Biodiversity & Conservation

IR.MIR.SedK.HalXK

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Removal of the substratum will result in removal of the entire community with the exception of mobile fish, which can probably avoid the factor. Therefore, an intolerance of high has been recorded. Recoverability has been assessed as high, although species diversity, especially epifauna may take longer to recover.
Smothering
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Halidrys siliquosa and laminarians are large and unlikely to be smothered by 5cm of sediment (see benchmark). Similarly, erect turf forming red and brown algae, e.g. Furcellaria lumbricalis, Ahnfeltia plicata, Chondrus crispus, Dilsea carnosa, Dictyota dichotoma and Delesseria sanguinea are probably large enough to be unaffected. For example, Ahnfeltia plicata and Furcellaria lumbricalis are tolerant of sand cover (Dixon & Irvine, 1977). However, smaller or low lying algae may be adversely affected. Algal spores and propagules are adversely affected by a layer of sediment, which can exclude up to 98% of light (Vadas et al., 1992), although the germlings of Halidrys siliquosa can survive darkness for up to 120 days. Germlings and juveniles are likely to be highly intolerant of smothering and any associated scour. A layer of sediment is likely to interfere with settlement and attachment of spores, especially if smothering occurred during winter reproductive maxima for the dominant species. Therefore, it is likely that while adult plants of most species will survive, smaller species and overall recruitment in the community may be adversely affected. Therefore, an intolerance of intermediate has been recorded. Algal recruitment within the community is likely to be rapid, so a recoverability of high has been recorded.
Increase in suspended sediment
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Increased suspended sediment levels will increase turbidity (see below). This biotope is exposed to sediment abrasion and, therefore, characterized by species tolerant of siltation and sediment scour. Most species within the biotope are, therefore, probably tolerant. For example, Johansson et al. (1998) reported that Furcellaria lumbricalis persisted in areas of the Baltic Sea where eutrophication resulted in high sediment loads. However, algal propagules and germlings are probably more intolerant (Vadas et al., 1992).
Adult Saccharina latissima (studied as Laminaria saccharina) plants appear to tolerate silt because they are found in areas of siltation (Birkett et al., 1998b), but they cannot tolerate heavy sand scour and the gametophytes and spores are probably more intolerant. An increase in the level of suspended sediment was found to reduce the growth rate of Saccharina latissima (studied as Laminaria saccharina) by 20% (Lyngby & Mortensen, 1996) and Norton (1978) found that siltation of settled spores inhibited development of gametophytes and spores failed to form an attachment when settling out on silty surfaces.
Overall, therefore most members of the community would probably survive increased suspended sediment levels, whereas a few species, most notably Saccharina latissima may be adversely affect, although in a months duration (see benchmark) probably not destroyed. Therefore, an intolerance of low has been recorded. Increased suspended sediment may interfere with suspension feeding apparatus of several epiphytic or sessile invertebrates, resulting in a reduction in species richness. Recoverability has been recorded as very high (see additional information below).
Decrease in suspended sediment
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Decreased suspended sediment levels will decrease turbidity (see below). This biotope is exposed to sediment abrasion and, therefore, characterized by species tolerant of siltation and sediment scour. A decrease in suspended sediment and hence scour may allow other species to invade the biotope, for example, laminarians. This biotope is similar to MIR.XKScrR, which suffers less scour and is characterized by lower abundance of Halidrys siliquosa but higher abundance of Saccharina latissima and Laminaria hyperborea. Long term decreases would probably result in an increase in laminarian abundance, eventually out-competing Halidrys siliquosa and the biotopes replacement by MIR.XKScrR or other laminarian dominated biotopes. The biotope is probably highly intolerant of changes in suspended sediment in the long term. However, a decrease in suspended sediment for a month (see benchmark) is likely to have little adverse effect and an intolerance of low has been recorded with a recoverability of very high.
Desiccation
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Macroalgal species that occur in the intertidal and subtidal are likely to be more tolerant of desiccation than species that occur in the subtidal. Spores and germlings are likely to be especially intolerant. For example, the germlings of the pool dwelling or subtidal red algae were less tolerant of desiccation than intertidal red algae (Kain & Norton, 1990). Subtidal species such as Furcellaria lumbricalis and Delesseria sanguinea are probably highly intolerant of desiccation (see reviews). Similarly, Saccharina latissima is probably highly intolerant of desiccation. Halidrys siliquosa may occur in the sublittoral fringe, exposed to the air at spring low tides and Chondrus crispus occurs in the low intertidal and can probably tolerate desiccation at the benchmark level (see benchmark). Several epiphytic species, especially the hydroids may be intolerant of aerial exposure.
However, this biotope occurs from 0 -20m depth (Connor et al.,1997) so that only the upper extent of the population is likely to be affected by desiccation, and only a proportion of the species in the community are likely to be affected at the benchmark level, therefore, an intolerance of intermediate has been recorded. Recoverably has been assessed as high (see additional information below).
Increase in emergence regime
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An increase in emergence will increase exposure of the biotope to air and hence desiccation (see above). Therefore, the upper extent of several species within the biotope, most notably Halidrys siliquosa, Furcellaria lumbricalis and Saccharina latissima and hence the upper extent of the biotope is likely to be reduced. Therefore, an intolerance of intermediate has been recorded. Recoverability has been assessed as high (see additional information below).
Decrease in emergence regime
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A decrease in emergence may allow the biotope to extent its range up the shore and out-complete other species adapted to higher levels of desiccation. Therefore, a rank of 'not sensitive*' has been recorded.
Increase in water flow rate
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Halidrys siliquosa communities were reported from the 'rapids approaches' in association with Himanthalia elongata and Saccharina latissima (studied as Laminaria saccharina), and may occur in association with Laminaria digitata in strongly flowing tidal streams (Lewis, 1964). Halidrys siliquosa decreases in abundance with increasing water flow, so that in tidal rapids with current speeds of 2-3m/sec (ca 6 knots), it is replaced by Laminaria digitata, Laminaria hyperborea and Saccorhiza polyschides communities (Lewis, 1964; Schwenke, 1971). The tolerance of red algae to water flow varies with species, so that some species may be lost, however, the understorey of red algae will probably survive but with an altered species composition.
This biotope is found in weak to moderately strong tidal streams (Connor et al., 1997a). An increase from moderately strong to very strong will probably result in loss of Halidrys siliquosa and its replacement as the dominant canopy algae by Laminaria hyperborea or Laminaria digitata (Lewis, 1964) resulting in loss of the biotope. Therefore, an intolerance of high has been recorded. Recoverability has been assessed as high (see additional information below).
Decrease in water flow rate
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This biotope occurs from moderately strong to weak tidal streams (Connor et al., 1997). Therefore, the biotope is tolerant of weak tidal flows. However, a further decrease to negligible water flow may result in stagnant conditions and increased siltation of fine sediments. Macroalgae are dependant on water flow to maintain a supply of nutrients and to remove waste products. Stagnant or negligible flow may be detrimental to some species, e.g. Chondrus crispus and Ahnfeltia plicata, whereas others are able to tolerate very weak or negligible water flow, e.g. Delesseria sanguinea and Furcellaria lumbricalis. In addition, passive suspension feeders may not be able to obtain adequate food while the suspension feeding apparatus of other species may be clogged by increased siltation (see above). Loss of suspension feeding epiphytes would result in a decrease in species richness. Many of the associated animals are likely to be lost. Overall, it is unlikely that the biotope will survive and an intolerance of high has been recorded. Recoverability has been assessed as high.
Increase in temperature
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Temperature affects photosynthetic rates, photosynthetic saturation points and growth in macroalgae, and may also show seasonal adaptation, with tolerance to high temperatures being lower in winter than summer in some species (e.g. laminarians), and photosynthetic rates of some red algae higher at low temperatures in winter or at high temperatures in summer (see Lüning, 1984, 1990 and Kain & Norton, 1990 for reviews). Refer to individual species reviews for details of temperature tolerance.
Overall, the majority of macroalgal species found in the biotope are widely distributed in British waters, and many are found further south. Some species, e.g. Chondrus crispus occurs in the lower intertidal, exposed to a wider range of temperatures than in the subtidal, while Halidrys siliquosa and Chondrus crispus also occur in rock pools that are potentially exposed to high temperatures in sunlight at low tide. Therefore, the biotope will probably be little affected by long term changes in temperature in British waters, and Halidrys siliquosa and other species that are also found in the intertidal are probably tolerant of acute temperature change at the benchmark level. For example Chondrus crispus did not suffer adverse effects as a result of an 4.8 -8.5 °C increase in temperature above average during the hot summer of 1983 (Hawkins & Hartnoll, 1985). However, to represent the physiological effects of temperature on growth and reproduction an intolerance of low has been recorded.
Decrease in temperature
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Temperature affects photosynthetic rates, photosynthetic saturation points and growth in macroalgae, and shows seasonal adaptation with photosynthetic rates of some red algae higher at low temperatures in winter or at high temperatures in summer (see Lüning, 1984, 1990 and Kain & Norton, 1990 for reviews). Refer to individual species reviews for details of temperature tolerance.
Overall, the majority of macroalgal species found in the biotope are widely distributed in British waters, and many are found in northern Norway or within the Arctic circle. Some species, e.g. Chondrus crispus occurs in the lower intertidal, exposed to a wider range of temperatures than in the subtidal, while Halidrys siliquosa and Chondrus crispus also occur inn rock pools that are potentially exposed to low temperatures at low tide. For example, Furcellaria lumbricalis tolerated -5 C for 3 months with no mortality and Bird et al. (1979) concluded that growth would not be inhibited at 0 C. Pearson & Davison (1993) recorded that Chondrus crispus froze at -7.59C when cooled slowly from 5 C and froze at -3.7 C when cooled rapidly. Therefore, the biotope will probably be little affected by long term changes in temperature in British waters, and Halidrys siliquosa and other species that are also found in the intertidal are probably tolerant of acute temperature change at the benchmark level. However, to represent the physiological effects of temperature on growth an intolerance of low has been recorded.
Increase in turbidity
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Increased turbidity reduces the light available for photosynthesis and hence growth and reproduction in macroalgae. For example, Saccharina latissima (studied as Laminaria saccharina) was shown to have a critical light requirement for gametophyte fertilization, and show a restricted distribution on the northeast coast of England in areas affected by light attenuating pollution (Fletcher, 1996). Understorey algae, especially red algae are shade tolerant. Birkett et al. (1998b) suggested that the reduced light under kelp canopies and, by inference, large macroalgae canopies, allowed red algae to colonize shallower waters. Some red algae, such as Delesseria sanguinea and Furcellaria lumbricalis tolerate turbid waters; Furcellaria lumbricalis being growth saturated at very low light levels. Similarly, Phyllophora truncata, Phycodrys rubens and Polysiphonia nigrescens apparently widely replaced Fucus spp. communities below 2m in the Kiel Bight, presumably due to increased turbidity (Fletcher, 1996).
The biotope occurs in shallow depths but Halidrys siliquosa often occurs as a usually dominant species deeper than the kelp forest suggesting tolerance of low light levels. While red algae are more tolerant, the species composition may change, favouring the most tolerant species, e.g. Furcellaria lumbricalis, however, some less tolerant algae may be lost. Therefore, an intolerance of intermediate has been recorded to represent a reduction in the downward extent of the biotope. Recoverability has been assessed as high (see additional information below).
Decrease in turbidity
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Decreased turbidity increases the light available for photosynthesis and potentially increases growth rates of macroalgae. Halidrys siliquosa and sublittoral fringe algae are probably tolerant of high light levels and would probably benefit form increased light, allowing the biotope to extent its range to shallower water where possible. Understorey red algae may be subject to increased competition from shallow water algae, so that the species composition may change, however, the understorey layer will survive. Therefore, the biotope may extend its range and a rank of 'not sensitive*' has been recorded.
Increase in wave exposure
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This biotope occurs in moderate to low wave exposure. An increase in wave exposure at the benchmark level may expose the biotope to wave exposed or very wave exposed conditions. Halidrys siliquosa develops as a short, stunted turf in wave exposed pools (Moss & Lacey, 1963; Lewis, 1964) and Lewis, (1964) suggested it could tolerate strong water movement. However, the stunted form does not occur in this biotope. Saccharina latissima is highly intolerant of wave exposure. However, with increasing wave exposure Halidrys siliquosa / Saccharina latissima communities are replaced by Laminaria digitata or Laminaria hyperborea communities (Lewis, 1964). Strong 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). Increased wave action is likely to turn and move boulders and cobbles within the biotope, removing macroalgae and some sessile invertebrates. Therefore, the biotope is likely to be lost and an intolerance of high has been recorded. After a period of a year (see benchmark) the biotope is likely to recover from the remaining plants remnants and attached holdfasts, and a rank if high has been recorded.
Decrease in wave exposure
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The biotope occurs in very wave sheltered situations and it is tidal flow that is important and it is likely that the biotope will survive a reduction in wave exposure. Therefore, a not sensitive has been recorded.
Noise
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Mobile fish species may be driven away by underwater noise, especially during their mating season. However, few other species within the biotope are likely to perceive noise. Therefore, a rank of not sensitive has been recorded.
Visual Presence
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Mobile fish species may be driven away by shading, especially during their mating season. However, few other species within the biotope are likely to perceive visual presence. Therefore, a rank of not sensitive has been recorded.
Abrasion & physical disturbance
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This biotope is characterized by species tolerant of sediment abrasion, suggesting a tolerance of abrasion. However, physical disturbance by, e.g., an anchor or mobile fishing gear is likely to damage fronds and may remove some individuals, especially large macroalgae such as Halidrys siliquosa and Saccharina latissima. Therefore, an intolerance of intermediate has been recorded. Loss of the distal parts of the plants may entail loss of the epiphytes, resulting in loss of species richness. Recovery may be rapid, especially where the holdfasts or encrusting forms of species remain (e.g. Chondrus crispus or Ahnfeltia plicata) and has been assessed as high. Large scale physical disturbance, such as dredging, will have an impact similar to substratum removal (see above).
Displacement
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Macroalgae are not capable of reattaching their holdfasts if removed. Plants may survive as drifting algae, e.g. Halidrys siliquosa, however many may be washed ashore. Therefore, an intolerance of high has been recorded. Recovery has been assessed as high (see additional information below).

Chemical Factors

Synthetic compound contamination
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Fucoids, are generally quite robust in terms of chemical pollution (Holt et al., 1995, 1997), e.g. Fucus sp. seems to thrive in TBT-polluted waters (Bryan & Gibbs, 1991). However, Rosemarin et al. (1994) stated that brown algae (Phaeophycota) were extraordinarily intolerant of chlorate, such as from pulp mill or brine electrolysis effluents (Holt et al., 1997). 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 reported 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, including Plocamium cartilagineum, 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 in urban or agricultural runoff, 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. Crustacean members of the fauna (mesoherbivores) are likely to be intolerant of pesticides, such as ivermecten, dichlorvos and synthetic pythrethroids (Cole et al., 1999), the exact toxicity varying with location (concentration) and species. Ascidian larval stages were reported to be intolerant of TBT (Mansueto et al., 1993 cited in Rees et al., 2001). Rees et al. (1999; 2001) reported that the epifauna of the inner Crouch estuary had largely recovered within 5 years (1987-1992) after the ban on the use of TBT on small boats in 1987. Increases in the abundance of Ascidiella sp. and Ciona intestinalis were especially noted.
Overall, the brown algae may be relatively robust, e.g. Halidrys siliquosa, to many but not all forms of synthetic chemical pollution, while the red algae and some fauna are probably particularly sensitive. Therefore, a proportion of the community is likely to be lost and an intolerance of intermediate has been recorded, although species richness may decline markedly. Recovery has been assessed as high (see additional information below).
Heavy metal contamination
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Holt et al., (1995, 1997) reported that fucoids and other algae were capable of retaining and concentrating heavy metals, so much so that Fucus spp. are used as indicators of heavy metal pollution. Alginates found in fucoids (and in Halidrys siliquosa) strip heavy metals and some radionuclides from seawater and store them in inert forms. Hence, adult plants are considered to be relatively tolerant of heavy metal contamination. However, younger stages may be more intolerant. For example iron ore dust interfered with the interaction between eggs and sperm in Fucus serratus (Boney, 1980; cited in Bryan, 1984). Bryan (1984) also reported that heavy metals retarded growth in brown algae and 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. Heavy metals have been shown to effects on sporophyte development, growth and respiration in Laminaria hyperborea (Hopkin & Kain, 1978) and in Laminaria digitata (Axelsson & Axelsson, 1987).
Cole et al. (1999) suggested that Cd was very toxic to Crustacea (amphipods, isopods, shrimp, mysids and crabs), and Hg, Cd, Pb, Cr, Zn, Cu, Ni, and As were very toxic to fish. Bryan (1984) reported sublethal effects of heavy metals in crustaceans at low (ppb) levels. Bryan (1984) suggested that polychaetes are fairly resistant to heavy metals, based on the species studied. Short term toxicity in polychaetes was highest to Hg, Cu and Ag, declined with Al, Cr, Zn and Pb whereas Cd, Ni, Co and Se were the least toxic. However, he suggested that gastropods were relatively tolerant of heavy metal pollution. Overall, there is little information specific to the species present in this biotope. Halidrys siliquosa is probably of low intolerance to heavy metals due to the presence of alginates, whereas laminarians may be more intolerant. Therefore, an intolerance of low has been recorded, albeit at very low confidence.
Hydrocarbon contamination
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This biotope is protected from the direct effects of oil spills due to its subtidal habit, although it may be exposed to water soluble components of the oil or oil adsorbed on to particulates. No information concerning the effects of oil on Halidrys siliquosa was found. However, Holt et al. (1997) suggested that other Fucales, Fucus sp. had limited intolerance to oil but noted that studies on long-term exposure were limited. Saccharina latissima (studied as Laminaria saccharina) was observed to show no discernible effects from oil spills, largely due to poor dispersion into the water column and high levels of dilution (Holt et al., 1995).
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, including Delesseria sanguinea and Plocamium cartilagineum, 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). Long term effects 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 g/l and 129.4 g/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. O'Brien & Dixon (1976) also noted that hydrocarbon exposure reduced photosynthesis in algae.
Oil spills and hydrocarbon exposure in the intertidal results in loss of gastropod or crustacean grazers (Southward, 1982; Suchanek, 1993). Loss of grazers may allow development of more ephemeral green algae and a change in the algal community. However, although Bokn et al., (1993) could not demonstrate direct effects of chronic hydrocarbon contamination in their mesocosms, they concluded that chronic effects of oil on Littorina littorea and perhaps other herbivores may require more than 2 years to develop.
Overall, while the dominant brown algae is probably of low intolerance to hydrocarbon contamination, most red algae are probably highly intolerant. In addition, crustacean and gastropod grazers may be lost reducing species richness. Therefore, an intolerance of intermediate has been recorded to represent loss of a proportion of the community and probable changes in the algal composition. Recoverability has been assessed as high (see additional information).
Radionuclide contamination
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No information found.
Changes in nutrient levels
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Macroalgae are probably nutrient, particularly nitrogen, limited during summer or high temperatures. Nutrients are generally abundant in the winter months in temperate climates. Slow growing species, such as Furcellaria lumbricalis and species that store nutrients in winter for growth in summer, such as Delesseria sanguinea and laminarians, are probably nutrient limited. However, moderate nutrient enrichment may stimulate macroalgal growth, e.g. Halidrys dioica and other algae exposed to 10% untreated sewage effluent in the field, resulted in increased gross productivity (Kindig & Littler, 1980).

Increased nutrient enrichment and eutrophication can also result in increased sedimentation and turbidity (see above) due to increased suspended sediment and/or increased phytoplankton productivity. Studies of changes in the benthic algal community of the Skagerrak coast in the Baltic Sea, an area heavily affected by eutrophication, between 1960 and 1997, noted the disappearance of the red alga, Polyides rotundus, 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. (Johansson et al., 1998). 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.

Eutrophication also results in an increase in opportunistic, fast growing, ephemeral green algae (e.g. Ulva, spp.) and some brown algae (e.g. Ectocarpus spp.) at the expense of fleshy and/or perennial red algae resulting in dominance by relatively few algae and hence a reduction in species richness (see Fletcher, 1996 for review). Localities characterized by excess loading of nutrients exhibit a general reduction in the diversity and occurrence of brown and red algae and a corresponding increase in green algae, such as Ulva sp. (Fletcher, 1996). Epiphytic algae growing on Halidrys siliquosa may also increase in abundance resulting in smothering of their host algae.
Overall, while moderate nutrient enrichment may be beneficial, the above evidence suggests that eutrophication could result in marked changes in the macroalgae and the associated community, and in extreme cases potentially resulting in loss of the biotope. Therefore, an intolerance of intermediate has been recorded. Recoverability has been assessed as high (see below).
Increase in salinity
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This subtidal biotope is unlikely to be exposed to hypersaline conditions or effluents.
Decrease in salinity
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Reduced salinity affects rates of photosynthesis and respiration and influences temperature tolerance in macroalgae, depending on species. Saccharina latissima (studied as Laminaria saccharina) was the most tolerant of the laminarians, surviving down to 17psu, although its growth was severely retarded at 16 psu and plants did not survive below 8 psu (Kain, 1979). Halidrys siliquosa occurs in rock pools exposed to rainfall and is probably tolerant of short term reductions in salinity.
Red algae vary in their ability to tolerant low salinities, e.g. Chondrus crispus grows optimally at 25-40psu but did not grow at 10psu (Kain & Norton, 1990). Furcellaria lumbricalis forms extensive populations in the main basin of the Baltic Sea where salinity is 6-8 psu in the upper 60-70 m and its extension into the Gulfs of Bothnia and Finland is limited by the 4 psu isohaline (see review by Bird et al., 1991). Rietema (1993) examined ecotypic differences between North Sea and Baltic populations of Delesseria sanguinea. Optimal growth occurred in Baltic specimens at 19 -23 psu and North Sea specimens at 33 psu. North Sea specimens died at 7.5 - 11 psu. Ahnfeltia plicata 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).
However, demographic evidence suggests that number of species of red algae declines with decreasing salinity (sooner than brown or green algae), with a marked decline below 20 psu (Kain & Norton, 1990).
Botryllus schlosseri lives in enclosed waters including docks and in estuaries where salinity is variable. However, its absence from low salinity conditions in upper estuaries and lagoons suggests that colonies will be intolerant of low salinities. Gastropods that extend their range into the intertidal are probably tolerant of reduced salinities e.g. Lacuna vincta is found in a range of salinities and has been recorded in salinities as low as 12-13 psu. However, gastropods that are primarily subtidal (e.g. Helcion pellucidum and Tectura spp.) probably have a more limited tolerance of low salinities and may be lost from the biotope. Overall, the dominant macroalgal species within this biotope would probably survive exposure to variable salinity in the long term or reduced salinity in the short term (see benchmark). However, several species of red algae in particular may be lost as a result of a long term reduction in salinity. Similarly, a reduction in faunal diversity is associated with reduced salinity, so that a marked reduction in overall species richness may occur. The biotope will, however, probably survive. Therefore, an intolerance of low has been recorded. Recoverably has been assessed as very high, although species richness may take longer to return.
Changes in oxygenation
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The effects of reduced oxygen levels of plants has been little studied. Reduced oxygen concentrations inhibit both photosynthesis and respiration (see review by Vidaver, 1972). The effects of decreased oxygen concentration equivalent of the benchmark would be greatest during dark when the macroalgae are dependant on respiration. A study of the effects of anoxia on Delesseria sanguinea revealed that specimens died after 24 hours at 15C but that some survived at 5C (Hammer, 1972). However, no other information was found.

Biological Factors

Introduction of microbial pathogens/parasites
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Halidrys siliquosa supports a number of epiphytic species, which use it as a substratum but are not parasitic on the plant. Gall formation may occur in response to bacterial or nematode infection in Ahnfeltia plicata and Furcellaria lumbricalis respectively. Growth rates of Saccharina latissima may be reduced by Streblonema disease. Growth and reproduction of Chondrus crispus may be reduced by fungal infections, epiphytic algae and bacteria. Little other information was found regarding diseases in macroalgae, and their effects on the biotope as a whole are difficult to assess. However, given the potential reduction in growth and reproduction due to disease an intolerance of low has been recorded, albeit at low confidence. Recoverably is probably very high (see additional information below).
Introduction of non-native species
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Halidrys siliquosa has been reported to be displaced as the dominant species in rock pools by the non-native Sargassum muticum on the south coast of England (Eno et al., 1997). Staehr et al. (2000) reported that an increase in the abundance of Sargassum muticum in Limfjorden, Denmark had resulted in a significant decline of the cover of large brown algae, especially Saccharina latissima (studied as Laminaria saccharina), Halidrys siliquosa, Codium fragile and Fucus vesiculosus.
It seems that Sargassum muticum occurs in similar locations to Halidrys siliquosa and even attracts similar epibiota. However, although it may be more vigorous than Halidrys siliquosa it dies back in winter whereas Halidrys siliquosa persists. Although, Halidrys siliquosa plants are likely to remain, Sargassum muticum appears to able to significantly reduce the extent of Halidrys siliquosa and other algae, particularly in shallow waters. Therefore, an intolerance of intermediate has been recorded. Recoverability has been assessed as high (see additional information below).
Extraction
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No evidence of the extraction or harvesting of Halidrys siliquosa was found. Svendsen (1972; summary only) reported that Halidrys siliquosa became one of the dominant macroalgae, 3 years after kelp harvesting in Norway. This suggests that removal of other algae species that compete with Halidrys siliquosa for space and light would be beneficial.

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 t p.a. 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, through changes in community structure and physical disturbance of the other species present.

On balance, intolerance has been assessed as intermediate because even though the important characterizing species (Halidrys siliquosa) may benefit from the loss of other species, the other species may experience a decline. Recoverability has been assessed as high (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
Halidrys dioica was shown to recruit to cleared areas within 3-4 months in the absence of sea urchins on the California coast (Sousa et al., 1981). Similarly, Halidrys siliquosa became a dominant algae in 3 years after the removal of kelps in Norway (summary only, Svendsen, 1972). Several fucoids have been shown to recolonize cleared areas readily, especially in the absence of grazers (Holt et al., 1995, 1997). For example, Fucus dominated areas may take 1-3 years to recolonize in British waters (Holt et al., 1995).

Kain (1975) reported that Saccharina latissima (studied as Laminaria saccharina) and Saccorhiza polyschides were initial colonizers, colonizing of cleared blocks in the shallow subtidal within 25-28 weeks (ca 6 months). Saccorhiza polyschides colonized within the winter months only whereas Saccharina latissima recruited throughout the year (Kain, 1975).

Delesseria sanguinea was shown to colonize cleared blocks within 56-59 days or 41 weeks (ca 10 months) depending on depth and time of year. Similarly, Chondrus crispus recovered prior biomass after its substratum was denuded by ice scour within 5 years, and if holdfasts remained was able to recover cover within 18 months. However, Furcellaria lumbricalis is slow growing, takes 5 year to reach maturity and has limited dispersal and would probably take between 5 and 10 years to recover.

Overall, macroalgal recruitment, and hence recovery is likely to be good within cleared areas in the proximity of reproductive parent plants, depending on season and species. The limited evidence suggests that Halidrys siliquosa may recover within 5 years. Similarly, most red algal species appear to be capable of rapid recovery, probably less than 5 years, with the exception of Furcellaria lumbricalis. Epiphytic species are also widespread and ubiquitous so that, although their dispersal is limited, they are likely to recruit quickly. However, the most luxuriant epiphytic growth occur on old plants (e.g. Halidrys siliquosa) may take many years to recover, perhaps up to 10 years.

It is likely that some sort of succession would occur in colonization of bare substratum, with establishment of the balance of species in the biotope taking several years to establish. Nevertheless, it is likely that the biotope will re-establish within 5 years. Isolated population may take longer to recover their macroalgal cover, due to the poor dispersal capabilities of most fucoids or red algae, again dependant of hydrography.

This review can be cited as follows:

Tyler-Walters, H. 2002. Halidrys siliquosa and mixed kelps on tide-swept infralittoral rock with coarse sediment.. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 21/09/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=258&code=1997>