The Marine Life Information Network

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

Evidence

Laminaria digitata is distributed from Brittany to the coast of Norway, while its UK distribution encompasses the whole of the UK coast (Blight and Thompson, 2008).  Its distribution suggests that the species would tolerate chronic temperature change (e.g. by 2°C for a year).  However, local populations may have acclimatized to local physical conditions meaning that populations at the extremes of the species’ range are less comparable than those populations in the middle of its range. Additionally, the distribution data of this species suggests it is a northern species, as such, it will be vulnerable to increases in temperature and may be out-competed at its southern limits by other kelp species.

The thermal optimum of Laminaria digitata is between 10-15°C, with reproductive ability impaired to 20% at 18°C (Arzel, 1998). Therefore while the current population may not be affected, recruitment may be reduced.  Spore production only occurs between 5-10°C and is the most temperature sensitive stage of reproduction in Laminaria digitata. Outside this temperature range, reproduction is severely reduced and the species is at risk from local extinction in the long-term. In addition, a temperature increases to 22-23°C causes cell damage and death (Sundene, 1964; Bolton & Lüning, 1982). The sensitivity of this species, therefore, relies on the current sea temperatures of the specific location (Bartsch et al., 2013). A minimum of 10 weeks a year between 5-18°C is needed in order to ensure spore formation and hence reproduction (Bartsch et al., 2013).

Combining predicted sea surface temperate over the next century with the current distribution of Laminaria digitata, Merzouk & Johnson (2011) predict an expansion of its northern limits and localised extinctions across its southern range edge (Mid Bay of Biscay, Northern France and southern England; Birkett et al., 1998). Suggesting at sites where sea temperature is artificially increased as a result of anthropogenic activity (e.g. effluent output) local extinction of the biotope may occur (Raybaud et al., 2013) especially if combined with high UK summer sea temperatures in southern examples of this biotope (Bartsch et al., 2013). In southern examples of IR.MIR.KT.LdigT, Laminaria digitata may also be out-competed by its Lusitanian competitor Laminaria ochroleuca which is regionally abundant across the south UK coastline (Smale et al., 2014).

Sensitivity assessment. Species within the habitat are likely to vary in response to temperature change, despite this, Laminaria digitata is likely to withstand an increase in temperature at the benchmark level, and this biotope is therefore assessed as having a ‘High’ resistance to this pressure.  Resilience is, therefore, also ‘High’. If the temperature increase is short-term (a 5°C increase in temp for one month period) this biotope is considered ‘Not sensitive’ at the benchmark level. However, because this pressure may affect the reproductive stage of the algae, over periods longer than one year, if the temperature does not facilitate spore production then a temperature increase of 5°C is likely to negatively affect the biotope and may result in its local loss at the extremes of its range, especially in the light of climate change which has seen the northward retreat of Laminaria digitata biotopes in Europe (Raybaud et al., 2013).

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

Evidence

The thermal optimum of Laminaria digitata is between 10-15°C, with reproductive ability impaired to 20% at 18°C (Arzel, 1998). Therefore while the current population may not be affected, recruitment may be reduced. Spore production only occurs between 5-10°C and is the most temperature sensitive stage of reproduction in Laminaria digitata. Outside this temperature range, reproduction is severely reduced and the species is at risk from local extinction in the short-term. In addition, a temperature increase to 22-23°C causes cell damage and death (Sundene, 1964; Bolton & Lüning, 1982). During an exceptionally warm summer in Norway, Sundene (1964) reported the destruction of Laminaria digitata plants exposed to temperatures of 22-23°C. The sensitivity of this species, therefore, relies on the current sea temperatures of the specific location (Bartsch et al., 2013). A minimum of 10 weeks a year between 5-18°C is needed in order to ensure spore formation and hence reproduction (Bartsch et al., 2013).

With a warming climate, Merzouk & Johnston (2011) predict an expansion of the northern limits of this species. The northward retreat of Laminaria digitata has been predicted using the IPCC climate change model, suggesting that in conditions of increased temperatures by anthropogenic means (e.g. effluent output) local extinction of the biotope may be likely (Raybaud et al., 2013) especially if the increases in summer temperatures inhibiting reproduction recorded by Bartsch et al. (2013) continues.

Sensitivity assessment.  The dominant kelp Laminaria digitata is thought to be a northern species and likely to retreat north as a result of climate change. Therefore, it is unlikely to be sensitive to a reduction in temperature at the benchmark level. Several members of the red algal community are found from the lower shore to the sublittoral, and probably not sensitive to changes at the benchmark level, as if some individual species may reduce in abundance or be lost, the diverse red algal community will remain.  Therefore, a resistance of 'High' is suggested, with a resilience of 'High' and 'the biotope is regarded as 'Not sensitive' at the benchmark level.

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Kelps are tolerant to short-term daily fluctuation in salinity and are recorded as tolerating 5-60 psu; however, they are much less tolerant to long-term changes with growth and photosynthetic rates declining either side of 20-45 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). Therefore, other species probably out-compete Laminaria digitata at the limits of its salinity tolerance, meaning that despite the biotope’s tolerance under conditions of no competition, under natural conditions this biotope is unlikely to occur in conditions above 40 psu.

Sensitivity assessment. This biotope is only recorded in conditions of full salinity and a change from full to hypersaline conditions (>40 psu) for a year is likely to result in major changes in the character of the biotope, including loss of the dominant kelp and members of the red algal community. Therefore, a resistance of 'Low' is suggested, with a resilience of 'High' and a resultant sensitivity of 'Low'.  However, in the absence of direct evidence, the overall confidence in the assessment is 'Low'.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further 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 30-35 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 (5-60 psu; Karsten, 2007).  At the extremes of this range; decreases in photosynthetic rates are evident, particularly at low salinities (Gordillo et al., 2002).  In the study by Karsten (2007), kelp thalli were kept at constant salinities for 5 days, with their photosynthetic rates measured after 2 and 5 days. 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 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).

Red algae vary in their resistance to reduced or variable salinity, so that changes in salinity may change the species composition of the red algal community. In laboratory experiments, maximum rates of photosynthesis and respiration in Palmaria palmata were observed at a salinity 32 psu (Robbins, 1978) although photosynthetic rates were high down to a salinity of 21 psu. Palmaria palmata is likely to be tolerant of small changes in salinity because as an intertidal species it is regularly exposed to precipitation. Corallina officinalis inhabits rock pools and gullies from mid to low water. Therefore, it is likely to be exposed to short-term hyposaline (freshwater runoff and rainfall) and hypersaline (evaporation) events. However, its distribution in the Baltic is restricted to increasingly deep water as the surface salinity decreases, suggesting that it requires full salinity in the long-term (Kinne, 1971).  Some of the fauna, including Halichondria panicea, are tolerant of a wide variety of salinity habitats from reduced to full salinity and are therefore unlikely to be affected by a drop in salinity at the benchmark level.

Sensitivity assessment.  The main studies which this assessment is based on are laboratory studies.  More weight is lent to studies carried out in the field as they provide more realistic estimates of how this species is likely to respond to pressures. Although Laminaria digitata cold survive reduced salinity conditions (i.e. 18-30) this biotope is only found in full salinity conditions (Connor et al., 2004), this biotope is considered to have a resistance of ‘None’ at the benchmark level as the biotope could be lost or replaced by variable salinity biotopes (e.g. IR.MIR.KR.Slat.Ldig) due to an increased abundance of Saccharina latissima. Resilience is considered ‘Medium’ (2-10 years) due to the scale of the impact, giving a sensitivity of ‘Medium’.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

This biotope occurs in a range of water flow conditions from moderately strong (0.5-1.5 meters /second) to very weak (negligible) (Connor et al., 2004). Its flexible stipe and low profile holdfast allow it to flourish in areas with strong water currents. In Lough Ine in Ireland, Laminaria digitata forms dense forests in the fast flowing water of the Rapids where water speeds vary from 4-6 knots (ca 2-3 m/s) (Bassingdale et al., 1948). Laminaria digitata is also found in very strong flows (>3.87 m/s) although it is often out-competed 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 typically of this biotope is also likely to reduce water flow by increasing frictional drag, providing some inherent resistance within the biotope.  Therefore, the Laminaria digitata and its 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 6 weeks resulting in narrower blades and a significantly higher rate of cell elongation, compared to those 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).

Larval 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 larvae 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 larval loss from the local biotope, which if not balanced by a larval influx from another geographically different population, could result in the demise of the local biotope’s health; with a shift in the age structure of the population and a dearth of young alga. 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 and Marshall, 2007).

Sensitivity Assessment.  ‘High’ resistance and by default a ‘High’ resilience to this pressure. Therefore, this biotope is regarded as ‘Not sensitive’ at the benchmark level’, although a prolonged increase in flow could result in loss of the biotope due to reduced local recruitment and competition from other species.

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. Further detail

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 a further 2 cm into the eulittoral zone where grazing pressure is removed (Southward & Southward, 1978). When exposed to the air, kelp canopies buffer the effects of high temperatures and water loss on organisms below their fronds with substratum temperatures on average 8-10°C lower under the canopy than bare rock, additionally decreasing water loss by >45% (Bertness et al, 1999).

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 out-competed by Fucus serratus at its upper limit. The kelp is able to resist both an increase and 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.

Sensitivity assessment. This pressure is a key driver of biotope extent because the upper and lower limits of this species are set by inter-species competition.  In the direct footprint of the impact resistance is, therefore, probably ‘Low’ (loss of 25-75%). Resilience is suggested as ‘Medium’ (2-10 years) due to the scale of the impact. 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. Further detail

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.

A flexible stipe and low profile holdfast allow Laminaria digitata to flourish in moderately to strongly wave exposed areas (as defined by MNCR, Connor et al., 2004).  The absence of this biotope from extreme wave exposure and sheltered sites indicates a failure of the biotope to compete with other biotopes for space.  The biotope IR.HIR.KFaR.Ala.Ldig typically replaces this biotope under conditions of extreme wave exposure, while in sheltered conditions, IR.LIR.K.Slat.Ldig becomes prevalent.

The physiology of seaweeds grown at exposed sites differs morphologically to those at sheltered sites with those exposed to greater wave action. A transplant experiment of Laminaria digitata, from exposed to sheltered sites resulted in a changed morphology with the frond widening, while individuals transplanted 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.

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

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 strong or higher, and a reduction in exposure to sheltered condition would result in a change in the community and biotope. However, a 3-5% 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 contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

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

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

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

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 tonnes 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 the 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 relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

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

Several of the species characteristic of the biotope are reported as having a high intolerance to synthetic chemicals. For instance, Cole et al. (1999) suggested that herbicides such as Simazine and Atrazine were very toxic to macrophytic algae. Hiscock & Hoare (1974) noted that almost all red algal species and many animal species were absent from Amlwch Bay in North Wales adjacent to an acidified halogenated effluent. Red algae have also been found to be sensitive to oil spill dispersants (O'Brien & Dixon 1976).

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

No evidence

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

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). Further detail

Evidence

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).  A rapid recovery from a state of low oxygen is expected if the environmental conditions are transient. If levels do drop below 4 mg/l negative effects on these organisms can be expected with adverse effects occurring below 2 mg/l (Cole et al., 1999).

Sensitivity Assessment.  Reduced oxygen levels are likely to inhibit photosynthesis and respiration but not cause a loss of the macroalgae population directly but small invertebrate epifauna may be lost, causing a reduction in species richness. However, in wave exposed conditions, and hence high mixing, the effects of deoxygenation are likely to be short-lived. Therefore, resistance is assessed as ‘High’.  Hence resilience is likely to be ‘High’, and the biotope is considered ‘Not sensitive’ at the pressure benchmark.

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

Evidence

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 alga 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 alga to photosynthesis and withstand pressure from water movement (Scheibling et al., 1999).

Sensitivity Assessment. The benchmark of this pressure (compliance with WFD ‘good’ status) allows for a slightly less diverse community of red, green and brown seaweeds with the greatest reduction in red species and an increase in the proportion of short-lived species under the WFD criteria for good status. However, this biotope is considered to be 'Not sensitive' at the pressure benchmark, that assumes compliance with good status as defined by the WFD. 

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr. Further detail

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). The use of some kelp species in conjunction with fish aquaculture (to buffer the effects of organic enrichment in the local area) suggests that many commercial kelps  (including Laminaria digitata) are tolerant to local increases in organic enrichment, although the level of enrichment experienced will be dependent on fish species and aquaculture design (Troell et al., 2003). In California, investigations into the effects of sewage effluent into coastal waters concluded that kelp was negatively affected by the organic enrichment of the surrounding waters (North & Schaefer, 1964). The addition of organic matter may also 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’.

Johnston & Roberts (2009) conducted a meta-analysis, which reviewed 216 papers to assess how a variety of contaminants (including sewage and nutrient loading) affected 6 marine habitats (including subtidal reefs). A 30-50% reduction in species diversity and richness were identified from all habitats exposed to the contaminant types. Johnston & Roberts (2009) however also highlighted that macroalgal communities are relatively tolerant to contamination, but that contaminated communities can have low diversity assemblages which are dominated by opportunistic and fast growing species (Johnston & Roberts, 2009 and references therein).

Sensitivity Assessment. Therefore, the biotope is probably resistant of the direct effects of organic enrichment but may be sensitive to the effects of increased turbidity (see water clarity above) and eutrophication. Therefore, a resistance of 'Medium' is suggested to represent a reduction in species richness and an increase in ephemeral green algae. Resilience is probably ‘High' and the biotope is, therefore, considered to have a 'Low' sensitivity at the pressure benchmark.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

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. Further detail

Evidence

A change in substratum type from bedrock to sedimentary is likely to have a similar effect as a ‘physical loss (to land or freshwater habitat) because kelp requires a stable, hard, substratum on which to settle. No evidence of this biotope occurring on sedimentary substratum was found in the literature. Also, it is scarce on the south-east coast of Ireland, in particular, Counties Wicklow and Wexford, due to lack of hard substratum.

Sensitivity assessment.  This biotope is considered to have a resistance of 'None' to this pressure as sedimentary habitats are unsuitable for Laminaria digitata and the associated attached biological assemblage. It would be unable to recover from this permanent change of habitat so resilience is ‘Very low' and its sensitivity within the direct spatial footprint of this pressure is, therefore ‘High’.  

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

'Not relevant' to bedrock biotopes typical of IR.MIR.KT.Ldig.  Please note the soft rock sub-biotope IR.MIR.KT.Ldig.Pid and the under-boulder sub-biotope IR.MIR.KT.Ldig.Bo are assessed as 'High' sensitivity to this pressure.

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). Further detail

Evidence

The species characterizing this biotope are epifauna or epiflora occurring on rock and would be sensitive to the removal of the habitat. However, extraction of the substratum is considered unlikely and this pressure is considered to be ‘Not relevant’ to hard substratum habitats.  Please note the soft rock sub-biotope IR.MIR.KT.Ldig.Pid is assessed as 'High' sensitivity and the under-boulder sub-biotope IR.MIR.KT.Ldig.Bo is assessed as 'Medium' sensitivity to this pressure.

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Low-level disturbance (e.g. solitary anchors) are 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 its 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 18-24 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 & Harkins, 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.

Sensitivity Assessment.  Abrasion of the substratum could remove a proportion of the kelp and associated red algal community. Therefore, the biotope is considered to have ‘Low’ resistance in the footprint of the pressure (loss of 25-75%) while resilience is ‘High’ (within 2 years). Hence, the biotope is assessed as ‘Low’ sensitivity. Please note the soft rock sub-biotope IR.MIR.KT.Ldig.Pid and the under-boulder sub-biotope IR.MIR.KT.Ldig.Bo are assessed as 'Medium' sensitivity to this pressure.

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

The species characterizing this biotope group are epifauna or epiflora occurring on hard rock, which is resistant to subsurface penetration. Therefore, ‘penetration’ is 'Not relevant'. The assessment for abrasion at the surface only is, therefore, considered to equally represent sensitivity to this pressure’. Please refer to ‘abrasion’ above.  Please note the soft rock sub-biotope IR.MIR.KT.Ldig.Pid is assessed as 'High' sensitivity and the under-boulder sub-biotope IR.MIR.KT.Ldig.Bo is assessed as 'Medium' sensitivity to this pressure.

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate for one year. Further detail

Evidence

Suspended Particle Matter (SPM) concentration has a linear relationship with subsurface light attenuation (Kd) (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 6-7 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). Laminaria spp. show a decrease of 50% photosynthetic activity when turbidity increases by 0.1/m (light attenuation coefficient = 0.1-0.2/m; Staehr & Wernberg, 2009). An increase in water turbidity will likely affect the photosynthetic ability of Laminaria digitata, decrease kelp abundance and density.

An increase in SPM results in an increase in subsurface light attenuation. 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) absorbs 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).

Sensitivity Assessment. A decrease in turbidity is likely to support enhanced growth (and possible habitat expansion) and is therefore not considered in this assessment. However, an increase in turbidity is likely to result in loss of the deeper extent of the biotope. Therefore, resistance to this pressure is defined as ‘Medium’ and resilience to this pressure is defined as ‘High’. Hence, sensitivity is assessed as ‘Low‘.

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. Further detail

Evidence

No direct evidence of the effects of this pressure was found for this biotope although some studies were found for the sensitivity of Saccharina latissima to this pressure. An experiment by Roleda & Dethleff (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 PSII function were exhibited (Roleda and Dethleff, 2011).  A layer of fine grained sediment (0.1-0.2 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, therefore 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 disk 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, additionally it is unclear how a whole plant would respond to siltation, however, the findings are still worth considering.

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 spores 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 and Connell, 2001; cited in Airoldi, 2003).

Sensitivity Assessment.  The resistance of this biotope to this pressure is assessed as ‘High’ because despite the dramatic effect of siltation on Laminaria digitata under conditions of no water motion (Lyngby & Mortensen 1996), this biotope exists in water regimes which should favour the rapid exportation of sediment from the biotope, giving it an inherent level of protection against this pressure.  Resilience to this pressure is regarded as ‘High’. This biotope is regarded as ‘Not sensitive’ to this pressure at the benchmark. Please note the soft rock sub-biotope IR.MIR.KT.Ldig.Pid is assessed as 'Medium' sensitivity to this pressure.

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. Further detail

Evidence

The evidence suggests that Laminaria digitata is sensitive to prolonged smothering (Roleda & Dethleff, 2011) (see ‘light‘ deposition above ). 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.  Therefore, the biotope is likely to have a 'Medium’ resistance to this pressure.  Resilience to this pressure is regarded as ‘High’ and sensitivity to this pressure is therefore assessed as ‘Low’. Please note the soft rock sub-biotope IR.MIR.KT.Ldig.Pid and the under-boulder sub-biotope IR.MIR.KT.Ldig.Bo are assessed as 'Medium' sensitivity to this pressure.

Litter

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

Evidence

Not assessed. It is feasible that discarded fishing line, plastic netting, or similar discards could tangle on kelp fronds and potentially damage or remove individuals.  However, no documented evidence was found.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

Evidence

No 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. Further detail

Evidence

It is feasible that localised light sources (e.g. post or harbour side lighting) might increase the length of time available for photosynthesis in a shallow example of the biotope, where overnight lighting and the biotope abut.  However, no evidence was found to qualify any effect and sensitivity is not assessed. 

Shading of shallow examples of the biotope (e.g. by the construction of pontoons or jetties) would limit the availability of light and have similar effects to that of increased turbidity (see above) in the affected area. The dominant kelp is likely to be excluded while shade-tolerant red algae may increase in abundance or be reduced to encrusting corallines and become dominated by faunal turfs, depending on the degree of shading.  The biotope may be lost in the affected area, hence a resistance of 'Low' is suggested, with a resilience of 'High' and sensitivity of 'Low'. 

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. Further detail

Evidence

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

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

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. Further detail

Evidence

Not relevant

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

No evidence regarding the genetic modification of this species was found. Harvesting is carried out on wild kelp stands in a sustainable 5 year cycle (Vea and Ask, 2011), therefore, translocation of this species is unlikely.  In addition, if translocation of populations does occur, a loss in genetic diversity is not regarded as an issue for this species, unless additional pressures result in the isolation and fragmentation of wild coastal populations (Valero et al., 2011). Genetic differentiation in wild populations occurs within 10 km with genetic flow occurring between adjacent species (Billot et al., 2003).  No evidence was found to suggest that Laminaria digitata hybridizes with other species.

Introduction or spread of invasive non-indigenous species

Benchmark. The introduction of one or more invasive non-indigenous species (INIS). Further detail

Evidence

Competition with invasive macroalgae may be a potential threat to this biotope. Potential invasives include Undaria pinnatifida, Sargassum muticum and Codium fragile. Sargassum muticum has been shown to competitively replace Laminaria spp. in Denmark (Staehr et al., 2000). In Nova Scotia, Codium fragile competes successfully with native kelps for space including Laminaria digitata, exploiting gaps within the kelp beds. Once established the algal mat created by Codium fragile prevents re-colonization by other macro-algae (Scheibling et al., 2006).

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

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

Strong & Dring (2011) used canopy removal experiments to investigate inter- and intra-species competition between Sargassum muticum and Saccharina latissima in the Dorn, Strangford Lough, N. Ireland. The Dorn consists of tidal pools, very sheltered from wave action but with moderately strong tidal streams (1-2 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 San Juan Islands, Washington State, USA, showed that Sargassum muticum reduced the abundance of native macroalgae, including the kelp Laminaria bongardiana due to shading. However, the experimental removal of Sargassum resulted in the recovery of native species within about 1 year (Britton-Simmons, 2004; Engelen et al., 2015). The negative effects of Sargassum muticum on native macroalgae are mainly due to competition for light, rather than changes in nutrient availability, sedimentation, or water flow (Britton-Simmons, 2004; Engelen et al., 2015). Cosson (1999) reported a significant decline in Laminaria digitata at two sites between 1983 and 1997 on the coast of Normandy, France, due to an increase in Sargassum muticum abundance in the same areas. For example, on the Grandcamp rocks, Laminaria digitata has almost disappeared while Sargassum muticum had covered 80% of the lower intertidal and subtidal zone in summer.

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

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

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

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

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

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

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

The carpet sea squirt Didemnum vexillum (syn. Didemnum vestitum; Didemnum vestum) is a colonial ascidian with rapidly expanding populations that have invaded most temperate coastal regions around the world (Kleeman, 2009; Stefaniak et al., 2012; Tillin et al., 2020). It is an ‘ecosystem engineer’ that can change or modify invaded habitats and alter biodiversity (Griffith et al., 2009; Mercer et al., 2009). Didemnum vexillum has colonized and established populations in the northeast Pacific, Canadian and USA coast; New Zealand; France, Spain, and the Wadden Sea, Netherlands; the Mediterranean Sea and Adriatic Sea (Bullard et al., 2007; Coutts & Forrest, 2007; Dijkstra et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Lambert, 2009; Hitchin, 2012; Tagliapietra et al., 2012; Gittenberger et al., 2015; Vercaemer et al., 2015; Mckenzie et al., 2017; Cinar & Ozgul, 2023; Holt, 2024). In the UK, Didemnum vexillum has colonized Holyhead marina and Milford Haven, Wales; the west coast of Scotland (marinas around Largs, Clyde, Loch Creran and Loch Fyne), South Devon (Plymouth, Yealm, and Dartmouth estuaries), the Solent, northern Kent, Essex, and Suffolk coasts (Griffith et al., 2009; Lambert, 2009; Hitchin, 2012; Michin & Nunn, 2013; Bishop et al., 2015; Mckenzie et al., 2017; Tillin et al., 2020, Holt, 2024; NBN, 2024).

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

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

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

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

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

Didemnum vexillum requires suitable hard substrata for successful settlement and the establishment of colonies. It can grow quickly and establish large colonies of dense encrusting mats on a variety of hard substrata (Valentine et al., 2007a; Griffith et al., 2009; Lambert, 2009; Groner et al., 2011; Cinar & Ozgul, 2023). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems because of its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock gravel, pebble, cobble, or boulders or artificial substrata such as a variety of maritime structures such as pontoons, docks, wood and metal pilings, chains, ropes and moorings, plastic and ship hulls and at aquaculture facilities (Valentine et al., 2007 a&b; Bullard et al., 2007; Griffith et al., 2009; Lambert, 2009; Tagliapietra et al., 2012; Tillin et al., 2020). Didemnum vexillum has been reported colonizing these types of hard substrata in the USA, Canada, northern Kent, and the Solent (Bullard et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Hitchin, 2012; Vercaemer et al., 2015; Tillin et al., 2020).

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

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

Sensitivity Assessment. The above evidence suggests that Undaria pinnatifida can co-exist with native kelp species within its depth range (-1 to 4 m) e.g., Laminaria digitata as shown in Plymouth Sound, UK.  The dense kelp canopy may restrict or slow the proliferation of Undaria pinnatifida. But Laminaria digitata has been shown to out-compete Undaria pinnatifida. For example, removal of the Laminaria digitata canopy resulted in the increase of Undaria pinnatifida abundance but recovery of Laminaria digitata in the following year resulted in a concurrent decline in Undaria pinnatifida.

In addition, this Laminaria digitata dominated biotope (IR.MIR.KR.Ldig) is found in the sublittoral fringe with a wave exposure ranging from exposed to sheltered and is structured by moderately strong to weak tidal streams. The evidence above suggests that Undaria prefers sheltered conditions, with low tidal flow, and is less tolerant of desiccation than Laminaria digitata.  It is unlikely to out-compete or replace Laminaria digitata under the physical conditions that characterize this biotope. Therefore, resistance is assessed as ‘Medium’ to represent the possibility of short-term colonization by Undaria after a disturbance. Resilience is ‘High’ due to the high fitness nature of Laminaria digitata (Epstein et al., 2019b), so sensitivity is assessed as ‘Low’.

However, while Sargassum muticum prefers wave-sheltered, shallow sites in the sublittoral fringe, it has been reported to invade and out-compete Laminaria digitata beds in Normandy, France (Cosson, 1999; de Bettignies et al., 2021). Therefore, resistance is assessed as ‘Low’ to represent the possibility of colonization by Sargassum. While Laminaria digitata may out-compete Undaria, the recovery after invasion by Sargassum, although rapid, would require direct intervention (removal). Hence, resilience is probably ‘Very low’ so sensitivity is assessed as ‘High’. Overall, confidence is assessed as ‘Low’ due to evidence of variation and the site-specific nature of competition between native kelps and both Undaria pinnatifida and Sargassum muticum and the limited direct evidence of the effect of Sargassum on Laminaria digitata.

There is no evidence of Didemnum vexillum colonizing this biotope in the UK. However, it has been recorded in similar kelp habitats in Norway (Järnegren et al., 2023). Didemnum vexillum requires hard substrata for successful colonization, therefore, it could colonize the bedrock and boulders that characterize this biotope. Also, Didemnum vexillum prefers wave-sheltered conditions and has been recorded in the lower intertidal. Didemnum vexillum can overgrow sessile organisms, including kelp Laminaria sp. However, no direct evidence was found on how Didemnum vexillum affects kelp or if it contributes to Laminaria sp. mortality, 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). 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. 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'.

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). Further detail

Evidence

Symptoms of disease are regularly seen on Laminaria species, however little evidence in the literature is apparent. Infection of Laminaria japonica sporophytes by Pseudoalteromonas, Vibrio and Halomonas result 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 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). Based on the lack of reported mortalities of the characterizing and associated species, the resistance is assessed as ‘High’ resistance to this pressure. Hence, resilience is assessed as ‘High’ and the biotope is assessed as ‘Not sensitive’. 

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

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). If Laminaria digitata, the key characterizing and structuring species of this biotope is removed then the biotope is considered lost and a significant alteration to the biotope classification and character of the habitat is likely.

Debate exists on whether kelp harvesting is detrimental to fish stocks. While some state that no negative consequences of harvesting have been documented (Vea & Ask, 2011); others suggest that as important foraging and nursery grounds for birds and fish, removal will inevitably result in negative consequences (Lorentsen et al., 2010). 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, it is therefore not surprising that no evidence of how wild UK kelp populations would react to commercial harvesting was found in the literature (Netalgae, 2012).

Sensitivity Assessment. This biotope has a resistance of ‘None’ to this pressure in the footprint of the pressure, as the removal of its key characterizing and structural species, Laminaria digitata would result in the loss of the biotope. However, resilience is assessed as ‘Medium’, giving a sensitivity of ‘Medium’ 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. Further detail

Evidence

Removal of Laminaria digitata as by-catch would result in the loss of the biotope.  In healthy macroalgae communities, many species contribute to the balanced condition of the ecosystem. Disrupting this balance may cause top-down consequences for the biotope; for example, overfishing of top predators in Norwegian waters is thought to have caused the urchin bloom, subsequent overgrazing and proliferation of urchin barrens (Steneck et al., 2004).

Sensitivity assessment. Because of the nature of the pressure, resistance to it is considered as ‘None’, because it potentially involves the removal of Laminaria digitata from the biotope, which would result in its loss. Resilience to this pressure is assessed as ‘Medium’. The sensitivity of this biotope to the removal of the non-targeted catch is assessed as ‘Medium’.