Biodiversity & Conservation

IR.MIR.KR.Ldig.Ldig

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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intolerance is very high because the key structuring species within the biotope, Laminaria digitata and Lithophyllum incrustans, as well as a large proportion of the fauna, such as hydroids, ascidians and sponges, are permanently attached to hard substrata and would be completely removed by any substratum loss. They cannot re-attach once removed and would be swept away. Other sedentary and mobile species associated with the kelps would also be lost significantly reducing faunal diversity. However, recovery is likely to be high because Laminaria digitata plants can recolonize cleared rock within two years (Kain, 1979) and most other characterizing species have planktonic larvae and/or are mobile and so can migrate into the affected area. Although the main species covering rock, Lithophyllum incrustans, grows at a rate of only <7mm a year (Irvine & Chamberlain 1995) and will take much longer to colonize the available space the biotope will be much as before after five years and so recovery is assessed as high.
Smothering
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The key species in the biotope, Laminaria digitata, is likely to protrude above smothering material and so will probably tolerate smothering at the benchmark level. However, smothering is likely to affect sporeling and gametophyte survival. Other species such as the active suspension feeders and foliose algae are likely to be killed by smothering. Holdfast fauna may be particularly affected and there may be a switch to more siltation tolerant species, particularly deposit feeders. Recovery is likely to be high because most characterizing species have a planktonic larva and/or are mobile and so can migrate into the affected area and recolonization of Laminaria digitata plants in cleared areas takes about two years.
Increase in suspended sediment
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Laminaria digitata can be found in areas of siltation although in very high silt environments the species may be out-competed by Saccharina latissima. Encrusting calcareous algae are frequently subject to cover by sediment and appear to survive well. However, silt may clog respiratory and feeding organs of suspension feeders such as sea squirts and may result in a decline in faunal species diversity. However, increased siltation is unlikely to have a significant effect in terms of smothering by settlement in the regime of strong water flow typical of this biotope.
Decrease in suspended sediment
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Desiccation
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Laminaria digitata biotopes are predominantly sublittoral but extend into the lower eulittoral and therefore have some ability to resist desiccation. Laminaria digitata, in particular, is moderately tolerant of desiccation and in the absence of grazing molluscs, killed by the Torrey Canyon oil spill, was able to extend 2m upwards on wave exposed shores (Southward & Southward, 1978). Species living below the kelp fronds, such as Lithophyllum incrustans and sponges, such as Halichondria panicea, are likely to be protected from the worst effects of desiccation by the kelp canopy.
Increase in emergence regime
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The biotope is predominantly sublittoral and so a change in the emergence regime at the benchmark level (one hour in the time covered or not covered by the sea for a period of 1 year) is likely to result in a depression of the upper limit of the biotope. Some sessile species, such as sea squirts, are unlikely to survive a long term increase in emergence. However, in the presence of a suitable substratum the biotope is likely to re-establish further down the shore. Kain (1975) recorded that Laminaria digitata had recolonized cleared rocks within 2 years so recovery should be high. Most other characterizing species have planktonic larvae and/or are mobile and so can migrate into the affected area. Growth rates of sessile species in the biotope are generally rapid. For instance, Halichondria panicea increases by about 5% per week (Barthel, 1988).
Decrease in emergence regime
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Increase in water flow rate
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The biotope occurs in a wide range of water flow environments, from very weak to moderately strong and so will be relatively tolerant of changes. Laminaria digitata is not found in areas subject to sand scouring which may occur if water flow rate increases.
Decrease in water flow rate
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Increase in temperature
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The biotope occurs in warmer and colder parts of Britain and Ireland and similar assemblages of species are known to occur in Scandinavia and in Brittany so that long-term temperature change is unlikely to cause a significant impact. 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 not likely to be intolerant of a long term, chronic change in temperature. However, the biotope may be intolerant of rapid changes in temperature outside its tolerance range. During an exceptionally warm summer in Norway, Sundene (1964) reported the destruction of Laminaria digitata plants exposed to temperatures of 22-23 °C. In Scotland, when spring low tides coincided with night time extreme air frosts on several consecutive days mortality of all but the lowest shore adult Laminaria digitata plants occurred (Todd & Lewis, 1984). Other species within the biotope may be intolerant of changes in temperature. Therefore, the biotope is likely to be of intermediate intolerance to short term acute temperature change.
Decrease in temperature
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Increase in turbidity
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Turbidity of the water can determine the depths to which kelp species can be found. However, limiting conditions for the depth to which Laminaria digitata can grow are not usually to do with light, but due to competition with the truly sublittoral kelp Laminaria hyperborea. Therefore, reduced turbidity may not extend the range of the biotope but kelp and other algal productivity may increase because of additional light. 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. An increase in turbidity will reduce photosynthesis and growth of plants. In Narragansett Bay, Rhode Island growth rates of Laminaria digitata fell during a summer bloom of microalgae that dramatically reduced downward irradiance. On return to normal turbidity levels the growth rate would be quickly return to normal. Increased turbidity may alter the benthic species, with holdfasts for example becoming dominated by species like Pomatoceros triqueter.
Decrease in turbidity
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Increase in wave exposure
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The biotope is found in a range of wave exposures, from sheltered to exposed, and so is likely to be tolerant of changes in exposure. Laminaria digitata flourishes in moderately to strongly exposed areas due to a flexible stipe and low profile holdfast. The species may extend its upper limit upwards into the lower eulittoral in exposed areas with a great deal of wave action. In extreme wave exposure the Alaria esculenta dominated biotope EIR.Ala.Ldig generally replaces MIR.Ldig.Ldig and at sheltered sites Saccharina latissima biotope £SIR.Lsac£ will dominate.
Decrease in wave exposure
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Noise
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The macroalgae characterizing the biotope have no known sound or vibration sensors. The response of macroinvertebrates is not known.
Visual Presence
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Macrophytes have no known visual sensors. Most macroinvertebrates have poor or short range perception and although some are likely to respond to shading caused by predators the biotope as a whole is unlikely to be sensitive to visual disturbance.
Abrasion & physical disturbance
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The fronds of Laminaria digitata are leathery and the whole plant is very flexible so a force equivalent to a scallop dredge or an anchor landing on or being dragged across the seabed, is unlikely to cause significant damage to the kelp bed as a whole. However, some plants may be fatally damaged or ripped off the substratum. Other algae and sessile species such as sponges and large solitary tunicates are likely to be especially intolerant of physical disturbance and so the biotope has been assessed as having intermediate intolerance.
Displacement
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Displacement of Laminaria digitata, the key species, will result in loss of the biotope. Species abundance and diversity will be significantly reduced because the additional habitats and refugia provided by kelp fronds, stipes and particularly the holdfast will be lost. However, recovery is good because recolonization of kelp plants to previous densities takes place within about 2 years (Kain, 1975) and most other characterizing species have planktonic larvae and/or are mobile and so can migrate into the affected area and a likely return to full abundance and diversity within five years.

Chemical Factors

Synthetic compound contamination
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Several of the species characteristic of the biotope are reported as having high intolerance to synthetic chemicals. For instance, Cole et al. (1999) suggested that herbicides such as Simazina and Atrazine were very toxic to macrophytic algae. Hiscock and Hoare (1974) noted that almost all red algal species and many animal species were absent from Amlwch Bay in North Wales adjacent to an acidified halogenated effluent. Red algae have also been found to be sensitive to oil spill dispersants (O'Brien & Dixon 1976). Recovery is good because recolonization of algae takes place within 2 years and most fauna have pelagic larvae and so can recolonize rapidly.
Heavy metal contamination
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intolerance is assessed as intermediate because all the representative species have intermediate intolerance.
Hydrocarbon contamination
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Laminaria digitata is less susceptible to coating than some other seaweeds because of its preference for exposed locations where wave action will rapidly dissipate oil. The effects of oil accumulation on the thalli are mitigated by the perennial growth of kelps. No significant effects of the Amoco Cadiz spill were observed for Laminaria populations and the World Prodigy spill of 922 tons of oil in Narragansett Bay had no discernible effects on Laminaria digitata (Peckol et al., 1990). Mesocosm studies in Norwegian waters showed that chronic low level oil pollution (25µg/L) reduced growth rates in Laminaria digitata but only in the second and third years of growth (Bokn, 1985). Where exposed to direct contact with fresh hydrocarbons, encrusting calcareous algae have a high intolerance. The sensitivities of the faunal components of the kelp bed are not known although amphipods normally suffer high mortality in oil affected areas. Analysis of kelp holdfast fauna after the Sea Empress oil spill in Milford Haven illustrated decreases in number of species, diversity and abundance at sites nearest the spill (SEEEC, 1998).
Radionuclide contamination
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Insufficient information.
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 so an increase in nutrients is likely to result in higher growth rates. In Helgoland, where ambient nutrient concentrations are enriched, growth of Laminaria digitata also occurs in summer months. However, increased nutrient levels are often associated with eutrophication, increased siltation and turbidity. After removal of sewage pollution in the Firth of Forth, Laminaria digitata became abundant on rocky shores from which they had previously been absent. 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 and mussels. High levels of ephemeral algae can cause smothering of species changing the character of the biotope 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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Kelps are stenohaline seaweeds, in that they do not tolerate wide fluctuations in salinity (Birkett et al.,1998b). The growth rate may be adversely affected if the kelp plant is subjected to periodic salinity stress. However, the biotope is essentially marine and localized, long term reductions in salinity may result in the loss of the kelp beds in the affected area and thus loss of the biotope (Birkett et al., 1998b).
Decrease in salinity
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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 2mg/l. Kinne (1972) reports that reduced oxygen concentrations inhibit both photosynthesis and respiration although macroalgae may tolerate at level of deoxygenation since they can produce their own oxygen. On return to oxygenated conditions, rapid recovery is likely. Laminaria digitata, colonizes cleared areas of the substratum within two years (Kain, 1975) and most other characterizing species have a planktonic larva and/or are mobile and so can migrate into the affected area.

Biological Factors

Introduction of microbial pathogens/parasites
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There is very little information available on microbial pathogens infecting the characterizing species of the biotope. However the occurrence of hyperplasia or gall growths, seen as dark spots, on Laminaria digitata is well known and may be associated with the presence of endophytic brown filamentous algae. Fronds of Palmaria palmata frequently bear algal epiphytes and endophytes, a number of marine fungi and galls produced by nematodes, copepods and bacteria. Growth rates of algae may be impaired by such infections. However, no evidence of losses of this biotope due to disease were found and it is likely that microbial pathogens will have only a minor possible impact on this biotope.
Introduction of non-native species
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The Northwest Pacific kelp Undaria pinnatifida has been introduced into Britain, primarily in the south-west, in recent years. The species may cause displacement of native kelps including Laminaria digitata although in Brittany Undaria pinnatifida was seen to colonize areas normally inhabited by Saccorhiza polyschides rather than Laminaria digitata or Laminaria hyperborea. The Japanese brown algae Sargassum muticum has not been documented to directly displace Laminaria digitata but in France it has replaced Saccharina latissima (studied as Laminaria saccharina) through over-growing and shading of underlying species (Eno et al., 1997).
Extraction
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Of the species indicative of sensitivity only the oarweed Laminaria digitata and dulse Palmaria palmata are known to be targeted for extraction. Laminaria digitata 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. Dulse is used as a vegetable substitute or animal fodder although harvesting on a commercial scale only takes place in Ireland and in France. 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 position of this biotope it is unlikely that vast areas of Laminaria digitata 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). After Laminaria digitata cutting the standing crop was re-established within 18-20 months (Kain, 1979). In macroalgae clearance experiments at Port Erin, Isle of Man (Kain, 1975) recolonization of Laminaria digitata on concrete blocks took place within 2 years. 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.

This review can be cited as follows:

Hill, J.M. 2000. Laminaria digitata on moderately exposed 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 02/09/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=297&code=1997>