Biodiversity & Conservation

IR.MIR.SedK.PolAhn

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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The majority of species in the biotope live permanently attached to the substratum or to algae growing on the substratum. Substratum loss would result in loss of these populations and therefore intolerance is assessed as high. Loss of the entire population of Furcellaria lumbricalis would limit the recovery of the biotope (see additional information below) and so recoverability is recorded as moderate. Substratum loss would result in the eradication of entire populations and hence, it is expected that there would be a major decline in species richness in the biotope.
Smothering
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The algal turf in the biotope comprises erect species which are unlikely to be affected by smothering with 5 cm of sediment. Ahnfeltia plicata and Furcellaria lumbricalis are tolerant of sand cover (Dixon & Irvine, 1977) and Furcellaria lumbricalis persisted in areas of the Baltic Sea where eutrophication resulted in high sediment loads (Johansson et al., 1998). However, recently settled propagules and small developing plants would be buried by 5 cm of sediment and be unable to photosynthesise. For example, Vadas et al. (1992) stated that algal spores and propagules are adversely affected by a layer of sediment, which can exclude up to 98% of light. There is therefore likely to be mortality of some portion of the characterizing algal community and intolerance is assessed as intermediate. As some portion of the populations are likely to remain, recoverability is assessed as high (see additional information below). Urticina felina lives in situations where it is frequently buried by mobile sand (Holme & Wilson, 1985) and is likely to be tolerant of smothering with 5 cm of sediment. Epilithic species such as the tube worm Pomatoceros triqueter would be unable to feed and respire under 5 cm of sediment and would be likely to suffer high mortality. There is likely therefore to be a minor decline in species richness.
Increase in suspended sediment
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The algal species which characterize the biotope are not likely to be affected directly by an increase in suspended sediment. However, increased suspended sediment will decrease light attenuation (considered in 'turbidity') and increase siltation. As discussed above in 'smothering', increased rate of siltation may inhibit development of algal spores and propagules resulting in some mortality. Intolerance is therefore assessed as intermediate. As some portion of the populations are likely to remain, recoverability is assessed as high (see additional information below). Epilithic and suspension feeding fauna may be smothered by an increase in suspended sediment so there may be a minor decline in species richness.
Decrease in suspended sediment
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The algal species which characterize the biotope are unlikely to be affected directly by a decrease in suspended sediment. The consequent effect of decreased turbidity is discussed below. Similarly, Urticina felina is unlikely to be affected by a decrease in suspended sediment.
Desiccation
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The biotope occurs sublittorally and is characterized by a number of algal species which are intolerant of emersion and therefore most likely desiccation. Gessner & Schramm (1971) (reviewed by Bird et al., 1991) recorded that at 18-20°C, the critical saturation deficit for Furcellaria lumbricalis was 60-70% of total water content, as contrasted with 10% for the intertidal species Fucus vesiculosus. On desiccation to 65% total water content, photosynthetic rate was depressed to 60% of the norm and recovery following reimmersion took 7 hours. Desiccation to 42% resulted in only 50% recovery in 7 hours and there was no recovery of photosynthesis in thalli dried to 7% of their original water content. Growth experiments by Indergaard et al. (1986) revealed that growth of Furcellaria lumbricalis in a continuous spray regime was over 3 times faster (227 µm/day vs. 61 µm/day) than growth in an intermittent spray regime. The benchmark level of desiccation is exposure to air and sun for one hour. It is difficult to determine how this level relates to the recorded reactions, but it is likely that desiccation would cause at least loss of fronds of intolerant species. Ahnfeltia plicata and Chondrus crispus, however, both occur in the intertidal and so are likely to be more tolerant of desiccation, although photosynthesis may be inhibited. For example, Mathieson & Burns (1971) measured the photosynthetic rate of Chondrus crispus at varying degrees of desiccation and concluded that apparent photosynthesis always decreases with dehydration. After loss of 65% of its water content, rate of photosynthesis in Chondrus crispus was 55% of the control rate. Since only the most intolerant species are likely to be adversely affected and the whole plant seems unlikely to be killed, an intolerance of Intermediate seems likely. Also, since many species in the biotope attached to fronds may be adversely affected either directly or as a result of loss of fronds, there would be a decline in species richness in the biotope.
Increase in emergence regime
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MIR.PolAhn is a subtidal biotope, occurring in the 5-10 m depth band (Connor et al., 1997a). Increase in emergence is therefore not a relevant factor.
Decrease in emergence regime
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MIR.PolAhn is a subtidal biotope, occurring in the 5-10 m depth band (Connor et al., 1997a). Decrease in emergence is therefore not a relevant factor.
Increase in water flow rate
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MIR.PolAhn typically occurs in areas with 'moderately strong' or 'weak' water flow (Connor et al., 1997a). Moderate water movement is beneficial to seaweeds as it carries a supply of nutrients and gases to the plants, removes waste products, and prevents settling of silt. However, if flow becomes too strong, plants may be damaged and growth stunted. Additionally, an increase to stronger flows may inhibit settlement of spores and remove adults or germlings. It is likely therefore that the benchmark increase in water flow rate to 'strong' or 'very strong' flow would result in some mortality of the algal species which characterize the biotope, particularly of older individuals or those attached to the least stable substrata. Biotope intolerance is therefore assessed as intermediate. As some portion of the populations are likely to remain, recoverability is assessed as high (see additional information below).
Decrease in water flow rate
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MIR.PolAhn typically occurs in areas with 'moderately strong' or 'weak' water flow (Connor et al., 1997a). The benchmark decrease in water flow would place the species in areas of 'weak' water flow. Seaweeds in still water rapidly deplete the nutrients in the immediate vicinity (Kain & Norton, 1990) and are likely to be more vulnerable to depletion of essential dissolved gases and accumulation of waste products. Furthermore, decreased water flow would result in deposition of fine sediments and possible smothering of low growing forms, such as the encrusting tetrasporophyte phase of Ahnfeltia plicata. Some mortality of species such as Ahnfeltia plicata and Chondrus crispus is likely to result and so intolerance is assessed as intermediate. As some portion of the populations are likely to remain, recoverability is assessed as high (see additional information below). Some species of algae are more tolerant of low flow conditions. Gessner (1955) (cited in Schwenke, 1971) stated that deeper growing species of the benthos near Helgoland, including Furcellaria lumbricalis, had a smaller stagnation-caused respiratory inhibition than surface living species, which enabled them to thrive in low flow conditions. Furthermore, Austin (1960b) noted that Furcellaria lumbricalis in areas of low water flow lived longer and grew larger than specimens from areas with high flow.
Increase in temperature
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The important species in the biotope have wide geographic ranges and are likely to be tolerant of higher temperatures than those experienced in the British Isles. The European range of Furcellaria lumbricalis, for example, is from northern Norway to the Bay of Biscay. Novaczek & Breeman (1990) recorded that specimens of Furcellaria lumbricalis grew well in the laboratory from 0-25°C with optimal growth between 10 and 15°C. Growth ceased at 25°C and 100% mortality resulted after 3 months exposure to 27°C. Similarly, Bird et al. (1979) recorded optimum growth at 15°C and cessation of growth at 25°C with associated necrosis of apical segments.
Chondrus crispus also has a wide geographical range, occurring in Europe from northern Russia to southern Spain (Dixon & Irvine, 1977). In New Hampshire, USA, Chondrus crispus grows abundantly in waters with an annual variation in surface temperature from -1 to +19°C (Mathieson & Burns, 1975).
Lüning & Freshwater (1988) incubated Ahnfeltia plicata from British Columbia at a range of temperatures for 1 week and tested their survivability by ability to photosynthesize at the end of the incubation period. The species survived from the coldest temperature tested (-1.5°C) to 28°C. Total mortality occurred at 30°C. Lüning & Freshwater (1988) suggested that Ahnfeltia plicata was therefore amongst the group of most eurythermal heat tolerant algae. Considering that maximum sea surface temperatures around the British Isles rarely exceed 20°C (Hiscock, 1998), it is unlikely that the important species would suffer mortality due to the benchmark increase in temperature. However, elevated temperatures would probably result in inhibition of growth, particularly if the change was acute. Biotope intolerance is therefore assessed as low. Growth rates should quickly return to normal when temperatures return to their original levels so recoverability is assessed as very high. None of the species which characterize the biotope are expected to experience mortality due to temperature increases so species richness is not likely to change.
Decrease in temperature
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Ahnfeltia plicata has a very wide geographic range, occurring from northern Russia to Portugal (Dixon & Irvine, 1977). The species is therefore likely to be tolerant of higher temperatures than it experiences in Britain and Ireland.
Dudgeon et al. (1990) investigated the effects of freezing on Chondrus crispus. Plants from Maine, USA, were frozen at -5°C for 3 hours a day for 30 days. Photosynthesis was reduced to 55% of control values, growth rates were reduced and fronds were eventually bleached and fragmented resulting in biomass losses. Additionally, fronds of Chondrus crispus which were frozen daily had higher photosynthetic rates following subsequent freezing events than unfrozen controls, indicating that the species is able to acclimate to freezing conditions (Dudgeon et al., 1990). Furcellaria lumbricalis is a northern species whose range extends to northern Norway. Novaczek & Breeman (1990) recorded that specimens of Furcellaria lumbricalis grew well in the laboratory from 0-25°C with optimal growth between 10 and 15°C. The species tolerated -5°C for 3 months with no mortality. Minimum surface seawater temperatures rarely fall below 5C around the British Isles (Hiscock, 1998) so it is unlikely that the important species would suffer mortality due to the benchmark increase in temperature. However, reduced temperatures would probably result in inhibition of growth, particularly if the change was acute. Biotope intolerance is therefore assessed as low. Growth rates should quickly return to normal when temperatures return to their original levels so recoverability is assessed as very high. None of the species which characterize the biotope are expected to experience mortality due to temperature decreases so species richness is not likely to change.
Increase in turbidity
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In general, an increase in turbidity and therefore decreased light penetration would be expected to negatively affect growth of photoautotrophs. However, the algae which characterize the biotope are relatively tolerant of low light conditions. For example, laboratory experiments by Bird et al. (1979) revealed that Furcellaria lumbricalis was growth saturated at very low light levels (ca 20 µE/m²/s) compared to other algae and suggested that this may explain why Furcellaria lumbricalis is able to proliferate in relatively deep and turbid waters. Chondrus crispus is growth saturated at light levels of 60-70 µE/m²/s and is not photoinhibited at 250 µE/m²/s (Bird et al., 1979; Fortes & Lüning, 1980). Over the course of a year, the decreased light availability is likely to result in a reduction in growth of the more intolerant species and so biotope intolerance is assessed as low. Growth should quickly return to normal when turbidity returns to its original levels and so recoverability is assessed as very high. Algae which require more light for growth, such as ephemeral greens, would be unlikely to cope with increased turbidity so there would be a minor decline in species richness.
Decrease in turbidity
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Chondrus crispus is growth saturated at light levels of 60-70 µE/m²/s and is not photoinhibited at 250 µE/m²/s (Bird et al., 1979; Fortes & Lüning, 1980). Furcellaria lumbricalis is growth saturated at very low light levels compared to other macroalgae (Bird et al., 1979; Bird et al., 1991) and will not be affected directly by increased light availability. Haglund et al. (1987) reported no inhibition of photosynthesis up to 500 µE/m²/s in Ahnfeltia plicata and suggested the species had a high potential for growth provided no other factors were limiting. The algal turf which characterizes the biotope is therefore probably relatively tolerant of a decrease in turbidity. The community composition may shift towards species which were previously light limited such as Ahnfeltia plicata.
Increase in wave exposure
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The biotope typically occurs in 'exposed ' or 'moderately' areas (Connor et al., 1997a). The benchmark increase would place the biotope in 'extremely exposed' or 'very exposed' areas for 1 year. Increased wave action is likely to cause some damage to fronds resulting in reduced photosynthesis and compromised growth. Furthermore, individuals may be damaged or dislodged by scouring from sand and gravel mobilized by increased wave action (Hiscock, 1983). Austin (1960b) noted that Furcellaria lumbricalis from extremely exposed sites have smaller dimensions than individuals from semi-exposed sites and that fronds may be lost due to storm action. Sharp et al. (1993) reported Furcellaria lumbricalis found cast ashore following storms. Dudgeon & Johnson (1992) noted wave induced disturbance of intertidal Chondrus crispus on shores of the Gulf of Maine during winter. 25-30% of cover of large Chondrus crispus thalli was lost in one winter. They also noted that Chondrus crispus suffered more heavily than Mastocarpus stellatus, probably because the drag on the thallus was greater. Increased wave action is therefore likely to result in some mortality and so biotope intolerance is assessed as intermediate. As some portion of the populations are likely to remain, recoverability is assessed as high (see additional information below). The more delicate algal species, such as Cryptopleura ramosa, are unlikely to be able to persist in extremely exposed areas so there will be a minor decline in species richness.
Decrease in wave exposure
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A decrease in wave exposure is unlikely to affect the algae which characterize the biotope directly. However, the consequent effects of decreased wave action are likely to include increased deposition of fine sediment and increased risk of stagnation. Species more tolerant of these factors, e.g. Polyides rotundus and Furcellaria lumbricalis (Gessner, 1955, cited in Schwenke, 1971) , are more likely to proliferate in these conditions, eventually at the expense of Ahnfeltia plicata and Chondrus crispus. Over the course of a year (the benchmark level), some mortality may be expected so intolerance is assessed as intermediate but without loss of diversity. Over a longer period or permanently, siltation would be likely to destroy the biotope and, in such a case, intolerance would be high and recoverability very low. Growth and reproduction should quickly return to normal when wave exposure returns to typical levels so recoverability is assessed as very high. Gutierrez & Fernandez (1992) described morphological variability of Chondrus crispus according to wave exposure and emersion. They identified 2 well defined morphotypes; filiform and planiform. The filiform morphotype had fewer dichotomies per unit length, a circular cross section, narrow fronds and was abundant in the low intertidal and at more exposed sites. The planiform morphotype had more dichotomies, was smaller, with a flattened cross section, broader fronds and was abundant higher up the shore and in more sheltered areas. A decrease in wave exposure is likely to precipitate a shift towards a community of the planiform morphotype.
Noise
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The characterizing species in the biotope have no auditory mechanisms and therefore are likely to be not sensitive to noise.
Visual Presence
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The majority of the species in the biotope have little or no visual acuity and would therefore be not sensitive to visual disturbance.
Abrasion & physical disturbance
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The Polyides / Furcellaria red algal turf is likely to be tolerant of abrasion as the fronds are flexible and cartilaginous. The plants' point of attachment to the substratum, the holdfast, is a potential point of weakness. For example, Taylor (1970) (cited in Sharp et al., 1993) stated that clumps of fronds of Furcellaria lumbricalis were easily removed from the substratum by drag raking, but only where the plant had a sufficient number of dichotomies (usually more than 3) to snag in the rake. It is likely therefore that the benchmark level of abrasion would cause detachment and/or damage. Sharp et al. (1993) noted that, following detachment, Furcellaria lumbricalis plants were capable of reattachment. The holdfast of Ahnfeltia plicata is similar in form to the encrusting phase (Dickinson, 1963) and would be unlikely to be damaged by physical abrasion. Daly & Mathieson (1977) noted that regeneration of upright fronds occurred from the holdfast. Worm & Chapman (1998) suggested that Chondrus crispus was highly resistant to intense physical and herbivore induced disturbance, ensuring competitive dominance on the lower shore. However, the benchmark level of abrasion, for example a scallop dredge, would be expected to remove or damage some fronds, although the holdfasts are likely to escape unscathed. Severe physical disturbance would be more akin to substratum removal where intolerance is high and recoverability moderate. Chondrus crispus is capable of regenerating from its holdfasts (e.g. Dudgeon & Johnson, 1992) and so no mortality is expected, although growth and reproduction of the algae would be compromised during regeneration. However, a proportion of the fronds would be removed, together with any associated mesoherbivores, meiofauna, and epiphytes. In addition, epilithic species on the rock surface may be damaged, and potential competitive species may recruit to the cleared areas. Therefore, an overall intolerance of intermediate has been recorded. Fronds may take up to 18 months to regrow (see additional information below), so recoverability is assessed as high. Soft bodied fauna, including the anemone, Urticina felina, are most likely to be impacted by physical abrasion.
Displacement
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The holdfast of Ahnfeltia plicata is a thin crust which grows epilithically (Dickinson, 1963). It is unlikely that the holdfast, or the encrusting tetrasporophyte phase, would survive removal from the substratum and be able to reattach to a new substratum. No information was found concerning displacement of Chondrus crispus. It seems unlikely that the holdfast could remain in situ for long enough on a rocky shore to re-establish a bond with the substratum. Sharp et al. (1993) noted that, following detachment, Furcellaria lumbricalis plants were capable of reattachment. However, in the light of the likely intolerance of Ahnfeltia plicata and Chondrus crispus, biotope intolerance is assessed as high and there is likely to be a decline in species richness. In the absence of holdfasts for regeneration, recovery is likely to take up to 5 years (see additional information below) so recoverability is recorded as high.

Chemical Factors

Synthetic compound contamination
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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. They also report that red algae are effective indicators of detergent damage since they undergo colour changes when exposed to relatively low concentration of detergent. Smith (1968) reported that 10 ppm of the detergent BP 1002 killed the majority of specimens in 24hrs in toxicity tests, although Ahnfeltia plicata and Chondrus crispus were amongst the algal species least affected by the detergent used to clean up the Torrey Canyon oil spill. 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). Cole et al. (1999) suggested that herbicides, such as simazine and atrazine were very toxic to macrophytes. Hoare & Hiscock (1974) noted that all red algae except Phyllophora sp. were excluded from Amlwch Bay, Anglesey, by acidified halogenated effluent discharge. The evidence suggests that, in general, red algae are very intolerant of synthetic chemicals and biotope intolerance is assessed as high. Loss of the entire population of Furcellaria lumbricalis would limit the recovery of the biotope (see additional information below) and so recoverability is recorded as moderate. Hoare & Hiscock (1974) found that Urticina felina survived fairly near to an acidified halogenated effluent discharge in a transition zone where many other species were unable to survive, suggesting a tolerance to chemical contamination.
Heavy metal contamination
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Bryan (1984) suggested that the general order for heavy metal toxicity in seaweeds is: organic Hg > inorganic Hg > Cu > Ag > Zn > Cd > Pb. Cole et al. (1999) reported that Hg was very toxic to macrophytes. The sub-lethal effects of Hg (organic and inorganic) on the sporelings of an intertidal red algae, Plumaria elegans, were reported by Boney (1971). 100% growth inhibition was caused by 1 ppm Hg. Burdin & Bird (1994) reported that both gametophyte and tetrasporophyte forms of Chondrus crispus accumulated Cu, Cd, Ni, Zn, Mn and Pb when immersed in 0.5 mg/l solutions for 24 hours. No effects were reported however, and no relationship was detected between hydrocolloid characteristics and heavy metal accumulation. Due to the lack of evidence, an intolerance assessment has not been attempted.
Hydrocarbon contamination
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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). Chondrus crispus however, is apparently relatively tolerant of hydrocarbon contamination. 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. 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. However, it was suggested that the development of young stages to adult plants was slow, with biomass still reduced 2 years after the event.
As a group, red algae are apparently highly intolerant of hydrocarbons, so, adopting a precautionary principle, biotope intolerance is assessed as high, but the decision is made with very low confidence. Loss of the entire population of Furcellaria lumbricalis would limit the recovery of the biotope (see additional information below) and seems unlikely. Overall, survival of at least the perennial holdfasts seems likely and so recoverability is recorded as high. One month after the Torrey Canyon oil spill the anemone, Urticina felina, was found to be one of the most resistant animals on the shore, being commonly found alive in pools between the tide-marks which appeared to be devoid of all other animals (Smith, 1968).
Radionuclide contamination
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No information was found concerning the sensitivity of the biotope to radionuclides.
Changes in nutrient levels
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In studies of Chondrus crispus from Prince Edward Island, Canada, Juanes & McLachlan (1992) concluded that primary production was limited by temperature during the autumn to the spring period and by nitrogen availability when production was maximal in the summer. They suggested that growth of Chondrus crispus became nutrient limited at approximately 14°C. Haglund et al. (1987) reported no inhibition of photosynthesis in Ahnfeltia plicata up to 500 µE/m²/s and suggested the species had a high potential for growth provided no other factors were limiting. To a certain degree, therefore, an increase in the level of nutrients would be likely to enhance growth of algae in the biotope
Johansson et al. (1998) suggested that one of the symptoms of large scale eutrophication is the deterioration of benthic algal vegetation in areas not directly affected by land-runoff or a point source of nutrient discharge. Altered depth distributions of algal species caused by decreased light penetration and/or increased sedimentation through higher pelagic production have been reported in the Baltic Sea (Kautsky et al., 1986; Vogt & Schramm, 1991). Johansson et al. (1998) studied changes in the benthic algal community of the Skagerrak coast in the Baltic Sea, an area heavily affected by eutrophication, between 1960 and 1997. They noted the disappearance of the red alga, Polyides rotundus, but commented that problems existed in their sampling method. They also noted the increase of delicate red algae with foliaceous thalli, e.g. Delesseria sanguinea and Phycodrys rubens, and tougher red algae with foliaceous thalli, e.g. Phyllophora sp. Increases in the delicate algae were most pronounced at the more wave exposed sites, while increases in the tougher algae occurred at the more sheltered sites with high sedimentation. They commented that these results suggest that the increase of delicate species with large growth potential may have been caused by eutrophication, but that the effect is counteracted when eutrophication results in high sedimentation, in which case the tougher Phyllophora sp. thrive. Additionally, Chondrus crispus and Furcellaria lumbricalis, both species with tough thalli, decreased at the wave exposed sites, possibly due to competition from the more vigorous Phycodrys rubens and Delesseria sanguinea, but persisted at the sites with high sedimentation. These findings suggest that the dominant red algal turf which characterizes the biotope is likely to decline following increases in nutrient levels and faster growing species are likely to proliferate. Biotope intolerance is therefore assessed as intermediate. As some portion of the populations are likely to remain, recoverability is assessed as high (see additional information below).
Increase in salinity
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MIR.PolAhn typically occurs in areas of 'full' salinity (Connor et al., 1997a) and therefore, increase in salinity is not a relevant factor. Mathieson & Burns (1971) recorded maximum photosynthesis of Chondrus crispus in culture at 24 psu, but rates were comparable at 8, 16 and 32 psu. Photosynthesis continued up to 60 psu. Bird et al. (1979) recorded growth of Canadian Chondrus crispus in culture between 10 and 50 psu, with a maximum at 30 psu. The species would therefore appear to be extremely tolerant of hypersaline conditions Growth experiments in the laboratory revealed that optimum growth of Furcellaria lumbricalis occurred at 20 psu, the species grew well at 10 psu and 30 psu, but that growth declined above 30 psu to negligible levels at 50 psu (Bird et al., 1979). Growth of Furcellaria lumbricalis is therefore likely to be inhibited in hypersaline conditions.
Decrease in salinity
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MIR.PolAhn typically occurs in areas of 'full' salinity (Connor et al., 1997a). The benchmark decrease in salinity would place the biotope in areas of reduced salinity for a week or variable salinity for a year. The algae which characterize the biotope are all found in reduced salinity conditions. Ahnfeltia plicata, for example, occurs over a very wide range of salinities. The species penetrates almost to the innermost part of Hardanger Fjord in Norway where it experiences very low salinity values and large salinity fluctuations due to the influence of snowmelt in spring (Jorde & Klavestad, 1963). Ahnfeltia plicata penetrates further than the euryhaline species, Polyides rotundus, and probably has a similar salinity tolerance to Furcellaria lumbricalis, which is limited only by the 4 psu isohaline (see review by Bird et al., 1991). Chondrus crispus occurs in estuaries in New Hampshire, USA, where surface water salinity varies from 16-32 psu (Mathieson & Burns, 1975). Mathieson & Burns (1971) recorded maximum photosynthesis of Chondrus crispus in culture at 24 psu, but rates were comparable at 8, 16 and 32 psu. The benchmark reduction in salinity is therefore unlikely to cause mortality, but may suppress growth of the algae in the biotope. Intolerance is therefore assessed as low. When salinity returns to original levels, growth should quickly return to normal so recoverability is assessed as very high. Marine species, such as the anemone, Urticina felina, are likely to be most intolerant of decreases in salinity and there may be a minor decline in species richness.
Changes in oxygenation
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Aerobic organisms in the biotope are certain to be intolerant of anoxia to some degree, and it is expected that, at the very least, growth and reproduction would be compromised by the benchmark decrease in oxygen levels. Biotope intolerance is therefore recorded as low. Growth should quickly return to normal levels when normoxia returns so recoverability is recorded as very high.
The effects of reduced oxygenation on algae are not well studied. Plants require oxygen for respiration, but this may be provided by production of oxygen during periods of photosynthesis. Lack of oxygen may impair both respiration and photosynthesis (see review by Vidaver, 1972).

Biological Factors

Introduction of microbial pathogens/parasites
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Craigie & Correa (1996) described 'green spot' disease in Chondrus crispus, caused by the interaction of several biotic agents including fungi, bacteria, algal endophytes and grazers, and resulting in tissue necrosis. Correa & McLachlan (1992) infected Chondrus crispus with the green algal endophytes Acrochaete operculata and Acrochaete heteroclada. Infections resulted in detrimental effects on host performance, including slower growth, reduced carrageenan yield, reduced generation capacity and tissue damage. Stanley (1992) described the fungus Lautita danica being parasitic on cystocarpic Chondrus crispus and Molina (1986) was the first to report Petersenia pollagaster, a fungal invasive pathogen of cultivated Chondrus crispus. Dixon & Irvine (1977) noted that galls, probably formed as a reaction to bacterial infection, were common in older plants of Ahnfeltia plicata. Barton (1901) noted that Furcellaria lumbricalis may become infested with nematode worms and reacts by gall formation. Pathogenic infections have the potential to cause mortality in red algae and so intolerance is assessed as intermediate. As some portion of the population is likely to remain for vegetative regrowth, recovery is likely to occur within 18 months (see additional information below) and recoverability is therefore assessed as high.
Introduction of non-native species
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The brown seaweed Sargassum muticum is a non native species which could potentially invade the biotope.
Extraction
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Ahnfeltia plicata is one of the world's principal commercial agarophytes. It is harvested mainly on the Russian coast of the White Sea as a source of high quality, low sulphate agar (Chapman & Chapman, 1980). In Britain and Ireland, however, Ahnfeltia plicata does not occur in sufficient quantities to harvest on a commercial scale (Dickinson, 1963).
Commercial utilization of Furcellaria lumbricalis is based on the gelling properties of its extracted structural polysaccharide, furcellaran (Bird et al., 1991). Extraction of Furcellaria lumbricalis was reviewed by Guiry & Blunden (1991). Commercial beds of Furcellaria lumbricalis occur in Denmark where the algae are harvested with purpose built trawl nets, whereas in the rest of Europe, the biomass is not sufficient for harvesting. In Denmark, harvesting reached its highest level of 31,000 t p.a. in 1962, but over-exploitation has led to a fall in production and the current harvest is about 10,000 tonnes/ annum. Christensen (1971) (cited in Bird et al., 1991) and Plinski & Florczyk (1984) noted that over-exploitation of Furcellaria lumbricalis has resulted in severe depletion of stocks. A sustainable harvest of Furcellaria lumbricalis occurs in Canada on the shores of the Gulf of St Lawrence where dredging and raking are prohibited and only storm cast plants may be gathered. However, no commercial harvest as yet occurs in Britain or Ireland.
Chondrus crispus is extracted commercially in Ireland, but the harvest has declined since its peak in the early 1960s (Pybus, 1977). The effect of harvesting has been best studied in Canada. Sharp et al. (1986) reported that the first drag rake harvest of the season on a Nova Scotian Chondrus crispus bed removed 11% of the fronds and 40% of the biomass. Efficiency declined as the harvesting season progressed. Chopin et al. (1988) noted that non-drag raked beds of Chondrus crispus in the Gulf of St Lawrence showed greater year round carposporangial reproductive capacity than a drag raked bed.
Commercial exploitation of the red seaweeds which characterize the biotope has the potential to impact the community greatly. Intolerance is therefore assessed as high. Biotope recovery would be limited by the recovery of Furcellaria lumbricalis and is hence recorded as moderate (see additional information below). It should be noted that large scale commercial harvesting in the biotope does not currently occur in Britain or Ireland.

Additional information icon Additional information

Recoverability
The most important factor to consider when assessing the recoverability of the biotope is the recoverability of the important characterizing species.
The life history characteristics of Ahnfeltia plicata suggest that the species is likely to recover within 5 years if local populations exist, but that recovery of remote populations will be more protracted and dependent upon hydrodynamic regime.
Recovery of a population of Chondrus crispus following a perturbation is likely to be largely dependent on whether holdfasts remain, from which new thalli can regenerate (Holt et al., 1995). Following experimental harvesting by drag raking in New Hampshire, USA, populations recovered to 1/3 of their original biomass after 6 months and totally recovered after 12 months (Mathieson & Burns, 1975). Raking is designed to remove the large fronds but leave the small upright shoots and holdfasts. The authors suggested that control levels of biomass and reproductive capacity are probably re-established after 18 months of regrowth. It was noted however, that time to recovery was much extended if harvesting occurred in the winter, rather than the spring or summer (Mathieson & Burns, 1975). Minchinton et al. (1997) documented the recovery of Chondrus crispus after a rocky shore in Nova Scotia, Canada, was totally denuded by an ice scouring event. Initial recolonization was dominated by diatoms and ephemeral macroalgae, followed by fucoids and then perennial red seaweeds. After 2 years, Chondrus crispus had re-established approximately 50% cover on the lower shore and after 5 years it was the dominant macroalga at this height, with approximately 100% cover. The authors pointed out that although Chondrus crispus was a poor colonizer, it was the best competitor. Therefore, recovery by Chondrus crispus will be relatively rapid (approximately 18 months) in situations where intolerance to a factor is intermediate and some holdfasts remain for regeneration of fronds. In situations of high intolerance, where the entire population of Chondrus crispus is removed, recovery will be limited by recruitment from a remote population and would be likely to take up to 5 years.
Furcellaria lumbricalis is highly fecund, an average sized gametophyte being able to produce approximately 1 million carpospores, or a tetrasporophyte, up to 2 million tetraspores (Austin, 1960a). However, the species grows very slowly compared to other red algae (Bird et al., 1979) and takes a long time to reach maturity. For example, Austin (1960b) reported that in Wales, Furcellaria lumbricalis typically takes 5 years to attain fertility. Christensen (1971) (cited in Bird et al., 1991) noted that following harvesting of Furcellaria lumbricalis forma aegagropila in the Baltic Sea, harvestable biomass had not been regained 5 years after the suspension of harvesting. In view of its slow growth, time to maturity and limited dispersal, recovery of Furcellaria lumbricalis is likely to take between 5 and 10 years in situations where intolerance to a factor is high. Where a portion of the population remains for vegetative regrowth, recovery is likely to occur within 5 years.
The anemone, Urticina felina, has poor powers of recoverability due to poor dispersal (Sole-Cava et al., 1994 for the similar Tealia crassicornis) and slow growth (Chia & Spaulding, 1972), though populations should recover within 5 years.

This review can be cited as follows:

Rayment, W.J. 2002. Polyides rotundus, Ahnfeltia plicata and Chondrus crispus on sand-covered infralittoral rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 18/12/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=222&code=1997>