Biodiversity & Conservation

Dead maerl beds

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Loss of the substratum (dead maerl) e.g. by extraction, channelization etc., would result in loss of the entire habitat and its associated community. Deep maerl beds are several thousands of years old and dead maerl cannot be replenished (effectively a non-renewable resource) so that recovery of a maerl bed is unlikely to occur naturally. Therefore, a high intolerance and ‘none’ recovery are recorded. Sensitivity is likely to be very high.
Smothering
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Smothering results from the rapid deposition of sediment or spoil, which may occur after dredging (suction or scallop), capital dredging (channelization), extreme runoff, spoil dumping etc. The effects depend on the nature of the smothering sediment. For example, live maerl were found to survive burial in coarse sediment (Wilson et al., 2004) but to die in fine sediments. In addition, detrimental effects on Fucus embryos were reported in fine sediments, presumably as fine sediment restricts water flow. Similarly, fine sediment is likely to prevent settlement of algal propagules, so that the effects are potentially greater during their settlement period. Kranz (1972; cited in Maurer et al. (1986)) reported that shallow burying siphonate suspension feeders are typically able to escape smothering with 10-50 cm of their native sediment and relocate to their preferred depth by burrowing. Dow & Wallace (1961) noted that large mortalities in clam beds resulted from smothering by blankets of algae (Ulva sp.) or mussels (Mytilus edulis). In addition, clam beds have been lost due to smothering by 6 cm of sawdust, thin layers of eroded clay material, and shifting sand (moved by water flow or storms) in the intertidal.

Smothering by 5 cm of sediment (the benchmark) is likely to clog or reduce water flow through the surface of the bed, and directly smother small non-mobile members of the epifauna and epiflora, while larger species e.g. sea squirts, anemones, some sponges and macroalgae would protrude above the smothering sediment. Mobile small burrowing species (e.g. amphipods and polychaetes) would probably burrow to safety. However non-motile epifauna (e.g. encrusting bryozoans and small hydroids) and small or prostrate algal will probably be reduced in abundance. Deep burrowing bivalves may experience some mortality due to loss of water flow through the bed, deoxygenating and lack of food depending on their depth. But large burrowing anemones and mud shrimp would probably just burrow through the smothering material. Overall, a proportion of the community may be lost and an intolerance of intermediate is suggested. Recoverability is probably high.

Increase in suspended sediment
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Increased suspended sediment levels will increase turbidity (see below), scour and siltation. Scour induces high mortality in early post settlement algal stages and prevents the settlement of propagules owing to accumulation of silt on the substratum (Vadas et al., 1992). But, increased particulates may provide additional food for filter feeders. However, an increase in suspended sediment may increase the fines within the bed, decreasing water flow and oxygenation through the bed, and hence the depth of the surface epifauna. It may result in an increase in burrowing species compared to filter feeding species. However, De Grave (1999) noted that sedimentary heterogeneity within maerl beds (including maerl debris with mud, sand or gravel) resulted in only minor changes in the community of amphipods and crustaceans present. Overall, a proportion of the epifauna and epiflora may be reduced and an intolerance of ‘intermediate’ is suggested. Recovery is likely to be high.
Decrease in suspended sediment
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Maerl beds occur in strong currents in bays and inlets. A further decrease in suspended sediment levels is unlikely.
Desiccation
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In the UK, maerl beds do not occur in the intertidal, as maerl itself is highly sensitive to desiccation (Wilson et al., 2004). Therefore, it is very unlikely that a maerl bed would be exposed at low water as a result of human activities or natural events.
Increase in emergence regime
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In the UK, maerl beds do not occur in the intertidal, as maerl itself is highly sensitive to desiccation (Wilson et al., 2004). Therefore, it is very unlikely that a maerl bed would be exposed at low water as a result of human activities or natural events.
Decrease in emergence regime
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In the UK, maerl beds do not occur in the intertidal, as maerl itself is highly sensitive to desiccation (Wilson et al., 2004). Therefore, it is very unlikely that a maerl bed would be exposed at low water as a result of human activities or natural events.
Increase in water flow rate
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Maerl beds are restricted to areas of strong tidal currents or wave oscillation (Birkett et al. 1998). For example, Birkett et al. (1998) quote a flow rate of 10 cm/s across the maerl bed at spring tides in Greatman’s Bay, Galway, while the UK Biotope classification (Connor et al., 2004) reports maerl beds occurring at sites with between moderately strong to very weak tidal streams. As Birkett et al. (1998) note, local topography and wave generated oscillation probably result in stronger local currents at the position of the bed.

An increase in water flow from moderately strong to very strong is likely to modify the substratum, removing fines and potentially mobilizing the surface of the bed, perhaps even resulting in winnowing away of the bed. Stronger water flow may favour filter feeders and suspension feeders but adversely affect the deposit or surface deposit feeders. Mobilization of the maerl bed surface is also likely to result in a reduced sessile epifauna and epiflora (macroalgae, sponges, and sea anemones). Overall, the community composition is likely to change but probably remain characteristic of maerl beds. Therefore, an intolerance of intermediate is suggested with a recoverability of high.

Decrease in water flow rate
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Maerl beds are restricted to areas of strong tidal currents or wave oscillation (Birkett et al. 1998). A decrease in water flow is likely to be extremely detrimental to the maerl community. The resultant increase in siltation and deposition of fines is likely to significantly reduce the epiflora, and change the epifaunal community in favour of deposit feeders, with the loss of surface filter feeders, especially passive suspension feeders. Fines would fill the open structure of the bed, restricting the depth to with much of the deep burrowing fauna can live, except normally deep burrowing mud shrimp, and large bivalves (e.g. Mya sp.). For example, Neopentadactyla mixta probably only survives at depth in maerl/gravel beds due to the good oxygenation, and would probably be lost. Overall, the diverse maerl bed community would probably be replaced by a mud and mixed sediment community. Therefore, an intolerance of high has been suggested, while recovery is probably high once water flow returns to its moderately strong or higher
Increase in temperature
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Maerl beds in the north east Atlantic range from Norway to the African coast, although the component maerl species vary in temperature tolerance (Birkett et al., 1998, Wilson et al., 2004). Similarly, the associated communities occur from Shetland to the Isles of Scilly, and represent a diverse sample of species within the local area. Therefore, long-term temperature change may cause a shift in the associated community to more northern or more southern species but the overall community is likely to remain. Short term acute changes (e.g. from thermal discharges) could potentially affect the surface of the bed as it has an open structure, while the deeper species will probably be unaffected. However, many of the species that occur in Scottish waters are also recorded from southern maerl beds, and have a wide geographic range. While subtidal algae are probably intolerant of acute temperature change, a three day exposure (the benchmark) is unlikely to result in death but will adversely affect photosynthesis and growth. Mobile epifauna could also avoid temperature change by retreating further into the bed. However, larval stages of bivalves and other invertebrates are likely to be more sensitive and thermal discharges could adversely affect recruitment depending on the time of year. Overall, an increase in temperature is unlikely to significantly affect the community. Therefore, an intolerance of low is suggested with a recoverability of very high.
Decrease in temperature
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See increase in temperature above.
Increase in turbidity
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An increase in turbidity (light attenuation) is liable to reduce the growth of the epiflora, especially green and brown algal species but less so for the shade tolerant red algae, depending on the depth of the bed. Also, increased competition for light and overall reduced light is likely to favour ubiquitous species (e.g. Ceramium spp. and Ulva spp. (Birkett et al., 1998). There may be a decrease in the overall primary productivity of macroalgae and microalgae, as well as reduced phytoplankton productivity. As a result the food supply for some filter feeders and grazers may be reduced. However, the effects are unlikely to have significant effects on the community, even after one year, so an intolerance of low has been given. Recoverability is likely to be very high.
Decrease in turbidity
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An increase in light (decreased turbidity) is likely to increase benthic microalgae and macroalgal growth, and hence increase available primary productivity. An increase in algal cover was noted due to eutrophication in the Rade de Brest (Grall & Glemarec 1997; cited in Birkett et al., 1998), which resulted in a slight decrease in the diversity of carnivores, detritivores and scavengers. Birkett et al., (1998) note that shading/smothering by other algae is potentially detrimental to live maerl beds, as it impairs the growth of the maerl. In dead maerl beds the growth of maerl itself is not a concern. However, grazing community is likely to increase. Therefore, an intolerance of low with a recoverably of very high is suggested.
Increase in wave exposure
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Maerl beds develop in strong currents but are restricted to areas of low wave action. For example, in Mannin Bay dense maerl beds were restricted to less wave exposed parts of the bay (Birkett et al., 1998). Areas of maerl subject to wave action often show mobile areas in the form of ripples or mega-ripples (Hall-Spencer & Atkinson, 1999; Keegan, 1974). In Galway Bay, Keegan (1974) noted the formation of ripples due to wave action and storms, where the ripples were flattened over time by tidal currents. However, he reported that the rippled area (average crest height 20 cm) had a poor faunal diversity with heavy macroalgal settlement on any firm substratum, including the tubes of Chaetopterus. However, the infauna was a typical ‘Venus’ community, the majority of which was found at depths of more than 20 cm. Hall-Spencer & Atkinson (1999) noted that mega-ripples at their wave exposed site were relatively stable but underwent large shifts due to storms. However, the mixed sediments of the subsurface of the bed (>12 cm) were unaffected so that the burrows of the mud shrimp remained in place. Similarly, Birkett et al. (1998) note that in areas where storms affected the maerl at a depth of 10 m, only the coarse upper layer of maerl was moved while the underlying layers were stable. Following storms infaunal species renewed burrow linings within a week. However, the epiflora of maerl beds was severely disturbed by storms in Galway Bay with a marked drop in abundance in winter months. Deep beds are less likely to be affected by storm damage. Overall, therefore, an increase in wave action is likely to mobilise the surface of the bed, reduce the abundance of epiflora, promoting opportunistic epiflora, reduce the abundance of sessile epifauna, but probably have only limited effect on infauna, especially deep burrowing infauna. Therefore, a reduction in the diversity of the bed is likely and an intolerance of intermediate is given. Recoverability is likely to be high.
Decrease in wave exposure
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Maerl beds develop in strong currents but are restricted to areas of low wave action. For example, in Mannin Bay dense maerl beds were restricted to less wave exposed parts of the bay (Birkett et al., 1998). Therefore, where beds occur in areas exposed to wave action, a reduction in wave exposure may benefit the diversity of the bed. Otherwise, further reduction in wave action is unlikely to be detrimental.
Noise
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There is little information on the effects of noise on invertebrates and plants. Some invertebrates may react to vibration and stop feeding. Otherwise, noise is unlikely to have any adverse effect.
Visual Presence
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None of the resident species are likely to have the visual acuity to respond to ‘visual disturbance’.
Abrasion & physical disturbance
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Physical disturbance can result from e.g. channelization (capital dredging), suction dredging for bivalves, extraction of maerl, scallop dredging or demersal trawling. The effects of physical disturbance were summarised by Birkett et al. (1998) and Hall-Spencer et al. (2010), and documented by Hall-Spencer and co-authors (Hall-Spencer, 1998, Hall-Spencer et al., 2003, Hall-Spencer & Moore, 2000a, Hall-Spencer & Moore, 2000b), Hauton et al. (2003) and others.

For example, in experimental studies, Hall-Spencer & Moore (2000a, c) reported that the passage of a single scallop dredge through a maerl bed could bury and kill 70 of living maerl in its path. The passing dredge also re-suspended sand and silt that settled over a wide area (up to 15 m from the dredged track), and smothered the living maerl. Abrasion may break up maerl nodules into smaller pieces resulting in easier displacement by wave action, resulting in a reduced structural heterogeneity and lower diversity of species (Kamenos et al., 2003). The dredge left a ca 2.5 m track and damaged or removed most megafauna within the top 10 cm of maerl (Hall-Spencer & Moore, 2000a). For example; crabs, Ensis species, the bivalve Laevicardium crassum, and sea urchins. Deep burrowing species such as the tube anemone Cerianthus lloydii and the crustacean Upogebia deltaura were protected by depth, although torn tubes of Cerianthus lloydii were present in the scallop dredge tracks (Hall-Spencer & Moore, 2000a). Neopentadactyla mixta may also escape damage due to the depth of its burrow, especially during winter torpor. Hall-Spencer & Moore (2000a) reported that sessile epifauna or shallow infauna such as Modiolus modiolus or Limaria hians, sponges and the anemone Metridium senile where present, were significantly reduced in abundance in dredged areas for 4 years post-dredging. Other epifaunal species, such as hydroids (e.g. Nemertesia species) and red seaweeds are likely to be removed by a passing dredge.

The tracks remained visible for up to 2.5 years. In pristine live beds experimental scallop dredging reduced the population densities of epibenthic species for over 4 years. However, in previously dredged maerl beds, the benthic communities recovered in 1-2 years.

Hauton et al. (2003) undertook experimental suction (hydraulic dredging) in Stravanan Bay, Scotland, a site subject to scallop dredging and recorded as impacted dead maerl by Kamenos et al. (2003). The suction dredge removed epiflora (burrowing algae and macroalgae), maerl, slow moving epifauna (e.g. starfish, gastropods and clingfish) and mainly infauna. Large or fragile polychaetes (e.g. Chaetopterus) and Cerianthus lloydii were removed and damaged, while polychaetes with tough bodies or strong tubes survived. Large infaunal bivalves dominated the catch, including Dosinia exoleta, Tapes rhomboides, Abra alba, and Ensis arcuatus but, while Mya truncata and Lutraria angustior% were not caught because of their depth, the catch did include torn siphons from these species; an injury they are unlikely to survive. The dredge resulted in a visible track that left numerous damaged megafauna, which in turn attracted scavengers. In addition, the dredging fragmented maerl and resulted in a large plume of fine sediment that settled over the surrounding area. However, recovery was not examined.

Hall-Spencer et al. (2003) drew attention to the dangers of suction dredging for bivalves in maerl beds, especially as many of the larger infaunal bivalves are long-lived (e.g. Dosinia exoleta), suggesting that the population would take a long time to recover. Hall-Spencer et al. (2003) also note that certain maerl beds in the Bay of Brest have been dredged from scallops and Venus verrucosa for over 40 years, yet remain productive with high levels of live maerl. Although they suggest that this is due to local restrictions that limit the activity to one scallop dredge per boat. Nevertheless, scallop dredging, demersal trawling and extraction have been reported to contribute to declines in the condition of maerl beds in the north east Atlantic and the UK (Barbera et al., 2003, Hall-Spencer et al., 2010, Hall-Spencer et al., 2003).

Therefore, physical disturbance is likely to result is drastic changes in and loss of components of the community and an intolerance of high is suggested. Although recoverability is likely to be high, long-lived bivalves may take a longer period to regain their original abundance and population structure, and a precautionary recoverability of moderate is suggested
Displacement
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Mobilization of the surface of the bed is likely to reduce the diversity of epiflora and epifauna. Species displaced due to physical disturbance, especially infauna, may re-burrow or may be damaged and/or subject to increased predation. Therefore, an intolerance of intermediate is suggested.

Chemical Factors

Synthetic compound contamination
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Dead maerl beds host a diverse community of epiflora, epifauna and infauna, including many groups of algae and invertebrates and some fish. The different major groups of species will show a wide range of responses to different synthetic chemicals, heavy metals and hydrocarbons. As no specific study has examined the effects of these contaminants on maerl beds, and as the species list is so long, general summaries of the effects of contaminants have be used. Examples follow.
  • 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).
  • Beaumont et al. (1989) concluded that bivalves (especially larvae) are particularly intolerant of tri-butyl tin (TBT), the toxic component of many antifouling paints.
  • Generally, polychaetes (see Bryan, 1984), gastropods and macroalgae (see Strömgren, 1979a, Strömgren, 1979b) are regarded as being tolerant of heavy metal contamination, while the larval and embryonic stages of bivalves are particularly intolerant of heavy metal contamination. 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.
  • Suchanek (1993) reviewed the effects of oil on bivalves. Sublethal concentrations may produce substantially reduced feeding rates and/or food detection ability, probably due to ciliary inhibition. Respiration rates have increased at low concentrations and decreased at high concentrations. Generally, contact with oil causes an increase in energy expenditure and a decrease in feeding rate, resulting in less energy available for growth and reproduction. Sublethal concentrations of hydrocarbons also reduce byssal thread production (thus weakening attachment) and infaunal burrowing rates. Mortality following oil spills has been recorded in Mya arenaria, Ensis sp. and Cerastoderma edule. Suchanek (1993) reported that infaunal polychaetes were also vulnerable to hydrocarbon contamination.
  • Echinoderms also seem to be especially sensitive to the toxic effects of oil, probably because of the large amount of exposed epidermis (Suchanek, 1993). The high intolerance of Echinocardium cordatum to hydrocarbons was seen by the mass mortality of animals, down to about 20 m, shortly after the Amoco Cadiz oil spill (Cabioch et al., 1978). Dauvin (1998) reported the effects of the Amoco Cadiz oil spill on the fine sand Abra alba community in the Bay of Morlaix. Reductions in abundance, biomass and production of the community were very evident through the disappearance of the dominant populations of the amphipods Ampelisca sp. which are very sensitive to oil contamination.
Overall, numerous components of the maerl bed community are likely to be intolerant of contaminants to varying degrees, depending on the contaminant, its concentration and duration and the species in question. Any effluent discharge or spill is, therefore, likely to result in loss of a proportion of the community, and an intolerance of intermediate is suggested. Recovery is likely to be high once the contaminant or discharge has been removed.
Heavy metal contamination
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Explanation as above
Hydrocarbon contamination
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Explanation as above
Radionuclide contamination
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Insufficient information
Changes in nutrient levels
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Birkett et al. (1998) noted that increased turbidity and eutrophication due to agricultural runoff in Brittany presented the establishment of many algal species resulting in domination of ubiquitous species (e.g. Ceramium sp. and Ulva sp.), while localised eutrophication due to fish and mussel farming (aquaculture) in a sheltered area resulted in a covering fungi and the bacterial mats of Begetonia. Hall-Spencer et al. (2006) examined maerl beds in the vicinity of fish farms in strongly tidal areas. They noted a build-up of waste organic materials up to 100 m from the farms examined and a 10-100 fold increase in scavenging fauna (e.g. crabs). In the vicinity of the farm cages the biodiversity was reduced, particularly of small crustaceans, with significant increases in species tolerant of organic enrichment (e.g. Capitella). Again eutrophication resulting from aquaculture is cited as one reason for the decline of some beds in the north east Atlantic (Hall-Spencer et al., 2010).

In Brittany, numerous maerl beds were affected by sewage outfalls and urban effluents, resulting in increases in contaminants, suspended solids, microbes and organic matter with resultant deoxygenation (Grall & Hall-Spencer, 2003). This resulted in increased siltation, higher abundance and biomass of opportunistic species, loss of sensitive species and reduction in biodiversity. Grall & Hall-Spencer (2003) note that two maerl beds directly under sewage outfalls were converted from dense deposits of live maerl in the 1950s to heterogeneous mud with maerl fragments buried under several centimetres of fine sediment with species poor communities. These maerl beds were effectively lost.

Therefore, increased nutrient levels and eutrophication can lead to major changes in the associated community and an intolerance of high is suggested. Recoverability of the community associated with dead maerl could recover quickly once the nutrient levels return to prior levels, although this assumes that any deposited sediment is winnowed away by currents. Therefore a recoverability of moderate is suggested.
Increase in salinity
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The majority of maerl beds occur in full salinity. An increase in salinity above full is unlikely, except via the discharge of hyper-saline effluents from desalination plants, none of which occur in the UK. However, Wilson et al. (2004) note that Phymatolithon calcareum and Lithothamnion corallioides were tolerant up to 40 psu while most subtidal seaweeds can survive up to 50 psu. Where the bed was found in areas of reduced or variable salinity, an increase in salinity may result in an increase in biodiversity and a shift in the community to one more representative of full salinity. Therefore, an intolerance of low is suggested with a recoverability of very high.
Decrease in salinity
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The majority of maerl beds occur in full salinity although some occur in areas of reduced salinity (Birkett et al. 1998). However, where the surface water may be of reduced salinity the bottom water is likely to be full salinity. A short term reduction in salinity from full to low (the benchmark), e.g. from freshwater runoff, will affect the epifauna and epiflora directly, and may cause the temporary loss of mobile species, and death of some members of the community, e.g. echinoderms which are particularly stenohaline. Long term decrease in salinity from ‘full’ to ‘reduced’ will probably result in a shift in the community composition towards reduced salinity tolerant species, with a resultant reduction in biodiversity but increase in abundance of tolerant species. In addition, the epiflora /fauna are most likely to be effected while infauna will be protected to a degree by their depth, depending on the depth of the bed. Therefore, an intolerance of intermediate is suggested, while recoverability is likely to be high.
Changes in oxygenation
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Deoxygenation can occur as a result of eutrophication (see nutrient levels above), effluents with high BOD/COD or due to the sudden death (and resultant settlement and decay) of algal blooms. The effects of hypoxia on marine benthos has been well documented (Diaz & Rosenberg, 1995, Pearson & Rosenberg, 1978, Rosenberg & Loo, 1988) and species vary in their tolerance of low oxygen levels. For example, echinoderms such as Asterias rubens and Echinocardium cordatum are highly intolerant of anoxic conditions; the barnacle Balanus crenatus is considered to be highly intolerant of anoxia; while Crustacea are probably intolerant of hypoxia but mobile species would be able to migrate to more suitable conditions. However, most polychaetes are capable of anaerobic respiration and Capitella capitata, Hediste diversicolor and were considered to be resistant of moderate hypoxia while Nephtys hombergii and Heteromastus filiformis were thought to be resistant of severe hypoxia (Diaz & Rosenberg, 1995).

The dinoflagellate bloom on the south coast of England in 1978, resulted in hypoxia of the seabed as a result of sudden mortality and decay (Boalch, 1979, Forster, 1979, Griffiths et al., 1979). As a result numerous fish and invertebrate species were reported dead on the seabed. For example, mortality was observed in Echinus esculentus, Marthasterias glacialis, Echinocardium cordatum, Labidoplax digitata, Cancer pagurus, Ensis siliqua, Lutraria lutraria, and some polychaetes while bryozoans, soft corals, and Lutraria spp. and other species were moribund.

Overall, sudden hypoxia is likely to result in immediate mortality, while prolonged hypoxia is likely to shift the community to species tolerant of low oxygen conditions, resulting in a change in the community and loss of biodiversity. However, where deoxygenation is the result of eutrophication and/or sewage effluent intolerance is likely to be high and recoverability moderate (see nutrient levels above).

Biological Factors

Introduction of microbial pathogens/parasites
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No evidence of the effects of diseases and pathogens on maerl beds was found. Many of the species that make up the community will be susceptible to disease in the form of viruses or parasites. Overall, diseases are likely to lower the viability of affected populations and an intolerance of low is suggested. Recovery is probably very high.
Introduction of non-native species
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No evidence of the effects of non-native species in the UK was found. However, Grall & Hall-spencer (2003) note that beds of invasive slipper limpet Crepidula fornicata grew across maerl beds in Brittany. As a result, the maerl thalli were killed, and the bed clogged with silt and pseudo-faeces, so that the associated community was drastically changed. Bivalve fishing was also rendered impossible. A ‘dead’ maerl bed would also suffer modification of the bed by silt and pseudo-faeces, with resultant changes in the resident community. Removal of the surface layer of Crepidula is possible but only with the removal of the surface layer of maerl itself, which would be extremely destructive on live beds. Overall, therefore, an intolerance of high is suggested. Recovery would depend on the removal of the cover of Crepidula which is unlikely to occur naturally.
Extraction
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Dead maerl beds have been subject to extraction for the coralline maerl itself. The likely impacts are similar to those described under physical disturbance above but remain one of the major threats to maerl beds (Hall-Spencer et al., 2010).

Birkett et al. (1998) noted that although maerl beds subject to extraction in the Fal estuary exhibit a diverse flora and fauna, they were less species-rich than those in Galway Bay, although direct correlation with dredging was unclear. Grall & Glemarec (1997; cited in Birkett et al., 1998) reported few differences in biological composition between exploited and control beds in Brittany.

Dyer & Worsfold (1998) showed differences in the communities present in exploited, previously exploited and unexploited areas of maerl bed in the Fal Estuary but it was unclear if the differences were due to extraction or the hydrography and depth of the maerl beds sampled.

In Brittany, many of the maerl beds are subject to extraction (Grall & Hall-Spencer, 2003). For example the clean maerl gravel of the Glenan maerl bank described in 1969, was degraded to muddy sand dominated by deposit feeders and omnivores within 30 years. Grall & Hall-Spencer (2003) noted that the bed would be completed removed within 50-100 years at the rates reported in their study. Hall-Spencer et al. (2010) note that maerl extraction was banned in the Fal in 2005.

The impact of extraction of maerl beds depends on the intensity of the activity, and low level activity may allow the community to recover in the meantime. The ‘dead’ maerl beds in the Fal have been reported to have a high species richness, even though they are targeted for extraction. Nevertheless, it is clear that extraction could have significantly detrimental effects on maerl habitats (dead or live). Therefore, an intolerance of high has been recorded. Extraction results in the permanent removal of maerl, which in ‘dead’ beds in never going to be replaced. Continued extraction must ultimately result in loss of the bed. Therefore, a recoverability of ‘none’ is suggested.

Additional information icon Additional information

Recoverability
No information was found concerning the time taken for the dead maerl communities to reach maturity, and where recovery has been examined, no distinction between ‘live’ and ‘dead’ was made. However, several studies examined undredged, fallow and dredged sites.

De Grave & Whitaker (1999) compared a dredged (extracted) maerl bed with one that been left ‘fallow’ for six months in Bantry Bay, Ireland. They noted that the dredged bed had significantly fewer molluscs than the fallow bed, but significantly more crustaceans and oligochaetes.

Hall-Spencer & Moore (2000a, 2000b) examined the recovery of maerl community after scallop dredging in previously un-dredged and dredged sites in Scotland. In comparison with control plots, mobile epibenthos returned within one month; fleshy macroalgae within six months; the abundance of Cerianthus lloydii was not significantly different after 14 months; other epifauna (e.g. Lanice conchilega and Ascidiella aspersa) returned after 1-2 years; but some of the larger sessile surface species (e.g. sponges, Metridium senile, Modiolus modiolus and Limaria hians) exhibited lower abundances on dredged plots after four years. Deep burrowing species (mud shrimp, large bivalves e.g. Mya truncata and the gravel sea cucumber Neopentadactyla mixta) were not impacted and their abundance changed little over the four year period. Hall-Spencer et al. (2003) noted that long lived (>10 years) species (e.g. Dosinia exoleta) can occur at high abundances in maerl beds but that the sustainability of stocks are unknown at present. Hall-Spencer (2000a) noted that there was no significant difference between controls and experimentally dredged sites after 1-2 years at the sites previously subject to dredging.

Overall, it appears that most of the maerl related community could develop within five years, although long-lived and/or large sessile species (e.g. bivalves, anemones, and sponges) would take longer.

This review can be cited as follows:

Tyler-Walters, H. 2013. Beds of dead maerl. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 20/04/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=999&code=2004>