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

Saccharina latissima is found in the north west Atlantic and north Pacific (Wilce 1965; Druehl, 1970; Lüning 1990), typically occurring between 40°N and 80°N.  In Europe, Saccharina latissima occurs from Portugal to Spitsbergen (Van den Hoek & Donze, 1967, Lüning, 1990).  Its distribution suggests a resistance to a chronic temperature change (e.g. by 2°C for a year).  Exposure to high short-term temperature increases are likely to result in stress.  There is a general consensus in the literature that increases in temperature are likely to have a more detrimental effect than decreases in temperature (Andersen et al., 2013, Nielsen et al., 2014).

Temperature ecotypes have been suggested for Saccharina latissima populations near its southern limit off the USA coastline.  Algae from New York, which experience water temperatures in excess of 20°C each summer, exhibit greater temperature tolerance than algae from Maine, where temperatures rarely exceed 17°C (Gerard & Du Bois, 1988).  Three weeks of exposure to temperatures greater than 20°C in the field resulted in 50% mortality of algae from New York, while 100% of the algae from Maine died (Gerard & Du Bois, 1988).  In comparison, individual Saccharina latissima from Helgoland in the southern North Sea undergo disintegration of blade tissue after three months at 15°C (Lüning, 1988).

Sporogenesis in Saccharina latissima requires a minimum period of four weeks at or below 15°C combined with short day lengths (Müller et al., 2009).  Germination of zoospores is also sensitive to temperature and may be population specific, with germination inhibited at 20°C in the laboratory, but exceeding 90% in field populations collected in July when photofluence rates were 5 µE m^-2 sec^-1 (Lee & Brinkhuis, 1988). The same study found that gametophyte growth improved with increasing water temperatures between 4-17°C and that fecundity was greatest between 7-17°C.

Sporophyte growth has been recorded between 10-15°C with 50-70% reduction in growth at 20°C (Bolton & Lüning, 1982).  For the gametophytes and young sporophytes of Saccharina latissima, the upper temperature tolerance is 22°C with exceptions including the growth of gametophytes in Long Island Sound at 23°C (Lee & Brinkhuis, 1988).  A temperature of 23°C is also considered to be the maximum survival temperature for gametophytes from three European populations of Saccharina latissima, with disintegration occurring after three weeks (Bolton & Lüning, 1982).  However, Lüning (1990) estimated the upper temperature limit to be 20°C.  In the summer of 1983 (the hottest on record before July 2009), bleaching of Saccharina latissima sporophytes was evident in Plymouth Sound and on the Isle of Man (Hawkins & Hartnoll, 1985).  Research showed that growth reduction was evident at only 5°C above the optimum temperature range for Saccharina latissima (10-15°C) (Kain 1979; Bolton & Lüning 1982; Andersen et al., 2013).

If environmental conditions for spore survival are not favourable, then development of the gametophytes can be delayed for a short period, creating a level of resistance against short-term environmental changes (Van den Hoek et al., 1995).  Despite this ability, seaweeds in general are considered particularly vulnerable to short-term warming events (Dayton & Tegner, 1984; Smale & Wernberg, 2013; Wernberg et al., 2013; from Smale et al., 2013).  Recruitment of Saccharina latissima generally occurs in the highest numbers from December to January (Andersen et al., 2011).

In an experiment observing gene expression in Saccharina latissima, a greater representation of genes associated with high temperature response than those for low temperatures was evident, suggesting that higher temperatures are more detrimental to Saccharina latissima (and therefore the biotope) than low temperatures (Heinrich et al., 2012).  

A permanent change to the local temperature regime may result in a shift to ephemeral algae that then form a barrier to future settlement of Saccharina latissima slowing or stopping recovery of the biotope (Moy & Christie, 2012).  Increased temperatures bring with them increased growth of epiphytic ephemeral algae.  Excessive epiphyte growth on kelp has been reported to result in high mortality rates within the kelp populations on the north American coast (Lee & Brinkhuis, 1988, Levin et al., 2002, Scheibling & Gagnon, 2006).  Krumhansl & Scheibling (2011) also found negative effects in growth in conjunction with increasing temperatures.  However, they also highlighted the role which epiphytic loading enhances blade tissue loss (Andersen et al., 2013).

Bishop (1985) suggested that Echinus esculentus cannot tolerate high temperatures for prolonged periods due to increased respiration rate and resultant metabolic stress.  Ursin (1960) reported Echinus esculentus occurred at temperatures between 0-18°C in Limfjord, Denmark.  Bishop (1985) noted that gametogenesis occurred at 11-19°C, however, continued exposure to 19°C disrupted gametogenesis.  Embryos and larvae developed abnormally after 24hr exposure to 15°C but normally at 4, 7 and 11°C (Tyler & Young 1998).

Sensitivity assessment.  Responses of this biotope to an increase in temperature are clearly population specific.  Those at the extremes of the biotope’s temperature range are likely to be more affected than those at the centre of their range.  An increase of 5°C for one month may affect the fecundity of Saccharina latissima for that year depending on when the increase occurs, because sporogenesis in Saccharina latissima requires a minimum period of four weeks at or below 15°C combined with short day lengths.  An increase of 2°C is more likely to affect those at the extremes of the biotope’s range, but the plasticity of Saccharina latissima may allow for populations to adapt to the new conditions over time.  Minor declines in Echinus esculentus may occur within this biotope.  Therefore, resistance has been assessed as ‘Medium’, and resilience has been assessed as ‘High’ so that sensitivity has been assessed as ‘Low’.

Temperature decrease (local)

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

Evidence

Saccharina latissima is found in the north west Atlantic and north Pacific (Wilce 1965; Druehl, 1970; Lüning 1990), typically occurring between 40°N and 80°N.  In Europe, Saccharina latissima occurs from Portugal to Spitbergen (Van den Hoek & Donze, 1967, Lüning, 1990).  Its distribution suggest that the species would tolerate a chronic temperature change (e.g. by 2°C for a year).  Other species found in this biotope may fair less well.  For example, the urchin Psammechinus miliaris was adversely affected by the 1962/63 winter, while the crinoid (rosy feather star) Antedon bifida may have been lost from the Menai Straits following winter 1947 (D.J. Crisp pers. comm. to K. Hiscock).

The life cycle of kelps, in particular their spore production stage is considered to be sensitive to temperature.  The gametophytes of Saccharina latissima suppress growth below 10°C (Lüning, 1990).  In a laboratory experiment with an Arctic population of Saccharina latissima, embryos achieved 100% germination at 0°C, but expressed lower rates of primary cell growth in comparison to those grown at 10°C.  These lower rates of growth do not seem to impede the kelp’s ability to compete successfully, with the species occurring in year round temperatures lower than 0°C in a high-arctic Fjord, Greenland (Borum et al., 2002).  Sjotun & Schoschina (2002) cultivated Saccharina latissima from embryospores at 0°C in the laboratory and showed that oogonia were produced 18-20 days after sporulation in comparison to a minimum of 20-24 days for Laminaria hyperborea, and 34 days for Laminaria digitata.

Under laboratory conditions chronic exposure to 5°C, after being maintained at 15°C, resulted in the adult sporophytes stage requiring a higher photon fluence rate to maintain net and light-saturated photosynthesis (studied as Laminaria saccharina, Davison et al., 1991).  This response is short-term, with the acclimation of growth temperatures over time buffering the depression in compensation point and light-saturated photosynthesis, allowing the alga to achieve similar rates of light-limited photosynthesis at both 5 and 15°C (Davison et al., 1991).  At 2°C, Saccharina latissima up-regulates the production of amino acids associated with Glutathione, an antioxidant, which suggests that below 2°C lowered growth rates are related to an increased energy expenditure on decreasing the effects of photo-oxidative stress (Heinrich et al., 2012).

Echinus esculentus has been recorded from the Murmansk Coast, Russia. Due to the high latitude at which Echinus esculentus can occur, it is unlikely to be affected at the pressure benchmark.

Sensitivity assessment.  A decrease in temperature at the benchmark is not likely to impact examples of this biotope at the centre of their temperature range.  However, those at its temperature limit are likely to experience decreases in abundance of Saccharina latissima (due to reduced reproduction and growth) if the temperature is lowered to 2°C for one year.  If decreases of 5°C for one month occur, then the time of the year is vital in determining the response of this biotope as it may affect the fecundity of the Saccharina latissima population.  Echinus esculentus is not thought to be sensitive to this pressure at the benchmark.  This biotope is considered to have a resistance of ‘High’, a resilience of ‘High’ and an overall sensitivity of ‘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

Saccharina latissima can be found in full, variable or reduced salinity conditions (Connor et al., 2004).  In a laboratory experiment, Saccharina latissima (studied as Laminaria saccharina) survived successfully between 17-32 psu (Druehl, 1967).  However, Gerard & Du Bois (1988) reported that Saccharina latissima had a salinity tolerance of 23-31 psu.  Karsten (2007) tested the photosynthetic ability of Saccharina latissima under acute two and five day exposure to salinity treatments ranging from 5-60 psu.  A control experiment was also carried at 34 psu. Saccharina latissima showed high photosynthetic ability at >80% of the control levels between 25-55 psu.  Decreases in salinity to 5 psu for Saccharina latissima from Arctic Kongsfjorden (Spitsbergen) induced bleaching, indicative of cell damage after five days of incubation in the laboratory, while treatments decreasing from 20-10 psu were associated with decreasing photosynthetic performance (Karsen, 2007).

Optimum growth rates in algae cultured from UK waters were achieved at 31 psu, while 16 psu dramatically decreased growth rates and 8 psu resulted in the death of the alga (Burrows & Pybus 1971).  In contrast, Saccharina latissima from the White Sea responded with decreased photosynthetic rates at 6-8 psu, while severe growth reductions were noted at 2 psu (Drobyshev, 1971).  Juvenile sporophytes of Saccharina latissima can survive salinities of 13 for 3 weeks.  However, at 10 psu, the juveniles become severely stressed and the majority die (Spurkland & Iken, 2011a).  In the Arctic, decreases in Saccharina latissima growth were associated with decreasing salinity (Spurkland & Iken 2011a).  Nielsen et al. (2014) also associated low growth, with decreases in salinity in a field experiment in Danish waters while Weile (1996), recorded low growth (5.4 mm/day) in areas <14 psu.  Responses of Saccharina latissima to salinity changes are population specific.  Exposure to salinities outside a kelp’s tolerance range causes osmotic and ionic stress (Kirst 1990) resulting in decreased efficiency of their photosynthetic apparatus (<20-25%, Kirst & Wiencke, 1995).

Echinoderms are generally stenohaline owing to the lack of an excretory organ and a poor ability to osmo- and ion-regulate (Boolootian, 1966; Stickle & Diehl, 1987; Russell, 2013).  Therefore, an increase in salinity may cause both Echinus esculentus and the brittlestars within this biotope mortality.  A review by Russell (2013) confirmed that none of the brittlestar species within this biotope occurs in hypersaline conditions, although Pagett (1981) suggested that localised physiological adaption to reduced or variable salinities may occur in near shore areas subject to freshwater runoffs.  

Sensitivity assessment.  IR.LIR.K.Slat.Gz is found within variable salinity conditions.  An increase in this pressure at the benchmark would mean that conditions would become fully saline, and the physical characteristics of the biotope would be changed.  The characterizing species within this biotope would all be able to survive within this new salinity regime.  The new salinity may even provide conditions in which more species would be able to survive and cause a shift to a biotope such as IR.LIR.K.Slat.Pk.  This shift in biotope would not result in a loss of the characterizing species.  However, there would be a shift of the biotope.  Therefore, a resilience of ‘None’ is recorded.  However, when the pressure abated the biotope would rapidly return, which means resilience is assessed as ‘High’. Therefore, sensitivity is assessed as ‘Medium’.  

Salinity decrease (local)

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

Evidence

Saccharina latissima can be found within full, variable or reduced salinity conditions (Connor et al., 2004).  In a laboratory experiment, Saccharina latissima (studied as Laminaria saccharina) survived successfully between 17-32 psu (Druehl, 1967).  However, Gerard & DuBois (1988) reported that Saccharina latissima had a salinity tolerance of 23-31 psu.  Optimum growth rates in algae cultured from UK waters were achieved at 31 psu, while 16 psu dramatically decreased growth rates and 8 psu resulted in the death of the alga (Burrows & Pybus 1971).  In contrast, Saccharina latissima from the White Sea responded with decreased photosynthetic rates at 6-8 psu, while severe growth reductions were noted at 2 psu (Drobyshev, 1971).  Decreases in salinity to 5 psu for Saccharina latissima from Arctic Kongsfjorden (Spitsbergen) induced bleaching, indicative of cell damage after 5 days of incubation in the laboratory, while treatments decreasing from 20-10psu were associated with decreasing photosynthetic performance (Karsen, 2007).  Juvenile sporophytes of Saccharina latissima can survive salinities of 13 for 3 weeks however, at 10 psu, the juveniles become severely stressed and the majority die (Spurkland & Iken, 2011a).  In Arctic kelp, decreases in Saccharina latissima growth were associated with decreasing salinity (Spurkland & Iken 2011a).  Nielsen et al., (2014) also associated low growth, with decreases in salinity in a field experiment in Danish waters while Weile (1996), recorded low growths (5.4 mm/day) in areas <14 psu.  Responses of Saccharina latissima to salinity changes are population specific.  Exposure to salinities outside a kelp’s tolerance range causes osmotic and ionic stress (Kirst 1990) resulting in decreased efficiency of their photosynthetic apparatus (<20-25%, Kirst & Wiencke, 1995).

Echinoderms are generally unable to tolerate low salinity (stenohaline) owing to the lack of an excretory organ and a consequent poor ability to osmo- and ion-regulate (Boolootian, 1966; Stickle & Diehl, 1987; Russell, 2013).  At low salinity, urchins gain weight, and the epidermis loses its pigment as patches are destroyed; prolonged exposure is fatal.  However, evidence shows some resilience to reduced salinity within Echinus esculentus through intracellular regulation of osmotic pressure due to increased amino acid concentrations.  Furthermore, as Echinus esculentus is found within a number of variable and reduced salinity biotopes, e.g. IR.LIR.KVS.SlatPsaVS (Connor et al., 2004).

Sensitivity assessment.  A reduction in this pressure at the benchmark within this biotope would create a reduced salinity regime.  Saccharina latissima may be able to cope with this reduction as this species is found within other biotopes with reduced salinity regimes.  However, the evidence suggests that a decrease in salinity may result in significant mortality of some of the other characterizing species within this biotope.  The lack of osmoregulatory organ within echinoderms means that Echinus esculentus is likely to suffer significant mortality.  The loss of this characterizing grazer will lead to a significant change in the biotope.  Resistance is therefore assessed as ‘Low’ and resilience as ‘Medium’. Sensitivity is therefore 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 to 0.2 m/s for more than one year. Further detail

Evidence

Although Saccharina latissima is unlikely to be directly affected by this pressure at the benchmark.  However, comparisons between biomass yields (dry weight) from two sites found significantly higher yields of Saccharina latissima at the moderately wave exposed site over the sheltered site, with light exposure and water velocity cited as the determining factors of both populations health (Peteiro & Freire, 2013).  In conditions which lack water motion, the transportation of dissolved gases and nutrients within the boundary layer may be significantly reduced, leading to diminished growth (Wheeler, 1980, Parker, 1981, 1982); although conditions of no water motion are rare in the field (Gerard, 1982).  Water activity (wave, tidal and current mediated) may also be important for reducing sedimentation and the growth of filamentous algae which may compete with the key species in this biotope (Norton, 1978; Pihl et al., 1994; Isæus, 2004; Moy et al., 2006).  These factors are also the suggested reason for the absence of this biotope from extremely sheltered Norwegian waters (Bekkby & Moy, 2011).  This infers that Saccharina latissima needs a minimum amount of water movement in order to survive.  Perhaps because of decreased competition from filamentous algae and sedimentation.  Also, because water flow maintains a nutrient flux and enhances light penetration to juvenile sporophytes by moving the fronds (Norton, 1978; Pihl et al., 1995; Lobban & Harrison, 1994; Hurd, 2000; Isæus, 2004; Moy et al., 2006; Bekkby & Moy, 2011).  Decreased wave exposure also causes localised stagnation and de-oxygenation of the water column that would decrease survivorship in the area.

Saccharina latissima’s morphology was noted to differ between a moderately exposed and sheltered site, with those at the moderately exposed site exhibiting a large surface area than those at the sheltered site (Peteiro & Freire, 2013).  Kelps typically have a plastic morphology. In controlled laboratory experiments juvenile Saccharina latissima (studied as Laminaria saccharina) altered their morphology under different water flow exposures; mechanical longitudinal stress resulted in narrower blades of increased cell elongation, while a lack of tension lead to greater blade widths after 6 weeks (Gerard, 1987).  This plasticity is likely to protect thallus damage in areas of greater exposure or in stormier conditions. Stronger water currents may dislodge the kelp from bedrock or cause damage by moving boulders and cobbles.

Echinus esculentus occurred in kelp beds on the west coast of Scotland in currents of about 0.5 m/sec.  Outside these beds, specimens were occasionally seen being rolled by the current (Comely & Ansell, 1988), which may have been up to 1.4 m/sec.  Urchins are removed from the stipe of kelps by wave and current action.  Echinus esculentus are also displaced by storm action.  After disturbance, Echinus esculentus migrates up the shore, an adaptation to being washed to deeper water by wave action (Lewis & Nichols, 1979).  Therefore, increased water flow may remove the population from the affected area and into deeper water.  However, individuals not killed in the process could recolonize the area quickly.

Sensitivity assessment.  This biotope is recorded in areas subject to weak to negligible water flow.  Therefore, an increase in water flow would probably result in a change in the biotope.  The characterizing species Saccharina latissima and Echinus esculentus are unlikely to be directly affected by this pressure at the prescribed benchmark.  Therefore, this biotope is considered to have ‘High’ resistance and ‘High’ resilience resulting in an overall sensitivity assessment score of, ‘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. Further detail

Evidence

This biotope does not occur in the intertidal, and consequently an increase in emergence is considered not relevant to this biotope. 

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

Saccharina latissima rarely grows in wave exposed conditions, as it is vulnerable to dislodgement by wave action and, is typically attached to cobbles and boulders in this biotope, which may be overturned in conditions of increased wave action.  In conditions of increased wave action, Saccharina latissima may gradually change position, shifting into the lower eulittoral (Birkett et al., 1998b).  Competition from other species such as Laminaria digitata, able to withstand higher levels of wave action, may out-compete Saccharina latissima.  Saccharina latissima has been cultivated in the presence of 6.4 m high waves (Buck & Buchholz, 2005), indicating that this competition is the likely driver of Saccharina latissima’s absence from exposed shores.  In conditions of greater wave action, Saccharina latissima productivity (studied as Laminaria saccharina) was less than that of sheltered population; this may have been due to greater nutrient availability in the sheltered site from a current of 0.5 meters/second/second (Gerard & Mann, 1979).  Urchins have been noted to migrate out of kelp biotopes during storms and periods of high wave action, probably to avoid damage by algal whiplash.  Hence, grazing pressure would be temporarily decreased (Lauzon-Gauy, 2007).  When considered in conjunction with emergence, wave exposure is beneficial to Saccharina latissima, with wave spray acting to hydrate individual alga that would otherwise suffer from desiccation and decreased growth rates (Kain, 1979).

While Saccharina latissima is generally absent from wave swept shores, it is also absent from extremely sheltered conditions in Norway.  This suggests that Saccharina latissima needs a minimum amount of water movement in order to survive.  Perhaps because of decreased competition from filamentous algae and sedimentation, but also because wave action maintains a nutrient flux and enhance light penetration to juvenile and smaller sporophytes by moving the fronds (Norton, 1978, Pihl et al., 1995, Lobban & Harrison, 1994, Hurd, 2000, Isæus, 2004, Moy et al., 2006 Bekkby & Moy, 2011).  Decreasing wave exposure also causes localised stagnation and de-oxygenation of the water column that would decrease survivorship in the area.

Sheltered conditions favour the growth of epiphytes, which decrease Saccharina latissima’s ability to withstand storm events and increased wave action, potentially increasing the vulnerability of this biotope to the pressure. Growth of the epiphytic bryozoan, Membranipora membranacea reduces the ability of individual alga to withstand wave action, increasing frond breakages by making them brittle and reducing the maximum stress, toughness and extensibility of the kelp blade (Krumhansl et al., 2011).  Andersen et al. (2011) suggested that in conditions of increased wave activity, water movement may act to clear the fronds’ surface of epibiota, thus improving the health of the population in comparison to those in deeper and more wave sheltered areas.

Echinus esculentus occurred in kelp beds on the west coast of Scotland in currents of about 0.5 m/sec.  Outside these beds, specimens were occasionally seen being rolled by the current (Comely & Ansell, 1988), which may have been up to 1.4 m/sec.  Urchins are removed from the stipe of kelps by wave and current action.  Echinus esculentus are also displaced by storm action.  After disturbance, Echinus esculentus migrates up the shore, an adaptation to being washed to deeper water by wave action (Lewis & Nichols, 1979).  Therefore, increased water flow may remove the population from the affected area and into deeper water.  However, individuals not killed in the process could recolonize the area quickly.

Sensitivity assessment.  This biotope is recorded from very wave sheltered sites.  A decrease in wave exposure, to extremely sheltered, may be detrimental and Saccharina latissima requires some water movement.  However, an increase I wave action would probably result in a change in the biotope due either to a decrease in urchin grazing and/or completion form other kelp species. However, a change in water flow at the benchmark level is not likely to have an effect on this biotope.  Therefore, this biotope is considered to have ‘High’ resistance and ‘High’ resilience resulting in an overall sensitivity assessment score of, ‘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.

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.

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

At the time of writing there was insufficient evidence on which to assess the effect of this pressure on the characterizing species within this biotope.  No direct evidence on the effects of deoxygenation for Saccharina latissima were found in the literature, but reduced oxygen concentrations have been shown to inhibiting both photosynthesis and respiration in macroalgae (Kinne, 1977).  Despite this, macroalgae are thought to buffer the environmental conditions of low oxygen, thereby acting as a refuge for organisms in oxygen depleted regions, especially if the oxygen depletion is short-term (Frieder et al., 2012).  In general, respiration in most marine invertebrates does not appear to be significantly affected until extremely low concentrations are reached.  For many benthic invertebrates this concentration is about 2 ml l^-1, or even less (Herreid, 1980; Rosenberg et al., 1991; Diaz & Rosenberg, 1995).

A rapid recovery from a state of low oxygen is expected if the environmental conditions are transient.  However, this biotope occurs in areas of low water movement, which suggests that a degree of hypoxia may be inherent in the system.  If levels do drop below 4 mg/l, negative effects on these organisms can be expected with adverse effects occurring below 2mg/l (Cole et al., 1999).

Sensitivity assessment:  Due to the lack of information regarding the effect of this pressure on the characterizing species within this biotope has led to an assessment of ‘No evidence’.

Nutrient enrichment

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

Evidence

The nutrient enrichment of a marine environment leads to organisms no longer being limited by the availability of certain nutrients.  The consequent changes in ecosystem functions can lead to the progression of eutrophic symptoms (Bricker et al., 2008), changes in species diversity and evenness (Johnston & Roberts, 2009) decreases in dissolved oxygen and uncharacteristic microalgae blooms (Bricker et al., 1999, 2008).

Johnston & Roberts (2009) undertook a review and meta-analysis of the effect of contaminants on species richness and evenness in the marine environment.  Of the 47 papers reviewed relating to nutrients as a contaminants, over 75% found that it had a negative impact on species diversity, <5% found increased diversity, and the remaining papers finding no detectable effect.  Although the papers reviewed within this report did not consider this exact biotope this finding is still relevant as the meta-analysis revealed that the effect of marine pollutants on species diversity were ‘remarkably consistent’ between habitats (Johnston & Roberts, 2009). It was found that any single pollutant reduced species richness by 30-50% within any of the marine habitats considered (Johnston & Roberts, 2009).  Throughout their investigation there were only a few examples where species richness was increased due to the anthropogenic introduction of a contaminant.  These examples were almost entirely from the introduction of nutrients, either from aquaculture or sewage outfalls.  

High ambient levels of phosphate and nitrogen enhance spore formation in Saccharina latissima (Nimura et al., 2002), but will eventually inhibit spore production, particularly at the extremes of the alga’s temperature tolerance (studied as Laminaria saccharina; Yarish et al., 1990).  Saccharina latissima from the east coast of Scotland, showed increased growth rates in the laboratory when nutrient levels were enhanced by 25% (Conolly & Drew, 1985).  Enhancement of coastal nutrients is likely to favour those species with more rapid growth rates including turf forming algae (Gorgula & Connell, 2004).  Epiphytic abundance and biomass on Laminaria longicruris for example increased under a eutrophic regime (Scheibling et al., 1999) and resulted in a shift from kelp dominated biotopes to an ephemeral algae dominated biotope in Norway (Moy & Christie, 2012).

It was suggested by Comely & Ansell (1988) that Echinus esculentus could absorb dissolved organic material for the purposes of nutrition.  Nutrient enrichment may encourage the growth of ephemeral and epiphytic algae and therefore increase sea-urchin food availability.  Lawrence (1975) reported that sea urchins had persisted over 13 years on barren grounds near sewage outfalls, presumably feeding on dissolved organic material, detritus, plankton and microalgae, although individuals died at an early age.  No information was available on the effects of nutrient enrichment on other parts of the Echinus esculentus life cycle.

Sensitivity assessment.  The evidence suggests that nutrient enrichment cold benefit the macroalgae in the biotope, in the short-term, but that long-term or high levels of nutrient enrichment would probably be detrimental due to an increase competition form in epiphytes and opportunistic green algae.

However, the biotope is recorded as ‘Not sensitive’ at the pressure benchmark of compliance with good status as defined by the WFD.

Organic enrichment

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

Evidence

Johnston & Roberts (2009) undertook a review and meta-analysis of the effect of contaminants on species richness and evenness in the marine environment.  Of the 49 papers reviewed relating to sewage as a contaminant, over 70% found that it had a negative impact on species diversity, <5% found increased diversity, and the remaining papers finding no detectable effect.  Although the papers reviewed within this report did not consider this exact biotope this finding is still relevant as the meta-analysis revealed that the effect of marine pollutants on species diversity were ‘remarkably consistent’ between habitats (Johnston & Roberts, 2009).  It was found that any single pollutant reduced species richness by 30-50% within any of the marine habitats considered (Johnston & Roberts, 2009).  Throughout their investigation there were only a few examples where species richness was increased due to the anthropogenic introduction of a contaminant. These examples were almost entirely from the introduction of nutrients, either from aquaculture or sewage outfalls.

The amount of organic nitrogen a Saccharina latissima stand may be able to uptake varies with location.  Saccharina latissima’s nitrogen uptake by a fish farm in Tristein, Central Norway estimated as 1.2 t of nitrogen per hectare of kelp over one growth season (Wang et al., 2014).  While a similar setup in north-western Scotland predicted the removal of 5% waste nitrogen from 500 tonnes salmon over 2 years (Sanderson et al., 2012).  The excrement and unused feed for fish farms increases the levels of organic matter in their local vicinity.  Evidence from experimental culture of Saccharina latissima around fish farms show enhanced growth rates by up to 61% at certain times in the year (Sanderson et al., 2012).  The quality of the nutrient source is also important with depressed growth rates associated with Saccharina latissima growing near a sewage sludge dumping ground in Liverpool Bay, Irish Sea (Burrows, 1971).

It was suggested by Comely & Ansell (1988) that Echinus esculentus could absorb dissolved organic material for the purposes of nutrition.  Organic enrichment may encourage the growth of ephemeral and epiphytic algae and therefore increase sea-urchin food availability.  Lawrence (1975) reported that sea urchins had persisted over 13 years on barren grounds near sewage outfalls, presumably feeding on dissolved organic material, detritus, plankton and microalgae, although individuals died at an early age.

Sensitivity assessment: There is little empirical evidence to determine the effect of the pressure at the benchmark to the characterizing species within this biotope.  Organic enrichment can cause significant negative effects on marine species, however, at the benchmark it is not considered to be excessively damaging.  Therefore, resistance and resilience are assessed as ‘High’, giving the biotope an overall assessment of ‘Not sensitive’. 

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

If rock were replaced with sediment, this would represent a fundamental change to the physical character of the biotope and the species would be unlikely to recover. The biotope would be lost.

Sensitivity assessment.  Resistance to the pressure is considered ‘None’, and resilience ‘Very low’.  Sensitivity has been assessed as ‘High’.

Physical change (to another sediment type)

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

Evidence

This biotope occurs only on bedrock and boulders, therefore, this pressure is not relevant to this biotope. 

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 rock substratum is considered unlikely and this pressure is considered to be ‘Not relevant’ to hard substratum habitats.

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

No direct evidence was found for this pressure on Saccharina latissima.  Jennings & Kaiser (1998) suggested that the main direct effects of fishing on marine ecosystems usually include scraping, scouring and re-suspension of substratum.  Low level disturbances (e.g. solitary anchors) are unlikely to cause harm to the biotope as a whole, due to the impact’s small footprint.  Natural abrasion of the Saccharina latissima tips occurs continuously, even in calm conditions (Krumhansl, 2012) as a result of water friction, although this erosion may be beneficial to the plants, reducing drag on the thalli (Reed et al., 2008, Krumhansl & Scheibling, 2011; Gunnill, 1985).  While Saccharina latissima is usually permanently attached to the substratum, Burrows (1958) suggests that re-attachment to the substratum after dislodgement is possible as individuals can re-growing hapeteron (root) branches.  It is therefore possible that individuals may be able to withstand dislodgement and abrasion.  Survival of Saccharina latissima in areas where high levels of abrasion occur (a glacial influenced shore) indicate the phenotypic plasticity of the species and suggest that this species, and therefore the habitat, may be resistant to a higher degree of abrasion than other kelp biotopes (Spurkland & Iken, 2011a).  Additionally Saccharina latissima was the only kelp species present on an exposed glacial shore, where high levels of abrasion, inorganic sediment and siltation occurred, while an adjacent sheltered site boasted five kelp species (Spurkland & Iken, 2011b).

Species with fragile tests, such as Echinus esculentus were reported to suffer badly as a result of scallop or queen scallop dredging (Bradshaw et al., 2000; Hall-Spencer & Moore, 2000).  Kaiser et al. (2000) reported that Echinus esculentus were less abundant in areas subject to high trawling disturbance in the Irish Sea.  Jenkins et al. (2001) conducted experimental scallop trawling in the North Irish sea and recorded the damage caused to several conspicuous megafauna species, both when caught as bycatch and when left on the seabed.  The authors recorded 16.4% of Echinus esculentus were crushed/dead, 29.3% would have >50% spine loss/minor cracks, 1.1% would have <50% spine loss and the remaining 53.3% would be in good condition.  The trawling examples mentioned above were conducted on sedimentary habitats and thus the evidence is not directly relevant to the rock based biotopes.  However, it does indicate the likely effects of abrasion on Echinus esculentus.

Sensitivity assessment.  Epifaunal species and communities are considered to be amongst the most vulnerable to bottom gears (Jennings & Kaiser, 1998) and the impact of surface abrasion will depend on the footprint, duration and magnitude of the pressure.  While Saccharina latissima may recover quickly, the urchin population may take longer to return to its prior abundance, so that a different biotope would form. Therefore, based on the evidence, resistance to a single abrasion event is assessed as ‘Low’ and resilience as ‘Medium’, so that sensitivity is assessed as ‘Medium’.  However, Veale et al. (2000) suggested that the abundance, biomass and production of epifaunal assemblages decreased with increasing fishing effort suggesting that, resistance and recovery of the biotope’s species are likely to vary with pressure intensity.  Resistance and resilience will therefore be lower (and hence sensitivity greater) to repeated abrasion events.

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 rock, which is resistant to subsurface penetration.  The assessment for abrasion at the surface only is considered to equally represent sensitivity to this pressure.  This pressure is not thought relevant to hard rock biotopes.

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

Next to wave exposure, light was a key descriptor of Saccharina latissima’s distribution along the Norwegian coast, indicating its importance to the biotope (Bekkby & Moy, 2011).  This biotope typically occurs in silty conditions, with Saccharina latissima able to maintain a positive carbon budget in very low light conditions (Andersen et al., 2011).  As a photosynthetic organism, ultimately Saccharina latissima’s depth distribution is reliant on light availability (Lüning, 1979; Lüning & Dring, 1979; Gerard, 1988).  Therefore, an increase in turbidity may lead to the mortality of alga at the biotope’s deeper range limit and may limit the biotope to shallower waters.

Blue light is crucial for the gametophytic stages of Saccharina latissima, and several other congenic species (Lüning, 1980).  Without blue light (1-4 nE cm/s) and in the presence of red light, female gametophytes do not become fertile or produce eggs (Lüning & Dring, 1975).  In comparison to Laminaria digitata and Laminaria hyperborea, Saccharina latissima exhibits a higher level of tolerance to UV light (indicative of its opportunistic nature, (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.  Populations of Saccharina latissima exhibit different rates of carbon assimilation and growth when exposed to different light acclimation levels in laboratory conditions with alga from turbid sites possessing the fastest growth across treatments (Gerard, 1988).  Deep water populations also exhibit adaptation, so that growth is reduced if daily irradiances exceeding an average of 20 E (radiant flux) /m² /day The resistance of a particular population to this pressure, therefore, varies with location..

Decreases in suspended solids are initially likely to increase photosynthesis and productivity of Saccharina latissima.  However, in conditions of greater water clarity (reduced suspended solids) Laminaria digitata typically out-competes Saccharina latissima, which would result in the loss of the biotope (Norton, 1978).  An absence of this biotope in low silt environments is therefore expected, although with greater water clarity, it may be able to shift its range to deeper waters.

Increases in the levels of suspended sediment were found to reduce growth rates in Saccharina latissima (studied as Laminaria saccharina) by 20% (Lyngby & Mortensen, 1996).  Suspended Particle Matter (SPM) concentration has a linear relationship with sub surface light attenuation (Kd) (Devlin et al., 2008).  Laminarians 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).

Burrow & Pybus (1971) found that the mean thalli thickness of Saccharina latissima (studied as Laminaria saccharina) that had grown in the silted waters of Redcar, Souter Point and Robin Hood's Bay (North-East England) were significantly smaller than those grown in the clearer waters of St Abbs (North-East England) and Port Erin (Isle of Man).  Due to the low water movement associated with this biotope, suspended solids are not likely to be removed by water currents or turbulence and subsequent siltation of the biotope is therefore likely.  Decreases in siltation may also cause a shift in the identity of the associated assemblage, as suspension and deposit feeders receive fewer nutrients, due to the lower carbon input and suspension feeders benefit as their feeding apparatus suffer less from clogging by silt.

Moore (1977) suggested that Echinus esculentus was unaffected by turbid conditions.  Echinus esculentus is an important grazer of red macro-algae in this biotope.  Increased turbidity and resultant reduced light penetration is likely to negatively affect algal growth.  However, Echinus esculentus can feed on alternative prey, detritus or dissolved organic material (Lawrence, 1975, Comely & Ansell, 1988).

Sensitivity Assessment:  The evidence presented suggests that an increase in suspended organic matter may be beneficial to the filter feeding brittlestar species by providing increased food material.  While for Saccharina latissima and other algae an increase in suspended solids will decrease light availability, and consequently lead to a reduction in the suitability of some of the habitat.  The opposite is true for a decrease in suspended sediment.  From the evidence available it is unlikely that a change in the pressure at the benchmark will have an effect on Echinus esculentus.  At the benchmark a change in suspended solids may see a change in abundance of algae and brittlestars.  Consequently, resistance is assessed as ‘Medium’ and resilience as ‘Medium’, so that the biotope is assessed as ‘Medium’ sensitivity to a change in turbidity at the pressure benchmark.

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

Low levels of siltation have been shown to initially offer protection to Saccharina latissima from UVR in laboratory experiments with thallus samples (Roleda et al., 2008).  However, after burial under a variety of sediment types, for over seven days, symptoms of degradation, bleaching, tissue loss and reduced PSII function, were evident (Roleda & Dethleff, 2011).  Laboratory experiments show that even a very thin deposit of fine grained sediment (0.1-0.2 cm thick) caused rotting of Saccharina latissima, resulting in 25% mortality if covered for 4 weeks, in conditions of no water movement (Lyngby & Mortensen 1996).  In the field, these conditions (no water movement) rarely exist and might explain the survival of Saccharina latissima sporophytes in areas of siltation (Birkett et al., 1998b).

The gametophytic and zoospore stages are more vulnerable than their adult counterpart.  Laboratory experiments indicated the adverse effects of siltation on Saccharina latissima, including abnormal development of the zoospore (Burrows, 1971).  Other studies have indicated that siltation inhibits spore settlement with spores failing to form attachments to the fine sediment or the hard bedrock beneath, resulting in their subsequent loss from the biotope by water activity (Devinny & Volse, 1978, Norton, 1978; Bartsch et al., 2008).

Smothering of the whole sporophyte is unlikely to last for long, if deposition is light (<5 cm) silt is likely to fall from the fronds to the substratum, even in conditions of weak water movement. Therefore, the rates of photosynthesis and growth are likely to return to normal within a few days of the deposition event. 

Comely & Ansell (1988) recorded large Echinus esculentus from kelp beds on the west coast of Scotland in which the substratum was seasonally covered with "high levels" of silt.  This suggests that Echinus esculentus is unlikely to be killed by smothering, however, smaller specimens and juveniles may be less resistant.  A layer of sediment may interfere with larval settlement.  If retained within the biotope for extended periods a layer of 5 cm of the sediment may negatively affect successive recruitment events.

Sensitivity assessment.  Where smothering is short-term (less than seven days), then the characterizing species, and therefore the biotope, should be relatively resistant to this pressure at the benchmark.  As this biotope is recorded from low energy habitats (wave sheltered and weak tidal streams) deposited sediment may remain for some time, depending on the local conditions and topography.  Therefore, a resistance of 'High’ is suggested, although long-term smothering would be detrimental.  The resilience of the biotope is considered to be ‘High’ and the sensitivity of this biotope is therefore ‘Not sensitive' at the benchmark, although confidence is low and local hydrography may increase or decrease the resistance.

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

Low levels of siltation have been shown to initially offer protection to Saccharina latissima from UVR in laboratory experiments with thallus samples (Roleda et al., 2008).  However, after burial under a variety of sediment types, for over seven days, symptoms of degradation, bleaching, tissue loss and reduced PSII function, were evident (Roleda & Dethleff, 2011).  Laboratory experiments show that even a very thin deposit of fine grained sediment (0.1-0.2 cm thick) caused rotting of Saccharina latissima, resulting in 25% mortality if covered for 4 weeks, in conditions of no water movement (Lyngby & Mortensen 1996).  In the field, these conditions (no water movement) rarely exist and might explain the survival of Saccharina latissima sporophytes in areas of siltation (Birkett et al., 1998b).

The gametophytic and zoospore stages are more vulnerable than their adult counterpart.  Laboratory experiments indicated the adverse effects of siltation on Saccharina latissima, including abnormal development of the zoospore (Burrows, 1971).  Other studies have indicated that siltation inhibits spore settlement with spores failing to form attachments to the fine sediment or the hard bedrock beneath, resulting in their subsequent loss from the biotope by water activity (Devinny & Volse, 1978, Norton, 1978; Bartsch et al., 2008).  The low levels of water movement within this biotope (Connor et al., 2004) mean that for the sediment to be re-suspended and removed from the biotope might take in excess of a week, after which it is likely that smothered organisms have suffered mortality through asphyxia. 

Sensitivity assessment.  The level of sediment deposition and the amount of time that it might take for the sediment to be removed from the habitat mean that there is likely to be a considerable amount of mortality of the characterizing species, and resistance has been assessed as ‘Low’.  Resilience is assessed as ‘Medium’, and takes into consideration the amount of time it might take for the sediment to be removed from the habitat. Therefore, the overall sensitivity of the biotope is assessed as ‘Medium’. 

Litter

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

Evidence

Not assessed.

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

Species characterizing this habitat do not have hearing perception but vibrations may cause an impact, however, no studies exist to support an assessment.

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

Next to wave exposure, light was a key descriptor of Saccharina latissima’s distribution along the Norwegian coast, indicating its importance to this biotope’s identity (Bekkby & Moy, 2011).  Light availability is very important to this photosynthesising organism.  However, there is no information on how unnatural light sources may affect Saccharina latissima.

There is a lack of evidence relating to the effect of light on Echinus esculentus.  Although some brittlestar species are able to perceive differences in light and dark, visual perception is limited (Tillin & Tyler-Walters, 2014) and this suggests that the brittlestars are unlikely to be directly affected by change in light.

Sensitivity assessment.  Due to the lack of evidence regarding all of the characterizing species within this biotope an assessment of ‘No evidence’ is given.

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 propagule dispersal.  However, propagule 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 – this pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit propagule dispersal.  However, propagule dispersal is not considered under the pressure definition and benchmark.

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

Echinus esculentus was identified by Kelly & Pantazis (2001) as a species suitable for culture for the urchin roe industry.  However, no evidence was found to suggest that significant Echinus esculentus mariculture was present in the UK.  If industrially cultivated, it is feasible that Echinus esculentus individuals could be translocated.  Translocation also has the potential to transport pathogens to uninfected areas (see pressure ‘introduction of microbial pathogens’). The sensitivity of the ‘donor’ population to harvesting to supply stock for translocation is assessed for the pressure ‘removal of target species’. No evidence can be found regarding the cultivation of either Saccharina latissima or brittlestars within the British Isles at the time of writing

Sensitivity assessment.  Due to the lack of evidence regarding the cultivation of any of the characterizing species from this biotope the pressure is considered ‘Not relevant’.

Introduction or spread of invasive non-indigenous species

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

Evidence

The effects of invasive species on Saccharina latissima appear to be limited, or not fully documented in the literature. Strong & Dring (2011) found that the invasive macroalgae Sargassum muticum did not pose a threat to Saccharina latissima stands in Strangford Lough, Northern Ireland. However, the grazer, Lacuna vincta preferentially grazes on Saccharina latissima over the invasive macroalgae Codium fragile in the Gulf of Maine, USA (Chavanich & Harris, 2004).  If similar conditions exist in UK waters, where native grazers preferentially feed on the native Saccharina latissima, then the invasive species will have an initial advantage, and may potentially out-compete Saccharina latissima, leading to the loss of the biotope.

The survival of Saccharina latissima in harbours and docks despite heavy fouling by epibionts has been documented in the southwest of England (Johnston et al., 2011).  While the health of the kelp was undetermined its presence illustrated the resistance of this biotope against this pressure. However, if Saccharina latissima is out-competed by invasive macroalgae, its recolonization could be prevented by heavy fouling of non-native origin, in a similar way that native fouling organisms have prevented recolonization and recovery of Saccharina latissima beds in the Skagerrak area (Andersen et al., 2011).  If an invasion of ephemeral turf algae is coupled with a large-scale disturbance event (e.g. a storm) Saccharina latissima is likely to be vulnerable and, consequently, the whole biotope could be at risk (O’Brien et al., 2015).

There is no evidence on the presence of non-indigenous species or impacts of non-indigenous species on either Echinus esculentus or the brittlestar species within this biotope.

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 that is influenced by temperature (Valentine et al., 2007a). In warmer months (June and July) colonies may be large and well-developed encrusting mats. Populations experience more rapid growth from July to September sometimes continuing into December. Colonies begin to decline in health and ‘die-off’ when temperatures drop below 5°C during winter months from around October to April (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). Cold water months cause colonies to regress and reduce in size, yet they often regenerate as temperatures warm (Griffith et al., 2009; Kleeman, 2009, Mercer et al., 2009), although some populations may not survive winter at all (Dijkstra et al., 2007). The early growth phase, from May to July, is initiated by smaller colonies developing from remnants of colonies that survived the cold water (Valentine et al., 2007a). The seasonal growth cycle is also likely influenced by location. For example, the Didemnum sp. growth cycle for colonies in Sandwich tide pool (temperature range from -1 °C to 24 °C, with daily fluctuations), probably does not occur in deep offshore subtidal habitats in Georges Bank (annual temperature range from 4 °C to 15°C, and daily fluctuations are minimal) (Valentine et al., 2007a).  Larval release and recruitment typically occur between 14 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.  At present there is little evidence to suggest that this biotope would be at significant risk of any known INIS.  Therefore, the resistance and resilience are assessed as ‘High’ and the biotope is considered ‘Not sensitive’ at the pressure benchmark. However, due to the constant threat of further species being introduced to British Isles a search for new evidence should be made regularly. 

There is no evidence of Didmenum vexillum colonizing this biotope in the UK. Didemnum vexillum requires hard substrata for successful colonization, therefore, it could colonize the bedrock and boulders which 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 species Laminaria sp. and Agarum sp. However, no evidence was found on how Didemnum vexillum affects kelp or if it contributes to kelp mortality. Didemnum vexillum may compete for light and space with kelp and epifauna, and potentially interfere with recruitment, which could lead to mortality and a reduction in biodiversity.  However, this biotope is characterized by the grazing of omnivorous sea urchins. There is no direct evidence on how Didemnum vexillum would be affected by urchin grazing. It is assumed that heavy grazing pressure would likely prevent the establishment of Didemnum vexillum, limiting its abundance and preventing it from forming extensive mats. Therefore, the resistance and resilience are accessed as 'High' and this biotope is considered 'Not sensitive', due to the level of grazing. 

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

Saccharina latissima (studied as Laminaria saccharina) may be infected by the microscopic brown alga Streblonema aecidioides that may manifest to different degrees from dark spots to heavy deformations, crippled thalli, and reduce growth rates.  Infection rates have been recorded as 87% (±13%) in Kiel Bay, Western Baltic (Peters & Scaffelke, 1996).  Association of Saccharina latissima with a marine bacterium, Pseudomonads in the Baltic Sea protects the algae from two algal pathogens, Pseudoalteromonas elyakovii and Algicola bacteriolytica.  Pseudomonads produces antibiotics that prevent Saccharina latissima’s infection, suggesting that this biotope’s resistance to disease is population and location specific (Nagel et al., 2012).

There is no evidence in the literature that infection by microbial pathogens results in mass death of kelp populations and the kelp themselves are known to regulate bacterial infections through iodine metabolism (Cosse et al., 2009).  No evidence was found regarding introduction of pathogens or metazoan disease vectors to kelp biotopes.

Echinus esculentus is susceptible to 'Bald-sea-urchin disease', which causes lesions, loss of spines, tube feet, pedicellariae, destruction of the upper layer of skeletal tissue and death.  It is thought to be caused by the bacteria Vibrio anguillarum and Aeromonas salmonicida.  Bald sea-urchin disease was recorded from Echinus esculentus on the Brittany Coast.  Although associated with mass mortalities of Strongylocentrotus franciscanus in California and Paracentrotus lividus in the French Mediterranean it is not known if the disease induces mass mortality (Bower, 1996).

Sensitivity assessment.  There are no recorded mass mortalities of the characterizing species within this biotope through infection by microbial pathogens.  This suggests that although the species are affected by pathogens the danger that they cause to the biotope are low, especially where certain species such as Saccharina latissima can self-regulate bacterial infection.  Based on the available evidence the biotope is judged to have ‘High’ resistance and resilience that results in the biotope being classed 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

In the UK, harvesting of Saccharina latissima is confined to manual harvesting on a small scale, and farming.  Manual harvesting may involve individual blade or whole alga removal.  Only two seaweed leases exist in the UK, so that the impact of harvesting is localised.  Mechanical harvesting of Saccharina latissima occurs in Italy, but the preferred method of commercial harvesting in Europe is by farming on ropes (Seaweed Industry in Europe, Netalgae, 2012).  Low level removal of individuals from the shoreline is unlikely to have an effect on the local biotope.  However, if gathering by diving also increased there would be little resistance to the pressure. Associated species are unlikely to be affected by low level removal of Saccharina latissima unless protection from desiccation on the lower shore is important.

Apex predators (in particular fin fish) have been overfished in UK and Irish waters for centuries.  This has resulted in habitats dominated by invertebrates and commercially undesirable fish such as the lesser spotted cat shark (Molfese et al., 2014).  It has also caused an ecosystem level shift in the way these food webs function.  The urchin barrens recorded off the coast of Norway and in the north west Atlantic are not common to UK waters.  The deforestation by urchins is restricted and patchy (although some have been noted in Scotland; Smale et al., 2013) but could be a result of a shift in apex predators, and a temporary more stable, less dynamic biotope.

No information could be found on the commercial harvest of Echinus esculetnus in the British Isles.  Kelly et al. (2001) investigated the commercial potential of Echinus esculentus and found that the wild capture of this urchin for commercial gain would be ‘impractical economically’.

Sensitivity assessment.  Due to the methods of harvesting used for Saccharina latissima, with the emphasis on aquaculture rather than wild harvesting, little evidence for the resilience of this biotope to harvesting exist.  It is not though that Echinus esculentus is harvested in the British Isles, as wild capture is not economically viable.  The removal of Saccharina latissima from this biotope would remove the main source of food for the other characterizing species, Echinus esculentus.  Therefore, the removal of Saccharina latissima is likely to cause a loss of the biotope.  Resistance is regarded as ‘None’ as the pressure is defined as the removal of key characterizing species from the biotope.  Nevertheless, resilience is probably ‘Medium’, so that sensitivity to this pressure is assessed as ‘Medium’.

Removal of non-target species

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

Evidence

Direct, physical impacts from harvesting are assessed through the abrasion and penetration of the seabed pressures.  The characterizing species within n this biotope could easily be incidentally removed from this biotope as by-catch when other species are being targeted.  The loss of these species and other associated species would decrease species richness and negatively impact on the ecosystem function.

Sensitivity assessment. Removal of a large percentage of the characterizing species would alter the character of the biotope. The resistance to removal is ‘Low’ due to the easy accessibility of the biotopes location and the inability of these species to evade collection. The resilience is ‘Medium’, with recovery only being able to begin when the harvesting pressure is removed altogether. This gives an overall sensitivity score of ‘Medium’.