Biodiversity & Conservation


Explanation of sensitivity and recoverability

Physical Factors

Substratum Loss
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All the algal species of the biotope and several faunal species are either attached to the substratum or are associated with species attached to the substratum, so loss of the substratum would also result in loss of algal and faunal populations, therefore intolerance has been assessed to be high. Recoverability has been assessed to be moderate (see additional in formation below).
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Smothering by 5cm of sediment would completely cover the algal species in the biotope, preventing photosynthesis and respiration. Several important characterizing species have high intolerance to smothering e.g. Patella vulgata whose locomotion, grazing and respiration would be affected. Algae may rot under smothering material and sessile fauna such as Mytilus edulis and Semibalanus balanoides would be unable to feed and may suffocate, unable to sustain anaerobic respiration for the period of one month. Sediment would have an especially adverse effect on young germling algae and on the settlement of larvae and spat. Intolerance has been assessed to be high as several characterizing species would be adversely affected by smothering. Recoverability has been assessed to be high (see additional information below).
Increase in suspended sediment
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Increased suspended sediment levels will increase turbidity (see later), scour and siltation. Scour induces high mortality in early post settlement (EPS) algal stages and prevents the settlement of propagules owing to accumulation of silt on the substratum (Vadas et al., 1992). Young stages of Himanthalia elongata were found to be very intolerance of resultant siltation (Moss et al., 1973), hence the impact of increased suspended sediment would depend on the time of year. If increased siltation occurred from June to December, when gametes are released, the population would be highly intolerant because zygotes cannot grow on silt (Moss et al., 1973). Adult plants may be less intolerant than EPS stages. For instance, Irvine (1983) observed morphological adaptation of Palmaria palmata in silty conditions, the form of which presumably reduced the likelihood of an accumulation of silt and the potential for smothering. Increased suspended sediment may reduce growth rate in Semibalanus balanoides due to the energetic costs of cleaning sediment particles from feeding apparatus although if the organic content is high suspension feeders could also benefit. Patella vulgata and Mytilus edulis were assessed to have a low intolerance to an increase in suspended sediment because they are found in turbid estuaries where suspended sediment levels are high. At the benchmark level, the biotope is considered to have an intermediate intolerance as the population of Himanthalia elongata may be degraded by the factor as a result of poor recruitment/ survival of early post settlement stages. Recoverability has been assessed to be high as mature plants are likely to remain in situ (see additional information below).
Decrease in suspended sediment
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A decrease in suspended sediment, especially organic particulates, could potentially reduce the food available to suspension feeders such as Semibalanus balanoides and Mytilus edulis and hence growth rates. For a period of a month however, the effect is not likely to be significant. None of the other species in the biotope require a supply of suspended sediment particles for feeding or for activities such as tube building. Therefore, an intolerance of low has been recorded. Recoverability has been assessed to be immediate as optimal feeding would resume on return to prior conditions.
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Algal species of the biotope occur in the intertidal where they experience regular periods of emersion and therefore tolerate some desiccation. Hawkins & Hartnoll (1985) observed no damage in Himanthalia elongata during the unusually hot summer of 1983. However, vegetative and reproductive plants died during hot weather during spring tides in Co. Clare (Stengel, pers. comm. to White, 2000). Fronds of Corallina officinalis are highly intolerance of desiccation and do not recover from 15 % water loss, which might occur within 40-45 minutes during a spring tide in summer (Wiedemann 1994). Lubchenco (1980) reviewed the available evidence and suggested that the upper limit of Chondrus crispus distribution was determined by physical factors, most likely desiccation. Mathieson & Burns (1971) measured the photosynthetic rate of Chondrus crispus at varying degrees of desiccation and concluded that apparent photosynthesis always decreased with dehydration. Algal spores and developing germlings are particularly susceptible to desiccation as they have very large surface-to-volume ratios, although they do benefit from the film of water that persists in concavities on the substratum (Kain & Norton, 1990). Faunal species such as Semibalanus balanoides and Patella vulgata are likely to be relatively tolerant of an increase in desiccation as they can suspend feeding/grazing activity to prevent moisture loss. An increase in desiccation at the benchmark level is likely to result in damage to and reduced viability of important characterizing species, especially algae. Mortality of some seaweed may occur in a severe desiccation event (e.g. a hot summer day when low spring tides occur at midday). Intolerance has been assessed to be intermediate. Recovery has been assessed to be high because sub-tidal populations of the species are likely to remain unaffected and will constitute a reservoir from which recruitment can occur or where refuge could be sought by mobile species.
Increase in emergence regime
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An increase in the period of emergence over a period of one year would probably result in a depression of the algal species upper limit on the shore owing to desiccation and radiation stress and some mortality is likely. For example, in Porphyra, 80-95 % of attached spores developed abnormally or died when exposed to direct insolation and evaporation stress (Boney, 1978).Mobile fauna would be able to avoid intolerable conditions by seeking refuge in crevices or under foliose algae, whilst sessile species such as Semibalanus balanoides and Mytilus edulis would 'close up' to avoid desiccation, but over a period of one year the resultant reduction in the length of time spent feeding could affect growth. An increase in desiccation at the benchmark level is likely to result in a reduction of the upper limit of both algal and faunal species. The extent of the biotope would be reduced as it is squeezed seaward so intolerance has been assessed to be intermediate. Recovery is high because sub-tidal populations of the species are likely to remain unaffected and will constitute a reservoir from which recruitment can occur.
Decrease in emergence regime
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A decrease in the time exposed to the air would reduce the likelihood of desiccation and the upper limit of the biotope may extend up the shore. However, algal species within the biotope may not necessarily benefit as grazing species would probably be more active and epiphytic species which may be kept in check by periodic exposure to air may increase in abundance and have a smothering effect on the host plant. Also species from the zone / community below may become more dominant in the biotope. However, because the extent of the biotope is unlikely to change but to shift up shore, the biotope has been assessed as being tolerant* to a decrease in emergence.
Increase in water flow rate
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The biotope is characteristic of wave exposed conditions where water movement from wave action will greatly exceed the strength of any possible tidal flow. The biotope is therefore considered to be not sensitive.
Decrease in water flow rate
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Moderate water movement is beneficial to seaweed. It carries a supply of nutrients and gases to the plants, transports spores, removes waste products, and prevents settling of silt. Seaweed in still water rapidly deplete the nutrients in the immediate vicinity (Kain & Norton, 1990). During periods of calm weather, the biotope may be kept clear of silt and supplied with oxygenated water and food, only by tidal flow, so that a reduction may lead to short term stress and loss of condition. Intolerance has been assessed to be low and recoverability very high as species would remain in situ (see additional information below).
Increase in temperature
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Most of the species in the biotope have a distribution that extends well to the south of the British Isles suggesting that a long-term chronic increase in temperature of 2 °C would not affect the types of species dominating the biotope. Himanthalia elongata survived the unusually hot summer of 1983 apart from a slight bleaching of buttons (Hawkins & Hartnoll, 1985). Semibalanus balanoides is pre-eminently a boreal species, adapted to cool environments. Higher temperatures are therefore likely to have more adverse effects on it than lower temperatures. Reproduction in Semibalanus balanoides is inhibited by temperatures greater than 10 °C (Barnes, 1989). Increased temperature is therefore likely to favour chthamalid barnacles rather than Semibalanus balanoides (Southward et al. 1995). Chthamalus spp. are warm water species, with a northern limit of distribution in the British Isles, so are likely to be tolerant of or favourably affected by long term increases in temperature. However, a change in the species of barnacle will not change the nature of the biotope. Patella vulgata is a hardy intertidal species that tolerates long periods of exposure to the air and consequently wide variations in temperature. Therefore, the impact on the biotope of temperature increases at the benchmark level are likely to be sub-lethal effects on growth and fecundity of characterizing algae and species. Thus, the biotope has been assessed to have a low intolerance to the benchmark increases in temperature. Recoverability has been assessed to be high (see additional information below). However, long-term climate change may allow species such as Bifurcaria bifurcata to become more abundant in the biotope which could then change.
Decrease in temperature
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The biotope is recorded in northernmost Britain. However, germination and vegetative growth in Himanthalia elongata are highly intolerance of reductions in temperature and are limited at temperatures below 15 °C (Stengel, 2000 in prep.). Palmaria palmata does well in low temperatures, with an optimum growth between 6 and 15 °C, consistent with a distribution in northern temperate and arctic waters. Semibalanus balanoides was not affected during the severe winter of 1962-63 in most areas, except the south east coast where it suffered 20-100 % mortality (Crisp, 1964). Intolerance has been assessed to be intermediate as the population of important characterizing species may become degraded as a consequence of acute temperature decreases. Recoverability has been assessed to be high (see additional information below).
Increase in turbidity
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The biotope is predominantly an assemblage of algal species so the light attenuation effects of an increase in turbidity could significantly affect the community. Himanthalia elongata which normally occurs on open coasts is absent from the more turbid eastern half of the Channel, but can be luxuriant in extreme shelter elsewhere provided tidal scour ensures clear water (Lewis, 1964). An increase in turbidity would reduce the light available for photosynthesis and therefore lower growth rates. A prolonged occurrence of increased turbidity might delay vegetative growth and Himanthalia elongata may fail to become fertile. However, red algae are well adapted to low light conditions and so may be tolerant of increased turbidity. Intolerance to an increase in turbidity has been assessed to be low as effects would most likely be sub-lethal. On return to prior conditions improved light penetration would stimulate photosynthesis and recoverability has been assessed to be very high.
Decrease in turbidity
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The biotope is predominantly an assemblage of algal species dependent on light for photosynthesis so a decrease in turbidity would be beneficial and an assessment of not sensitive* has been made.
Increase in wave exposure
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The biotope is typically found in locations that are exposed to wave action. At the benchmark level the biotope would experience conditions of extreme wave exposure. Algal fronds would be ripped from holdfasts. Algae and faunal species both sessile and mobile would probably experience problems with settlement and adhesion to the substratum. The biotope would probably begin to change in to another one inhabited only by species able to maintain a firm attachment to the substratum and whose morphology offers least resistance. Intolerance has therefore been assessed to be high and recoverability moderate on return to prior conditions (see additional information below).
Decrease in wave exposure
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Himanthalia elongata is more tolerant of wave action than Fucus serratus. A decrease in the level of wave action could result in Himanthalia elongata being displaced by faster growing fucoids such as Fucus serratus. In the absence of Himanthalia elongata the biotope would not necessarily be recognized so intolerance has been assessed to be high. On return to prior conditions species which displaced important characterizing species of the ELR.Him biotope would probably be competitively inferior and decline. Recovery has been assessed to be moderate, as in the worst case adult specimens of the displaced algal species may not remain in the vicinity (see additional information below).
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Seaweeds have no known mechanism for the perception of noise. Faunal species may be able to detect vibrations caused by noise but at the benchmark level the biotope has been assessed to be not sensitive to this factor.
Visual Presence
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Faunal species within the biotope may have some vision but probably lack the visual acuity to detect objects not normally found in the marine environment. Therefore the biotope has been assessed to be not sensitive to the factor.
Abrasion & physical disturbance
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Abrasion may damage fronds of established seaweed and crush germlings and faunal species. However, Patella vulgata, for instance has a tough shell which offers protection from any abrading factors and any near vibration causes the shell muscles to contract vigorously, clamping the animal to the rock. However, a short, sharp knock may dislodge an individual leaving it vulnerable to predation. In the eulittoral zone, abrasion caused by human trampling has been shown to reduce algal cover on shores (Holt et al., 1997), and may be of more relevance to the biotope than the dropping and dragging of an anchor as an abrasive factor. Brosnan & Crumrine (1994) found that the foliose red algae Mastocarpus papillatus was intolerant of moderate levels of trampling. However, trampling pressure may not be particularly intense either considering that the biotope occurs within the vicinity of the low water mark and therefore may be inaccessible for much of the time. At the benchmark level intolerance has been assessed to be low and recovery high as a population would remain in situ (see additional information below).
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Many of the important characterizing algal species of the biotope are permanently attached to the substratum and if displaced would not be able to reattach to suitable substratum, e.g. Himanthalia elongata . Faunal species such as Patella vulgata can re-attach to the rock but may be damaged as a result of removal. Displaced individuals with the foot exposed to the air, are likely to become prone to predation and desiccation and may die. Individuals removed several feet from their scars do not appear to make their way home again (Fretter & Graham, 1996) and so may be more vulnerable to desiccation without the tight fit to their 'home scar'. Intolerance to displacement has been assessed to be high and recovery moderate (see additional information below).

Chemical Factors

Synthetic compound contamination
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Cole et al. (1999) suggested that herbicides, such as simazine and atrazine, were very toxic to macrophytes and evidence of O'Brien & Dixon (1976) suggests that red algae in particular are very sensitive to synthetic chemicals The effects of tributyl tin (TBT), used in anti-fouling paints, on Nucella lapillus have been extensively documented and represent one of the best known examples of the effects of chemical pollution. Intolerance has been assessed to be high and recovery moderate assuming that populations of some species might be completely lost (see additional information below).
Heavy metal contamination
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Uptake of heavy metals from solution by seaweed is influenced by factors such as light, algal nitrogen content, frond age, length of emersion, temperature, salinity, season of the year and presence of other pollutants in the surrounding water (see Lobban & Harrison, 1997) and consequently seaweed may not accurately reflect metal concentrations in the surrounding water. The order of metal toxicity to algae varies with the algal species and the experimental conditions, but generally the order is Hg>Cu>Cd>Ag>Pb>Zn (Rice et al., 1973; Rai et al., 1981), however insufficient information was available to comment further on the particular intolerance of algal species within the biotope. Several faunal species have been assessed to have an intermediate intolerance to heavy metal pollution, e.g. Mytilus edulis and Patella vulgata (see full MarLIN reviews). Intolerance has therefore been assessed to be intermediate and recoverability high.
Hydrocarbon contamination
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O'Brien & Dixon (1976) stated that red algae were the most sensitive group of algae to oil contamination especially in combination with dispersant contamination, possibly due to the susceptibility of the photosynthetic pigment phycoerythrin to chemical damage. Filamentous forms are considered to be most sensitive. Observations following oil spills indicate that grazing species are particularly intolerance of oil pollution. Thick layers of deposited oil would probably interfere with respiration and spoil food supplies for Patella vulgata. Limpets are unable to remain 'closed-off' from the environment for very long, and the adductor muscles relax occasionally, lifting the shell very slightly exposing the animal to contaminants. After the Braer oil spill, in common with many other oil spills, the major impact in the intertidal zone was on the population of limpets and other grazers. 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). In the case of the Torrey Canyon spill the quantity and toxicity of the oil dispersants applied to the shore caused more mortalities than the oil alone, Patella vulgata being particularly susceptible, although all animals and many algae were killed in areas heavily sprayed (Raffaelli & Hawkins, 1996). Following oil pollution rocky shore communities are highly disturbed owing to the loss of structuring species. The recovery period can be extensive owing to both loss of species and the subsequent extreme fluctuations in abundance. In the Torrey Canyon incident, following the death of grazing species, a dense green flush of ephemeral algae (Blidingia & Ulva) developed and lasted for nearly a year, whilst after six months Fucus vesiculosus and Fucus serratus began to colonize the shore and persisted in dense stands for between 1 to 3 years. Patella vulgata colonized affected shores within the year and thrived in damp conditions under the fucoids, its grazing inhibited further extensive fucoid recruitment. Abnormal numbers of limpets accrued and cleared rocky substrata of much of the algae, allowing, after a period of 4 years (in dispersant treated areas), increased barnacle recruitment. Fucoid cover remained abnormal for the first 11 years following the spill and fluctuated for 15 years, whilst the population structure of Patella vulgata remained abnormal for at least 10 years (Smith, 1968; Southward & Southward, 1978; Hawkins & Southward, 1992). Intolerance has been assessed to be high as major changes of species composition would probably occur in the biotope. Recoverability has been assessed to be moderate, as whilst species normally found in the biotope may be found within 5 years, full recovery (in terms of recruitment of red algae and establishment of small-scale spatial and temporal fluctuations in the major components of fucoids, barnacles and limpets) is likely to take 10 years or longer.
Radionuclide contamination
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Insufficient information.
Changes in nutrient levels
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Nutrients are required for algal growth. A small increase in nutrient levels may enhance growth rates but large increases have a detrimental effect by leading to overgrowth of brown seaweed by green algae (Fletcher, 1996). Decreases in nutrient levels may slow down algal growth. Nucella lapillus is the species within the biotope with the highest intolerance attributed to nutrient enrichment. Gibbs et al. (1999) reported a massive kill of Nucella lapillus in Bude Bay, north Cornwall. Gibbs et al. (1999) suggested that the mass mortalities may have been caused by eutrophication and summer algal blooms due to a new sewage outfall in the area that received only primary treated sewage. However, intolerance of the biotope has been assessed to be intermediate as the biotope would not be destroyed, but the viability of a species population would possibly be reduced at the benchmark level. Recoverability has been assessed to be high. Gibbs et al. (1999) reported that the small numbers of dog whelks surviving a mass-kill in July -October 1995 were able to re-establish the population within two years. They also pointed out that in the worst affected parts of the area, the individuals were widely distributed so that breeding aggregations were not possible and suggested that it would take many years for dog whelks to regain their former abundance.
Increase in salinity
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The biotope occurs in conditions of full salinity so an assessment of an increase in salinity was not considered relevant.
Decrease in salinity
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In the eulittoral zone, species of the biotope would be exposed to conditions of reduced salinity following precipitation. Himanthalia elongata is intolerance of reduced salinity as the factor has an adverse effect on zygote development. When salinity is below 21 psu few eggs survive (Moss et al., 1973). Red algae such Palmaria palmata are not recorded as inhabiting reduced salinity habitats. A reduction in salinity below 18 psu is likely to adversely affect reproduction and feeding in Nucella lapillus. The community is unlikely to be particularly intolerance of reduced salinity for one week but over the period of one year the populations of important characterizing species would probably decline. Intolerance to decreased salinity has been assessed to be intermediate. Recoverability has been assessed to be high (see additional information below).
Changes in oxygenation
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Little information on the effects of oxygen depletion on macroalgae was found although Kinne (1972) reports that reduced oxygen concentrations inhibit both photosynthesis and respiration which may affect growth and reproduction. The effects of decreased oxygen concentration equivalent of the benchmark would be greatest during the dark when the algae require oxygen for respiration. Many faunal species in the biotope were assessed to have an intermediate intolerance to the benchmark decrease in oxygenation. For instance, Nucella lapillus and Patella vulgata can respire in air and therefore would only be intolerance of low oxygen concentrations in the water column during tidal immersion. Intolerance has been assessed to be intermediate and recoverability high as a proportion of populations would probably remain (see additional information below).

Biological Factors

Introduction of microbial pathogens/parasites
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The viability of seaweed species may be affected by a variety of biotic agents that include fungi, bacteria and algal endophytes. Symptoms include tissue necrosis, reduced reproductive output and growth. Barnacles are parasitised by a variety of organisms and, in particular, the cryptoniscid isopod Hemioniscus balani. Intertidal gastropods often act a secondary hosts for trematode parasites of sea birds. For instance, Nucella lapillus may be infected by cercaria larvae of the trematode Parorchis acanthus. Infestation causes castration and misformed growth (Feare, 1970b; Kinne, 1980; Crothers, 1985) (see full MarLIN review). Mytilus species host a wide variety of disease organisms, parasites and commensals from many animal and plant groups (see full MarLIN review). Intolerance has been assessed to be low owing to the fact that through reduced viability pathogens and parasites have the potential to reduce the abundance of one or two species within the biotope. Individuals may not necessarily recover, but recolonization of important characterizing species is likely to occur as recruitment could occur from distant populations (see additional information below).
Introduction of non-native species
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The only species identified as potentially threatened by non-native species was the barnacle, Semibalanus balanoides by the Australasian barnacle Elminius modestus. Elminius modestus was introduced to British waters on ships during the second world war and thrives in sheltered estuaries and bays, where it can displace Semibalanus balanoides and Chthamalus montagui. However, the native species are not necessarily displaced completely in all locations because they out-compete Elminius modestus on exposed shores (Raffaelli & Hawkins, 1996) such as in this biotope. An assessment of not sensitive has therefore been made.
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Extraction of grazers from semi-exposed shores, such as limpets and littorinids, might allow the upper limit of Himanthalia elongata to extend up the shore. For instance, Himanthalia elongata survived the 'Torrey Canyon' oil spill and extended its local distribution 2 m vertically up the shore, due to the absence of grazers (Southward & Southward, 1978). Several species of algae characteristic of the biotope are exploited e.g. Himanthalia elongata and Corallina officinalis (see exploitation). A 50 % removal of characterizing algae that occupy space on the substratum would represent a significant loss of the dominant species and intolerance has been assessed to be intermediate. Recovery from extraction of 50 % of the species is likely to be high because remaining plants constitute a reservoir from which recruitment can occur (see additional information below).

Additional information icon Additional information

For all algal species within the biotope, recovery periods would vary with season owing to the availability of spores and the proximity of fertile specimens to denuded areas, and /or the presence of vegetative material from which new plants could propagate. Furthermore, spores of red algae are not motile so algal dispersal is wholly a passive process (Fletcher & Callow, 1992) and colonization may rely on the presence of reproducing plants only a few metres away.
  • Himanthalia elongata recruited to a suitable substratum (placed in the eulittoral amongst adjacent species) within one year but initial plant densities declined to three or four holdfasts owing to the lack of protection from established adults or other foliose algae which would have provided protection from desiccation, wave action and high irradiance. However, the number of holdfasts rose to 1500 buttons per block by March of the second year (Stengel et al., 1999).
  • A recovery time of more than three years was reported by MacFarlane (1952) and Mathieson & Burns (1975) following total removal of Chondrus crispus by scraping. However, based on experimental evidence, regrowth of Mastocarpus stellatus and Chondrus crispus is likely to be good in instances where some holdfast is left intact, with recovery to pre-removal abundance of both species occurring within one year, and often considerably less, in lightly harvested areas, but taking 1-2 years in areas more heavily cleared (but with some material remaining) (Marshall et al., 1949 and other citations in Holt et al., 1995).
  • In kelp canopy removal experiments in the Isle of Man, Hawkins & Harkin (1985) observed a rapid increase in the number of Palmaria palmata sporelings on bare rock and the species came to dominate cleared plots within five months. Recolonization from distant populations would probably take longer, however, because dispersal distances are limited, with spores sinking and attaching close to adult plants.
  • Recolonization of Patella vulgata on rocky shores is likely to be rapid as it is a cosmopolitan species with planktonic dispersal. For instance, limpet spat recruited to suitable substratum within 6 months after the Torrey Canyon oil spill, peak abundance of the species was noted within 4-5 years after the spill.
  • Bennell (1981) observed that Semibalanus balanoides were removed when the surface rock was scraped off in a barge accident at Amlwch, North Wales. Barnacle populations returned to pre-accident levels within 3 years. However, barnacle recruitment can be very variable because it is dependent on a suite of environmental and biological factors (see full MarLIN review: reproduction), therefore barnacle populations may take longer to recover.
  • Many other faunal species in the biotope are widespread and have planktonic life stages which would aid recovery.
  • Mobile species such as Nucella lapillus (which lays eggs producing mobile young) are capable of recovering with about 2-5 years if survivors are present nearby intertidally or below low water. However, should a population need to recruit from distant locations recovery may take significantly longer as the species are relatively slow crawlers.
Recoverability of the biotope has been assessed to be high if fertile populations of both faunal and floral species are in the immediate vicinity or if, in the case of algae, vegetative material remains in situ. In the absence of either of the aforementioned, partial recovery may occur within 5 years but full recovery could take longer.

This review can be cited as follows:

Budd, G.C. 2002. Himanthalia elongata and red seaweeds on exposed lower eulittoral rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 25/11/2015]. Available from: <>