, 2003 references therein). However, this sediment is associated with the holdfasts of the kelp and not the fronds. At higher levels of wave exposure, whiplash by kelp is common, and this, in turn, reduces sediment accumulation at these sites (Kennelly 1989; Melville & Connell, 2001; cited in Airoldi, 2003).

While 5 cm of sediment coverage may be transported from the biotope relatively quickly, a deposition of 30 cm is likely to remain in place for a longer period of time, especially in wave sheltered examples of the biotope. Therefore, heavy siltation may have a greater effect on the health of the biotope, resulting in smothering of the epifauna and flora, the red algae community and holdfast fauna in particular.

The burrowing mechanisms of the piddocks Pholas dactylus and Barnea candida and other Pholads, mean that the burrows have a narrow entrance excavated by the juvenile. As the individual grows and excavates deeper the burrow widens resulting in a conical burrow from which the adult cannot emerge. Piddocks cannot therefore emerge from layers of deposited silt as other more mobile bivalves can.

Indirect indications for the impacts of siltation are provided by Witt et al., (2004), who studied the impacts of harbour dredge disposal. Petricolaria (syn. Petricola) pholadiformis was absent from the disposal area, and Witt et al., (2004) cite reports by Essink (1996, not seen) which suggested that smothering of Petricolaria pholadiformis from siltation could lead to mortality within a few hours. Hebda (2011) also identified that sedimentation may be one of the key threats to Barnea truncata populations. At Agigea, smothering of clay beds by sand and finer sediments had removed populations of Pholas dactylus. In this area sand banks up to 1 m thick frequently shift position due to storm events and currents (Micu, 2007). Similar smothering was described in the case of Barnea candida populations boring into clay beds (Gomoiu & Muller 1962, cited from Micu, 2007).

Sensitivity assessment. Siltation at the pressure benchmark level is considered to remove most or all of the piddocks and the surface algae and fauna. Resistance to siltation is therefore assessed as ‘None’ although effects could be mitigated where water currents and wave exposure rapidly removed the overburden and this will depend on shore height and local hydrodynamic conditions. Resilience is assessed as ‘Medium’ (2 to 10 years) for piddocks, so sensitivity is therefore assessed as ‘Medium’. 

Litter

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

Evidence

Not assessed.

Electromagnetic changes

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

Evidence

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

Barrier to species movement

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

Evidence

Not relevant.

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

Pholas dactylus reacts quickly to changes in light intensity, after a couple of seconds, by withdrawing its siphon (Knight, 1984). This reaction is ultimately an adaptation to reduce the risk of predation by, for example, approaching birds (Knight, 1984). However, its visual acuity is probably very limited and it is unlikely to be sensitive to visual disturbance. Birds are highly intolerant of visual presence and are likely to be scared away by increased human activity, therefore reducing the predation pressure on piddocks. Therefore, visual disturbance may be of indirect benefit to piddock populations. Resistance and resilience are, therefore, assessed as ‘High’ and the biotope is considered to be ‘Not sensitive’.

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

The species characterizing this biotope are not farmed or translocated and therefore this pressure is 'Not relevant' to this biotope.

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

Evidence for a new group of phaeoviruses (viruses which affect macroalgae) infecting kelps was first reported by McKeown et al. (2017). Within this group (sub-group C) is the type species referred to as LdigV, which targets the nucleus in the cells of Laminaria digitata for its genome replication. This infection results in the degradation of chloroplasts and the assembly of virions in the cytoplasm of vegetative and reproductive cells. McKeown et al. (2017) found that 64.7% of sporophytes and 23.2% of gametophyte mixes (both from all three kelp species sampled) tested positive for the phaeovirus.

Infection of Laminaria japonica sporophytes by Pseudoalteromonas, Vibrio and Halomonas results in the characteristic symptoms of hole-rotten disease (Wang et al., 2008). Additionally, red spot disease may be caused by bacteria of the genus Alteromonas (Sawabe et al., 1998). Hyperplasia or gall growths are often seen as dark spots on Laminaria digitata and have been associated with endophytic brown filamentous algae. It can be inferred from these observations that microbial pathogens may impact the growth rates of individuals.

There is no evidence in the literature that infection by microbial pathogens results in the mass death of Laminaria populations, and the kelp themselves are known to regulate bacterial infections through iodine metabolism (Cosse et al., 2009). Evidence of fungal pathogens has been reported for Laminaria digitata. Kononenko et al. (2022) reported traces of mycotoxins (toxic compounds produced by fungi) such as ergot alkaloids (produced by Claviceps spp.) and aflatoxins (produced by Aspergillus spp.). However, no signs of infection were reported.

No evidence of microbial infection and mortality in piddocks or Hiatella arctica was found.

Sensitivity Assessment. While Laminaria digitata is somewhat resistant to this pressure, there is Insufficient evidence to assess the sensitivity of the whole biotope to microbial pathogens, due to the lack of evidence for piddock and Hiatella arctica sensitivity.

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

Traditionally Laminaria digitata was added to agricultural lands as fertilizers; now Laminaria species are used in a range of different products, with its alginates used in the cosmetic, pharmaceutical and agri-food industries (Kervarec et al., 1999; McHugh, 2003). In France, Laminaria digitata is harvested with a ‘Scoubidou’ (a curved iron hook which is mechanically operated). This device is considered to be selective; only harvesting individuals older than 2 years (Arzel, 2002). France reportedly harvests 75,000t kelp, mainly consisting of Laminaria digitata annually (FAO, 2007). In France, Laminaria digitata is harvested with a ‘Scoubidou’, a curved iron hook which is mechanically operated. This device is considered to be selective; only harvesting individuals older than 2 years (Arzel, 2002). France reportedly harvests 75,000t kelp, mainly consisting of Laminaria digitata annually (FAO, 2007). Davoult et al. (2011) suggested that the maintenance of a sustainable crop of Laminaria digitata was possible if the industry continues employing small vessels evenly dispersed along the coastline. This would protect against habitat fragmentation and buffer over exploitation (Davoult et al., 2011). A fallow period of 18-24 months has been suggested for Laminaria digitata in France, where competition between the juvenile sporophytes of Laminaria digitata and Saccorhiza polyschides was indicated as a threat to the continued harvesting effort of Laminaria digitata (Engelen et al., 2011). If Laminaria digitata, the key characterizing and structuring species of this biotope is removed then the biotope is considered lost due to the significant alteration to the biotope classification and character of the habitat is likely.

Canopy removal of Laminaria digitata has been shown to reduce shading, resulting in the bleaching of sub canopy algae (Hawkins & Harkin, 1985). Harvesting may also result in habitat fragmentation, a major threat to this biotope’s ecosystem functioning (Valero et al., 2011).  In the UK harvesting of Laminaria digitata is currently restricted to manual removal and farming on small scales. Red algae that occur in this biotope may also be harvested. Palmaria palmata (known as dulse) is harvested from the wild both commercially and recreationally. Garbary et al., (2012) studied harvested and non-harvested shores in Nova Scotia, Canada containing stands of Palmaria palmata. They also conducted experimental removal of Palmaria palmata and assessed simulated removal of Palmaria palmata by an experienced commercial harvester. Simulated commercial harvesting reduced cover of Palmaria palmata from 70% to 40%, although experimental removal on shores not usually harvested reduced cover to 20% (Garbary et al., 2012).

Piddocks may be removed as bait and across Europe they have traditionally been harvested for food, however high levels of habitat damage are associated with the removal of boring molluscs (Fanelli et al., 1994) and this practice has largely been banned. The most sensitive component of this biotope to targeted harvesting is the chalk substratum which may be damaged and removed if piddocks were excavated from their burrows, this effect is considered through the physical damage pressures, abrasion and penetration and sub-surface damage.

Sensitivity assessment. Removal of the Laminaria digitata canopy, piddocks and red algae will have a negative impact on the primary and secondary productivity of the area and alter the character of the biotope. As these species are attached and easy to select and remove, resistance is assessed as ‘Low’. If some Laminaria digitata and the bases of red algae remain recovery will be fairly rapid. Resilience is assessed as ‘Medium’ (based on recovery of piddocks if these were targeted). The biotope is therefore considered to have ‘Medium’ sensitivity to this pressure.

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

Direct, physical impacts from harvesting are assessed through the abrasion and penetration of the seabed pressures. The sensitivity assessment for this pressure considers any biological/ecological effects resulting from the removal of non-target species on this biotope. The loss of characterizing and associated species due to incidental removal as by-catch would alter the character of the habitat from the biotope description. The ecological services such as primary production and habitat structure would also be lost.

Sensitivity assessment. Although piddocks and Hiatella arctica are unlikely to be affected in their burrows, harvesting on the surface of the substratum could significantly remove the Laminaria digitata canopy and result in a reclassification of the biotope. Resistance is therefore assessed as ‘Low’ and resilience as ‘Medium’, (based on the loss of holdfasts, but see resilience section for caveats), so sensitivity is assessed as ‘Medium’. If a high proportion of holdfasts remained, recovery would be assessed as ‘High’ and sensitivity would be assessed as ‘Low’.

The American slipper limpet, Crepidula fornicata

Evidence

The carpet sea squirt, Didemnum vexillum

Evidence

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

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

Didemnum vexillum has colonized and established populations in the northeast Pacific, Canadian and USA coast, New Zealand, France, Spain, and the Wadden Sea, Netherlands, the Mediterranean Sea, and the 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 localised 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 from 11 to 19 times after 30 days. Valentine et al. (2007a) concluded fragments could successfully grow, survive, and help to spread Didemnum vexillum.

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 & Grosholz, 2015; Vercaemer et al., 2015).

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

This species requires suitable hard substrata for successful settlement and the establishment of colonies. It can grow quickly and can establish large colonies of dense encrusting mats on a variety of hard substrata (Valentine et al., 2007a; Griffith et al., 2009; Lambert, 2009; Groner et al., 2011; Cinar & Ozgul, 2023). Mats can be up to several meters in area, covering large portions of the seafloor (Mercer et al., 2009). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems by its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock gravel, pebble, cobble, or boulders (Tillin et al., 2020). Didemnum vexillum has been reported colonizing these types of hard substrata in the USA, Canada, northern Kent and the Solent (Bullard et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Hitchin, 2012; Vercaemer et al., 2015; Tillin et al., 2020). It is therefore likely that the substrate in this biotope is suitable for Didemnum vexillum colonisation. In addition, the depth range at which this Laminaria digitata biotope is found (low intertidal to 5 m) overlaps with the depth range that is suitable for Didemnum vexillum colonization. Didemnum vexillum has been recorded from less than 1 m to at least 81 m deep (Bullard et al., 2007; Tagliapietra et al., 2012; Tillin et al., 2020).

There are few observations of Didemnum vexillum on soft bottom habitats as evidence suggests it is unable to establish or grow easily on mud, mobile sand or other unstable substrata, and it is vulnerable to smothering by fine sediment (Bullard et al., 2007; Valentine et al., 2007a; Griffith et al., 2009). The species is usually found in areas where the colony is protected from sedimentation and wave action (Valentine et al., 2007b; McKenzie et al., 2017; Tillin et al., 2020). For example, at Georges Bank, USA, the Didemnum vexillum mats were limited to gravelly areas and unable to colonize the surrounding sand ridges, which have a mobile surface that is moved daily by the strong tidal currents (Valentine et al., 2007b). Evidence also indicates that the species cannot survive being buried or smothered by coarse or fine-grained sediment. Furthermore, in Holyhead Marina, Didemnum vexillum colonies were contained in the harbour and established on artificial pontoons; they were absent from the natural seabed beneath the pontoon, composed of silty mud, and from deeper sections of mooring chains that became immersed in mud at low spring tides (Griffiths et al., 2009).

In northern Kent, Didemnum vexillum has been recorded covering London clay boulders on Whitstable Flats, West Beach; tabulate sandstone boulders (0.5 to 2 m across) on the mid shore; and sediment mounds on the low shore, characterized by larger areas of sand, mud, and low-lying sediment at Reculver and Bishopstone (Hitchin, 2012). It was also recorded in muddy substrata at that site. Hitchin (2012) noted that the site was exposed to enough waves and currents to cause sedimentation. However, Didemnum vexillum grew hanging from the underside of sandstone boulders nestled on sediment, on consolidated sediment mounds and firm clays, hence burial may prevent colonization and its survival rather than sedimentation alone.

The Sandwich tide pools were subject to air exposure at low tide, daily changes in water depth and temperature (Valentine et al., 2007a). Didemnum vexillum colonies can survive exposure to air at low tides during rapid colony growth in summer months July to September (Valentine et al., 2007a). However, parts of the large established colonies, which were artificially exposed to air for two to three hours in October, were observed desiccated or predated on by grazing periwinkles 30 days later, in the winter month November (Valentine et al., 2007a). They suggested that the invasive tunicates’ ability to tolerate exposure to air varies with the seasonal growth cycle. Didemnum vexillum also tolerated emersion in Kent, as colonies on the mid-shore at Reculver flourish and survive in air exposure for up to three hours per cycle during spring tides (Hitchin, 2012). Hitchin (2012) suggested the porous nature of the sandstone boulders the species colonized retained water. The Kent shore was sheltered but held water due to its shallow slope and flats, which may allow Didemnum sp. to survive in the low to mid-shore. There is evidence that Didemnum vexillum died when exposed to air for more than 6 hours (Laing et al., 2010).

Didemnum vexillum tolerates a wide range of environmental conditions, including temperature and salinity (Herborg et al., 2009; Tillin et al., 2020). Didemnum vexillum can withstand a wide range of salinities from 20 to 44 PSU, is commonly found in marine waters around 33 PSU, but is unable to survive in salinities below 20 PSU (Bullard & Whitlatch, 2009; Groner et al., 2011; Tillin et al., 2020). It has been recorded in estuarine conditions and tidal lagoons (Dijkstra et al., 2007; Tillin et al., 2020). In the Lagoon of Venice, Mediterranean, Didemnum vexillum is found in a mean salinity value of 30 PSU. It was absent in low salinity, such as the estuary and around the salt marshes, but well established in the euhaline and tidally well-flushed zones of the Lagoon of Venice (Tagliapietra et al., 2012). Similar results were found in Connecticut and Rhode Island, where Didemnum vexillum was not found in environments with salinity less than 20 PSU (Bullard & Whitlatch, 2009). However, in the Wadden Sea, colonies of Didemnum vexillum were abundant in salinities between 17.91 to 25.97 PSU (Gittenberger, 2007; Gittenberger et al., 2015).

Didemnum vexillum is a temperate species that can survive a broad temperature range of -2 to 24°C, with an upper survival limit suggested to be 25°C (Bullard et al., 2007; Valentine et al., 2007a; Herborg et al., 2009; Kleeman, 2009; McKenzie et al., 2017; Holt, 2024). It thrives best at 14 to 20°C, with optimal growth temperature between 14 to 18°C during summer months (May, June, September, October) (Gittenberger, 2007; Kleeman, 2009; McKenzie et al., 2017).

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

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

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

Sensitivity assessment. There is no evidence of Didemnum vexillum colonizing this biotope in the UK. However, it has been recorded in similar kelp habitats in Norway (Järnegren et al., 2023). Didemnum vexillum can overgrow sessile organisms, including Laminaria sp. However, no direct evidence was found on how Didemnum vexillum affects kelp or if it contributes to Laminaria sp. mortality (Järnegren et al., 2023), although epifaunal growth by Membranacea membrancea was reported to reduce the physical strength of kelp fronds (inc. Laminaria digitata) and make them susceptible to removal by wave action (Krumhansl et al., 2011). In addition, overgrowth by epiphytes contributed to the decline of Saccharina latissima in Norway (Andersen et al., 2011). However, Didemnum vexillum may compete for light and space with kelp and epifauna and could interfere with recruitment, which could lead to the mortality of some epifauna, the loss of kelp, and a reduction in biodiversity. Didemnum vexillum has been recorded on clay boulders (Hitchin, 2012). Therefore, a resistance of 'Medium' (some mortality, <25%) is suggested as a precaution. Resilience is likely to be 'Very low' as Didemnum vexillum would need to be physically removed to allow recovery. Hence, sensitivity to invasion by Didemnum is assessed as 'Medium'. However, confidence in the assessment is ‘Low’ due to the lack of direct evidence of damage to kelp beds.

The Pacific oyster, Magallana gigas

Evidence

Wireweed, Sargassum muticum

Evidence

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

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

Strong & Dring (2011) used canopy removal experiments to investigate inter- and intra-species competition between Sargassum muticum and Saccharina latissima in the Dorn, Strangford Lough, Northern Ireland. The Dorn consists of tide pools, very sheltered from wave action but with moderately strong tidal streams (1 to 2 knots). Sargassum muticum grew better in mixed stands with Saccharina latissima than in the highest-density monospecific stands examined. However, the growth of Saccharina was not affected by the proportion of Sargassum in mixed stands. They concluded that Saccharina was not impacted significantly by the alien species while Sargassum benefited from growth in mixed stands. Experimental manipulation of subtidal algal canopies in the San Juan Islands, Washington State, USA, showed that Sargassum muticum reduced the abundance of native macroalgae, including the kelp Laminaria bongardiana due to shading. However, the experimental removal of Sargassum resulted in the recovery of native species within one year (Britton-Simmons, 2004; Engelen et al., 2015). The negative effects of Sargassum muticum on native macroalgae are mainly due to competition for light, rather than changes in nutrient availability, sedimentation or water flow (Britton-Simmons, 2004; Engelen et al., 2015).

Sensitivity Assessment. No evidence of the effects of Sargassum on Laminaria digitata beds was found. Competition with Sargassum is probably site-specific and dependent on local conditions, so it is highly unlikely that Sargassum will be able to colonize and survive in this biotope due to the high degree of wave exposure. In addition, no evidence was found to show that Sargassum muticum can colonise chalk or clay. Therefore, resistance is assessed as ‘High’, resilience as 'High', and sensitivity is assessed as ‘Not Sensitive’. Overall, confidence is assessed as ‘Low’ due to evidence of variation and the site-specific nature of competition between native kelps and Sargassum muticum. 

Wakame, Undaria pinnatifida

Evidence

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

Undaria pinnatifida seems to settle better on artificial substrata (e.g., floats, marinas or piers) than on natural rocky shores among local kelps (Vaz-Pinto et al., 2014). It is found predominantly in low intertidal to shallow subtidal habitats (Epstein et al., 2019b) and is significantly more abundant on artificial substrata compared to natural rocky substrata (Heiser et al., 2014; Epstein & Smale, 2018). James (2017) suggested that Undaria pinnatifida could outcompete native species on artificial substrata (such as marinas and wharf structures).

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

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

In St Malo, France, there was evidence that Undaria pinnatifida co-existed with Laminaria hyperborea under certain conditions (Castric-Fey et al., 1993). Epstein & Smale (2018) also observed that Undaria pinnatifida was relatively common (abundance of >70 individuals per 25 m transect) at three sites in Devon, UK (Jennycliff, Bovisand and Beacon Cove) where Laminaria spp. were abundant (40 to 79%) or superabundant (>80%), which suggested that Undaria pinnatifida could co-exist within refugia amongst areas with dense Laminaria spp.. 

In Plymouth Sound, UK, Heiser et al. (2014) observed that Laminaria hyperborea was significantly less abundant at sites with the presence of Undaria pinnatifida, with only approx. 0.5 Laminaria hyperborea individuals per m2 present compared to approx. 8 individuals per m2 at sites without the presence of Undaria pinnatifida. However, the results from their correlation study only showed that the species were not found together (pers. comm., Epstein, 2021). Whereas exclusion and succession experiments on reefs tell us that Laminaria spp. exclude Undaria pinnatifida, not the other way around. Epstein & Smale (2018) reported that in Devon, UK, persistent, dense, and intact Laminaria spp. canopies in rocky reef habitats exerted a strong influence over the presence/absence, abundance, and percentage cover of Undaria pinnatifida. A dense canopy of native kelp restricts the proliferation of Undaria pinnatifida and disturbance of the canopy is often the key to the recruitment of Undaria pinnatifida. Epstein et al. (2019b) reported that Undaria pinnatifida density and biomass were significantly negatively correlated with the sum of all Laminaria spp. in Plymouth, UK. The evidence indicated that native Laminaria spp. canopies in the UK inhibited Undaria pinnatifida and implied that Undaria pinnatifida was opportunistic but competitively inferior (Farrell & Fletcher, 2006; Heiser et al., 2014; Minchin & Nunn, 2014; De Leij et al., 2017; Epstein & Smale, 2018; Epstein et al., 2019b). However, Epstein et al. (2019b) also noted that Laminaria hyperborea had a non-significant positive relationship with Undaria pinnatifida due to low densities of Laminaria hyperborea across the study area, resulting in insufficient data. 

Epstein et al. (2019b) found that within its depth range (+1 to -4 m), Undaria pinnatifida co-existed with seven species of canopy-forming brown macroalgae, including Laminaria hyperborea. De Leij et al. (2017) found that natural habitats with dense native macroalgal canopies had more resistance to Undaria pinnatifida invasion than disturbed or sparse canopies, due to limited space and light availability for Undaria recruits. However, the dense canopies will not prevent the invasion of Undaria, as sporophytes were still recorded within dense Laminaria canopies, and this suggests that canopy disturbance is not always required (De Leij et al., 2017; Epstein & Smale, 2018). 

Undaria pinnatifida was successfully eradicated on a sunken ship in Chatham Islands, New Zealand, by applying a heat treatment of 70°C (Wotton et al., 2004). However, numerous other eradication attempts have failed and as noted by Fletcher & Farrell (1998), once established Undaria pinnatifida resists most attempts at long-term removal.

Sensitivity Assessment. Undaria pinnatifida has the potential to colonize and co-exist in refugia within Laminaria sp. dominated habitats, especially in shallow examples of their biotopes that are within its depth range (1 to 4 m) and sheltered from wave action. A dense, native kelp canopy may restrict or slow the proliferation of Undaria pinnatifida. In addition, it is highly unlikely that Undaria pinnatifida will be able to colonize or survive the degree of wave exposure that characterize this biotope. Furthermore, no evidence was found of Undaria pinnatifida presence in chalk or clay habitats. Therefore, sensitivity is assessed as ‘Not Sensitive’. Overall, confidence is assessed as ‘Low’ due to evidence of variation and the site-specific nature of competition between native kelps and Undaria pinnatifida. 

Other INIS

Evidence