Biodiversity & Conservation

IR.MIR.KR.Ldig.T

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Loss of the substratum will result in loss of the entire biotope and therefore intolerance has been assessed as high. Some species may survive as epiphytes on the Laminaria digitata but the kelp will be washed away from the area very quickly due to the strong currents. Recoverability is likely to be high (see additional information).
Smothering
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Smothering by a 5 cm layer of sediment is unlikely to adversely affect this biotope given that it is associated with areas of moderately strong to very strong tidal flow rates. The sediment will soon be washed away and 'normal' conditions will resume almost immediately. The abundance of filter feeders may experience some short lived interference with their feeding apparatus and respiratory flows. Furthermore, the holdfasts and lower end of the stipes of Laminaria digitata may experience some mild sand scour. However, at the benchmark level this is unlikely to adversely affect the species and therefore tolerant has been recorded.
Increase in suspended sediment
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Given the strong currents with which this biotope is usually associated, an increase in the amount of suspended sediment could lead to a scouring affect which could damage both the fronds of Laminaria digitata and their resident epiphytes. Corallina officinalis and Lithophyllum incrustans are likely to be afforded protection from scouring from the calcium carbonate within the plants and both are found on wave exposed shores. The current will prevent the sediment from settling and there are, therefore, unlikely to be any effects associated with siltation. However, increased suspended sediment, combined with the strong water flow, may interfere with the respiratory currents and feeding apparatus of the suspension feeders. Over the course of one month this interference and reduced total ingestion is likely to result in a reduced scope for growth and reproductive success for resident invertebrates. For short-lived species such as the bryozoans and sea squirts, this may prove fatal. In contrast, an increase in suspended sediment could also lead to an increase in the availability of food for the suspension feeders, especially if the organic fraction of the suspended sediment were to increase. This biotope supports a highly diverse suspension feeding community including sponges, annelida, bryozoans, ascidians and hydroids. There may be a small decline in faunal species diversity but on balance, intolerance has been assessed as low. Recovery is likely to be very high due to the fact that most of the intolerant species produce planktonic larvae and are therefore likely to be able to recolonize the area quickly from surrounding areas.
Decrease in suspended sediment
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A decrease in the amount of suspended sediment could reduce the scour effect these particles may have on the flora and fauna. Furthermore, the suspension feeding community may become more efficient as there would be fewer inorganic particles to clog and interfere with feeding apparatus. An increase in light availability may occur as a result of a decrease in suspended sediment (see turbidity). Assuming that the decrease in suspended sediment refers to inorganic particles, a reduction in total ingestion in the suspension feeding community is not expected. Therefore, tolerant has been assessed.
Desiccation
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Due to the fact that MIR.Ldig.T is in the sublittoral fringe, the benchmark level for desiccation would involve the biotope moving up the shore into the lower eulittoral for one year. This change is likely to have an adverse affect on the biotope, especially if the change in position coincided with hot or windy weather. At the upper extent of the biotope, Laminaria digitata plants may perish and the death of the understory species below would most likely follow. Hawkins & Harkin (1985) found that the removal of the Laminaria digitata canopy lower on the shore resulted in the bleaching of encrusting corallines. Furthermore, they found that during the hot summer of 1983, extensive damage was observed to encrusting corallines in several sites in Britain. Most of the associated fauna within this biotope are fixed to the substratum and therefore unable to escape an increase in desiccation. In addition, the fauna are dominated by suspension feeders and immersion is therefore a prerequisite to feeding. Over the course of the year, the reduction in available feeding time could prove fatal for short-lived species such as the bryozoans, sponges and ascidians. Laminaria digitata at the upper shore extent of the biotope are likely to be replaced by Fucus serratus that is more tolerant to desiccation. It is likely that the extent of the biotope would be reduced over the course of the year and, accordingly, intolerance has been assessed as high. Recoverability is expected to be high (see additional information).

Increase in emergence regime
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Laminaria digitata biotopes are predominantly sublittoral but extend into the lower eulittoral and therefore have some ability to resist desiccation. At the sublittoral fringe Laminaria digitata regularly becomes exposed to air at low water. Dring & Brown (1982) found that plants that lost up 40-50% of their initial water content were still able to return to their original photosynthetic rate on re-immersion. Many species living beneath the kelp canopy, such as Halichondria panicea and Botryllus schlosseri are also found further up the shore and are therefore likely to be tolerant to a certain degree of desiccation. Furthermore, the kelp canopy is likely to protect the algal understory and benthic fauna from the worst effects of desiccation by the kelp canopy. However, at the benchmark level, some Laminaria digitata plants at the upper extent of the biotope may perish from the effects of desiccation. In turn, flora and fauna in the understory may die since the canopy offers protection from desiccation, wind and insolation. The upper extent of the biotope may be reduced although this may be counteracted by an extension of the biotope at the lower limit. On balance, an intolerance of intermediate has been assessed with a high recovery (see additional information).
Decrease in emergence regime
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MIR.Ldig.T is found in the sublittoral fringe with depths on the lower shore ranging from 0 to 0.5 m above ELWS (chart datum). A decrease in emergence would reduce any emersion experienced by this biotope thus increasing feeding opportunities for all filter feeders, the dominant fauna within this biotope. At the lower shore extent of the biotope the Laminaria digitata may be replaced by Laminaria hyperborea and consequently the biotope per se could be reduced. However, this loss is likely to be counteracted by an increase in this upper shore extent of the biotope. Therefore this biotope is likely to be tolerant to a decrease in emergence.
Increase in water flow rate
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The high species richness associated with this biotope is, in part, due to strong flow rates which enable a wide variety of filter feeders to flourish. Strong flow rates prevent siltation in this otherwise very sheltered environment, bring suspended food, and are also likely to reduce the abundance of grazers. Where water flow rate increases, the biotope will be enhanced in character.
Decrease in water flow rate
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The high species richness associated with this biotope is, in part, due to strong flow rates which prevent siltation in wave sheltered habitats and enable a wide variety of filter feeders to flourish. A decrease in water flow rate, similar to that in the benchmark, would mean the biotope could experience negligible water flow rates for one year. This would be catastrophic for the faunal component of the biotope and would probably result in a major decline in species diversity and richness. The fauna in this biotope is dominated by low lying and encrusting filter feeders, the majority of which are highly intolerant to sedimentation. Under normal conditions, sedimentation would not be a problem, due to the high flow rates, but with reduced rates, sedimentation could smother the animals. Some filter feeders have the ability to cope with siltation and excess suspended material. For example, the breadcrumb sponge Halichondria panicea has a mechanism for sloughing off their complete outer tissue layer together with any debris (Barthel & Wolfrath, 1989). However, there is an energetic cost of cleaning their tissues, and this species, together with other filter feeders, would probably experience reduced growth over the benchmark. For annual species, including the star ascidian Botryllus schlosseri, this could prove fatal. The keel worm Pomatoceros triqueter and the bryozoans Electra pilosa and Membranipora membranacea are also highly intolerant to siltation and smothering (see MarLIN reviews) and probably represent but a few suspension feeders commonly associated with this biotope that are highly intolerant of siltation. The hydroid Dynamena pumila experienced marked decline in areas with increased silt content in Strangford Lough, Northern Ireland (Seed et al., 1983). Round et al. (1961, cited in Hiscock, 1983) transferred colonies of the hydroid Sertularia operculata (studied as Amphisbetia operculata, from rocks in the tidal rapids at Lough Ine, Ireland, to a nearby sheltered bay. The colonies quickly became clogged with silt and most had died within 9 weeks of the transfer. The encrusting algae Lithophyllum incrustans is also likely to be highly intolerant at the benchmark level.

Laminaria digitata, with its flexible stipes, is likely to 'collapse' onto the underlying flora and fauna. This kelp blanket could completely overshadow the underlying seaweeds which would ultimately reduce their photosynthetic capabilities. Furthermore, the feeding efficiency of the underlying filter feeders may be reduced.

With respect to predation, reduced flow rates may increase the abundance of the grazer Gibbula cineraria as strong water flow rates are likely to dislodge these animals from the blades of the kelp and other plants. Not only will this increase potential damage to the kelp plants and red seaweeds, the grazing activities of this species have also been shown to have a deleterious affect on the post-settlement establishment of larvae of ctenostome and cheilostome bryozoans, hydroids and ascidians (Turner & Todd, 1991).

The Laminaria digitata will be outside their preferred tidal flow range (strong to very strong). Furthermore, Duggins et al. (2001) found that the mortality of the kelp Nereocystis leutkeana in the San Juan archipelago, Washington, was higher in populations that experienced periods of calm flow punctuated by intermediate to strong flow episodes. This was thought to be due to increased grazing efficiency in calmer periods. The grazers would attack the blades and stipes of the kelp so that when water flow rate increased again, they would more likely be torn off. In the case of this biotope, the resumption of 'normal' flow rates will have the same effect.

All the key and important species associated with this biotope would be adversely affected by a reduction in tidal flow rate at the benchmark level and accordingly, intolerance has bee assessed as high. Recoverability is likely to be high (see additional information). The sublittoral fringe of similarly sheltered shores that are not tide-swept are generally characterized by mixed Saccharina latissima and Laminaria digitata (£MIR.Lsac.Ldig£) or Saccharina latissima (JNCC, 1999) and it is possible that one of these two biotopes will prevail should decreased flow rates persist.
Increase in temperature
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Laminaria digitata is a eurythermal species with sporophytes growing over a wide temperature range. Lüning (1984) detected a seasonal shift in heat tolerance of Laminaria digitata plants in Helgoland of 2 °C between spring and summer so the species is likely to be tolerant of a long term, chronic change in temperature at the benchmark level.

Corallina officinalis from Helgoland survived temperatures between 0 °C and 28 °C when exposed for 1 week (Lüning, 1990). However, severe damage was noted in Corallina officinalis as a result of desiccation during unusually hot and sunny weather in summer 1983 (an increase of between 4.8 and 8.5 °C) (Hawkins & Hartnoll, 1985). In tank cultures of Palmaria palmata, all the plants were dead within a week at 20 °C and above, (Morgan et al., 1980). Such high temperatures, however, are unlikely in the subtidal fringes of Britain and Ireland. Increased temperatures are also likely to increase the risk of desiccation in the encrusting coralline algae Lithophyllum incrustans.

With regard to associated fauna, both the star ascidian Botryllus schlosseri and the breadcrumb sponge Halichondria panicea have large geographical ranges in which the UK is almost central. At the benchmark level these species are therefore likely to be tolerant of chronic temperature changes.

On balance, the biotope may be tolerant to long term chronic change in temperature but is likely to be of intermediate intolerance to short term acute temperature change. During an exceptionally warm summer in Norway, Sundene (1964) reported the destruction of Laminaria digitata plants exposed to temperatures of 22-23 °C. Littler & Kauker (1984) suggested that in Corallina officinalis, the crustose base was more resistant of desiccation or heating than fronds and that this species is probably intolerant of abrupt short term temperature increases. Therefore, an intolerance of intermediate has been recorded. Recovery is expected to be high (see additional information).

Decrease in temperature
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The flora within this biotope have a varying tolerance with regard to low temperatures. In Scotland, Todd & Lewis (1984) reported mortality of all but the lowest shore Laminaria digitata plants when low spring tides coincided with extreme air frosts on several consecutive days. This was probably due to damage to the exposed meristems. In contrast, New Zealand specimens of Corallina officinalis were found to tolerate -4 °C (Frazier et al. 1988, cited in Lüning, 1990). Palmaria palmata does well in low temperatures, with an optimum between 6 and 15 °C, consistent with its distribution in northern temperate and arctic waters. Furthermore, in the laboratory, plants only became fertile if left at temperatures between 5-7 ° C with a short light period (Meer van der, 1979). However, Kain & Norton (1990) suggest that a widely distributed species like Palmaria palmata reacts less strongly to temperature differences than some other red algae.

With regard to associated fauna, the star ascidian and breadcrumb sponge both have large geographical ranges in which the UK is almost central. At the benchmark level these species are therefore likely to be tolerant of chronic temperature changes. Acute temperature changes, however, are likely to be met with a higher intolerance, as illustrated by the work of Crisp (1964) following the severe winter of 1962-63. He noted damage to Halichondria panicea colonies during the severe winter but rapid recovery due to planktonic larvae. He reported the sponge Hymeniacidon perleve as unusually rare in abundance after the severe winter in North Wales. In South Wales, approximately half the population of Pomatoceros triqueter were reported to have died at Mumbles. Below a temperature of 7 °C, Pomatoceros triqueter is unable to build calcareous tubes (Thomas, 1940). This means that, even if the adults survived a decrease in temperature, larvae would not be able to attach to the substratum. In contrast, the hydroid Dynamena pumila was reported to have remained common in the habitats in which it was usually found in North Wales.

On balance, the biotope may be tolerant to long term chronic change in temperature but is likely to be of intermediate intolerance to short term acute temperature change. Recovery is expected to be high (see additional information).
Increase in turbidity
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The extent of the lower limit of kelp distribution on rocky shores is determined by the amount of light penetration. Increased turbidity around a sewage treatment plant was thought to be responsible for the absence of Laminaria digitata plants in the Firth of Forth (Read et al., 1983) and has been reported to result in reduced the depth range and the fewer new plants under the kelp canopy. In Narragansett Bay, Rhode Island growth rates of Laminaria digitata fell during a summer bloom of microalgae that dramatically reduced downward irradiance. An increase in turbidity will reduce photosynthetic capability and therefore growth of both the kelp and understory plants. The majority fauna are unlikely to be directly affected by an increase in turbidity although the symbiotic algae living within the breadcrumb sponge Halichondria panicea may decline in abundance. However this will not affect the survival of the sponge. On balance an intolerance of intermediate has been recorded. Recovery is expected to be high (see additional information).
Decrease in turbidity
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Reduced turbidity is likely to favour kelp growth and the growth of other micro and macroalgae. Although the majority of associated fauna are unlikely to be directly benefited by this change, there may be more food available for grazers such as Gibbula cineraria. Furthermore, increased water clarity may lead to an increase in the amount of green algae in the biotope. Overall, the biotope is likely to benefit from reduced turbidity and may experience a small rise in species diversity.
Increase in wave exposure
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Although, individually, the species in this biotope are generally tolerant of varying wave exposures, the species assemblage itself, in terms of the abundance of different species, is likely to change. Therefore this biotope is likely to be lost but is likely to be replaced by Laminaria digitata on moderately exposed sublittoral fringe rock (£MIR.Ldig.Ldig£) which is highly similar in terms of species composition but associated with more exposed locations. The MNCR recorded more than 700 species in records of this biotope (JNCC, 1999). However, nearly 350 records of the biotope were studied to produce that figure and it is, therefore, possible that MIR.Ldig.Ldig is less diverse than MIR.Ldig.T where 425 species were recorded in only 45 records of the biotope (JNCC, 1999).
Decrease in wave exposure
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This biotope occurs in extremely sheltered conditions and therefore a decrease in wave exposure is not relevant.
Noise
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The dominant flora (Laminaria digitata) and encrusting fauna (bryozoans, sponges and ascidians) are unlikely to have mechanisms for detecting noise and will therefore be tolerant to this factor.
Visual Presence
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The dominant flora (Laminaria digitata) and encrusting fauna (bryozoans, sponges and ascidians) are unlikely to have mechanisms for detecting visual presence and will therefore be tolerant to this factor.
Abrasion & physical disturbance
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The fronds of Laminaria digitata are soft and are likely to be damaged by abrasion at the benchmark level. Similarly, the encrusting sponges, encrusting algae bryozoans and colonial ascidians may be scraped off the surface of the bedrock, boulders or kelp fronds and small clumps of foliose red seaweeds and coral weed may also be lost. In general, however, a proportion of each species is likely to remain and therefore, intolerance has been assessed as intermediate. Recoverability is likely to be high (see additional information below).
Displacement
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The key structural species Laminaria digitata cannot reattach itself after having been removed from the substratum and it is unlikely that other important flora (Palmaria palmata and Corallina officinalis) can either. Kelp canopy removal experiments in the Isle of Man revealed that, after Laminaria digitata was removed, the underlying community changed dramatically (Hawkins & Harkin, 1985). For example, encrusting corallines became bleached and colonized by successive algal phases, including filamentous brown and green algae. Gibbula cineraria are mobile and will be tolerant of displacement but all other important characterizing fauna are encrusting and fixed to the substrata and will be highly intolerant to displacement. This factor is likely to have a similar affect as substratum loss and accordingly, intolerance has been assessed as high. Recovery is likely to be high (see additional information).

Chemical Factors

Synthetic compound contamination
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  • Laminaria digitata along with almost all red algal species and many animal species were found to be absent from sites close to acidified, halogenated effluent from a bromide extraction plant (Hoare & Hiscock, 1974).
  • Axelsson & Axelsson (1987) investigated the effect on Laminaria digitata of exposure to various chemicals for 24 hours by measuring ion leakage as an indication of plasma membrane damage. However, only limited ion loss was seen on exposure to two detergents, nonylphenol ethoxylate (NP-10) and linear alkylbenzene sulfonate (LAS).
  • Cole et al. (1999) suggested that herbicides such as Simazina and Atrazine were very toxic to macrophytic algae.
  • Laboratory studies of the effects of oil and dispersants on several red algae species, including Palmaria palmata (Grandy, 1984, cited in Holt et al., 1995) concluded that they were all intolerant of oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages.
  • Smith (1968) reported that, in areas of heavy spraying of oil and detergent dispersants, Corallina officinalis was killed, and was affected down to a depth of 6 m in one site, presumably due to wave action and mixing. However, regrowth of fronds had begun within 2 months after spraying ceased.
No information was found concerning the specific effects of synthetic chemicals on the important characterizing or important functional fauna. Given the evidence for the algal species however, an intolerance of high has been recorded. Assuming that the plants were able to reproduce and that the settlement of sporelings was not prohibited by residual chemicals, recovery is expected to be high. However, if the chemicals were bioaccumulated or resided in the biotope for a long time, this recovery may take significantly longer.
Heavy metal contamination
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The tolerance of Laminaria digitata to heavy metals is highly variable depending the on the metal concerned. Zinc was found to inhibit growth in Laminaria digitata at a concentration of 100 µg/L and at 515 µg/L, growth had almost completely ceased (Bryan, 1969). Axelsson & Axelsson (1987) investigated the effect of exposure to mercury (Hg), lead (Pb) and nickel (Ni) for 24 hours by measuring ion leakage to indicate plasma membrane damage. Inorganic and organic Hg concentrations of 1 mg/l resulted in the loss of ions equivalent to ion loss in seaweed that had been boiled for 5 minutes. Laminaria digitata was unaffected when subjected to Pb and Ni at concentrations up to 10 mg/l. The results also indicated that the species was intolerant of the tin compounds butyl-Sn and phenyl-Sn. Bryan (1984) suggested that the general order for heavy metal toxicity in seaweeds is: organic Hg > inorganic Hg > Cu > Ag > Zn > Cd > Pb. Cole et al. (1999) reported that Hg was very toxic to macrophytes and Boney (1971) reported that the red algae Plumaria elegans experienced 100% growth inhibition at 1 ppm Hg. However, no information was found concerning the specific effects of heavy metals on either Palmaria palmata or Corallina officinalis or any of the important faunal components of this biotope. Given the evidence for the algal species, however, intolerance has been assessed as low. Heavy metals have the potential to accumulate in plant tissue and therefore it may take some time for tissue levels to fall before recovery can begin. Assuming that the plants were able to reproduce and that the settlement of sporelings was not prohibited by residual metals, recovery is expected to be very high although this may take significantly longer depending on depuration of the metals from the biotope.
Hydrocarbon contamination
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The brown algae are thought to be largely protected from oil penetration damage by the presence of a mucilaginous coating (O'Brian & Dixon, 1976). In addition, effects of oil accumulation on the thalli are mitigated by the perennial growth of kelps. 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 strong tidal flow in this biotope may provide some protection to all seaweeds within this community. No significant effects of the Amoco Cadiz spill were observed for Laminaria populations and the World Prodigy spill of 922 tons of oil in Narragansett Bay had no discernible effects on Laminaria digitata (Peckol et al., 1990). Furthermore, the upper limit of distribution for Laminaria digitata moved up wave exposed shores by as much as 2 m during the first few years after the Torrey Canyon oil spill due to the death of animals that graze the plants (Southward & Southward, 1978). 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).

O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil contamination. Where exposed to direct contact with fresh hydrocarbons, encrusting calcareous algae have a high intolerance. Crump et al. (1999) noted a dramatic bleaching on encrusting corallines and signs of bleaching in Corallina officinalis, Chondrus crispus and Mastocarpus stellatus at West Angle Bay, Pembrokeshire after the Sea Empress oil spill. However, encrusting corallines recovered quickly and Corallina officinalis was not killed. Laboratory studies of the effects of oil and dispersants on several red algae species, including Palmaria palmata (Grandy 1984, cited in Holt et al., 1995) concluded that they were all sensitive to oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages. It is possible that Corallina officinalis and other algae are more intolerant of the dispersants used during oil spills than the oil itself. Where exposed to direct contact with fresh hydrocarbons, encrusting coralline algae appear to have a high intolerance. Crump et al. (1999) describe "dramatic and extensive bleaching" of 'Lithothamnia' following the Sea Empress oil spill. Observations following the Don Marika oil spill (K. Hiscock, own observations) were of rockpools with completely bleached coralline algae. However, Chamberlain (1996) observed that although Lithophyllum incrustans was quickly affected by oil during the Sea Empress spill, recovery occurred within about a year. The oil was found to have destroyed about one third of the thallus thickness but regeneration occurred from thallus filaments below the damaged area.

No information was found concerning the specific effects of hydrocarbon contamination on the important faunal component of this biotope. However, the intolerance of the sponges, ascidians and bryozoans is likely to be related to depth of the oil / tar deposition and the strong tidal flow associated with this biotope is likely to reduce this. Nevertheless, analysis of kelp holdfast fauna after the Sea Empress oil spill in Milford Haven illustrated decreases in number of species, diversity and abundance at sites nearest the spill (SEEEC, 1998). On balance, it is likely that some species within the biotope may be killed and accordingly, an intolerance of intermediate has been recorded. Recovery, assuming the environment was free from hydrocarbon pollution or at least back to 'normal' levels, should be high, especially since most of the intolerant species produce planktonic larvae and are therefore likely to be able to recolonize the area quickly from surrounding areas.
Radionuclide contamination
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No information was found regarding the actual effects of radionuclide contamination on this biotope.
Changes in nutrient levels
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The growth of macroalgae in temperate coastal waters is generally expected to be limited by nitrogen in the summer period. In the Bay of Fundy, for example, where there is a tidal flux of nutrients from the marshes there is luxurious growth of Palmaria palmata. A comparison of Laminaria digitata growth rates in Arbroath, Scotland with a more oligotrophic and a more eutrophic site appears to support this hypothesis (Davison et al., 1984) (see MarLIN review). Corallines seem to be tolerant and successful in polluted waters. Kindig & Littler (1980) demonstrated that Corallina officinalis var. chilensis in South California showed equivalent or enhanced health indices, highest productivity and lowest mortalities (amongst the species examined) when exposed to primary or secondary sewage effluent. Specimens from unpolluted areas were less tolerant, suggesting physiological adaptation to sewage pollution (Kindig & Littler 1980).

However, although nutrient enrichment may benefit Laminaria digitata, the indirect effects of eutrophication, such as increased light attenuation from suspended solids in the water column and interference with the settlement and growth of germlings, may be detrimental. 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). Eutrophication often results in the loss of perennial algae, reduction of the depth range of algae (due to turbidity), and increases in opportunistic species such as ephemeral algae. High levels of ephemeral algae can cause smothering of species changing the character of the biotope. In addition, very high levels of nutrients can be toxic to macroalgae. Palmaria palmata placed in tanks with continuous immersion in high nutrients over several weeks stopped growing (Morgan et al., 1980). and so intolerance is assessed as intermediate. Any recovery is likely to be high as species are unlikely to be completely lost and have planktonic larvae and high growth rates.

Increase in salinity
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This biotope occurs in the sublittoral fringe of full salinity environments. Consequently, an increase in salinity at the benchmark level is not relevant.
Decrease in salinity
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Birkett et al. (1998b) suggested that kelps are stenohaline, in that they do not tolerate wide fluctuations in salinity. Growth rate may be adversely affected if the kelp plant is subjected to periodic salinity stress. 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) investigations indicated damage of 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). However, at the benchmark level this unlikely to result in the loss of the biotope per se.

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 the breadcrumb sponge Halichondria panicea, tolerant of 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. However, given the evidence for kelp the intolerance has been assessed as intermediate. Recoverability is likely to be high as it is unlikely that many of the plants will be lost.
Changes in oxygenation
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The biotope occurs in areas where still water conditions do not occur and so some species may be intolerant of hypoxia. Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l. Kinne (1972) reports that reduced oxygen concentrations inhibit both photosynthesis and respiration although macroalgae may tolerate a level of deoxygenation since they can produce their own oxygen. However, the fauna that characterize the biotope may be more intolerant. The sponges and ascidians, for example, may experience some mortality and therefore intolerance to low oxygenation (2 mg/l) in seawater for a week has been assessed as intermediate. Effects are likely to be short lasting on return to normal oxygen concentrations so recovery is assessed as high.

Biological Factors

Introduction of microbial pathogens/parasites
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Insufficient information was found concerning the specific effects of microbial pathogens on the survival of this biotope.
Introduction of non-native species
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Insufficient information was found to suggest that alien and non-native species would outcompete or displace the key or important species within this biotope.
Extraction
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Of the key species within this biotope only the oarweed Laminaria digitata is known to be targeted for extraction. Plants cast up on the shore have historically been collected and used for fertilizer. More recently Laminaria digitata has been cultivated commercially for alginate in Britain and in Ireland and in France it is cultivated as a sea vegetable. The loss of canopy species including Laminariales (kelps) has been shown to be detrimental to encrusting corallines below which became bleached (Hawkins & Harkin, 1985). In addition, the loss of the kelp plants would lead to loss of the associated epiphytes including bryozoans, sponge, ascidians and the red algae Palmaria palmata, which are commonly found growing on the stipe. The increase in irradiance associated with a loss in canopy cover could lead to the growth of fast growing filamentous green and brown algae. A sustained extraction of the kelp could therefore lead to the loss if the biotope. However, due to the sublittoral fringe position of this biotope it is unlikely that vast areas of Laminaria digitata will be cleared.

Coral weed and dulse may also be targeted for extraction. Dulse (Palmaria palmata) is used as a vegetable substitute or animal fodder although harvesting on a commercial scale only takes place in Ireland and France. Recovery from extraction of the species is likely to be high because remaining plants constitute a reservoir from which recruitment can occur. In the Isle of Man recruitment of new plants to kelp cleared plots occurred within five months (Hawkins & Harkin, 1985). The coral weed Corallina officinalis is collected for medical purposes; the fronds are dried and converted to hydroxyapatite and used as bone forming material (Ewers et al., 1987). It is also sold as a powder for use in the cosmetic industry. An European research proposal for cultivation of Corallina officinalis is pending (Dr T. Wiedemann, pers. comm.).

Due to the sublittoral fringe position of this biotope it is unlikely that vast areas of any species will be cleared. Nevertheless there may be some extraction and accordingly, intolerance has been assessed as intermediate with a high recovery (see additional information below).

Additional information icon Additional information

  • Laminaria digitata plants are able to rapidly re-colonize any gaps in the upper infralittoral which result from storm damage (Birkett et al., 1998b) and have been shown, in various independent studies, to recovery rapidly after loss of population. On the Isle of Man, Laminaria digitata made a complete recovery within 8 months of being mowed just above the holdfast (Hawkins & Harkin, 1985). Kain (1979) reported that the standing crop was re-established within 18-20 months. Where the whole population was cleared (as opposed to just pruning the standing crop), recolonization of Laminaria digitata on concrete blocks on the Isle of Man took place within 2 years and red algae returned within a year (Kain, 1975). In Helgoland, recovery of cleared and burned plots to original density took 25 months but plants were smaller than those on undisturbed plots (Markham & Munda, 1980). This suggests that when all spores and germlings are also removed full population recovery is longer than 25 months.
  • Many other characterizing species have planktonic larvae and/or are mobile and so can migrate into the affected area. The dulse Palmaria palmata, for example, is commonly epiphytic on Laminaria digitata and is therefore probably recruited form a local source (Hawkins & Harkin, 1985).
  • Halichondria panicea increases by about 5% per week (Barthel, 1988) and exhibits high recovery following substratum loss.
  • In experimental plots, up to 15% cover of Corallina officinalis fronds returned within 3 months after removal of fronds and all other epiflora/fauna (Littler & Kauker, 1984).
However, although some species colonize the biotope quite rapidly, maturity of the overall community is likely to be slower. For example, encrusting coralline algae such as Lithophyllum incrustans are slow growing (2-7 mm per annum - see Irvine & Chamberlain 1994) and recruitment of other species to the kelp bed may take longer. In dredged kelp beds in Norway for example, although the rock between Laminaria hyperborea plants was uniformly covered with coralline algae after 3 years, the more diverse community of cnidarians, bryozoans and sponges associated with coralline algae seen on undredged plots was absent (Birkett et al., 1998b). In Massachusetts, Sebens (1986) found that red crustose algae, spirorbid worms, bryozoans and erect hydroids were quick to colonize cleared patches on subtidal vertical rock walls. These species usually achieved coverage on areas that have been scraped free of fauna within 4 months. The ascidian Dendrodoa carnea was found to appear towards the end of the first year after clearing whilst Halichondria panicea was slow to colonize the area. This sponge first appeared almost one year after clearance and only reached previous levels of cover after more than two years after scraping (Sebens, 1986). Within five years, however, this biotope is likely to have reached maturity.

This review can be cited as follows:

Marshall, C.E. 2005. Laminaria digitata, ascidians and bryozoans on tide-swept sublittoral fringe rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 22/12/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=273&code=1997>