Biodiversity & Conservation

SS.IGS.Mrl.Phy.HEc

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Phymatolithon calcareum is the key structural species within the biotope and is highly intolerant of substratum loss. Other obvious characterizing species in the biotope such as (Neopentadactyla mixta and Nemertesia ramosa) are also likely to be highly intolerant of substratum loss as will the more abundant but less obvious infaunal species. Phymatolithon calcareum has a very low recoverability from substratum loss. Loss of the substratum as well as the structural and characterizing species in the biotope will probably result in a major decline in species richness for the area.
Smothering
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Phymatolithon calcareum is the key structural species within the biotope and is highly intolerant of smothering. Scallop dredging is one of the main causes of smothering in maerl beds. A single passage of a dredge may bury and kill 70 percent of living maerl in their path. Phymatolithon calcareum has a very low recoverability from smothering. The loose and complex consistency of this biotope provides considerable structural diversity utilized by a wide range of species. Smothering of the main structural species, Phymatolithon calcareum, will probably result in a major decline in species richness for the area
Increase in suspended sediment
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Phymatolithon calcareum is the key structural species within the biotope and is highly intolerant of changes in siltation. Phymatolithon calcareum has a very low recoverability from changes in siltation. Many of the species in this biotope live between the maerl nodules. Some of these species will benefit whilst others will decline due to changes in siltation and subsequent changes in granulometry of the habitat.
Decrease in suspended sediment
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Desiccation
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Phymatolithon calcareum is the key structural species within the biotope and is highly intolerant of desiccation. Phymatolithon calcareum has a very low recoverability from desiccation. Most of the species associated with this deeper water maerl biotope are intolerant of desiccation. Exposure to desiccating influences for an hour is likely to cause many species to die.
Increase in emergence regime
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Phymatolithon calcareum is the key structural species within the biotope and is highly intolerant of changes in emergence regime. Phymatolithon calcareum has a very low recoverability from changes in emergence regime.
Decrease in emergence regime
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Increase in water flow rate
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Phymatolithon calcareum is the key structural species within the biotope and is intermediately intolerant of decreases in water flow rate. Neopentadactyla mixta, a species that has been chosen as representative of the intolerance of the echinoderms in the biotope (although not necessarily always present itself), is highly intolerant of changes in water flow rate. Phymatolithon calcareum has a moderate recoverability from changes in water flow rate. Many of the species in this biotope live within the structure provided by Phymatolithon calcareum, where there is protection from changes in water flow rate.
Decrease in water flow rate
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Increase in temperature
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Phymatolithon calcareum is the key structural species within the biotope and is intermediately intolerant of changes in temperature This maerl species dies below 2 degrees C and above 22 degrees C. Neopentadactyla mixta, a species that has been chosen as representative of the intolerance of the echinoderms in the biotope (although not necessarily always present itself), is also intermediately intolerant. Phymatolithon calcareum has a moderate recoverability from changes in temperature. This biotope potentially contains a wide variety of species, only some of which will be intolerant of changes in temperature.
Decrease in temperature
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Increase in turbidity
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Phymatolithon calcareum is the key structural species within the biotope and is intermediately intolerant of changes in turbidity. Being photosynthetic, this species is reliant on light availability. Consequently, increases in turbidity drastically reduce the lower depth limits of this species. It occurs to 105 m depth off Malta, to 32 m depth off western Ireland and to 18 m in the Clyde. Phymatolithon calcareum has a moderate recoverability from changes in turbidity. This biotope contains fewer algal species than other maerl biotopes. Faunal species tend to be less intolerant of changes in water clarity.
Decrease in turbidity
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Increase in wave exposure
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Phymatolithon calcareum is the key structural species within the biotope and is likely to be intermediately intolerant of changes in wave exposure. Strong wave action can cause live maerl thalli to be buried, broken into smaller pieces or dispersed. Neopentadactyla mixta, a species that has been chosen as representative of the intolerance of the echinoderms in the biotope (although not necessarily always present itself), is highly intolerant of changes in wave exposure. Both Phymatolithon calcareum and Neopentadactyla mixta have moderate recoverability from changes in wave exposure. Maerl biotopes can be highly mobile making it difficult for many species to establish themselves, increases in wave exposure may increase this mobility.
Decrease in wave exposure
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Noise
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Neopentadactyla mixta, a species that has been chosen as representative of the intolerance of the echinoderms in the biotope (although not necessarily always present itself), shows low intolerance to disturbance by noise. Few benthic species are highly intolerant of noise disturbance.
Visual Presence
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None of the key or important species in this biotope are sensitive to visual disturbance. It is unlikely that any of the infaunal and epifaunal species associated with this biotope are sensitive to visual disturbance.
Abrasion & physical disturbance
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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. Evidence from historic specimens of Phymatolithon calcareum collected between 1885 and 1891, before the onset of scallop fishing, showed that specimens collected from a scallop dredged area were smaller than those collected in the late 19th century (Hall-Spencer & Moore, 2000a, c). Abrasion may break up maerl nodules into smaller pieces resulting in easier displacement by wave action. Abrasion may also disrupt the physical integrity of accreted maerl beds. The dredge left a ca 2.5 m track and damaged or removed most megafauna within the top 10 cm of maerl. The tracks remained visible for up to 2.5 years. In pristine beds experimental scallop dredging reduced the population densities of epibenthic species for over 4 years, while the maerl species themselves may take decades to recover. In previously dredged maerl beds, the benthic communities recovered in 1-2 years. Maerl habitats are dependant on survival of slow-growing algae e.g. Phymatolithon calcareumand other maerl species, which cannot withstand prolonged burial, due to the lack of light, and die (Hall-Spencer & Moore, 2000a, c). Hall-Spencer & Moore, (2000a, c) concluded that maerl beds were particularly sensitive to the impacts of towed fishing gears. Boat moorings and dragging anchor chains have been noted to damage the surface of maerl beds, as has demersal fishing gear. Therefore, intolerance has been assessed as high.

However, megafauna on or in the top 10 cm of maerl were either removed or damaged and left on the dredge tracks, susceptible to subsequent predation (Hall-Spencer & Moore, 2000a). For example; crabs, Ensis species, the bivalve Laevicardium crassum, and sea urchins. Deep burrowing species such as the sea 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.

Overall, the key structural species, Phymatolithon calcareum, is recorded as being highly intolerant of physical disturbance, and an overall biotope intolerance of high has been recorded. Propagation of Phymatolithon calcareum in the British Isles is almost entirely vegetative so recruitment of new individuals to the population will not aid recovery. The very slow growth rate of Phymatolithon calcareum means that vegetative regeneration will take a long time. Therefore, recoverability has been assessed as very low.

Displacement
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The important characterizing species Nemertesia ramosa a species selected as being representative of the intolerance of hydroids (although not necessarily always present itself), is highly intolerant of and has a moderate recoverability from displacement. Many of the species in the biotope have an active, infaunal burying habit. Displacement of the key or important species in the biotope is not likely to cause many of the other species living within the biotope to die.

Chemical Factors

Synthetic compound contamination
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Insufficient information is available about the key and important species in this biotope to be able to make an assessment of intolerance to synthetic chemical contamination.
Heavy metal contamination
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Insufficient information is available about the key and important species in this biotope to be able to make an assessment of intolerance to heavy metal contamination.
Hydrocarbon contamination
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Insufficient information is available about the key and important species in this biotope to be able to make an assessment of intolerance to hydrocarbon contamination.
Radionuclide contamination
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Insufficient information is available about the key and important species in this biotope to be able to make an assessment of intolerance to radionuclide contamination.
Changes in nutrient levels
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The key structural species (Phymatolithon calcareum) in this biotope has a low intolerance to changes in nutrient concentration. King & Schramm, (1982) report that ionic calcium concentration is the main factor affecting growth of maerl in culture experiments rather than salinity per se (although this has not been shown in the field). Uptake of calcium carbonate occurs optimally at 30 psu. Increases in nutrient concentration can indirectly cause intermediate or high intolerance through smothering by fast growing algae that block light and cause anoxia (e.g. Rade de Brest, France, J. Hall-Spencer pers. comm.) This aspect is better covered under smothering.
Increase in salinity
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The key structural species, Phymatolithon calcareum, has a high intolerance to decreases in salinity (King & Schramm, 1976). Neopentadactyla mixta, a species selected as being representative of the intolerance of echinoderms in the biotope (although not necessarily always present itself) is also highly intolerant of decreases in salinity (Smith 1983). Phymatolithon calcareum has a very low recoverability from changes in salinity. This biotope is found deeper than shallow waters where salinity reductions from freshwater run-off occur. The biotope occurs in the more open parts of inlets where open coast salinity waters prevail. Changes from full salinity will probably cause a major decline in species richness.
Decrease in salinity
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Changes in oxygenation
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The important characterizing species Nemertesia ramosa is intermediately intolerant of and has a high recoverability from changes in oxygenation. Anoxia will kill live maerl (J. Hall-Spencer, pers. comm.). The loose structure of the maerl bed allows oxygenation to occur to considerable depth and this fact is exploited by many burrowing species. Changes in oxygenation are likely to cause a major decline in species richness.

Biological Factors

Introduction of microbial pathogens/parasites
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The key structural species of this biotope (Phymatolithon calcareum) has intermediate intolerance to the introduction of microbial pathogens. This refers to the potential effects of the western pacific disease 'coralline lethal orange disease'. Phymatolithon calcareum has a moderate recoverability from disease. This disease is specific to coralline algae and will not affect other taxa.
Introduction of non-native species
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The introduced species Crepidula fornicata has radically altered the ecology of maerl beds in the Rade de Brest, France through increasing siltation and provision of substrata (J. Hall-Spencer pers. comm.).
Extraction
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Phymatolithon calcareum, the only key structuring species for the biotope, is subject to commercial extraction although it is highly unlikely that either of the important characterizing species (Nemertesia ramosa or Neopentadactyla mixta) would be. The actual removal of Phymatolithon calcareum (usually by dredging) would also result in the removal of many other species associated with the biotope. Maerl beds are dredged to extract scallops and other molluscs which causes considerable structural damage. Dredging causes loss of sessile species such as Limaria hians and Modiolus modiolus and this can alter the stability and structural properties of the bed (Hall-Spencer & Moore, 2000(a)). These two species support their own suite of encrusting and epilithic species which will also be lost (see Physical Disturbance for further details). Limaria hians remains at significantly reduced levels for at least 4 years so recoverability will be moderate or worse (Hall-Spencer & Moore, 2000(a)). 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. Evidence from historic specimens of Phymatolithon calcareum collected between 1885 and 1891, before the onset of scallop fishing, showed that specimens collected from a scallop dredged area were smaller than those collected in the late 19th century (Hall-Spencer & Moore, 2000a, c). The dredging may break up maerl nodules into smaller pieces resulting in easier displacement by wave action. Furthermore, the physical integrity of accreted maerl beds may be disrupted. The dredge left a ca 2.5 m track and damaged or removed most megafauna within the top 10 cm of maerl. The tracks remained visible for up to 2.5 years. In pristine beds experimental scallop dredging reduced the population densities of epibenthic species for over 4 years, while the maerl species themselves may take decades to recover. In previously dredged maerl beds, the benthic communities recovered in 1-2 years. Maerl habitats are dependant on survival of slow-growing algae e.g. Phymatolithon calcareumand other maerl species, which cannot withstand prolonged burial, due to the lack of light, and die (Hall-Spencer & Moore, 2000a, c). Hall-Spencer & Moore, (2000a, c) concluded that maerl beds were particularly sensitive to the impacts of towed fishing gears and, therefore, intolerance has been assessed as high.

Propagation of Phymatolithon calcareum in the British Isles is almost entirely vegetative so recruitment of new individuals to the population will not aid recovery. The very slow growth rate of Phymatolithon calcareum means that vegetative regeneration will take a long time. Therefore, recoverability has been assessed as very low.

Additional information icon Additional information

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This review can be cited as follows:

Jackson, A. 2006. Phymatolithon calcareum maerl beds with hydroids and echinoderms in deeper infralittoral clean gravel or coarse sand. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 30/07/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=64&code=1997>