The Marine Life Information Network

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

Evidence

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., 1998a, Wilson et al., 2004).  Phymatolithon calcareum dominates maerl beds in the north of the UK.  In the south-west of England and Ireland, Phymatolithon calcareum is often found interspersed with Lithothamnion coralloides.  During laboratory experiments aquaria temperatures on Spanish specimens of Phymatolithon calcareum were increased from 15.1°C to and 17°C and then 19°C at one month intervals (Adey & McKibbin, 1970).  A steady decrease in growth rate was recorded from Phymatolithon calcareum in these tanks. However, Adey & McKibbin (1970) suggested that 12-13°C was the optimum temperature for growth in Phymatolithon calcareum. 

Specimens of Phymatolithon calcareum from Strangford Lough, Northern Ireland, showed no significant difference on photosynthetic activity at 9°C (the control), 17°C or 25°C for 4-5 weeks but were judged to be dead after 90 minutes at 40°C (Wilson et al., 2004).  The temperatures used in their study were typical of winter (9°C) and summer conditions (17°C) in the lough. Phymatolithon calcareum appear to have a wide geographic range (from Norway to the Mediterranean) and are likely to be tolerant of higher temperatures than those experienced in the British Isles.  The geographic range of Phymatolithon calcareum may not accurately describe its ability to withstand localized changes in temperature, as it may be acclimatized to local conditions.  However, the range of Phymatolithon calcareum may to some extent display the limits of the species genetic ability to acclimatize to temperatures. Phymatolithon calcareum is a cold temperate species that ranges from Norway to the Mediterranean (Wilson et al., 2004; Martin et al., 2006) and tolerates high temperatures better than many subtidal temperate red algae (Wilson et al., 2004). Martin & Hall-Spencer (2017) noted that a 3°C increase in temperature above that normally experienced by tropical or warm-temperate coralline algae caused bleaching and adversely affected health, rates of calcification and photosynthesis and survival. Current trends in climate change driven temperature increases have already caused shifts in seaweed biogeography, as the tropical regions widen polewards, to the detriment of the warm-temperate region, and the cold-temperate region shrinks (Martin & Hall-spencer, 2017). 

Sensitivity assessment.  An increase in temperature at the benchmark level is unlikely to affect Phymatolithon calcareum.  However, given the slow growth rates of Phymatolithon calcareum, no effect is likely to be perceived within the duration of the benchmark, although long-term climate change effects may be noticed in future.  Therefore, Phymatolithon calcareum probably has a ‘High’ resistance to an increase in temperature at the benchmark level.  Resilience is, therefore, ‘High’, and a sensitivity assessment of ‘Not sensitive’ is recorded.

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

Evidence

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., 1998a; Wilson et al., 2004).  During laboratory experiments on Spanish Phymatolithon calcareum, aquaria temperatures were reduced from 10°C to 0.4°C or 0.2°C (depending on light intensity and light: dark cycle) over a period of two to eight weeks (Adey & McKibbin, 1970). After the period of temperature reduction, all samples of Phymatolithon calcareum were dead or showed a reduction in pigmentation.  When temperatures were returned to 10°C over three weeks, six out of fifteen Phymatolithon calcareum specimens showed signs of recovery.  Specimens in aquaria where temperatures had only been reduced to 2.1°C or 5°C survived.  In the same study 13°C was recorded as the optimum temperature for Phymatolithon calcareum (Adey & McKibbin 1970).

Phymatolithon calcareum is a cold temperate species that ranges from Norway to the Mediterranean (Wilson et al., 2004; Martin et al., 2006) and tolerates high temperatures better than many subtidal temperate red algae (Wilson et al., 2004).  Hence, Phymatolithon calcareum is likely to be tolerant of lower temperatures than those experienced in the British Isles. Species ranges may not accurately describe their ability to withstand localized changes in temperature, as species will be acclimatized to local conditions.  However, the range of Phymatolithon calcareum may to some extent display the limits of the species genetic ability to acclimatize to temperatures.

Sensitivity assessment.  A decrease in temperature at the benchmark level is unlikely to affect Phymatolithon calcareum in UK waters.  However, given the slow growth rates exhibited by maerls, no effect is likely to be perceived within the duration of the benchmark, but long-term climate change effects may be noticed in future.  Therefore, Phymatolithon calcareum probably have a ‘High’ resistance to an increase in temperature at the benchmark level. Resilience is, therefore ‘High’, and a sensitivity assessment of ‘Not sensitive’ is recorded.

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Phymatolithon calcareum occurs in full salinity.  Joubin (1910 cited in Wilson et al., 2004) thought that maerl beds were only present in areas with lowered salinity.  Bosence (1976) found that although surface salinities could be low, the benthic water was mostly fully saline.  Wilson et al. (2004) noted that Phymatolithon calcareum tolerated up to 40 psu.  The growth of Phymatolithon calcareum is impaired at salinities <24 ppt (Adey & McKibbin, 1970; King & Schramm, 1982).

Sensitivity assessment.  An increase in salinity above full is unlikely, except via the discharge of hypersaline effluents from desalination plants, none of which occur in the UK.  Where Phymatolithon calcareum is found in areas of reduced or variable salinity, an increase in salinity is very unlikely to have an effect.  Maerl does not naturally occur within hypersaline areas, and although it may be able to tolerate a short-term increase in salinity, an increase to hypersaline conditions for a year would probably cause significant negative impacts. However, no evidence was found on which to base an assessment of maerl forming species. 

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Phymatolithon calcareum occurs in full salinity.  Although Joubin, (1910, cited in Wilson et al., 2004) thought that maerl beds were only present in areas with lowered salinity.  Bosence (1976) found that although surface salinities could be low, the benthic water was mostly fully saline.  The growth of Phymatolithon calcareum is impaired at salinities <24 ppt (Adey & McKibbin, 1970; King & Schramm, 1982).  Wilson et al. (2004) noted that Phymatolithon calcareum survived at 3 psu for five weeks, but showed significantly reduced photosynthetic activity.  At 15 psu, Phymatolithon calcareum was able to recover from the initial drop in photosynthetic activity.

Sensitivity assessment.  A reduction from a fully marine regime to one that is reduced for a short period of time is unlikely to have a significant negative impact on Phymatolithon calcareum.  However, a reduced salinity regime for an extended period of time would remove Phymatolithon calcareum from its optimum conditions and will cause a reduction in the growth rate, and may lead to mortality.  No long-term salinity experiments have been carried out on Phymatolithon calcareum.  Therefore, a precautionary resistance assessment of ‘Medium’ is suggested. , Therefore, he resilience is ‘Very low’ and sensitivity is assessed as ‘Medium’.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

Maerl species, including Phymatolithon calcareum, require a degree of shelter from wave action, to prevent burial and dispersal into deep water.  However, they also need enough water movement to prevent smothering with silt (Hall-Spencer, 1998).  Therefore, maerl is restricted to areas of strong tidal currents or wave oscillation (Birkett et al. 1998a).  For example, Birkett et al. (1998a) quote a flow rate of 0.1 m/s across the maerl bed at spring tides in Greatman’s Bay, Galway, while the UK biotope classification (Connor et al., 2004) reports maerl occurring at sites with between moderately strong to very weak tidal streams.  As Birkett et al. (1998a) note, local topography and wave generated oscillation probably result in stronger local currents at the position of the bed. However, Hall-Spencer et al. (2006) reported that maerls beds in the vicinity of fish farms became silted with particulates from fish farms even in areas of strong flow. Hall-Spencer et al. (2006) reported peak flow rates of 0.5 to 0.7 m/s at the sites studied, and one site experienced mean flows of 0.11 to 0.12 m/s and maxima of 0.21 to 0.47 m/s depending on depth above the seabed. 

Sensitivity assessment.  An increase in water flow to strong or very strong may result winnow away the surface of the bed and result in loss of the live layer of Phymatolithon calcareum.  However, beds in moderately strong water flow probably occur at greater depths than those in weaker flow.  Similarly, as a decrease in water flow may result in an increase in siltation, smothering maerl and filling the bed with silt, causing the death of maerl (see smothering/siltation below).  However, maerl beds in very weak tidal streams probably receive wave mediated flow.  So the effect will depend on local hydrography and the wave climate.  A change of 0.1-0.2 m/s may have a limited effect in areas of moderately strong flow but may be significant in areas of weak or negligible flow.  However, Hall-spencer (pers. comm.) noted that any change in water flow is likely to affect maerl beds. Therefore, a resistance of 'Low' is suggested but with 'Low' confidence. Hence, as resilience is likely to be 'Very low', sensitivity is assessed as 'High'. 

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year or 2) an increase in relative sea level or decrease in high water level for ≥1 year. Further detail

Evidence

Phymatolithon calcareum does not occur in the intertidal, as it is highly sensitive to desiccation (Wilson et al., 2004).  Also, it is very unlikely that Phymatolithon calcareum would be exposed at low water as a result of human activities or natural events (Tyler-Walters, 2013).  Therefore, this pressure is probably ‘Not relevant’.

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

Phymatolithon calcareum requires a degree of shelter from wave action, to prevent burial and dispersal into deep water.  However, enough water movement needed to prevent smothering with silt (Hall-Spencer, 1998).  Therefore, Phymatolithon calcareum 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., 1998a).  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.  Hall-Spencer & Atkinson (1999) noted that mega-ripples at their wave exposed site were relatively stable but underwent large shifts due to storms.  Similarly, Birkett et al. (1998a) noted 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.  Deep beds are less likely to be affected by an increase in wave exposure.

Sensitivity assessment.  Phymatolithon calcareum occurs in a range of wave exposures and can survive in areas subject to wave action and even storms, if deep enough.  Therefore, an increase in wave exposure is probably detrimental to Phymatolithon calcareum in shallow waters.  Similarly, a decrease in wave action may be detrimental where wave action is the main contribution to water movement through the bed, due to the potential increase of siltation.  However, a 3-5% change in significant wave height is unlikely to be significant. Both resistance and resilience are assessed as ‘High’, and Phymatolithon calcareum is assessed as ‘Not sensitive’ to this pressure at the benchmark.

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed.

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed.

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed.

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

‘No evidence’.

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status). Further detail

Evidence

Deoxygenation at the benchmark level is likely to be detrimental to the maerl beds and their infaunal community but mitigated.  Water flow experienced by these biotopes suggests that deoxygenating conditions may be short-lived.  However, 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). In addition, they reported less live maerl around all three of the fish farm sites studied than the 50-60% found at reference sites. Most of the maerl around fish farms in Orkney and South Uist was dead and clogged with black sulphurous anoxic silt. The Shetland farm had the most live maerl but this was formed into mega-ripples, indicating that the maerl had been transported to the site by rough weather (Hall-Spencer et al., 2006).   Eutrophication resulting from aquaculture is cited as one reason for the decline of maerl beds in the North East Atlantic (Hall-Spencer et al., 2010).  In the laboratory, Wilson et al. (2004) noted that burial in black muddy sand, smelling of hydrogen sulphide, was fatal to live maerl. Even thalli placed on the surface of the black muddy sand died within two weeks, together with thalli buried by 0.25 cm and 2 cm of the sediment (Wilson et al., 2004). A study of a phytoplankton bloom that killed herring eggs on a maerl bed in the Firth of Clyde found that the resultant anoxia caused mass mortalities of the burrowing infauna (Napier, in press, cited by Hall-Spencer pers comm.).

Sensitivity assessment. The available evidence suggests that maerl and its associated community are sensitive to the effects of deoxygenation and anoxia, even in areas of strong water movement. Therefore, resistance has been assessed as ‘Low’, resilience as ‘Very low’, and sensitivity is assessed as ‘High’.

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

Evidence

This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations.  The nutrient enrichment of a marine environment leads to organisms no longer being limited by the availability of certain nutrients.  The consequent changes in ecosystem functions can lead to the progression of eutrophic symptoms (Bricker et al., 2008), changes in species diversity and evenness (Johnston & Roberts, 2009) decreases in dissolved oxygen and uncharacteristic microalgal bloom (Bricker et al., 1999, 2008).

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, and loss of sensitive species.  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.

Sensitivity assessment.  The effect of eutrophication on maerl is difficult to disentangle from the effects of organic enrichment, and sedimentation.  However, the nutrient load from sewage outfalls and urban runoff is likely to be higher than the benchmark level. Hence, Phymatolithon calcareum is considered to be ‘Not sensitive’ at the pressure benchmark of compliance with WFD criteria for good status.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr. Further detail

Evidence

The organic enrichment of a marine environment at this pressure benchmark leads to organisms no longer being limited by the availability of organic carbon.  The consequent changes in ecosystem functions can lead to the progression of eutrophic symptoms (Bricker et al., 2008), changes in species diversity and evenness (Johnston & Roberts, 2009) and decreases in dissolved oxygen and uncharacteristic microalgal blooms (Bricker et al., 1999, 2008).  Grall & Hall-Spencer (2003) considered the impacts of eutrophication as a major threat to maerl beds.

Hall-Spencer et al. (2006) compared maerl beds under salmon farms with reference maerl beds.  It was found that maerl beds underneath salmon farms had visible signs of organic enrichment (feed pellets, fish faeces and/or Beggiatoa mats), and significantly lower biodiversity.  At the sites underneath the salmon nets, there were 10 – 100 times the number of scavenging species present compared to the reference sites.  Grall & Glémarec (1997) noted similar decreases in maerl bed biodiversity due to anthropogenic eutrophication in the Bay of Brest.  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. 

Sensitivity assessment.  Little empirical evidence was found to directly compare the benchmark organic enrichment to Phymatolithon calcareum.  However, the evidence suggests that organic enrichment and resultant increased in organic content, hydrogen sulphide levels and sedimentation may result in loss of maerl.  Resistance has been assessed as ‘None’ and resilience has been assessed as ‘Very low’.  This gives an overall sensitivity score of ‘High’.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

Evidence

All marine benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very Low’).  Sensitivity within the direct spatial footprint of this pressure is, therefore ‘High’.  Although no specific evidence is described, confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata or vice-versa. Further detail

Evidence

Phymatolithon calcareum can be found on top of a variety of sediment types including gravels, sand and mud, but never bedrock.  Therefore, if rock or an artificial substrate were to replace the normal substratum that this species is found on then the species would not be able to survive.  Resistance is likely to be ‘None’, resilience ‘Very low’ (permanent change) and sensitivity is assessed as ‘High’. 

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

The sediment often found underneath Phymatolithon calcareum can vary from shell and maerl gravel through to sand and mud.  Phymatolithon calcareum is also not attached to the substratum, and instead lies over the top of it.  Therefore, if the substratum were to change this would not have a negative effect on the species. 

Sensitivity assessment.  A change in this pressure at the benchmark will not affect Phymatolithon calcareum.  Resistance and resilience are assessed as ‘High’, resulting in a ‘Not Sensitive’ assessment.

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock). Further detail

Evidence

The extraction of the substratum to 30 cm would remove almost all of the Phymatolithon calcareum (live and dead), within the footprint of the activity.

Sensitivity assessment.  The resistance of Phymatolithon calcareum to this pressure at the benchmark is ‘None’ and resilience is assessed as ‘Very low’ so that sensitivity is assessed as ‘High’.

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Physical disturbance can result from; 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. (1998a) 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, b; Hauton et al., 2003).  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 Phymatolithon calcareum nodules into smaller pieces resulting in easier displacement by wave action, a reduced structural heterogeneity and reduced biodiversity of the bed (Kamenos et al., 2003).

Sensitivity assessment.  Physical disturbance can result in the fragmentation of Phymatolithon calcareum but will not kill it directly.  However, subsequent death is likely due to a reduction in water flow caused by compaction and sedimentation (Hall-spencer & Moore, 2000a; 2000c).  Dredging can create plumes of sediment that can settle on top of the maerl, and overturn and bury maerl, causing it to be smothered, a pressure to which maerl is not resistant (see 'smothering' and 'siltation' pressures).  The evidence from Hall-Spencer & Moore (2000a; 2000 c) alone strongly suggests that resistance to physical disturbance and abrasion is ‘Low’. Therefore, resilience is probably ‘Very low’, and sensitivity is assessed as ‘High’.

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

The fragmentation of Phymatolithon calcareum will not directly cause mortality of the organism.  However, the smaller pieces will be lighter and therefore more likely to be entrained and exported in the strong tidal flows characteristic of where this species is found.  Nevertheless, the evidence provided under the 'abrasion and disturbance' pressure suggests that maerl is not resistant of physical disturbance. Penetration of the maerl bed will only exacerbate the negative effect by damaging more of the underlying maerl.

Sensitivity assessment.  Based on the evidence provided by the 'abrasion and disturbance' assessment the resistance of Phymatolithon calcareum to this pressure at the benchmark is assessed as ‘None’, the resilience as ‘Very low’, and sensitivity assessed as ‘High’.

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate for one year. Further detail

Evidence

Phymatolithon calcareum requires light and is, therefore, restricted to depths shallower than 32 m in the relatively turbid waters of northern Europe (Hall-Spencer, 1998).  An increase in suspended sediments in the water column will increase light attenuation and decrease the availability of light.  A decrease in light availability will alter the ability of the maerl to photosynthesise.  This could be detrimental to maerl found towards the bottom of its depth limit in Europe (i.e. 32 m).  An increase in suspended solids is also likely to increase scour, as there are characteristically high levels of water movement in maerl habitat.  Scour is known to induce high mortality in early post-settlement algal stages and prevents the settlement of propagules owing to the accumulation of silt on the substratum (Vadas et al., 1992). A decrease in suspended solids may increase light levels, which could benefit maerl.

Sensitivity assessment.  Any factor that could decrease the ability for Phymatolithon calcareum to photosynthesise will have a negative impact.  Where this species is found at the bottom depth limit of itsange, it may experience high levels of mortality.  The resistance of this species is considered to be ‘Medium’ and the resilience is ‘Very low’.  This gives the species an overall sensitivity of ‘Medium’ to the pressure at the benchmark.

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

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 was found to survive burial in coarse sediment (Wilson et al., 2004) but to die in fine sediments.  Phymatolithon calcareum survived for four weeks buried under 4 and 8 cm of sand or gravel but died within 2 weeks under 2 cm of muddy sand.  Wilson et al. (2004) suggested that the hydrogen sulphide content of the muddy sand was the most detrimental aspect of burial since even those maerl nodules on the surface of the muddy sand died within two weeks. They also suggested that the high death rate of maerl observed after burial due to scallop dredging (Hall-Spencer & Moore, 2000a,c) was probably due to physical and chemical effects of burial rather than a lack of light (Wilson et al., 2004).

In Galicia, France, ongoing deterioration of maerl has been linked to mussel farming which increases sedimentation, reducing habitat complexity, lowering biodiversity, and killing maerl (Pena & Barbara, 2007a, b; cited in Hall-Spencer et al., 2010). Wilson et al. (2004) also point out that the toxic effect of fine organic sediment and associated hydrogen sulphide explain the detrimental effect on maerl beds of Crepidula fornicata in Brittany, sewage outfalls, and aquaculture (Grall & Hall-spencer, 2003).

Sensitivity assessment.  Even though Phymatolithon calcareum occurs in areas of tidal or wave mediated water flow, the fine smothering material could penetrate the open matrix of the maerl bed rather than sit on top of the bed. At the pressure benchmark Phymatolithon calcareum's resistance is assessed as ‘None’, and the resilience is ‘Very low’, resulting in an overall sensitivity of ‘High’.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

A deposit at the pressure benchmark would cover Phymatolithon calcareum with a thick layer of fine materials.  The pressure is significantly higher than light smothering discussed above.  Therefore, resistance is assessed as ‘None’, and the resilience is ‘Very low’, resulting in an overall sensitivity of ‘High’.

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline). Further detail

Evidence

‘Not assessed’.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

Evidence

‘No evidence’.

Underwater noise changes

Benchmark. MSFD indicator levels (SEL or peak SPL) exceeded for 20% of days in a calendar year. Further detail

Evidence

‘Not relevant’.

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

Phymatolithon calcareum has a requirement for light which restricts it to depths shallower than 32 m in the relatively turbid waters of northern Europe (Hall-Spencer, 1998), which suggests that maerl is not resistant of long-term reductions in light availability.  However, in the short-term maerl exhibits little stress after being kept in the dark for 4 weeks (Wilson et al., 2004). 

Sensitivity assessment.  Artificial light is unlikely to affect any but Phymatolithon calcareum in the shallowest conditions.  There is a possibility that shading by artificial structures could result in the loss if Phymatolithon calcareum is found at its most extreme depths, but only where shading was long-term or permanent.  There is insufficient information to assess the effect of this pressure at the benchmark on this species.  Therefore, ‘No evidence’ is recorded.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion. Further detail

Evidence

‘Not relevant’ – this pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than immobile benthic species. Physical and hydrographic barriers may limit propagule dispersal.  But propagule dispersal is not considered under the pressure definition and benchmark.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

Evidence

‘Not relevant’ to immobile benthic species.  NB. Collision by grounding vessels is addressed under ‘surface abrasion’.

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

Evidence

‘Not relevant’.

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

‘No evidence’ of genetic modification and/or translocation was found.

Introduction or spread of invasive non-indigenous species

Benchmark. The introduction of one or more invasive non-indigenous species (INIS). Further detail

Evidence

No evidence of the effects of non- indigenous invasive species in the UK was found.  However, Grall & Hall-spencer (2003) note that beds of invasive slipper limpet Crepidula fornicata grew across maerl in Brittany.  As a result, the maerl thalli were killed, and the bed clogged with silt and pseudofaeces so that the associated community was drastically changed.  Bivalve fishing was also rendered impossible. Peña et al. (2014) identified eleven invasive algal species found on maerl beds in the North East Atlantic.  The invasive species included Sargassum muticum, which causes habitat shading (Hall-Spencer pers. comm.).

Sensitivity assessment.  Removal of the surface layer of Crepidula fornicata is possible but only with the removal of the surface layer of Phymatolithon calcareum itself, which in itself would be extremely destructive. A resistance of ‘None’ and a resilience of ‘Very low’ have been recorded, resulting in an overall sensitivity assessment of ‘High’.

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus). Further detail

Evidence

Coralline lethal orange disease is found in the Pacific and could have devastating consequences for Phymatolithon calcareum.  However, this disease was not known to be in Europe (Birkett et al., 1998a) and ‘No evidence’ of the effects of diseases and pathogens on Phymatolithon calcareum could be found.

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

Phymatolithon calcareum can be sold dried as a soil additive but is also used in animal feed, water filtration systems, pharmaceuticals, cosmetics and bone surgery.  Maerl beds containing Phymatolithon calcareum are dredged for scallops (found in high densities compared with other scallop habitats) where extraction efficiency is very high.  This harvesting has serious detrimental effects on the diversity, species richness and abundance of maerl beds (Hall-Spencer & Moore, 2000c). 

Within Europe, there is a history of the commercial collection and sale of maerl.  Two notable sites from western Europe, from which maerl was collected, were off the coast of Brittany, where 300,000 – 500,000 t / annum were dredged (Blunden, 1991), and off Falmouth harbour in Cornwall where extraction was around 20,000 t / annum (Martin, 1994; Hall-Spencer, 1998). However, maerl extraction was banned in the Fal in 2005 Hall-Spencer et al. (2010).

Kamenos et al. (2003) reported that maerl grounds impacted by towed demersal fishing gears are structurally less heterogeneous than pristine, un-impacted maerl grounds, diminishing the biodiversity potential of these habitats. Birkett et al. (1998a) 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., 1998a) 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. 

Sensitivity assessment.  Maerl, including Phymatolithon calcareum, has historically been targeted for commercial collection.  Phymatolithon calcareum has no ability to avoid removal, and therefore, resistance is assessed as ‘None’, and the resilience is assessed as ‘Very low’, giving a sensitivity assessment of ‘High’.

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

Direct, physical impacts from harvesting are assessed through the abrasion and penetration of the seabed pressures.  Phymatolithon calcareum could easily be incidentally removed as by-catch when other species are being targeted (e.g. via scallop dredging), see ‘removal of target species’ and ‘abrasion’ pressures above.

Sensitivity assessment.  The resistance to removal is ‘Low’ due to the inability of Phymatolithon calcareum to evade collection. The resilience is ‘Very low’, with recovery only being able to begin when the harvesting pressure is removed altogether. This gives an overall sensitivity score of ‘High’.