Biodiversity & Conservation

LR.SLR.F.Asc

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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All key and important species in the biotope are highly intolerant of substratum loss. The algae and barnacles are permanently attached to the substratum so populations would be lost. Epifaunal grazers like Patella vulgata and littorinid snails are epifaunal and likely to be removed along with the substratum. Those that do remain will be subject to increased risk of desiccation and predation and so populations are unlikely to survive. Mobile species like the amphipod Hyale prevostii will be indirectly affected by the loss of fucoid plants as will sessile epiphytic flora and fauna. Recovery is low. See additional information for rationale.
Smothering
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A 5 cm layer of sediment or debris on a dense fucoid shore will reduce photosynthesis in algae that are covered and may cause some plants to rot. However, the dominant species, Ascophyllum nodosum, and its associated species would float above the layer of silt and almost certainly survive. Sediment will have an especially adverse effect on young germling algae and on the settlement of larvae and spat. Barnacle feeding may be affected and limpet locomotion and grazing may be impaired. Lower down the shore active suspension feeders such as sponges and mussels may be killed by smothering. However, as not all species are lost, and Ascophyllum nodosum in particular survives, intolerance is intermediate and as the slow recruiting keystone species survives recovery will be high. On sheltered shores there is not likely to be enough wave action to mobilise sediment to alleviate the effects of smothering. For recovery see additional information.
Increase in suspended sediment
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Ascophyllum nodosum, and the other macroalgal species in the biotope, are probably relatively tolerant of an increase in suspended sediment because they are primary producing species. Settlement out of the sediment may cover some surfaces of the plants, reducing photosynthesis rates which may reduce growth and in the sheltered conditions in which the biotope is found will probably not be removed by wave action. However, the direct effects of increased suspended sediment (see turbidity for indirect effects of light attenuation) on photosynthesising plants are not expected to be significant. Patella vulgata invade the lower reaches of estuaries where there is sufficient rock or stone on which it may live, and in such muddy habitats, with abundant silt and detritus, the growth rate is rapid (Fretter & Graham, 1994) although the species is absent from some sheltered shores where silt and algal turfs are likely to restrict space (Professor S.J. Hawkins, pers. comm.). Other species in the biotope, such as suspension feeding barnacles, sponges and tunicates (sea squirts), are likely to be more intolerant because an increase in suspended sediment may interfere with feeding, increase cleaning costs and result in lower growth rates. However, the impact of an increase in suspended sediment of 100mg/l for a month on the biotope as a whole will be sublethal effects such as reduced growth etc. so intolerance has been assessed as low. There may be a loss of a few very intolerant species. On return to pre-impact suspended sediment levels feeding rates of affected species and photosynthetic rates will return to normal very rapidly.
Decrease in suspended sediment
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Ascophyllum nodosum is not likely to be directly intolerant of a decrease in suspended sediment because the species is a primary producer. Other species in the biotope, in particular the suspension feeding barnacles, sponges and tunicates, are likely to be more intolerant because a decrease in suspended sediment may also result in a decrease in food supplies so growth may be affected. However, the impact of a decrease, of 100mg/l suspended sediment for a month, on the biotope as a whole will be sublethal effects (i.e. growth, fecundity etc.) so intolerance has been assessed as low. On return to pre-impact suspended sediment levels feeding of affected species and photosynthetic rates will return to normal very rapidly.
Desiccation
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Ascophyllum nodosum regularly becomes exposed to air during tidal cycles and so is tolerant of some desiccation. Brinkhuis et al. (1976) concluded that productivity is affected by desiccation, but only when water loss exceeds 50%. Stengel & Dring (1997) found that growth was lowest in those plants highest up the shore. 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. However, the photosynthetic and growth rates of those plants that survived 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. An increase in desiccation at the level of the benchmark, equivalent to a change in position of one vertical biological zone on the shore, will kill a large proportion of plants at the upper end of the populations range depressing the upper limit and so intolerance of the biotope is reported as intermediate. The range of the biotope may be able to extend further down the shore if desiccation increases. However, such recovery is likely to take a long time because Ascophyllum nodosum settles infrequently Macrofauna, such as the gammarid amphipod Hyale prevostii, that use the algae as a sheltered and humid habitat are also intolerant of increased desiccation and will be likely to move down the shore to avoid the factor. Other species such as the limpet Patella vulgata and the periwinkle Littorina littorea are able to tolerate some increase in desiccation and will be little affected.
Increase in emergence regime
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Ascophyllum nodosum is normally exposed to air for no more than a few hours in each tidal cycle (Lüning, 1990). An increase in the period of emersion of 1 hour would subject the species to greater desiccation and nutrient stress and may lead to the death of some organisms at the uppermost limit of species distribution on the shore. Thus, the biotope is likely to be lost at the upper limit of its range but may be able to extend further down the shore so that the overall impact is a shifting of the biotope downwards. However, an extension of the biotope is likely to be very slow because Ascophyllum nodosum has very poor recruitment, settling infrequently so that recolonization can take many years (see additional information). Thus, because a proportion of the biotope is likely to be lost a rank of intermediate is reported. Loss of the seaweed will have consequential effects such as the loss of other species using the weed as substratum, including Littorina littorea or as food and shelter, such as Hyale prevosti. Areas previously covered by algae may become dominated by more emergence tolerant species such as barnacles.
Decrease in emergence regime
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A reduction in the period of emersion may result in Ascophyllum nodosum being competitively displaced by faster growing species at the bottom of its range and may allow the upper limit of the population and hence the biotope to extend up the shore. However, Ascophyllum nodosum settles infrequently and recruitment to colonize new areas and thus compensate for loss of plants would be very slow (see additional information).
Increase in water flow rate
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Significant increases in water flow rate may cause some of the macroalgal populations to be torn off the substratum. However, the biotope is found in strong tidal streams, such as experienced in the narrows of sea lochs and so it seems likely that the biotope will have low intolerance to an increase in water movement. Patella vulgata and attached species such as the barnacles will remain attached to the rock even in strong water flow although feeding may be impaired. On the lower shore the increased water movement encourages several filter feeding faunal groups, such as sponges and ascidians, to occur and so species richness would probably increase and could lead to the development of the sub-biotope SLR.Asc.T.
Decrease in water flow rate
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The effect of a decrease in water flow rate is likely to be low because the biotope is also found on shores with low water flow. However, a certain degree of water flow is required to supply nutrients and remove waste products so a reduction in the water flow below a certain level may have an adverse effect on the species an hence the biotope. Barnacle growth rates are lower in reduced water flow and this may promote additional fucoid coverage.
Increase in temperature
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Ascophyllum nodosum occurs in waters warmer than those around Britain and Ireland (e.g. Portugal) and similar assemblages of species are known to occur in Brittany so that long-term temperature change is unlikely to have a significant impact on the biotope. Schonbeck & Norton (1979) demonstrated that fucoids can increase tolerance in response to gradual change in a process known as 'drought hardening'. Hawkins & Hartnoll (1985) report that fucoids are more intolerant of sudden changes in temperature and relative humidity observing the bleaching and death of plants during periods of hot weather (Hawkins & Hartnoll, 1985). However, intertidal algae, such as Ascophyllum nodosum, are regularly exposed to rapid and short-term variations in temperature. Both exposure at low tide or rising tide on a sun-heated shore may involve considerable temperature changes, and during winter the air temperature may be far below freezing point. Growth of Ascophyllum nodosum has been measured between 2.5 and 35°C with an optimum between 10 and 17°C (Strömgren, 1977). All other key species are moderately tolerant of temperature changes at the benchmark level. Larvae and juvenile individuals are likely to be more intolerant of changes in temperature than adults. The balance of interactions between fucoids and limpets plus barnacles changes with geographical location. Warmer conditions (e.g. Spain and Portugal) favour greater penetration of limpets and barnacles into sheltered locations (Ballantine, 1961 cited in Raffaelli & Hawkins, 1996). Warmer conditions are also likely to favour Chthamalus spp. rather than Semibalanus balanoides although a change of species will not alter the function of the biotope. However Ascophyllum nodosum has been found to be damaged by thermal pollution if the water temperature is above 24°C for several weeks (Lobban & Harrison, 1997) and the southern limit of the species distribution is controlled by the maximum summer temperature of about 22°C (Baardseth, 1970). Provided the temperature has not exceeded the critical limits it will soon recover on return to normal conditions.
Decrease in temperature
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Ascophyllum nodosum occurs in waters cooler than those around Britain and Ireland and similar assemblages of species are known to occur in Norway so that long-term temperature decrease is unlikely to have a significant impact on the biotope. Ascophyllum nodosum can tolerate freezing as it has been observed to survive in a block of ice for several days. However, temperature is important for reproduction in Ascophyllum nodosum. David (1943) suggests that temperature could provide the stimulus for gamete release. Studies in Maine, USA (Bacon & Vadas, 1991) and in Norway (Printz, 1959) have shown that gamete release in both countries commences at 6°C and in Maine terminated at about 15°C. Colder conditions (e.g. Norway) favour expansion of fucoids into exposed conditions at the expense of limpets and barnacles (Ballantine, 1961 cited in Raffaelli & Hawkins, 1996). Cooler temperatures also favour Semibalanus balanoides rather than the chthamalid barnacles although a change of species is not likely to change the overall nature of the biotope. Provided the temperature has not exceeded the critical limits it will soon recover on return to normal conditions.
Increase in turbidity
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An increase in turbidity would reduce the light available for photosynthesis during immersion which could result in reduced biomass of plants. However, the biotope is found at the upper and mid-tide levels and so is subject to long periods of emersion during which time macroalgae can continue to photosynthesize as long as plants have a sufficiently high water content. Therefore, photosynthesis and consequently growth will be unaffected during this period and so intolerance of the macroalgal species, and hence the biotope, is considered to be low. Upon return to previous turbidity levels the photosynthesis rate would return immediately to normal.
Decrease in turbidity
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A decrease in turbidity would increase the light available for photosynthesis during immersion which may increase growth rates of all the algae in the biotope. Upon return to previous turbidity levels the photosynthesis rate would return immediately to normal.
Increase in wave exposure
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Ascophyllum nodosum cannot resist very heavy wave action so exposure to wave action is an important factor controlling the distribution of the species, and therefore the biotope. Work in New England has suggested that the distribution of Ascophyllum nodosum may be directly set by wave action preventing settlement of propagules (Vadas et al., 1990). In moving from protected sites to the open sea the number of plants become progressively reduced, and individual plants become increasingly short and stumpy (Baardseth, 1970) and with a higher percentage of injured tissue (Levin & Mathieson, 1991). Thus, the species is only present in sheltered or moderately exposed locations and increased wave exposure causes plants to be torn off the substratum and replaced by Fucus vesiculosus. The dense Ascophyllum beds of the SLR.Asc biotopes can only develop in sheltered to extremely sheltered conditions. Thus, an increase in wave exposure of two ranks on the exposure scale, e.g. from sheltered to exposed, is likely to result in the removal of many plants from the substratum and the loss of the biotope and so intolerance is considered to be high. On return to normal conditions recovery is likely to be low because of poor recruitment and slow growth - see additional information for full rationale.
Decrease in wave exposure
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Dense canopies of Ascophyllum nodosum can only develop in sheltered to extremely sheltered conditions where wave exposure is negligible so a decrease in wave exposure at the level of the benchmark is not relevant to the biotope SLR.Asc and its sub-biotopes.
Noise
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None of the selected key or important species in the biotope are recorded as sensitive to noise although limpets and amphipods do respond to vibration. However, the biotope as a whole is not likely to be sensitive to changes in noise levels.
Visual Presence
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Algae have no visual perception. Most macroinvertebrates have poor or short range perception and are unlikely to be affected by visual disturbance such as boats or humans.
Abrasion & physical disturbance
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Trampling on the rocky shore has been observed to reduce fucoid cover which decreased the microhabitat available for epiphytic species, increased bare space and increased cover of opportunistic species such as Ulva (Fletcher & Frid, 1996). Ascophyllum nodosum seems to be particularly intolerant of damage from trampling (Flavell, 1995 cited in Holt et al., 1997). It is also likely to be removed if shores are mechanically cleaned following oil spills. Light trampling pressure has also been shown to damage and remove barnacles (Brosnan & Crumrine, 1994). Thus, trampling can significantly affect community structure and intolerance has, therefore, been assessed as high. Ascophyllum nodosum, has poor recruitment rates and is slow growing, limiting recovery (Holt et al., 1997). The lack of recovery of Ascophyllum nodosum from harvesting is well documented. For example, in their work on fucoid recolonization of cleared areas at Port Erin, Knight and Parke (1950) observed that even eight years after the original clearance there was still no sign of the establishment of an Ascophyllum nodosum population. Therefore, recovery is likely to be low.
Displacement
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intolerance to displacement is high because many of the key and important species in the biotope, including Ascophyllum nodosum, Fucus vesiculosus and Semibalanus balanoides are permanently attached to the substratum and cannot re-establish themselves if detached. Loss of the key species, Ascophyllum nodosum results in loss of the biotope. Removal of the fucoid canopy would create an increased risk of desiccation (see above) for the understory macroinvertebrates resulting in a significantly reduced species diversity. Recovery is low - see additional information for full rationale.

Chemical Factors

Synthetic compound contamination
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Adult fucoids are generally quite tolerant of chemical pollutants. However, brown algae have an extraordinarily high intolerance to chlorate and pulp mill effluents containing chlorate. Also, 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 100m of an acidified, halogenated effluent discharge, although plants had abnormal and retarded growth. Limpets are extremely intolerant of aromatic solvent based dispersants used in oil spill clean-up. During the clean-up response to the Torrey Canyon oil spill nearly all the limpets were killed in areas close to dispersant spraying. Gastropod molluscs are known to be intolerant of endocrine disruption from synthetic chemicals such as tri-butyl tin (Cole et al., 1999) although no information on the specific effects of tri-butyl tin on Patella vulgata was found. Hoare & Hiscock (1974) reported that in Amlwch Bay Patella vulgata was excluded from sites within 100-150m of the discharge of acidified, halogenated effluent. A loss of limpets may result in increased fucoid density as Patella vulgata is the major grazer of fucoid sporelings.
Heavy metal contamination
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The disappearance of Ascophyllum nodosum from Oslofjord has been attributed to an increase in pollution and copper at concentrations of 1039nM (66µg/L) have been found to inhibit the growth of Ascophyllum nodosum (Strömgren, 1979a). However, adult plants appear to be fairly robust in the face of heavy metal pollution (Holt et al., 1997). For example, 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 1000-2000µg/g in the sediment and 10-100µg/l in seawater (Bryan & Gibbs, 1983). Fucoids are able to concentrate heavy metals in their tissues and have been used as bioindicators of heavy metals. The alginate within the fucoids is believed to have a role both in the uptake of the metals, and in storing them in fairly inert terms, so that plants do not seem to be harmed (Holt et al., 1997). Although Ascophyllum nodosum accumulates copper this can be removed because the species naturally sheds its epidermis at regular intervals (Stengel & Dring, 2000). The intolerance of the biotope is set at low. Other species in the biotope may be more intolerant of heavy metals and in areas of high metal pollution species diversity may decline. On return to pre-impact levels of heavy metal pollution there should be a return to normal algal growth levels within a year or two and so recoverability is considered to be high.
Hydrocarbon contamination
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The key characterizing species Ascophyllum nodosum is unlikely to be killed by oil. Long-term exposure to low levels of diesel have been shown to reduce growth rates in Ascophyllum nodosum (Bokn, 1987). However, growth rates recovered within a season after two years exposure so it appears the species has some tolerance to chronic levels of oil in seawater and so intolerance is assessed as intermediate and recovery will be high. Major impacts will however, result from oil spills. In Norway, heavy oil pollution from a grounded ship reduced both fucoid cover and the number of associated species. In sheltered conditions species number had not recovered after 4 years (Hjolman & Lein, 1994 cited in Holt et al., 1997). Amphipods in particular are known to be particularly intolerant of oil contamination. Limpets are also intolerant of oils and effects on populations will have an effect on the biotope because of reduced grazing.
Radionuclide contamination
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Insufficient information.
Changes in nutrient levels
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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). Loss of Ascophyllum nodosum will result in the loss of the biotope so intolerance is assessed as intermediate.
Increase in salinity
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The biotope occurs in full, reduced or variable salinity but there are no reports of the biotope occurring in hypersaline areas. Ascophyllum nodosum is euryhaline species with a salinity tolerance of about 15 to 37 psu (Baardseth, 1970). Therefore, it seems likely that the biotope will be highly intolerant of increases in salinity.
Decrease in salinity
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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 17.3psu and a minimum of 0psu. Further evidence is provided by Chock & Mathieson (1979) who found Ascophyllum nodosum plants in the laboratory photosynthesised at salinities from 0 to 40 psu although the long term effects within this range were not evaluated. In the Teign Estuary in South Devon Ascophyllum nodosum inhabits areas subject to salinities as low as 8psu (Laffoley & Hiscock, 1993). Semibalanus balanoides can tolerate salinities between 12 and 50 psu, below this cirral activity ceases (Foster, 1970). Some of the other characterizing species in the biotope are, however, more intolerant than Ascophyllum nodosum to a decrease in salinity. The distribution of Patella vulgata, for example, extends into the mouths of estuaries surviving in salinities down to about 20psu and Hyale prevostii is tolerant down to the 17psu isohaline. Therefore, the impact on the biotope is likely to be a loss of some species, resulting in a decline in diversity. However, since the key characterizing species, Ascophyllum nodosum is likely to survive the intolerance of the biotope as a whole is assessed as low. However, growth rates may be affected but once salinity levels have returned to pre-impact levels normal metabolic activity and growth rates should rapidly recover and so a recovery rate of very high is reported.
Changes in oxygenation
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The biotope occurs in areas where still water conditions do occur and so some species may be tolerant of slightly deoxygenated conditions. The effects of deoxygenation on macroalgae are poorly studied. Kinne (1972) reports that reduced oxygen concentrations inhibit both photosynthesis and respiration although macroalgae may not be very intolerant of deoxygenation since they can produce their own oxygen. Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l so some very intolerant species may be affected and an intolerance rank of intermediate is reported. However, the biotope occurs in the eulittoral zone where flora and fauna are regularly exposed to air so effects of deoxygenated water will only occur during periods of immersion. During emersion macroalgae can continue to respire so the biotope may not be particularly vulnerable. On return to oxygenated conditions, rapid recovery is likely.

Biological Factors

Introduction of microbial pathogens/parasites
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Although bacteria and fungi are associated with Ascophyllum nodosum no information could be found on any disease causing microbes in the biotope and so intolerance is assessed as low. However, there is always the potential for this to change.
Introduction of non-native species
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There are no records of any non-native species invading the biotope that may compete with or graze upon Ascophyllum nodosum and so the biotope is assessed as not sensitive. However, as several species have become established in British waters there is always the potential for this to occur. The Australasian barnacle Elminius modestus does well in estuaries and bays where it can displace the native Semibalanus balanoides. Its overall effect on the dynamics of rocky shores has however, been small as Elminius modestus has simply replaced some individuals of a group of co-occurring barnacles (Raffaelli & Hawkins, 1996).
Extraction
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Harvesting of Ascophyllum nodosum for alginate is commonly carried out in most areas of its distribution. In an area of Strangford Lough, where harvesting on a small scale was carried out and then stopped, ecological effects were noticed several years later (Boaden & Dring, 1980). The growth rate of Ascophyllum nodosum had increased but shore cover was less. Cover by green algae and Fucus vesiculosus had increased. Patella density had increased and mean size decreased. Microalgal cover of boulders had increased. Sediment median diameter had increased. Halichondria panicea, Hymeniacidon and to a lesser extent Balanus crenatus had decreased. Underboulder fauna remained impoverished by a factor of between one- and two-thirds. Removal of limpets, which graze upon fucoid sporelings, is likely to benefit fucoid plants.

Removal of other important species in the biotope, such as Hyale prevostii and Semibalanus balanoides may reduce grazing pressure on fucoid plants which may ameliorate the effects of removal of Ascophyllum nodosum to a certain extent. Littorina littorea is often a dominant grazing gastropod on the lower shore. The species has some commercial value and is gathered by hand at a number of localities, particularly in Scotland and in Ireland. Demand increases considerably over Christmas from the French market. Overall, intolerance has been assessed as intermediate to reflect the likelihood that the extent of the biotope will decrease. Recovery is likely to be moderate (see additional information).

Additional information icon Additional information

Recoverability
Where Ascophyllum nodosum is lost, recovery would be slow due to the slow growth rate and poor recruitment of this dominant species and so a rank of moderate recovery is reported. The lack of recovery of Ascophyllum nodosum from harvesting is well documented. For example, in their work on fucoid recolonization of cleared areas at Port Erin, Knight and Parke (1950) observed that even eight years after the original clearance there was still no sign of the establishment of an Ascophyllum nodosum population. The species is extremely fertile every year and Printz (1959) suggests it must be assumed that some special combination of climatic or environmental conditions is needed for an effective recolonization. However, most associated species are likely to recover fairly rapidly (within two to five years) due to recruitment from planktonic larvae or through immigration. Thus, if Ascophyllum nodosum remains recovery of the biotope will be much more rapid and a rank of high is reported.

This review can be cited as follows:

Hill, J.M. 2001. Ascophyllum nodosum on very sheltered 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 19/12/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=4&code=1997>