Biodiversity & Conservation

LR.SLR.F.Asc.T

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Substratum loss would result in the loss of the entire biotope and, accordingly, intolerance has been assessed as high. Ascophyllum nodosum has poor recruitment rates and is slow growing, therefore limiting recovery (Holt et al., 1997). Due to the fact that the Ascophyllum nodosum canopy is central to the survival and characterization of the biotope, recoverability is likely to be low (see additional information).
Smothering
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At the benchmark level, smothering is unlikely to adversely affect this biotope. Due to the fact that the biotope is associated with areas of moderately strong to very strong water flow, the sediment layer will be washed away and 'normal' conditions will resume rapidly. The suspension feeders may experience some short-lived interference with feeding but this is not likely to adversely affect their viability. Accordingly, SLR.Asc.T has been assessed as being tolerant to smothering at the benchmark level.
Increase in suspended sediment
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An increase in suspended sediment, in combination with the tide-swept nature of this biotope, could be of detriment to the associated community. The photoautotrophic component may experience a reduction in light available for photosynthesis (see turbidity). Furthermore, depending on the nature of the sediment, the fronds of the plants may suffer from a scouring effect. This may also serve to scour delicate species such as hydroids off the fronds and stipes. The feeding apparatus of suspension feeders may become bombarded with particles. Although some of the suspension feeders may be able to clear their feeding apparatus of the sediment, this is energetically expensive and will most probably result in a reduced scope for growth over the benchmark period. If they are unable to effectively clean their feeding apparatus, they will simply experience a reduced ingestion over the course of one month. For example, Robbins (1985b) undertook experiments to establish the possible effects of high inorganic particulate concentrations on the sea squirt Ascidiella scabra. He concluded that growth rate was likely to be reduced and mortality was possible in high levels of suspended sediment. For the characteristic species this is unlikely to prove fatal although short lived species such as the star ascidian Botryllus schlosseri may experience some mortality.

Conversely, an increase in suspended sediment could increase food availability for the suspension feeders, especially if the proportion of organic particulate matter were to increase. On balance, an intolerance of low has been assessed. Recoverability is likely to be very high.

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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SLR.Asc.T is an intertidal biotope and, at the benchmark level, desiccation is equivalent to a change in position of one biological vertical zone on the shore for one year. In this case, that would mean a transition from the mid eulittoral to the upper eulittoral. The most obvious effect of such a transition would be the risk of desiccation.

Ascophyllum nodosum regularly becomes exposed to air during tidal cycles and so is tolerant of some desiccation (see MarLIN review). However, in transplantation experiments, Stengel & Dring (1997) found that 80% of plants moved from the lower shore to the upper shore died within 3 months, whereas all transplants from the upper to the lower shore and all controls survived. Those plants that did survive on the upper shore had acclimated to the new conditions, but whether the plants survived or not seemed to be determined by thallus morphology which may be genetically fixed.

In terms of the other important algal species, the critical water content of Fucus vesiculosus and Fucus serratus are 30% and 40% respectively. If desiccation exceeds these levels, irreversible damage occurs. For Fucus serratus, only two hours of sunshine are required before this level is exceeded. Mathieson & Burns (1971) measured the photosynthetic rate of Chondrus crispus at varying degrees of desiccation and found that after loss of 65% of its water content, the rate of photosynthesis was only 55% of the control rate (see MarLIN review). It is likely that an increase in desiccation will kill some of these four species of plants at the upper end of the biotope thereby depressing the upper limit. Species more characteristic of the upper eulittoral may competitively displace these algae. However, some plants are likely to be able to acclimate to the new conditions and survive.

In terms of characterizing fauna, the tissue of the breadcrumb sponge Halichondria panicea holds some water and is tolerant of a certain degree of desiccation. However, at the benchmark level, bleaching and tissue death in colonies at the upper shore extent of the population is likely to occur.
The soft body of the sea squirt Ascidiella scabra makes it vulnerable to desiccation. It is afforded some protection to desiccation by its location under seaweed and in damp crevices. However, some individuals at the highest point on the shore may dry out and die. Grazing gastropods such as Patella vulgata and Littorina obtusata occur higher up the shore than Ascophyllum nodosum and will therefore most likely to be tolerant to desiccation at the benchmark level.

Overall it is likely that much of the associated community will experience some mortality and intolerance has, therefore, been assessed as intermediate. Recoverability will depend on the extent to which Ascophyllum nodosum was lost and due to the fact that a proportion of this species is likely to remain, recoverability has been assessed as moderate.
Increase in emergence regime
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Intertidal biotopes such as SLR.Asc.T are adapted to a certain degree of cyclical immersion and emersion and the associated changes in salinity, desiccation and temperature etc. At the benchmark level, the algal canopy is unlikely to experience significant mortality, especially since the density of the canopy is likely to delay water loss. However, some plants at the upper shore extent of the biotope may be lost. In contrast, the suspension feeding community is likely to be adversely affected by an increase in emergence. Feeding time will be reduced and over the course of one year, a reduction in total growth and reproduction is expected. Annual species are likely to be lost and a large decline in suspension feeding diversity is likely to occur. Intolerance has been assessed as high. Recoverability has been assessed as high because Ascophyllum nodosum (that has a low recoverability from factors to which it is highly intolerant) is not likely to experience significant changes in abundance and most of the intolerant species produce planktonic larvae and are therefore likely to be able to recolonize quickly from surrounding areas.
Decrease in emergence regime
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In contrast to an increase in emergence, a decrease in emergence will be highly beneficial to the suspension feeding community which will experience greater feeding opportunities. The stresses associated with aerial exposure such as desiccation and extremes of temperature will be reduced for the whole biotope. In consequence, the biotope may extend further up the shore although this extension is likely to be counteracted by a reduction in the lower shore extent of the biotope. Therefore, even though the biotope may just shift its position on the shore, the area it covers is not likely to change and therefore SLR.Asc.T has been assessed as being tolerant of this factor.
Increase in water flow rate
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This biotope is associated with areas of moderately strong to very strong water flow rates (>6 knots, the highest flow rate in the benchmark) and, therefore, an increase in water flow rate is not considered to be relevant.
Decrease in water flow rate
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The diverse and species rich suspension feeding community associated with this biotope is as a result of the strong tidal flows that provide these creatures with a continual supply of food and remove sediment that may otherwise clog their feeding apparatus. A decrease in water flow at the level of the benchmark could result in negligible flow rates. This would lead to siltation which is ultimately detrimental to suspension feeders. Although some filter feeders have the ability to cope with siltation by, for example, clearing their feeding apparatus, this is energetically expensive and could result in reduced scope for growth and reproductive capacity. The sea squirt Ascidiella scabra can extend its siphons, to a small extent, above silt and can also most likely maintain a passage through the silt to the siphons. It also attaches to other erect biota and, in such situations, may escape smothering effects. The breadcrumb sponge Halichondria panicea has a mechanism for sloughing off their complete outer tissue layer together with any debris (Barthel & Wolfrath, 1989). The algae may also suffer from siltation since the reduction in light penetration would lead to reduced photosynthetic capabilities. Over the course of one year, this is likely to result in reduced total growth and reproduction may be adversely affected, especially if algal spores are sensitive to the nature of the substratum on which they settle. An increase in deposit feeders may be observed although these species would most probably be rapidly lost on resumption of previous water flow rates. Recoverability has been assessed as high because Ascophyllum nodosum (that has a low recoverability from factors to which it is highly intolerant) is not likely to experience significant changes in abundance and most of the intolerant species produce planktonic larvae and are therefore likely to be able to recolonize quickly from surrounding areas.
Increase in temperature
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Intertidal algae, such as Ascophyllum nodosum, are regularly exposed to rapid and short-term variations in temperature. In the North Sea Ascophyllum nodosum can tolerate a maximum temperature of 28°C and the optimum growth rate is at 15°C (Lüning, 1990). Therefore, the species is likely to be quite tolerant of a long term change in temperature of 2°C. The species is unlikely to be affected by a short term change of 5°C, as it was not damaged during the unusually hot summer of 1983 when the average temperature was 8.3°C higher than normal (Hawkins & Hartnoll, 1985). This is also true of Fucus vesiculosus which can withstand temperatures as high as 30°C (Lüning, 1990). In fact, the distribution of most of the important, key and characterizing species within this biotope is such that a chronic increase in temperature of 2°C is unlikely to adversely affect them. An acute rise in temperature may have a deleterious affect on some species. For example, the soft bodied Ascidiella scabra may be more prone to desiccation. However, this is not thought to cause mass mortalities among these populations and, accordingly, an intolerance of low has been recorded. Recoverability is expected to be very high.
Decrease in temperature
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Intertidal algae, such as Ascophyllum nodosum, are regularly exposed to rapid and short-term variations in temperature. This species can tolerate freezing temperatures as it has been observed to survive in a block of ice for several days. Fucus vesiculosus have been found to tolerate -30°C in Maine for several weeks (Lüning, 1990). However, at these temperatures, intercellular and extracellular ice crystals form which would cause some damage to the plant (S. Kraan, pers. comm.). All of the dominant and characterizing algae and the important and characterizing fauna, are well within their temperature range in Britain and Ireland so would not be affected by a change of 2°C. However, evidence from Crisp (1964a) suggests that, during the very cold 1962-63 winter, at least part of the populations of Halichondria panicea and Patella vulgata were killed in some locations and the fronds of Ascophyllum nodosum, where they has become frozen to the bedrock, snapped off and floated away with the tide. However, at the benchmark level of three days, a decrease in temperature of 5 °C is unlikely to cause mortality in Ascophyllum nodosum. On balance, it has been suggested that the intolerance of this biotope to a reduction in temperature is low, with a very high recovery.
Increase in turbidity
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The suspension feeding community associated with this biotope would be unaffected by an increase in turbidity and may actually benefit from it if the increase was associated with an increase in the concentration of organic particulate matter. However, an increase in turbidity would alter the light available for photosynthesis during immersion. Over the benchmark period, two of the canopy species, Fucus serratus and Fucus vesiculosus, may experience negative growth rates. Ascophyllum nodosum, on the other hand, can continue to photosynthesize at low tide when the plants are emersed, as long as the plant has a sufficiently high water content and so will be unaffected during this period. Furthermore, the red algae / turf species, that are more tolerant of reduced light levels, may increase in abundance at the expense of small green and brown algal species, although such a change would not affect the recognizable biotope. SLR.Asc.T is considered to be tolerant of an increase in turbidity at the benchmark level.
Decrease in turbidity
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This biotope has been recorded in areas of very high turbidity, such as the Menai Strait, and the algal component of this biotope are likely to benefit from a decrease in turbidity. Over the benchmark period the algae may experience enhanced photosynthesis and increased total growth. An increase in the proportion of green algae may be observed which would increase species diversity in the biotope.
Increase in wave exposure
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This biotope occurs in very sheltered to extremely sheltered habitats. Accordingly, an increase in exposure at the benchmark level would be catastrophic for this biotope. The local distribution of Ascophyllum nodosum is largely determined by wave exposure. As exposure to wave action increases, the number of plants becomes progressively less and they consist increasingly of short and stumpy shoots (Baardseth, 1970) and with a higher percentage of injured tissue (Levin & Mathieson, 1991). This loss of the main canopy species would lead to the degeneration of the recognizable biotope. As Jenkins et al. (2004) observed, in Ascophyllum nodosum canopy clearance experiments, the turf species would become bleached and die and the substratum would become progressively dominated by limpet grazed areas. Species more tolerant of wave exposed areas, such as Fucus vesiculosus, may replace the Ascophyllum nodosum to some degree but the biotope, at this stage, will have been lost. Recoverability is likely to be low (see additional information) since Vadas et al. (1990) found that a single wave on a wave exposed shore was sufficient to remove 90% of newly settled Ascophyllum nodosum zygotes.
Decrease in wave exposure
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This biotope occurs in very sheltered and extremely sheltered habitats and therefore a decrease in wave exposure is not considered to be relevant.
Noise
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The key structural and important characterizing species are unlikely to have mechanisms for detecting noise. Patella vulgata may respond to the vibrations caused by noise by clamping down on the rock although at the benchmark level this is not likely to aversely affect the biotope. Therefore tolerant has been recorded.
Visual Presence
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The key structural and important characterizing species are unlikely to have mechanisms for detecting visual presence and, therefore, tolerant has been recorded.
Abrasion & physical disturbance
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The canopy species in this biotope are likely to be adversely affected by abrasion and physical disturbance at the benchmark level. Ascophyllum nodosum is particularly intolerant of abrasion from trampling (Holt et al., 1997). Abrasion may cause damage to the fronds and germlings of all the algal species in this biotope, although the turf species are likely to escape unscathed due to the protection provided by the canopy layer. Human trampling can significantly reduce the cover of fucoids on the shore (Holt et al., 1997). Severe abrasion, for instance through a vessel stranding, is likely to be considerably more detrimental to the biotope. The forces involved in such an event are likely to scour a large area of the rocky shore of macroalgae and sessile invertebrates. Furthermore, if the vessel remained on the shore, it would prevent light reaching any surviving algae under the vessel and therefore lead to their death.

Epifaunal species have also been found to be particularly adversely affected by trawling or dredging activities, either due to direct damage or modification of the substratum (Jennings & Kaiser, 1998). Patches of hydroids and bryozoans, and encrusting fauna such as colonial ascidians and sponges are likely to be scraped off the rock and the shells of limpets and periwinkles may be crushed by the weight and force of the abrasion.

Some epifaunal species have been reported to exhibit increased abundances on high fishing effort areas, probably due to their ability to colonize and grow rapidly (Bradshaw et al., 2000). The sea squirt Ascidiella scabra was found to have become more abundant in a study of the long term effects of scallop dredging (Bradshaw et al., 2002). Overall, many species associated with this biotope are likely to experience some mortality although depending on the size of the population, the majority of the population is likely to remain. Therefore, intolerance has been assessed as intermediate. Recoverability has been assessed as high because Ascophyllum nodosum (that has a low recoverability from factors to which it is highly intolerant) is not likely to experience significant changes in abundance and most of the intolerant species produce planktonic larvae and are therefore likely to be able to recolonize quickly from surrounding areas.
Displacement
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The majority of the characterizing and key species associated with this biotope are permanently attached to the substratum and would not survive displacement. The displacement of species including Ascophyllum nodosum, Ascidiella scabra and Halichondria panicea will have a similar effect to substratum loss and will result in the loss of the entire biotope. Accordingly, intolerance has been assessed as high. Ascophyllum nodosum has poor recruitment rates and is slow growing, therefore limiting recovery (Holt et al., 1997). Due to the fact that the Ascophyllum nodosum canopy is central to the survival and characterization of the biotope, recoverability is likely to be low (see additional information).

Chemical Factors

Synthetic compound contamination
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  • The disappearance of Ascophyllum nodosum from Oslofjord has been attributed to the reduced ability of germlings to recruit at highly polluted sites (Sjoetun & Lein, 1993). However, Hoare & Hiscock (1974) observed that Ascophyllum nodosum was found within 100 m of an acidified, halogenated effluent discharge, although plants had abnormal and retarded growth.
  • Fucus vesiculosus is extraordinarily highly intolerant of chlorate, such as from pulp mill effluents. In the Baltic, the species has disappeared in the vicinity of pulp mill discharge points and is affected even at immediate and remote distances (Kautsky, 1992).
  • Scanlan & Wilkinson (1987) found that the spermatozoa and newly fertilized eggs of Fucus serratus were the most intolerant of biocides, while adult plants were only just significantly affected at 5 ml/l of the biocides Dodigen v181-1, Dodigen v 2861-1 and ML-910.
  • O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil or dispersant contamination, possibly due to the susceptibility of phycoerythrins to destruction. Laboratory studies of the effects of oil and dispersants on several red algal species concluded that they were all sensitive to oil/dispersant mixtures, with little difference between adults, sporelings, diploid or haploid life stages (Grandy, 1984, cited in Holt et al., 1995).
  • Ascidians may be intolerant of synthetic chemicals such as tri-butyl-tin (TBT) anti-foulants. Rees et al. (2001), working in the Crouch estuary, observed that six ascidian species were recorded at one station in 1997 compared with only two at the same station in 1987, shortly following the banning of TBT in antifouling paints. Also, there was a marked increase in the abundance of ascidians especially Ascidiella aspersa and Ascidia conchilega in the estuary.
  • Limpets including Patella vulgata are extremely intolerant of aromatic solvent based dispersants used in oil spill clean-up (see MarLIN review). During the clean-up response to the Torrey Canyon oil spill nearly all the limpets were killed in areas close to dispersant spraying. A concentration of 5ppm killed half the limpets tested in 24 hours (Southward & Southward, 1978; Hawkins & Southward, 1992). Hoare & Hiscock (1974) reported that in Amlwch Bay Patella vulgata was excluded from sites within 100-150 m of the discharge of acidified, halogenated effluent.
  • Gastropod molluscs are known to be intolerant of endocrine disruption from synthetic chemicals such as tri-butyl tin (Cole et al., 1999). No specific information was found concerning the effects of synthetic chemicals of Littorina obtusata although Littorina littorea is thought to have only a low intolerance (see MarLIN review.
In summary, all the key and important characterizing species in this biotope are reported to have at least some intolerance to synthetic chemicals although the fast water flow may ameliorate initial intolerance and subsequent recoverability by flushing out the contaminant. On balance, an intolerance of intermediate has been assessed, with a high recovery.
Heavy metal contamination
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  • Fucoids accumulate heavy metals and may be used as indicators for monitoring. It is generally accepted that adult plants are relatively tolerant of heavy metal pollution (Holt et al., 1997). The disappearance of Ascophyllum nodosum from Oslofjord has been attributed to an increase in pollution and copper at concentrations of 1039 nM (66 µg/L) have been found to inhibit the growth of Ascophyllum nodosum (Strömgren, 1979a). However, the species penetrates into the metal polluted middle reaches of Restronguet Creek in the Fal estuary system where concentrations of both copper and zinc are in the region of 1000-2000 µg/g in the sediment and 10-100 µg/l in seawater (Bryan & Gibbs, 1983). Earlier life stages of Ascophyllum nodosum, and Fucus serratus (below), are probably more sensitive than adult plants.
  • The effect of heavy metals on the growth rate of adult Fucus serratus plants has been studied by Strömgren (1979b; 1980a & b). Copper significantly reduces the growth rate of vegetative apices at 25 µg/l over 10 days (Strömgren, 1979b). Zinc, lead, cadmium & mercury significantly reduce growth rate at 1400 µg/l, 810 µg/l, 450 µg/l and 5 µg/l respectively (Stromgren, 1980a, b).
  • Bryan (1984) suggested that gastropods are rather tolerant of heavy metals. In the Fal estuary Patella vulgata occurs at, or just outside, Restronguet Point at the end of the creek where metal concentrations are high, e.g. Zinc (Zn) exists at concentrations of 100-2000 µg/l (Bryan & Gibbs, 1983). However, Davies (1992) found that, in the laboratory, Patella vulgata was found to be intolerant of small changes in environmental concentrations of Cd and Zn. Furthermore, exposure to Cu at a concentration of 100 µg/l for one week resulted in progressive brachycardia (slowing of the heart beat) and the death of limpets (Marchan et al., 1999).
  • No specific information was found concerning the effects of heavy metals on Littorina obtusata although Littorina littorea was assessed as being of intermediate intolerance (see MarLIN review).
No information was found concerning the specific effects of heavy metals on the other important and characterizing species although an intolerance of intermediate has been recorded in light of the above evidence. Recovery is likely to be high although recovery from the effects of highly persistent metal and their compounds may take significantly longer.
Hydrocarbon contamination
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  • Experimental studies have found that long-term exposure to low levels of diesel reduces the growth rate in Ascophyllum nodosum. For example, in mesocosm experiments, Bokn (1987) observed growth inhibition at a diesel concentration of 130 ppb and that inhibition stops when the oil is removed. Thus, a limited amount of oil pollution need not be detrimental to a population with good recruitment (Sjoetun & Lein, 1993). However, Ascophyllum nodosum generally has poor recruitment and hydrocarbon contamination may also prevent fertilization and germination.
  • Adult Fucus serratus plants are tolerant of exposure to spills of crude oil although very young germlings are intolerant of relatively low concentrations of 'water soluble' extractions of crude oils. Exposure of eggs to these extractions (at 1.5 µg/ml for 96 hours) interferes with adhesion during settling and (at 0.1 µg/ml) prevents further development (Johnston, 1977).
  • Fucus vesiculosus shows limited intolerance to oil. After the Amoco Cadiz oil spill it was observed that Fucus vesiculosus suffered very little (Floc'h & Diouris, 1980). Indeed, Fucus vesiculosus may increase significantly in abundance on a shore if grazing gastropods are killed by the oil. However, very heavy fouling could reduce light available for photosynthesis, and in Norway, a heavy oil spill reduced fucoid cover. Recovery occurred within four years in sheltered conditions (Holt et al., 1997).
  • Observations have shown that filamentous red algae are among the most severely affected by the toxic properties of oil (O'Brien & Dixon, 1976). Effects including bleaching and loss of photosynthetic pigments have been observed in red algal species following contamination with fuel oil (O' Brien & Dixon, 1976). The long term effects on Chondrus crispus of continuous doses of the water accommodated fraction (WAF) of diesel oil were determined in experimental mesocosms (Bokn et al., 1993). Mean hydrocarbon concentrations tested were 30.1 µg/l and 129.4 µg/l. After 2 years, there were no demonstrable differences in the abundance patterns of Chondrus crispus. Furthermore, Kaas (1980, cited in Holt et al., 1995) reported that the reproduction of adult Chondrus crispus plants on the French coast was normal following the Amoco Cadiz oil spill.
  • It appears that Halichondria panicea survived in areas affected by the Torrey Canyon oil spill (Smith, 1968), although few observations were made.
  • In areas of moderate oil deposit, up to about 1/2 cm thick, on rocks after the Torrey Canyon oil spill, limpets had survived unscathed over a month after the event and feeding continued even though a coating of oil smothered their food source of algae and diatoms (Smith, 1968). However, thick layers of oil smothering individuals will interfere with respiration and feeding. In West Angle Bay, where fresh oil from the Sea Empress tanker reached rocky shores within one day of the spill, limpet mortality was 90% (Glegg et al., 1999). A significant reduction in the density of juvenile limpets was also observed at all sites known to have been oiled by the Sea Empress spill (Moore, 1997).
  • Experience of and observations from oil spills such as the Sea Empress and Amoco Cadiz suggest that gastropod molluscs are highly intolerant of hydrocarbon pollution but that recovery is usually rapid.
On balance, it is likely that many of the associated species will suffer reduced viability and, in some cases, some mortality. Therefore, intolerance has been assessed as high. Recoverability is likely to be high and the fast water flow associated with this biotope may help to flush the pollutants from the area.
Radionuclide contamination
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Insufficient information was found concerning the effects of radionuclides on the important and characterizing species to be able to assess sensitivity.
Changes in nutrient levels
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Plant growth is often limited by nutrient availability and an influx of nutrients into this biotope could serve to stimulate the growth of the associated macroalgae. However, phytoplankton are also likely to be stimulated and, under certain conditions, the formation of plankton blooms may occur. Not only do the presence of thick blooms and mats of algae reduce light penetration, which has the potential to reduce photosynthesis, but the eventual degradation of the blooms requires significant oxygen consumption. Despite the fact that fucoids appear relatively resistant to sewage, and have been observed to grow within 20m of an outfall discharging sewage in the Isle of Man (Holt et al., 1997), Steen & Rueness (2004) reported that along the Norwegian Skagerrak coast, Ascophyllum nodosum and Fucus serratus showed a decline in biomass in eutrophic areas, even disappearing in some places. Ascophyllum nodosum plants, when transplanted into sewage-stressed areas have become heavily infested with epiphytes and frequently overgrown by Ulva species and there are reports of a decline in populations of the species in the North Atlantic as a result of increased eutrophication (Fletcher, 1996). Chronic nutrient enrichment could potentially alter the recognizable biotope. However, in a tide-swept biotope such as this an influx of nutrients from a single event is unlikely to adversely affect it. On balance, intolerance has been assessed as low with a very high recovery.
Increase in salinity
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SLR.Asc.T is found in areas of full salinity and therefore an increase in salinity is not relevant.
Decrease in salinity
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SLR.Asc.T is an intertidal biotope and its associated flora and fauna will therefore be tolerant, to a certain extent, of cyclical changes in salinity.

Ascophyllum nodosum is euryhaline with a salinity tolerance of about 15 to 37 psu (Baardseth, 1970). The species can also withstand periodic emersion in freshwater (Baardseth, 1970) and frequently inhabits estuaries where salinity is variable. Doty & Newhouse (1954) reported Ascophyllum nodosum from estuarine waters with a maximum salinity of 17.3 psu and a minimum of 0 psu. Further evidence is provided by Chock & Mathieson (1979) who found Ascophyllum nodosum plants in the laboratory exhibited net photosynthesis at salinities from 0 to 40 psu although the long term effects within this range were not evaluated.

In the UK, Fucus vesiculosus tolerates salinity down to 11 psu, below which it is replaced by Fucus ceranoides (Suryono & Hardy, 1997).

Fucus serratus is able to compensate for these changes in salinity by adjusting internal ion concentrations. Salinity affects the photosynthetic rate and hence growth rate of seaweed. For Fucus serratus, growth rate is maximal at a salinity of 20 psu and therefore, a decrease in salinity into the 'low' salinity category would be of preference to the plant.

Other important / characterizing species associated with this biotope are also likely to be tolerant of a reduction in salinity. The breadcrumb sponge Halichondria panicea and carrageen Chondrus crispus are found in of low salinity and Ascidiella scabra and Patella vulgata can be found in reduced salinity conditions.

A rapid fall in salinity of two categories on the MNCR scale from variable to low salinity could result in more adverse effects on the whole community and therefore intolerance has been assessed as low but with a very high recovery.

Changes in oxygenation
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No specific information was found concerning the effects of reduced oxygenation on the key / characterizing species although Cole et al. (1999) suggested possible adverse effects on marine species below 4 mg/l and probably adverse effects below 2 mg/l. Some of the associated fauna may be tolerant of low oxygen environments and the strong water flow rates associated with this biotope are likely to ameliorate any reductions in oxygenation. Overall, an intolerance of low has been suggested although recoverability is expected to be very high on return to 'normal' conditions.

Biological Factors

Introduction of microbial pathogens/parasites
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No information was found concerning the effects of and occurrence of microbial pathogen infestations on the key and characterizing species. Therefore, insufficient information was available to assess sensitivity.
Introduction of non-native species
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There are no known alien or invasive species that are known to compete with or displace the important characterizing species associated with this biotope.
Extraction
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Ascophyllum nodosum is collected in several countries and it is likely to pose a significant threat to this biotope if the whole plant is removed. Removal of the entire plant has a considerable impact and recovery can take anything from 3-8 years (Norway) (Seip, 1980, cited in Holt et al., 1997) to more than 8 years (England) (Knight & Parke, 1950). If the plants are not fully removed, Ascophyllum nodosum can regenerate from the basal proportions that remain (Holt et al., 1997). Printz (1956, cited in Holt et al., 1997) found that, in Norway, plants cut at 15 cm recovered fully within 5 years. However, regular harvesting is reported to change the size distribution of the population (Ang et al., 1993, cited in Holt et al., 1997). Baardseth (1970) stated that if stumps 10-20 cm high are left, harvesting is possible again after 3 to 6 years. Due to the fact that the Ascophyllum nodosum canopy is central to the survival and characterization of the biotope, the loss of this main canopy species would lead to the generation of the recognizable biotope. As Jenkins et al. (2004) observed, in Ascophyllum nodosum canopy clearance experiments, the turf species would become bleached and die and the substratum would become progressively dominated by limpet grazed areas.

Three of the other important species (Fucus serratus, Fucus vesiculosus and Chondrus crispus) are also targeted for extraction. Although none of these species are characterizing species, the two fucoids provide substratum for various epiphytes, especially the hydroids Dynamena pumila and Clava multicornis. The reduction in the thickness of the canopy that would be caused by the loss of the fucoids may be detrimental to some of the underlying red algal species which may perish if bleached by the sun. Furthermore, the removal of the fucoids would remove a large source of potential food for Patella vulgata and this species may subsequently focus grazing on encrusting red algae, and the fine green and red algae. This also has the potential to reduce species diversity.

In light of the evidence surrounding removal of Ascophyllum nodosum, intolerance has been assessed as high with a low recovery (see additional information).

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Recoverability
The Ascophyllum nodosum canopy is paramount to the survival and characterization of this biotope. It provides shade for the diverse red algal understory and substratum for a profusion of filter feeders. Ascophyllum nodosum is a long lived, slow growing algae with poor recruitment rates that limit recovery (Holt et al., 1997). The lack of recovery of Ascophyllum nodosum from harvesting is well documented and recovery from the loss of this species from SLR.Asc.T is likely to be low, i.e. full recovery will take up to 25 years. Jenkins et al. (2004) studied the long term effects of Ascophyllum nodosum canopy removal on the whole understory community structure on a sheltered rocky shore on the Isle of Man. They reported that, even after twelve years, major effects of the canopy removal were still apparent. At the culmination of the study, mean cover of Ascophyllum nodosum was still only about 50% of its original level compared with control plots. However, the most alarming changes were evident in the understory community. Previously, this community was characterized by a balance between patches of red algal turf and patches grazed by Patella vulgata. Removal of the canopy layer broke down the balance between these two 'functional units' to the extent that the community showed no signs of reverting to its pre-disturbance state. In a cascade of events, the red algal turf deteriorated from the lack of canopy protection which in turn increased the available area for limpet grazing, thereby increasing the limpet population. An increase in the abundance of limpets would also hamper recolonization by Ascophyllum nodosum sporelings. Furthermore, Hawkins & Hartnoll (1985) found that when the Ascophyllum nodosum canopy was removed on a moderately sheltered shore on the Isle of Man, the Fucus vesiculosus canopy in the zone above it was able to achieve an 80% cover after just one year. The establishment of a canopy layer of another species is likely to make the re-establishment of the Ascophyllum nodosum canopy take even longer.

Ascophyllum nodosum takes by far the longest time to recover when compared to other important and characterizing species. Should any of the other important and characterizing species be lost from the biotope, recovery is likely to be high and should be complete within about five years.


This review can be cited as follows:

Marshall, C.E. 2005. Ascophyllum nodosum, sponges and ascidians on tide-swept mid eulittoral rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 22/10/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=100&code=1997>