Biodiversity & Conservation

IR.MIR.KR.Ldig.Pid

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Most of the species characteristic of this biotope are permanently attached to the substratum so would be removed upon substratum loss. These species are unable to re-attach and will be swept away so intolerance is assessed as high. The total population of Polydora ciliata is unlikely to be lost because it can reburrow into any remaining suitable substratum. Species diversity will be significantly reduced because many of the microhabitats provided by the characterizing species will be lost. Recovery of the main characterizing species Laminaria digitata is rapid with cleared rocks fully recolonized within two years (Kain, 1979). Most other characterizing species have a planktonic larva and/or are mobile and so can migrate into the affected area. Colonization of most species of fauna inhabiting kelp holdfast fauna in Norway were found as early as one year after kelp trawling (Christie et al., 1998) and on rocks the more diverse community of coralline algae joined by species of cnidarians, bryozoans and sponges seen on undredged plots was absent three years after kelp trawling (Birkett et al., 1998b). However, although full species richness and abundance may be reduced the appearance of the biotope will be much as before substratum loss and so recovery is high.
Smothering
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Some species, especially adult Laminaria digitata plants, are likely to protrude above any smothering material. The burrowing species such as Pholas dactylus and Polydora ciliata are able to tolerate high levels of smothering and sedimentation. However, others species such as the active suspension feeders and foliose algae are likely to be killed by smothering. Smothering can also be highly detrimental to kelp plants, in particular spores, gametophytes and young plants (Dayton, 1985) which will reduce the kelp population within the biotope and so intolerance has been assessed as intermediate. Species diversity within the biotope is likely to experience a major decline. Recovery is high because most characterizing species have a planktonic larva and/or are mobile and so can migrate into the affected area.
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. Since many of the species, Pholas dactylus and Polydora ciliata in particular, in this biotope live in areas of high silt content (turbid water) it is expected that they would survive increased levels of silt in the water. However, very high levels of silt may clog respiratory and feeding organs of some suspension feeders such as sea squirts and may result in a decline in faunal species diversity. 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. A significant decrease in siltation levels may reduce food input to the biotope resulting in reduced growth and fecundity.
Decrease in suspended sediment
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Desiccation
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The biotope is predominantly sublittoral but does extend onto the shore and therefore has some ability to resist desiccation. Laminaria digitata in particular can return to original photosynthetic rate on reimmersion after 40-50% water loss. Species living below the kelp, such as foliose red algae, sponges and ascidians are likely to be protected from the worst effects of desiccation by the kelp canopy. However, increased desiccation equivalent to moving one biological zone up the shore is likely to result in the death of some intolerant sessile species so intolerance of the biotope is assessed as intermediate.
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. Many of the subordinate species, especially solitary sea squirts, are unlikely to survive an increased emergence regime and mobile species are likely to move away so that species diversity will decline. 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. In areas of very strong water flow it is often out-competed by Alaria esculenta and in much slower water by Saccharina latissima. Laminaria digitata is not found in areas subject to sand scouring. Water motion affects light by moving canopies and influences the impact of sedimentation and scour and importantly water motion determines the availability of nutrients. It is unlikely that species in the biotope will be killed by an increase or decrease in flow rate. Existing organisms are likely to persist although conditions will not be ideal. Decreased water flow will lead to a reduced competitive advantage for suspension feeding animals especially sponges which will decline in growth rate.
Decrease in water flow rate
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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 relatively tolerant 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). Therefore, the biotope is likely to be of intermediate intolerance to short term acute temperature change. Loss of plants will result in reduces species diversity.
Decrease in temperature
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The cold winter of 1962/63 had a dramatic effect on intertidal fauna in southern Britain including piddocks which were killed at the sites investigated (Crisp, 1964). Subsequently piddocks are now abundant throughout much of the eastern English Channel (Pinn et al., in press).
Increase in turbidity
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Changes in turbidity may affect the distribution or growth rates of Laminaria digitata and other algae. Reduced turbidity may increase productivity of kelps and other algae but is not expected to increase the depth range to which the biotope extends because 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. Increases in 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 a reduced depth range and the fewer new plants under the kelp canopy. An increase in turbidity will reduce photosynthesis and growth of plants. On return to normal turbidity levels the growth rate would be quickly return to normal. In almost all cases not involving canopy competition, irradiance is most severely reduced by suspended particles in the water column (Dayton, 1985). There may be some clogging of suspension feeding apparatus in sea squirts, brittle stars and feather stars although those groups survive in occasionally very turbid conditions. Species richness may decline in the long-term.
Decrease in turbidity
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Increase in wave exposure
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The biotope occurs in areas of moderate wave exposure. Although the kelp Laminaria digitata can tolerate a wide range of wave exposures a significant increase in wave exposure will have a detrimental effect on the biotope because of the friable nature of the substratum resulting in a loss of sessile species. Changes in wave exposure may also interfere with feeding for the piddocks, Polydora ciliata and other suspension feeding organisms.
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 physical disturbance by 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 sensitive to abrasion and so the biotope as a whole 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 because plants are unable to re-attach. 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). Similarly, displaced piddocks will be lost but empty burrows will provide additional refugia for small invertebrates (Pinn et al., in press).

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) suggests that herbicides such as Simazina and Atrazine are very toxic to macrophytic algae. Hiscock and Hoare (1974) noted that almost all red algal species were absent from areas adjacent to an acidified halogenated effluent in Amlwch Bay, North Wales. Red algae have also been found to be sensitive to oil spill dispersants (O'Brien & Dixon, 1976). Bivalve molluscs, such as piddocks are reported to be very intolerant of TBT contamination (see Pholas dactylus review) with reduced abundance and growth reported in the field and laboratory. Other species in the biotope, in particular polychaete worms, are much more tolerant of chemical pollutants. The tube dwelling polychaetes Polydora ciliata and Pomatoceros triqueter, for example, flourished close to the Amlwch Bay bromide extraction plant effluent (Hoare & Hiscock, 1974). Therefore, the result of an increase in synthetic chemicals within the biotope is likely to be the death of several of the more intolerant species, including key species such as Pholas dactylus. Abundance of other more pollution tolerant species, especially polychaete worms, is likely to increase. The overall impact is one of the probable loss of key species and a major decline in species diversity and the intolerance of the biotope is therefore reported as high. 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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No information is available on the effect of heavy metals on the biotope. Intolerance of the key species is reported as intermediate, with likely effects on growth and fecundity, so biotope intolerance is assessed as intermediate. There may be a decline in overall species diversity with long term heavy metal pollution.
Hydrocarbon contamination
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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). However, 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). It is also expected that other species in the biotope will be intolerant of hydrocarbons. A proliferation of polychaete worms often follows oil spills. A major decline in species diversity within the biotope is likely and so intolerance is reported as intermediate.
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. In Helgoland, where ambient nutrient concentrations are double those of the Scotland site Laminaria digitata grows in the summer months. Higher growth rates have also been associated with plants situated close to sewage outfalls. However, after removal of sewage pollution in the Firth of Forth, Laminaria digitata became abundant on rocky shores from which they had previously been absent. Therefore, 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. Increased nutrients may increase the abundance of ephemeral algae and result in smothering or changing the character of the biotope.
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) although Laminaria digitata has been reported to grow in salinities of 25psu. The biotope occurs in situations that are naturally subject to fluctuating salinity because of precipitation but kelp growth rates may be adversely affected if subjected to periodic salinity stress. Localized, long term reductions in salinity, to below 20 psu, may result in the loss of kelp beds in affected areas and thus loss of the biotope (Birkett et al., 1998). Other species within the biotope may be intolerant of large salinity changes resulting in reduced species diversity.
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. For instance, death of a bloom of the phytoplankton Gyrodinium aureolum in Mounts Bay, Penzance in 1978 produced a layer of brown slime on the sea bottom. This resulted in the death of fish and invertebrates presumably due to anoxia caused by the decay of the dead dinoflagellates (Griffiths et al., 1979). Kinne (1972) reports that reduced oxygen concentrations inhibit both algal photosynthesis and respiration. However, on return to oxygenated conditions, rapid recovery is likely. The main characterizing species, 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 was 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 non-native species currently (October 2000) most likely to colonize this biotope are the Northwest Pacific kelp Undaria pinnatifida and the Japanese brown algae Sargassum muticum. Undaria pinnatifida, which has been introduced into south-west Britain in recent years, may cause displacement of native kelps including Laminaria digitata although in Brittany only areas inhabited by Saccorhiza polyschides have been affected. Sargassum muticum which is generally considered to be a 'gap-filler' 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). The American piddock Petricola pholadiformis has become established along south and east coasts of England from Lyme Regis in Dorset to the Humber although there is no documentary evidence that the species has displaced any native piddocks (Eno et al., 1997).
Extraction
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Extraction of Laminaria digitata does occur although there is no evidence available on the effects of Laminaria digitata harvesting on the biodiversity of kelp bed species. However, since the whole plant, including the holdfast is removed it is likely that faunal diversity will show a major decline. Given that MIR.Ldig.Pid occurs in the sublittoral fringe it is unlikely that vast amounts of Laminaria digitata will be collected although an intermediate intolerance has been suggested to reflect some loss. Palmaria palmata is used as a vegetable substitute or animal fodder although harvesting on a commercial scale only takes place in France. Recovery should be high because recolonization by Laminaria digitata on cleared rocks takes place within 2 years (Kain, 1979) and most other characterizing species have planktonic larvae and/or are mobile and so can migrate into the affected area. However, partially due to human collection for food, piddocks are no longer prevalent across the entire Meditterranean and the Atlantic coast of Europe, where they were once found (Michelson, 1978). They now have a reduced distribution.

This review can be cited as follows:

Hill, J.M. 2008. Laminaria digitata and piddocks on sublittoral fringe soft rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 26/11/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=26&code=1997>