Lithothamnion corallioides maerl beds on infralittoral muddy gravel

Summary

UK and Ireland classification

Description

Live maerl beds in sheltered, silty conditions that are dominated by Lithothamnion corallioides with a variety of foliose and filamentous seaweeds. Live maerl is at least common but there may be noticeable amounts of dead maerl gravel and pebbles. Other species of maerl, such as Phymatolithon calcareum and Phymatolithon purpureum, may also occur as a less abundant component. Species of seaweed such as Dictyota dichotoma, Halarachnion ligulatum and Ulva spp. are often present, although are not restricted to this biotope, whereas Dudresnaya verticillata tends not to occur on other types of maerl beds. The anemones Anemonia viridis and Cerianthus lloydii, the polychaetes Notomastus latericeus and Caulleriella alata, the isopod Janira maculosa and the bivalve Hiatella arctica are typically found in SMp.Mrl.Lcor whereas Echinus esculentus tends to occur more in other types of maerl. The seaweeds Saccharina latissima and Chorda filum may also be present in some habitats. SMp.Mrl.Lcor has a south-western distribution in Britain and Ireland. Sheltered, stable, fully saline maerl beds in the north of Great Britain (where Lithothamnion corallioides has not been confirmed to occur) may need to be described as an analogous biotope (see SMp.Mrl.Pcal). (Information from JNCC, 2015).

Depth range

0-5 m, 5-10 m, 10-20 m

Additional information

-

Sensitivity reviewHow is sensitivity assessed?

Sensitivity characteristics of the habitat and relevant characteristic species

Maerls beds are formed by calcareous red algae that grow as unattached nodules (occasionally crusts) forming dense but relatively open beds of coralline algal gravel.  Beds of maerl form on a variety of sediments and occur on the open coast and in tide-swept channels of marine inlets (the latter are often stony).  In fully marine conditions, the dominant maerl is typically Phymatolithon calcareum or Lithothamnion coralloides in England.  Maerl beds support diverse communities of burrowing infauna, especially bivalves, and interstitial invertebrates; including suspension feeding polychaetes and echinoderms. 

Long-lived maerl thalli and their dead remains build upon underlying sediments to produce deposits with a three-dimensional structure that is intermediate in character between hard and soft grounds (Jacquotte, 1962; Cabioch, 1969; Keegan, 1974; Hall-Spencer, 1998; Barbera et al., 2003).  Thicker maerl beds occur in areas of water movement (wave or current based) while sheltered beds tend to be thinner with more epiphytes.  The associated community varies with underlying and surrounding sediment type, water movement, depth of bed and salinity (Tyler-Walters, 2013).

Maerl beds are highly variable and range from a thin layer of living maerl on top of a thick deposit of dead maerl to a layer of live maerl on silty or variable substrata to a deposit of completely dead maerl or maerl debris of variable thickness. Live maerl beds vary in the depth and proportion of ‘live maerl’ present (Birkett et al., 1998a).  In areas subject to wave action, they may form wave ripples or mega ripples e.g. in Galway Bay (Keegan, 1974) and in Stravanan Bay (Hall-Spencer & Atkinson, 1999).  Maerl beds also show considerable variation in water depth, the depth of the bed, and biodiversity (see Birkett et al., 1998a).  They also vary in the dominant maerl forming species, with Phymatolithon calcareum dominating northern beds while both Phymatolithon calcareum and Lithothamnion coralloides occur in the south-west of England and Ireland.  Lithothamnion glaciale and Lithothamnion erinaceum also occur in northern waters and replaces Lithothamnion coralloides in Scotland (Birkett et al., 1998a; Melbourne et al., 2017).  Birkett et al. (1998a) list other minor maerl forming species in the UK, however, their taxonomic status remains unresolved (Pena et al., 2013).

Maerl has a complex three dimensional structure with interlocking thalli providing a wide range of niches for infaunal and epifaunal invertebrates (Birkett et al., 1998a).  Un-impacted maerl grounds are more structurally complex than those which have been affected by dredging (Kamenos et al., 2003).  The interstitial space provided by maerl beds allow water to flow through the bed, and oxygenated water to penetrate at depth so that other species can colonize the bed to greater depths than most other sediments.  Maerl forming species are the pivotal, ecosystem engineer and biogenic reef species in maerl beds (including this biotope and its sub-biotopes).  The integrity and survival of maerl beds are dependent on the thin surface layer of living maerl (Birkett et al., 1998a; Hall-Spencer & Moore, 2000a&b).  Therefore, maerl species are the single most important functional group with respect to the sensitivity of this habitat.  The other members of the community occur in other coarse substrata, although the maerl habitat supports a diverse community.  Where appropriate, the sensitivity of other members of the community is mentioned.  The biotopes assessed under this review are live maerl beds. The sensitivity of ‘dead’ maerl beds was reviewed by Tyler-Walters (2013).

Resilience and recovery rates of habitat

Maerl beds occur from the tropics to the poles (Foster, 2001; Hinojosa-Arango & Riosmena-Rodriquez, 2004).  Both dead and live maerl contribute to subtidal biotopes. Maerl thalli grow very slowly (Adey & McKibbin, 1970; Potin et al., 1990; Littler et al., 1991; Hall-Spencer, 1994; Birkett et al.,1998a  Hall-Spencer & Moore, 2000a,b) so that maerl deposits may take hundreds of years to develop, especially in high latitudes (BIOMAERL, 1998). Species of maerl are extremely slow growing.  Growth rates of European maerl species range between tenths of a millimetre to 1 millimetre per annum (Bosence & Wilson, 2003).  The growth rates of the three most abundant species of maerl in Europe (Phymatolithon calcareum, Lithothamnion glaciale and Lithothamnion coralloides) ranged between 0.5 to 1.5 mm per tip per year under a wide range of field and laboratory conditions (Blake & Maggs, 2003). 

Individual maerl thalli may live for >100 years (Foster, 2001).  Maerl beds off Brittany are over 5500 years old (Grall & Hall-Spencer, 2003) and the maerl bed at St Mawes Bank, Falmouth was estimated to have a maximum age of 4000 years (Bosence & Wilson, 2003) while carbon dating suggested that some established beds may be 4000 to 6000 years old (Birkett et al. (1998a).  A maerl bed in the Sound of Iona is up to 4000 years old (Hall-Spencer et al., 2003).  Maerl is highly sensitive to damage from any source due to this very slow rate of growth (Hall-Spencer, 1998).  Maerl is also very slow to recruit as it rarely produces reproductive spores.  Maerl is considered to be a non-renewable resource due to its very slow growth rate and its inability to sustain direct exploitation (Barbera et al., 2003; Wilson et al., 2004).

Maerl species in the UK  propagate mainly by fragmentation (Wilson et al., 2004).  There are no detailed studies about reproduction in Lithothamnion coralloides. However, recruitment of Phymatolithon calcareum is mainly through vegetative propagation.  Although spore bearing individuals of Phymatolithon calcareum thalli have been found in the British Isles, the crustose individuals that would result from sexual reproduction have yet to be recorded in the British Isles (Irvine & Chmberlain, 1994).  Recruitment may occur from distant populations that exhibit sexual reproduction and have crustose individuals (e.g. Brittany).  Hall-Spencer (pers. comm.) observed that colonization of new locations by maerl can be mediated by a 'rafting' process where maerl thalli are bound up with other sessile organisms that are displaced and carried by currents (e.g. Saccharina latissima holdfasts after storms). Cabioch (1969) suggested that Phymatolithon calcareum may have phasic reproduction with peaks every six years.  This may account for observed changes in the relative proportions of live Lithothamnion coralloides and Phymatolithon calcareum in maerl beds.  Dominance cycles with periods of about thirty years have been recorded on some of the maerl beds of northern Brittany.  Adey & McKibbin (1970) undertook growth studies of Phymatolithon calcareum in the field and under laboratory conditions.  Field studies in the Ria de Vigo, show that growth occurs predominantly in the summer and suggests an annual growth of about 0.55 mm/year for branch tips of Phymatolithon calcareum (Adey & McKibbin, 1970).  Newly settled maerl thalli have never been found in the British Isles (Irvine and Chamberlain, 1994).  In a report for the Port of Falmouth development initiative, Hall-spencer (2009) suggested that a live maerl bed would take 1000’s of years to return to the site of navigation channel after planned capital dredging in the Fal estuary.  He also suggested that it would take 100’s of years for live maerl to grow on a translocated bed, based on the growth and accumulation rates of maerl given by Blake et al. (2007) (Hall-Spencer, 2009).

The BIOMAERL project confirmed the high levels of biodiversity associated with maerl beds that had been recorded from numerous other projects (Barbera et al., 2003; BIOMAERL, 1998, 1999).  The maerl thalli are frequently loose and mobile preventing colonization by many species.  However, deep burrowing fauna (to 68 cm) are a notable feature of this biotope (Hall-Spencer & Atkinson, 1999).  Maerl is known as a particularly diverse habitat with over 150 macroalgal species and 500 benthic faunal species recorded (Birkett et al., 1998a).  To date, 349 macroalgal species have been recorded on maerl beds in the North East Atlantic (Peña et al., 2014). Around the UK there are several maerl specialists, e.g. Cruoria cruoriiformis, Cladophora rhodolithocola and Gelidiella calcicola (Peña et al., 2014). The sea cucumber Neopentadactyla mixta can reach densities of up to 400 per square metre in loose gravels such as maerl (Smith & Keegan, 1985).

In an analysis of re-colonization processes following cessation of maerl dredging in Ireland, De Grave & Whitaker (1999a) found clear differences in the benthos between dredged and fallow sites but they were unable to determine whether there had been a return to pre-dredging conditions as there were no pre-dredge data (Hall-Spencer, 2009).  The diverse nature of communities within maerl beds results in a high level of ecological function.  Hall-Spencer (2009) stated that within a translocated maerl bed, from which the long-lived species such as Dosinia exoleta and Mya truncata had been killed, could take 20 – 50 years to recover, assuming dead or live maerl remained.  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,b) 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 is 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.  A review of historical data and the current situation at a maerl bed on the west coast of Scotland (Firth of Clyde) revealed extensive damage over the last 100 years (Hall-Spencer et al., 2010).  A living maerl bed with abundant large thalli and nests of the gaping file shell Limaria hians had become a bed of predominately dead maerl with few, small, live maerl thalli and no Limaria hians (Hall-Spencer & Moore, 2003).

Resilience assessment.  The current evidence regarding the recovery of maerl suggests that if maerl is removed, fragmented or killed then it has almost no ability to recover.  Therefore, resilience is assessed as ‘Very low’ and probably far exceeds the minimum of 25 years for this category on the scale in cases where the resistance is 'Medium', 'Low' or 'None'.  If the maerl is killed but dead maerl remains then the resident community may recover within 2-10 years (Tyler-Walters, 2013), but where the maerl is fragmented, species richness will probably decrease. However, Hall-spencer (2009) suggested that large long-lived species such as Dosinia exoleta and Mya truncata may take 20-50 years to recover. In addition, for permanent or ongoing (long-term) pressures where recovery is not possible as the pressure is irreversible, resilience is assessed as ‘Very low’ by default. 

Note. The resilience and the ability to recover from human induced pressures is a combination of the environmental conditions of the site, the frequency (repeated disturbances versus a one-off event) and the intensity of the disturbance.  Recovery of impacted populations will always be mediated by stochastic events and processes acting over different scales including, but not limited to, local habitat conditions, further impacts and processes such as larval-supply and recruitment between populations. Full recovery is defined as the return to the state of the habitat that existed prior to impact.  This does not necessarily mean that every component species has returned to its prior condition, abundance or extent but that the relevant functional components are present and the habitat is structurally and functionally recognisable as the initial habitat of interest. It should be noted that the recovery rates are only indicative of the recovery potential.

Climate Change Pressures

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ResistanceResilienceSensitivity
Global warming (extreme) [Show more]

Global warming (extreme)

Extreme emission scenario (by the end of this century 2081-2100) benchmark of:

  • A 5°C rise in SST and NBT (coastal to the shelf seas),

  • A 6°C rise in surface air temperature (in eulittoral and supralittoral habitats).

  • A 1°C rise in Deep-sea habitats (>200 m) off the continental shelf, and

  • A 5°C rise in surface air temperature in intertidal habitats exclusive to Scotland. Further detail.

Evidence

The distribution of seaweeds is climatically defined (Breeman, 1990). Northern boundaries are set by lethal winter temperatures or summer temperatures too low for growth and/or reproduction, whilst southern limits are set by high lethal summer temperatures or winter temperatures too high for induction of a crucial step in the life cycle (Breeman, 1990). Maerl beds occur from the tropics to polar waters (Foster, 2001; Hinojosa-Arango & Riosmena-Rodriquez, 2004).  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).  Four species of maerl create maerl beds in the UK.  These species vary in their distribution within the UK, a phenomenon that is thought to be due to their temperature tolerances.  Lithothamnion coralloides is a warm temperate species that occurs in the Mediterranean and has a southern limit in the Canary Islands (Wilson et al., 2004). Its northern limit is Northern Ireland, or possibly Scotland, although currently, Lithothamnion glaciale generally replaces Lithothamnion coralloides as a companion to Phymatolithon calcareum in northern parts of the UK (Hiscock et al., 2001). This is due, either to winter temperatures dropping below the minimum survival temperature (between 2 and 5°C) or because the temperatures don’t allow a suitable growing season (Adey & McKibbin, 1970; cited in Wilson et al., 2004). 

Blake & Maggs (2003) reported that the growth rate of Phymatolithon calcareum in the laboratory was only slightly affected by temperature treatments (10, 14 and 18°C), with an optimum at 10°C. The growth rate of Lithothamnion corallioides in comparison was significantly affected by temperature with an optimum at 14°C, at which it grew faster than Phymatolithon calcareum. Martin et al. (2006) reported that primary productivity in Lithothamnion corallioides was twice as high in August compared to January to February in the Bay of Brest. They found that primary productivity, calcification and respiration rates of Lithothamnion corallioides increased as temperatures rose from 10 to 16°C (Martin et al., 2006).

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. Lithothamnion coralloides extends its biogeographic range down to the Canary Islands and the Mediterranean, where summer sea surface temperatures can reach up to 28°C (www.seatemperature.org) suggesting that this species may be reasonably tolerant to an increase in temperature. It must be noted that in the Mediterranean, Lithothamnion coralloides beds generally occur at greater depths than in the UK (20 – 150 m; EEA, 2016), where temperatures may be significantly lower than surface temperatures (Houpert et al., 2015). Furthermore, Carro et al. (2014) used DNA barcoding and found that further southwards away from the UK, the presence of Lithothamnion coralloides decreases and is replaced by a newly identified species of rhodophyte in Portugal, named Phymatolithon lusitanicum.

Under the middle and high emission and extreme scenarios seawater temperatures are expected to rise by 3-5°C to give potential southern summer temperatures of 23-24°C and northern summer temperatures of 17-19°C. Ocean warming is likely to be beneficial to Lithothamnion coralloides beds found towards the northern limits of its distribution and may lead to this species increasing its range across Scotland (Hiscock et al., 2001).

Judging by its biogeographic distribution, this species is likely to be able to withstand the temperatures predicted for the south of the UK by the end of this century, although genetic differences may accompany this higher thermal tolerance. While evolutionary change can occur within a few generations in plants (Rice & Emery, 2003), Lithothamnion coralloides is slow-growing and individual maerl nodules may live for >100 years (Foster, 2001). Furthermore, this species primarily reproduces through vegetative propagation, leading to low genetic diversity in some beds, and long-distance dispersal is uncommon (Pardo et al., 2019). Therefore, some mortality cannot be ruled out if individuals fail to acclimate to rising temperatures under all three scenarios. Therefore, this biotope is assessed as having a ‘Medium’ resistance to ocean warming, albeit with ‘Low’ confidence.  Resilience is assessed as ‘Very low’, due to the long-term nature of ocean warming. Therefore, sensitivity assessed as ‘Medium’ under the middle, high and extreme emission scenarios.

Medium
Medium
Medium
Medium
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Very Low
High
High
High
Help
Medium
Medium
Medium
Medium
Help
Global warming (high) [Show more]

Global warming (high)

High emission scenario (by the end of this century 2081-2100) benchmark of:

  • A 4°C rise in SST, NBT (coastal to the shelf seas) and surface air temperature (in eulittoral and supralittoral habitats).

  • A 1°C rise in Deep-sea habitats (>200 m) off the continental shelf, and

  • A 3°C rise in surface air temperature in intertidal habitats exclusive to Scotland. Further detail.

Evidence

The distribution of seaweeds is climatically defined (Breeman, 1990). Northern boundaries are set by lethal winter temperatures or summer temperatures too low for growth and/or reproduction, whilst southern limits are set by high lethal summer temperatures or winter temperatures too high for induction of a crucial step in the life cycle (Breeman, 1990). Maerl beds occur from the tropics to polar waters (Foster, 2001; Hinojosa-Arango & Riosmena-Rodriquez, 2004).  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).  Four species of maerl create maerl beds in the UK.  These species vary in their distribution within the UK, a phenomenon that is thought to be due to their temperature tolerances.  Lithothamnion coralloides is a warm temperate species that occurs in the Mediterranean and has a southern limit in the Canary Islands (Wilson et al., 2004). Its northern limit is Northern Ireland, or possibly Scotland, although currently, Lithothamnion glaciale generally replaces Lithothamnion coralloides as a companion to Phymatolithon calcareum in northern parts of the UK (Hiscock et al., 2001). This is due, either to winter temperatures dropping below the minimum survival temperature (between 2 and 5°C) or because the temperatures don’t allow a suitable growing season (Adey & McKibbin, 1970; cited in Wilson et al., 2004). 

Blake & Maggs (2003) reported that the growth rate of Phymatolithon calcareum in the laboratory was only slightly affected by temperature treatments (10, 14 and 18°C), with an optimum at 10°C. The growth rate of Lithothamnion corallioides in comparison was significantly affected by temperature with an optimum at 14°C, at which it grew faster than Phymatolithon calcareum. Martin et al. (2006) reported that primary productivity in Lithothamnion corallioides was twice as high in August compared to January to February in the Bay of Brest. They found that primary productivity, calcification and respiration rates of Lithothamnion corallioides increased as temperatures rose from 10 to 16°C (Martin et al., 2006).

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. Lithothamnion coralloides extends its biogeographic range down to the Canary Islands and the Mediterranean, where summer sea surface temperatures can reach up to 28°C (www.seatemperature.org) suggesting that this species may be reasonably tolerant to an increase in temperature. It must be noted that in the Mediterranean, Lithothamnion coralloides beds generally occur at greater depths than in the UK (20 – 150 m; EEA, 2016), where temperatures may be significantly lower than surface temperatures (Houpert et al., 2015). Furthermore, Carro et al. (2014) used DNA barcoding and found that further southwards away from the UK, the presence of Lithothamnion coralloides decreases and is replaced by a newly identified species of rhodophyte in Portugal, named Phymatolithon lusitanicum.

Under the middle and high emission and extreme scenarios seawater temperatures are expected to rise by 3-5°C to give potential southern summer temperatures of 23-24°C and northern summer temperatures of 17-19°C. Ocean warming is likely to be beneficial to Lithothamnion coralloides beds found towards the northern limits of its distribution and may lead to this species increasing its range across Scotland (Hiscock et al., 2001).

Judging by its biogeographic distribution, this species is likely to be able to withstand the temperatures predicted for the south of the UK by the end of this century, although genetic differences may accompany this higher thermal tolerance. While evolutionary change can occur within a few generations in plants (Rice & Emery, 2003), Lithothamnion coralloides is slow-growing and individual maerl nodules may live for >100 years (Foster, 2001). Furthermore, this species primarily reproduces through vegetative propagation, leading to low genetic diversity in some beds, and long-distance dispersal is uncommon (Pardo et al., 2019). Therefore, some mortality cannot be ruled out if individuals fail to acclimate to rising temperatures under all three scenarios. Therefore, this biotope is assessed as having a ‘Medium’ resistance to ocean warming, albeit with ‘Low’ confidence.  Resilience is assessed as ‘Very low’, due to the long-term nature of ocean warming. Therefore, sensitivity assessed as ‘Medium’ under the middle, high and extreme emission scenarios.

Medium
Medium
Medium
Medium
Help
Very Low
High
High
High
Help
Medium
Medium
Medium
Medium
Help
Global warming (middle) [Show more]

Global warming (middle)

Middle emission scenario (by the end of this century 2081-2100) benchmark of:

  • A 3°C rise in SST, NBT (coastal to the shelf seas) and surface air temperature (in eulittoral and supralittoral habitats).

  • A 1°C rise in Deep-sea habitats (>200 m) off the continental shelf.

  • A 2°C rise in surface air temperature in intertidal habitats exclusive to Scotland. Further detail.

Evidence

The distribution of seaweeds is climatically defined (Breeman, 1990). Northern boundaries are set by lethal winter temperatures or summer temperatures too low for growth and/or reproduction, whilst southern limits are set by high lethal summer temperatures or winter temperatures too high for induction of a crucial step in the life cycle (Breeman, 1990). Maerl beds occur from the tropics to polar waters (Foster, 2001; Hinojosa-Arango & Riosmena-Rodriquez, 2004).  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).  Four species of maerl create maerl beds in the UK.  These species vary in their distribution within the UK, a phenomenon that is thought to be due to their temperature tolerances.  Lithothamnion coralloides is a warm temperate species that occurs in the Mediterranean and has a southern limit in the Canary Islands (Wilson et al., 2004). Its northern limit is Northern Ireland, or possibly Scotland, although currently, Lithothamnion glaciale generally replaces Lithothamnion coralloides as a companion to Phymatolithon calcareum in northern parts of the UK (Hiscock et al., 2001). This is due, either to winter temperatures dropping below the minimum survival temperature (between 2 and 5°C) or because the temperatures don’t allow a suitable growing season (Adey & McKibbin, 1970; cited in Wilson et al., 2004). 

Blake & Maggs (2003) reported that the growth rate of Phymatolithon calcareum in the laboratory was only slightly affected by temperature treatments (10, 14 and 18°C), with an optimum at 10°C. The growth rate of Lithothamnion corallioides in comparison was significantly affected by temperature with an optimum at 14°C, at which it grew faster than Phymatolithon calcareum. Martin et al. (2006) reported that primary productivity in Lithothamnion corallioides was twice as high in August compared to January to February in the Bay of Brest. They found that primary productivity, calcification and respiration rates of Lithothamnion corallioides increased as temperatures rose from 10 to 16°C (Martin et al., 2006).

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. Lithothamnion coralloides extends its biogeographic range down to the Canary Islands and the Mediterranean, where summer sea surface temperatures can reach up to 28°C (www.seatemperature.org) suggesting that this species may be reasonably tolerant to an increase in temperature. It must be noted that in the Mediterranean, Lithothamnion coralloides beds generally occur at greater depths than in the UK (20 – 150 m; EEA, 2016), where temperatures may be significantly lower than surface temperatures (Houpert et al., 2015). Furthermore, Carro et al. (2014) used DNA barcoding and found that further southwards away from the UK, the presence of Lithothamnion coralloides decreases and is replaced by a newly identified species of rhodophyte in Portugal, named Phymatolithon lusitanicum.

Under the middle and high emission and extreme scenarios seawater temperatures are expected to rise by 3-5°C to give potential southern summer temperatures of 23-24°C and northern summer temperatures of 17-19°C. Ocean warming is likely to be beneficial to Lithothamnion coralloides beds found towards the northern limits of its distribution and may lead to this species increasing its range across Scotland (Hiscock et al., 2001).

Judging by its biogeographic distribution, this species is likely to be able to withstand the temperatures predicted for the south of the UK by the end of this century, although genetic differences may accompany this higher thermal tolerance. While evolutionary change can occur within a few generations in plants (Rice & Emery, 2003), Lithothamnion coralloides is slow-growing and individual maerl nodules may live for >100 years (Foster, 2001). Furthermore, this species primarily reproduces through vegetative propagation, leading to low genetic diversity in some beds, and long-distance dispersal is uncommon (Pardo et al., 2019). Therefore, some mortality cannot be ruled out if individuals fail to acclimate to rising temperatures under all three scenarios. Therefore, this biotope is assessed as having a ‘Medium’ resistance to ocean warming, albeit with ‘Low’ confidence.  Resilience is assessed as ‘Very low’, due to the long-term nature of ocean warming. Therefore, sensitivity assessed as ‘Medium’ under the middle, high and extreme emission scenarios.

Medium
Medium
Medium
Medium
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Very Low
High
High
High
Help
Medium
Medium
Medium
Medium
Help
Marine heatwaves (high) [Show more]

Marine heatwaves (high)

High emission scenario benchmark: A marine heatwave occurring every two years, with a mean duration of 120 days, and a maximum intensity of 3.5°C. Further detail.

Evidence

Marine heatwaves due to increased air-sea heat flux are predicted to occur more frequently, at an increased intensity and last for longer by the end of this century under both middle and high emission scenarios (Frölicher et al., 2018). Coralline algae are sensitive to marine heatwaves. 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. In Western Australia, anomalously high seawater temperatures in 2012-2013, which were up to 2°C above the long term average, led to significant mortality of coralline crustose algae (Short et al., 2015).

When the temperature of the co-occurring rhodolith species,  Phymatolithon calcareum, was experimentally increased from 9°C to 25°C for five weeks in the laboratory, photosynthesis was maintained (Wilson et al., 2004), suggesting this species can cope with these temperatures in the short term. In contrast, there was a 30% decrease in growth when  Lithothamnion coralloides was exposed to temperatures of 18°C compared to those exposed to 14°C (Blake & Maggs, 2003), which suggested that this species may be less tolerant to sharp increases in temperature. Shubert et al. (2019) investigated the impact of an experimental heatwave (an increase of 5°C from 23 - 28°C) on the photosynthesis and calcification of two species of maerl, Lithothamnion crispatum and Melyvonnea erubescens.  Whilst photosynthesis decreased only in Melyvonnea erubescens, calcification was negatively affected in both species.

Sensitivity assessment. Laboratory studies have shown that growth in Lithothamnion coralloides decreases with increasing temperature, suggesting that this species may be negatively impacted by heatwaves. Under the middle emission scenario, if heatwaves were occurring at a frequency of every three years by the end of this century, with heatwaves reaching a maximum intensity of 2°C for a period of 80 days, this could lead to sea temperatures reaching up to 21°C in Scotland and Ireland in the summer months and 24°C in the south of England. Under the high emission scenario, if heatwaves occur every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C in southern England and 21.5°C in Scotland. As Lithothamnion coralloides is found in the Canary Islands, this species may be able to cope with these temperatures, although some mortality cannot be ruled out for either of these scenarios, particularly in the south of the UK. Resistance is assessed as ‘Medium’ and resilience is assessed as ‘Very low’. Therefore, the sensitivity of this biotope to marine heatwaves has been assessed as ‘Medium’ at both the middle and high emission scenario benchmark levels, albeit with ‘Low’ confidence.

Medium
Medium
Low
Medium
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Very Low
High
High
High
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Medium
Medium
Low
Medium
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Marine heatwaves (middle) [Show more]

Marine heatwaves (middle)

Middle emission scenario benchmark:  A marine heatwave occurring every three years, with a mean duration of 80 days, with a maximum intensity of 2°C. Further detail.

Evidence

Marine heatwaves due to increased air-sea heat flux are predicted to occur more frequently, at an increased intensity and last for longer by the end of this century under both middle and high emission scenarios (Frölicher et al., 2018). Coralline algae are sensitive to marine heatwaves. 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. In Western Australia, anomalously high seawater temperatures in 2012-2013, which were up to 2°C above the long term average, led to significant mortality of coralline crustose algae (Short et al., 2015).

When the temperature of the co-occurring rhodolith species,  Phymatolithon calcareum, was experimentally increased from 9°C to 25°C for five weeks in the laboratory, photosynthesis was maintained (Wilson et al., 2004), suggesting this species can cope with these temperatures in the short term. In contrast, there was a 30% decrease in growth when  Lithothamnion coralloides was exposed to temperatures of 18°C compared to those exposed to 14°C (Blake & Maggs, 2003), which suggested that this species may be less tolerant to sharp increases in temperature. Shubert et al. (2019) investigated the impact of an experimental heatwave (an increase of 5°C from 23 - 28°C) on the photosynthesis and calcification of two species of maerl, Lithothamnion crispatum and Melyvonnea erubescens.  Whilst photosynthesis decreased only in Melyvonnea erubescens, calcification was negatively affected in both species.

Sensitivity assessment. Laboratory studies have shown that growth in Lithothamnion coralloides decreases with increasing temperature, suggesting that this species may be negatively impacted by heatwaves. Under the middle emission scenario, if heatwaves were occurring at a frequency of every three years by the end of this century, with heatwaves reaching a maximum intensity of 2°C for a period of 80 days, this could lead to sea temperatures reaching up to 21°C in Scotland and Ireland in the summer months and 24°C in the south of England. Under the high emission scenario, if heatwaves occur every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C in southern England and 21.5°C in Scotland. As Lithothamnion coralloides is found in the Canary Islands, this species may be able to cope with these temperatures, although some mortality cannot be ruled out for either of these scenarios, particularly in the south of the UK. Resistance is assessed as ‘Medium’ and resilience is assessed as ‘Very low’. Therefore, the sensitivity of this biotope to marine heatwaves has been assessed as ‘Medium’ at both the middle and high emission scenario benchmark levels, albeit with ‘Low’ confidence.

Medium
Medium
Low
Medium
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Very Low
High
High
High
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Medium
Medium
Low
Medium
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Ocean acidification (high) [Show more]

Ocean acidification (high)

High emission scenario benchmark: a further decrease in pH of 0.35 (annual mean) and corresponding 120% increase in H+ ions , seasonal aragonite saturation of 20% of UK coastal waters and North Sea bottom waters, and the aragonite saturation horizon in the NE Atlantic, off the continental shelf, occurring at a depth of 400 m by the end of this century 2081-2100. Further detail 

Evidence

Increasing levels of CO2 in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005), with it expected to drop by a further 0.35 units by the end of this century, dependent on the emission scenario. Approximately 20% of coastal areas (particularly around Scotland) are expected to suffer from seasonal aragonite undersaturation by the end of this century (Ostle et al., 2016).  Coralline algae are thought to be one of the groups of species most vulnerable to ocean acidification due to the solubility of their high magnesium-calcite skeletons (Martin & Hall-Spencer, 2017).

Noisette et al. (2013) found that at levels expected for the middle emission scenario (550 µatm pCO2), photosynthesis increased and calcification rates were maintained in Lithothamnion coralloides. At levels expected under the high emission scenario (1000 µatm pCO2), photosynthesis remained the same as the control but there was a sharp decrease in diel calcification. Diel calcification decreased to almost zero at 10°C, decreased by 75% at 16°C and by 50% at 19°C (Noisette et al., 2013), which suggested that increasing temperatures may ameliorate some of the negative effects of ocean acidification on this species. This may be because the aragonite saturation state (ΩAR) is higher at warmer temperatures. For example, during this experiment at a pCO2 of 1000 µatm, ΩAR was 1.00 at 10°C and 1.53 at 19°C (Noisette et al., 2013). Furthermore, the structural integrity of maerl skeletons weakened as a result of increasing carbon dioxide (Ragazzola et al., 2012, Kamenos et al., 2013). The weaker structural integrity of the maerl skeletons may lead to increased fragmentation and a decrease in ecosystem function (Kamenos et al., 2013).

Sensitivity assessment. Most species of rhodoliths/maerl appear to suffer negative consequences of ocean acidification (Martin & Hall-Spencer, 2017). Calcareous red algae is often conspicuously absent from CO2 vents with extremely low pH (<7) and significantly reduced in areas of pH expected for the end of this century (pH 7.8) (Hall-Spencer et al., 2008, Porzio et al., 2011), although some species are more tolerant of a decrease in pH than others (Porzio et al., 2011). This confirms the paradigm that species show variation in their response to pCO2 independent of their taxonomic group or habitat preferences (Widdicombe & Spicer, 2008, Kroeker et al., 2013). Under the middle emission scenario, aragonite undersaturation is not expected to occur around the coast of the UK by the end of this century, and therefore Lithothamnion coralloides is unlikely to suffer dissolution. In fact, the middle emission scenario for acidification has been shown to lead to an increase in photosynthesis (Noisette et al., 2013). Therefore, resistance to ocean acidification under the middle emission scenario has been assessed as ‘High’, whilst resilience is assessed as ‘High’. Lithothamnion coralloides is assessed as ‘Not sensitive’ to ocean acidification at this benchmark level.

Under the high emission scenario, 20% of coastal areas are expected to suffer from seasonal aragonite undersaturation. Aragonite undersaturation will primarily occur in Scotland, therefore, the majority of Lithothamnion coralloides beds, which generally do not occur in Scotland, will not be affected. It is expected that under this scenario, calcification will be negatively affected, and some loss of living maerl is expected to occur, together with the potential dissolution of dead maerl, especially in Scottish waters. Therefore, resistance has been assessed as ‘Medium’, whilst resilience is assessed as ‘Very Low’ due to the long-term nature of ocean acidification. Under the high emission scenario, sensitivity to ocean acidification is assessed as ‘Medium’.

Medium
Medium
Medium
Medium
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Very Low
High
High
High
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Medium
Medium
Medium
Medium
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Ocean acidification (middle) [Show more]

Ocean acidification (middle)

Middle emission scenario benchmark: a further decrease in pH of 0.15 (annual mean) and corresponding 35% increase in H+ ions with no coastal aragonite undersaturation and the aragonite saturation horizon in the NE Atlantic, off the continental shelf, at a depth of 800 m by the end of this century 2081-2100. Further detail.

Evidence

Increasing levels of CO2 in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005), with it expected to drop by a further 0.35 units by the end of this century, dependent on the emission scenario. Approximately 20% of coastal areas (particularly around Scotland) are expected to suffer from seasonal aragonite undersaturation by the end of this century (Ostle et al., 2016).  Coralline algae are thought to be one of the groups of species most vulnerable to ocean acidification due to the solubility of their high magnesium-calcite skeletons (Martin & Hall-Spencer, 2017).

Noisette et al. (2013) found that at levels expected for the middle emission scenario (550 µatm pCO2), photosynthesis increased and calcification rates were maintained in Lithothamnion coralloides. At levels expected under the high emission scenario (1000 µatm pCO2), photosynthesis remained the same as the control but there was a sharp decrease in diel calcification. Diel calcification decreased to almost zero at 10°C, decreased by 75% at 16°C and by 50% at 19°C (Noisette et al., 2013), which suggested that increasing temperatures may ameliorate some of the negative effects of ocean acidification on this species. This may be because the aragonite saturation state (ΩAR) is higher at warmer temperatures. For example, during this experiment at a pCO2 of 1000 µatm, ΩAR was 1.00 at 10°C and 1.53 at 19°C (Noisette et al., 2013). Furthermore, the structural integrity of maerl skeletons weakened as a result of increasing carbon dioxide (Ragazzola et al., 2012, Kamenos et al., 2013). The weaker structural integrity of the maerl skeletons may lead to increased fragmentation and a decrease in ecosystem function (Kamenos et al., 2013).

Sensitivity assessment. Most species of rhodoliths/maerl appear to suffer negative consequences of ocean acidification (Martin & Hall-Spencer, 2017). Calcareous red algae is often conspicuously absent from CO2 vents with extremely low pH (<7) and significantly reduced in areas of pH expected for the end of this century (pH 7.8) (Hall-Spencer et al., 2008, Porzio et al., 2011), although some species are more tolerant of a decrease in pH than others (Porzio et al., 2011). This confirms the paradigm that species show variation in their response to pCO2 independent of their taxonomic group or habitat preferences (Widdicombe & Spicer, 2008, Kroeker et al., 2013). Under the middle emission scenario, aragonite undersaturation is not expected to occur around the coast of the UK by the end of this century, and therefore Lithothamnion coralloides is unlikely to suffer dissolution. In fact, the middle emission scenario for acidification has been shown to lead to an increase in photosynthesis (Noisette et al., 2013). Therefore, resistance to ocean acidification under the middle emission scenario has been assessed as ‘High’, whilst resilience is assessed as ‘High’. Lithothamnion coralloides is assessed as ‘Not sensitive’ to ocean acidification at this benchmark level.

Under the high emission scenario, 20% of coastal areas are expected to suffer from seasonal aragonite undersaturation. Aragonite undersaturation will primarily occur in Scotland, therefore, the majority of Lithothamnion coralloides beds, which generally do not occur in Scotland, will not be affected. It is expected that under this scenario, calcification will be negatively affected, and some loss of living maerl is expected to occur, together with the potential dissolution of dead maerl, especially in Scottish waters. Therefore, resistance has been assessed as ‘Medium’, whilst resilience is assessed as ‘Very Low’ due to the long-term nature of ocean acidification. Under the high emission scenario, sensitivity to ocean acidification is assessed as ‘Medium’.

High
Medium
Medium
Medium
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High
High
High
High
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Not sensitive
Medium
Medium
Medium
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Sea level rise (extreme) [Show more]

Sea level rise (extreme)

Extreme scenario benchmark: a 107 cm rise in average UK by the end of this century (2018-2100). Further detail.

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr. in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). In the NE Atlantic, maerl beds generally occur in ocean-facing coastal waters <20-30 m deep, that are directly south-west of the approach of storm waves and have little terrigenous sediment supply (Bosence & Wilson, 2003). This biotope occurs in sheltered, silty conditions, with weak to moderately strong tidal streams (JNCC, 2015).

Understanding how sea-level rise will affect exposure or tidal energy, is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site-dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. The effects of sea-level rise and increased wave action may be increased further due to storms and storms surges. IPCC (2019) note that the frequency of extreme sea-level events (e.g. due to storms) are predicted to increase as sea-level rises, however, there is no consensus on the future storm and, hence, wave climate around UK coasts (Mossman et al., 2015; Lowe et al., 2018; Palmer et al., 2018).

Sensitivity assessment. It is difficult to assess the effect of sea-level rise scenario on exposure or tidal energy, as evidence predicts that any changes will be site-specific.  As this biotope can occur from 0-20 m in depth, it is assumed that a sea-level rise of 50 cm or 70 cm (middle to high emission scenarios) would have limited effect but that a 107 cm rise (the extreme scenario) might result in loss of part of the deeper extent of the biotope in some sites. Maerl is slow-growing and most populations are limited to vegetative reproduction so maerl beds are unlikely to be able to migrate in response to increasing depth.  Therefore, resistance is assessed as ‘High’ under the middle and high emission scenarios, so that resilience is ‘High’ and sensitivity is assessed as ‘Not sensitive’.  Resistance is possibly ‘Medium’ under the extreme scenario so that resilience is ‘Very low’ and sensitivity is assessed as ‘Medium’, albeit with ‘Low’ confidence.

Medium
Low
NR
NR
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Very Low
High
High
High
Help
Medium
Low
Low
Low
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Sea level rise (high) [Show more]

Sea level rise (high)

High emission scenario benchmark: a 70 cm rise in average UK by the end of this century (2018-2100). Further detail.

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr. in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). In the NE Atlantic, maerl beds generally occur in ocean-facing coastal waters <20-30 m deep, that are directly south-west of the approach of storm waves and have little terrigenous sediment supply (Bosence & Wilson, 2003). This biotope occurs in sheltered, silty conditions, with weak to moderately strong tidal streams (JNCC, 2015).

Understanding how sea-level rise will affect exposure or tidal energy, is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site-dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. The effects of sea-level rise and increased wave action may be increased further due to storms and storms surges. IPCC (2019) note that the frequency of extreme sea-level events (e.g. due to storms) are predicted to increase as sea-level rises, however, there is no consensus on the future storm and, hence, wave climate around UK coasts (Mossman et al., 2015; Lowe et al., 2018; Palmer et al., 2018).

Sensitivity assessment. It is difficult to assess the effect of sea-level rise scenario on exposure or tidal energy, as evidence predicts that any changes will be site-specific.  As this biotope can occur from 0-20 m in depth, it is assumed that a sea-level rise of 50 cm or 70 cm (middle to high emission scenarios) would have limited effect but that a 107 cm rise (the extreme scenario) might result in loss of part of the deeper extent of the biotope in some sites. Maerl is slow-growing and most populations are limited to vegetative reproduction so maerl beds are unlikely to be able to migrate in response to increasing depth.  Therefore, resistance is assessed as ‘High’ under the middle and high emission scenarios, so that resilience is ‘High’ and sensitivity is assessed as ‘Not sensitive’.  Resistance is possibly ‘Medium’ under the extreme scenario so that resilience is ‘Very low’ and sensitivity is assessed as ‘Medium’, albeit with ‘Low’ confidence.

High
Low
NR
NR
Help
High
High
High
High
Help
Not sensitive
Low
Low
Low
Help
Sea level rise (middle) [Show more]

Sea level rise (middle)

Middle emission scenario benchmark: a 50 cm rise in average UK sea-level rise by the end of this century (2081-2100). Further detail.

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr. in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). In the NE Atlantic, maerl beds generally occur in ocean-facing coastal waters <20-30 m deep, that are directly south-west of the approach of storm waves and have little terrigenous sediment supply (Bosence & Wilson, 2003). This biotope occurs in sheltered, silty conditions, with weak to moderately strong tidal streams (JNCC, 2015).

Understanding how sea-level rise will affect exposure or tidal energy, is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site-dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. The effects of sea-level rise and increased wave action may be increased further due to storms and storms surges. IPCC (2019) note that the frequency of extreme sea-level events (e.g. due to storms) are predicted to increase as sea-level rises, however, there is no consensus on the future storm and, hence, wave climate around UK coasts (Mossman et al., 2015; Lowe et al., 2018; Palmer et al., 2018).

Sensitivity assessment. It is difficult to assess the effect of sea-level rise scenario on exposure or tidal energy, as evidence predicts that any changes will be site-specific.  As this biotope can occur from 0-20 m in depth, it is assumed that a sea-level rise of 50 cm or 70 cm (middle to high emission scenarios) would have limited effect but that a 107 cm rise (the extreme scenario) might result in loss of part of the deeper extent of the biotope in some sites. Maerl is slow-growing and most populations are limited to vegetative reproduction so maerl beds are unlikely to be able to migrate in response to increasing depth.  Therefore, resistance is assessed as ‘High’ under the middle and high emission scenarios, so that resilience is ‘High’ and sensitivity is assessed as ‘Not sensitive’.  Resistance is possibly ‘Medium’ under the extreme scenario so that resilience is ‘Very low’ and sensitivity is assessed as ‘Medium’, albeit with ‘Low’ confidence.

High
Low
NR
NR
Help
High
High
High
High
Help
Not sensitive
Low
Low
Low
Help

Hydrological Pressures

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Temperature increase (local) [Show more]

Temperature increase (local)

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

Evidence

Maerl beds occur from the tropics to polar waters (Foster, 2001; Hinojosa-Arango & Riosmena-Rodriquez, 2004).  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).  There are four species of maerl that create maerl beds in the UK.  These species vary in their distribution within the UK, a phenomenon which is thought to be due to their temperature tolerances.  Similarly, the associated communities within the maerl habitat differ and represent a diverse sample of species within the local area.  Lithothamnion coralloides is absent from Scottish waters.  This is due, either to winter temperatures dropping below the minimum survival temperature (between 2 and 5°C) or because the temperatures don’t allow a suitable growing season (Adey & McKibbin, 1970; cited in Wilson et al., 2004).  Lithothamnion corallioides had a higher minimum survival temperature than Phymatolithon calcareum; dying at 2°C and surviving without growth at 5°C (Adey & McKibbin, 1970).  In laboratory conditions, Phymatolithon calcareum survived down to 2°C, died at 0.4°C, and had a recorded optimum temperature for growth of 12-13°C (Adey & McKibbin 1970 cited in Wilson et al., 2004).  Phymatolithon calcareum 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). Temperature appears to confine Lithothamnion glaciale to northern parts of the British Isles, possibly because reproductive conceptacles are only produced in winter when temperature fall below 9°C (Hall-Spencer, 1994 cited in Wilson et al., 2004).  In addition, Adey (1970) found optimal growth rates of Lithothamnion glaciale between 10-12°C and that development of reproductive conceptacles in Lithothamnion glaciale requires winter temperatures of between 1-5°C (Adey, 1970). Blake & Maggs (2003) reported that the growth rate of Phymatolithon calcareum in the laboratory was only slightly affected by temperature treatments (10, 14 and 18°C), with an optimum at 10°C while the growth rate of Lithothamnion corallioides was significantly affected by temperature with an optimum at 14°C, at which temperature it grew faster than Phymatolithon calcareum. Martin et al. (2006) reported that primary productivity in Lithothamnion corallioides was twice as high in August as in January to February in the Bay of Brest. They found that primary productivity, calcification and respiration rates of Lithothamnion corallioides increased as temperature rose from 10 to 16°C (Martin et al., 2006).

The main maerl forming species have wide geographic ranges and their range indicates the limits of their temperature tolerance. 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). Lithothamnion corallioides is a warm temperate species ranging from Ireland and the south of Britain to the Mediterranean, while Lithothamnion glaciale and Lithothamnion erinaceum are cold temperate species that replace Lithothamnion corallioides in northern waters of the UK and the North East Atlantic (Melbourne et al., 2017). 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, 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 may affect Lithothamnion coralloides.  It has a more southern distribution in the UK and may benefit from a localised temperature increase, so that the relative abundance of Lithothamnion coralloides may change in the long-term compared to other maerl forming species.  However, given the slow growth rates exhibited by Lithothamnion coralloides, 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, SS.SMp.Mrl.Lcor probably has a ‘High’ resistance to an increase in temperature at the benchmark level.  Resilience is, therefore, ‘High’ and an assessment of ‘Not sensitive’ at the benchmark level is recorded.

High
Low
NR
NR
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High
High
High
High
Help
Not sensitive
Low
Low
Low
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Temperature decrease (local) [Show more]

Temperature decrease (local)

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

Evidence

Maerl beds occur from the tropics to polar waters (Foster, 2001; Hinojosa-Arango & Riosmena-Rodriquez, 2004).  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).  There are four species of maerl that create maerl beds in the UK.  These species vary in their distribution within the UK, a phenomenon which is thought to be due to their temperature tolerances.  Similarly, the associated communities within the maerl habitat differ and represent a diverse sample of species within the local area.  Lithothamnion coralloides is absent from Scottish waters.  This is due, either to winter temperatures dropping below the minimum survival temperature (between 2 and 5°C) or because the temperatures don’t allow a suitable growing season (Adey & McKibbin, 1970; cited in Wilson et al., 2004).  Lithothamnion corallioides had a higher minimum survival temperature than Phymatolithon calcareum; dying at 2°C and surviving without growth at 5°C (Adey & McKibbin, 1970).  In laboratory conditions, Phymatolithon calcareum survived down to 2°C, died at 0.4°C, and had a recorded optimum temperature for growth of 12-13°C (Adey & McKibbin 1970 cited in Wilson et al., 2004).  Phymatolithon calcareum 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). Temperature appears to confine Lithothamnion glaciale to northern parts of the British Isles, possibly because reproductive conceptacles are only produced in winter when temperature fall below 9°C (Hall-Spencer, 1994 cited in Wilson et al., 2004).  In addition, Adey (1970) found optimal growth rates of Lithothamnion glaciale between 10-12°C and that development of reproductive conceptacles in Lithothamnion glaciale requires winter temperatures of between 1-5°C (Adey, 1970). Blake & Maggs (2003) reported that the growth rate of Phymatolithon calcareum in the laboratory was only slightly affected by temperature treatments (10, 14 and 18°C), with an optimum at 10°C while the growth rate of Lithothamnion corallioides was significantly affected by temperature with an optimum at 14°C, at which temperature it grew faster than Phymatolithon calcareum. Martin et al. (2006) reported that primary productivity in Lithothamnion corallioides was twice as high in August as in January to February in the Bay of Brest. They found that primary productivity, calcification and respiration rates of Lithothamnion corallioides increased as temperature rose from 10 to 16°C (Martin et al., 2006).

The main maerl forming species have wide geographic ranges and their range indicates the limits of their temperature tolerance. 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). Lithothamnion corallioides is a warm temperate species ranging from Ireland and the south of Britain to the Mediterranean, while Lithothamnion glaciale and Lithothamnion erinaceum are cold temperate species that replace Lithothamnion corallioides in northern waters of the UK and the North East Atlantic (Melbourne et al., 2017).

Sensitivity assessment.  A decrease in temperature at the benchmark level may be detrimental to Lithothamnion coralloides, which is restricted to southern waters in the UK, especially at the northernmost extent of its range.  A decrease in temperature of 2°C for a year is likely to reduce growth and reproduction in Lithothamnion coralloides, although there was no information on the effect of a decrease of 5°C for one month.  Therefore, a resistance of ‘Medium’ is suggested for SS.SMp.Mrl.Lcor at the benchmark level to represent the possible reduction in abundance at its northernmost extent, but with 'Low' confidence.  Resilience is, therefore ‘Very low’, and sensitivity is assessed as ‘Medium’.

Medium
Low
NR
NR
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Very Low
High
Medium
High
Help
Medium
Low
Low
Low
Help
Salinity increase (local) [Show more]

Salinity increase (local)

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

Evidence

The majority of maerl beds occur 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.  The only maerl species currently thought to create beds in biotopes with salinities below fully marine is Lithothamnion glaciale (Connor et al., 2004), although the recently described Lithothamnion erinaceum might also.  Wilson et al. (2004) noted that Phymatolithon calcareum and Lithothamnion coralloides were tolerant up to 40 psu while most subtidal seaweeds can survive up to 50 psu. The growth of Phymatolithon calcareum is impaired at salinities <24% (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 at present. Where the biotope 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. 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.

No evidence (NEv)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
No evidence (NEv)
NR
NR
NR
Help
Salinity decrease (local) [Show more]

Salinity decrease (local)

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

Evidence

The majority of maerl beds occur 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 only maerl species currently thought to survive in biotopes with salinities below fully marine is Lithothamnion glaciale (Connor et al., 2004).  The growth of Phymatolithon calcareum is impaired at salinities <24 ppt (Adey & McKibbin, 1970; King & Schramm, 1982). However, Wilson et al. (2004) noted that Phymatolithon calcareum was more tolerant of low salinity than Lithothamnion glaciale in their experiments.  Both species survived at 3 psu for five weeks but showed significantly reduced photosynthetic activity.  However, at 15 psu, Phymatolithon calcareum recovered from the initial drop in photosynthetic activity, while Lithothamnion glaciale did not.  

Lithothamnion coralloides is recorded from areas with ‘full’ salinity regimes.  Adey & McKibbin (1970) slowly lower the salinity of specimens of Lithothamnion coralloides (grown for two months at 33.5 ppt in the laboratory) to 23 ppt for a month, and then to 13 ppt for another two weeks. Lithothamnion coralloides stopped growing at 24 ppt but on return to full salinity, resumed growth after a month and appeared healthy. Adey & McKibbin (1970) suggested that low salinity had 'little lethal importance' but that low salinity may 'have an adverse effect on growth', especially in enclosed estuaries with 'large' streams.

Sensitivity assessment.  Therefore, a decrease in salinity from ‘full’ to reduced’ conditions’ is unlikely to have an adverse effect on the dominant Lithothamnion coralloides in this SS.SMp.Mrl.Lcor biotope. However, a decrease from ‘reduced’ to ‘low’ may result in reduced growth and potentially death of the maerl over a period of a year. Similarly, where present in the biotope, a reduced salinity for an extended period of time would stress Phymatolithon calcareum and could lead to mortality, although no long-term salinity experiments have been carried out on Phymatolithon calcareum. Hence, a resistance of ‘Medium’ is suggested but with 'Low' confidence. Resilience is probably ‘Very low’ so that a sensitivity of 'Medium' is recorded.

Medium
Medium
Medium
Medium
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Very Low
High
Medium
High
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Medium
Medium
Medium
Medium
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Water flow (tidal current) changes (local) [Show more]

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 requires enough water movement to prevent smothering with silt (Hall-Spencer, 1998).  Therefore, maerl beds are 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 beds 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 winnow away the surface of the bed and result in loss of the biotope. A decrease in water flow may result in increased siltation, smothering maerl, and causing the death of maerl and significant change in the associated community (see smothering/siltation below).  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'.

Low
Low
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Very Low
High
Medium
High
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High
Low
Low
Low
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Emergence regime changes [Show more]

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

In the UK, maerl beds do not occur in the intertidal, as maerl is highly sensitive to desiccation (Wilson et al., 2004).  Also, it is very unlikely that a maerl bed would be exposed at low water as a result of human activities or natural events.  Therefore, this pressure is ‘Not relevant’.

Not relevant (NR)
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Not relevant (NR)
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Not relevant (NR)
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Wave exposure changes (local) [Show more]

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

Maerl requires enough water movement to prevent smothering with silt (Hall-Spencer, 1998).  Therefore, 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., 1998a). 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 sp.  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. (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.  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 an increase in wave exposure.

Sensitivity assessment.  Lithothamnion coralloides 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 Lithothamnion coralloides in shallow waters.  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 and reduction in infaunal diversity.  Similarly, SS.SMp.Mrl.Lcor occurs in wave sheltered conditions and 'moderately strong' to 'very weak' water flow. However, a 3-5% change in significant wave height is unlikely to damage the maerl bed. Both resistance and resilience are assessed as ‘High’, and the biotope is assessed as ‘Not sensitive’ to this pressure at the benchmark.

High
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Medium
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High
High
High
High
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Not sensitive
High
Medium
Medium
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Chemical Pressures

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ResistanceResilienceSensitivity
Transition elements & organo-metal contamination [Show more]

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

The results of the Rapid Evidence Assessment on the effects of contaminants on maerl-forming species are summarized below.  The full 'Maerl evidence review' should be consulted for details of the studies examined. 

Only sublethal effects were reported after exposure of calcareous red algae to heavy metals (Amiard, 1973 (data only); Wilson et al., 2004). In particular, Wilson et al. (2004) exposed the maerl-forming species Phymatolithon calcareum to a single dose of a mixture of heavy metals in the ratio 37:16:14:11:1, Zn: Pb: Ni: Cu: Cd, where the Cd concentration of ranges from 0.174 to 174 ppb and 1.74 ppb represented standard industrial effluent. Phymatolithon calcareum experienced a significant reduction in photosynthetic capacity depending on concentration but recovered quickly. However, the authors noted that longer-term exposure to heavy metals may have chronic effects. Nevertheless, only sublethal effects were reported. ‘Insufficient evidence’ is recorded as it is imprudent to suggest that all maerl-forming species are ‘Not’ sensitive’ without further evidence.

Insufficient evidence (IEv)
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Not relevant (NR)
NR
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Insufficient evidence (IEv)
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Hydrocarbon & PAH contamination [Show more]

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

The results of the Rapid Evidence Assessment on the effects of contaminants on maerl-forming species are summarized below.  The full 'Maerl evidence review' should be consulted for details of the studies examined. 

The effect of petrochemical hydrocarbons on encrusting coralline algae was only reported by a single paper using intertidal field mesocosms (Bokn et al., 1993). They reported that the encrusting coralline Phymatolithon lenormandii exhibited a significant decrease in cover in the upper shore mesocosm after exposure to 129.4 µg/l Diesel WAF for two years but an increase in cover in the mid and lower shore mesocosms. This suggests that diesel WAF only affect the species in the more physiologically demanding upper shore. However, the resultant worst-case resistance of Phymatolithon lenormandii could be assessed as ‘Medium’. No information on its recovery was available. If it is similar to other encrusting corallines then its resilience is probably ‘High’ and its sensitivity ‘Low’. However, it is difficult the extrapolate to maerl-forming species with any confidence, especially based on a single study.

The effect of oil spills on coralline species was reported in several papers but no evidence of the effects on maerl-forming species was found. Diaz et al. (2009a) examined changes in macroalgal abundance (inc. Corallina elongata and Lithophyllum incrustans) along the Basque coast after the Prestige oil spill but did not find any significant differences between oiled and non-oiled sites. However, Bowman et al. (1978) reported that 100% of the cover of Lithothamnia was bleached and dead rims of Lithothamnia in lower shore rock pools after the Dounreay oil spill and treatment with BP100X. Similarly, Newey & Seed (1995) reported bleached and dead coralline algae (no species were specified) in mid-shore rockpools close to the wreck of the Braer oil tanker. Jackson et al. (1989) also reported that crustose corallines (no species were specified) and other fleshy algae decreased in cover after the Panamanian oil spill, to levels below those observed before the spill. Crump et al., (1999) reported that encrusting coralline algae, Lithothamnion incrustans, Phymatolithon purpureum, and Corallina officinalis were bleached in West Angle Bay immediately after the Sea Empress oil spill but recovered quickly, which suggested only the surface layers were affected rather than individuals were killed. Crump et al. (1999) also stated that previous literature has shown oil and dispersants to have harmful effects on the pigmentation of red algae in experimental conditions. Smith et al. (1968) reported that the dispersants used to treat the Torrey Canyon oil spill killed Corallina and Lithophyllum in shallow rock pools, while those in deep pools survived, and that Corallina was killed in high shore pools but appeared healthy in mid-shore pools. Overall, the effects of detergents depended on duration and concentration of exposure, assuming lower shore populations were exposed for shorter periods than higher-shore populations as the tide returned.

Felder et al. (2014) and Fredericq et al. (2014) both examined the effects of the DWH oil spill on deep (50-77 m) ‘rhodolith’ beds in the Gulf of Mexico. They both reported that the ‘rubble’ and ‘rhodoliths’ were visibly bleached and where the encrusting calcareous algae were alive, the diverse seaweed community was lost. They suggested that the rhodolith and encrusting species included Lithophyllum sp., Lithothamnion sp., Mesophyllum sp., and Porolithon sp.

Sensitivity assessment. The above evidence suggests that exposure to oil spills and/or their dispersants can result in bleaching or death of calcareous coralline algae, especially encrusting corallines, depending on the length of exposure, shore height, and type of oil. Therefore, the resistance of encrusting corallines or Corallina sp. to exposure to oil spills and dispersants is assessed as ‘Low’ based on the worst-case scenario reported by Bowman et al. (1978). Hence, resilience is assessed as ‘Medium’ and sensitivity as ‘Medium’. Confidence in the assessment is ‘Medium’ due to the variation in the effect between studies. No direct evidence of the effect of petrochemical hydrocarbons on maerl-forming species or their beds was found. However, the evidence of effects on similar species, e.g. Phymatolithon spp. and Lithothamnion spp. above suggests that maerl-forming species and their beds may experience similar bleaching and possible death depending on exposure to oil spills. As maerl beds are sublittoral they may be protected from direct exposure, although the evidence from the DWH spill at 50 to 77 metres suggests they are not immune. The potential for smothering by oil was not discussed. However, Tuya et al. (2023) suggested oil spills were a potential threat to ‘rhodoliths’ beds worldwide. Therefore, the resistance of maerl-forming species and their beds to exposure to oil spills and/or dispersants is assessed as ‘Low’ as a precaution, albeit with ‘Low’ confidence due to the lack of direct evidence. Hence, resilience is assessed as ‘Very low’ and sensitivity as ‘High’

 

Low
Low
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Very Low
High
Medium
High
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High
Low
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Synthetic compound contamination [Show more]

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

The results of the Rapid Evidence Assessment on the effects of contaminants on maerl-forming species are summarized below.  The full Maerl evidence review should be consulted for details of the studies examined. 

The herbicides Diuron, Atrazine and Hexazinone were found to inhibit photosynthesis in crustose coralline algae (Harrington et al. 2005; Negri et al., 2011; McCoy & Kamenos, 2015). Negri et al. (2011) reported that diuron was the most toxic after 24 hours, then hexazione, and then atrazine in Neogoniolithon fosliei. Harrington et al. (2005) reported that diuron was also toxic to Porolithon onkodes when applied alone. Visible bleaching of Porolithon onkodes was observed at 29 µg/l of diuron, likely due to the destruction of chloroplasts and carotenoids that causes its colouration to lighten. However, when exposure to diuron was combined with sedimentation, the toxicity of diuron was greater and the time taken to recover in clean water was increased. However, MacVicar et al. (2022) reported no effects of exposure of Lithothamnion spp. to a high concentration of the UV filter oxybenzone (benzophenone-3; BP-3) after 15 days.

A reduction in photosynthesis is likely to reduce growth rates and increase an individual’s susceptibility to other stresses, such as sedimentation. Nevertheless, only sublethal effects were reported based on short-term experiments and no evidence of long-term effects were found. ‘Insufficient evidence’ is recorded as it is imprudent to suggest that all coralline algae and maerl-forming species are ‘Not’ sensitive’ to exposure to herbicides without further evidence.

Insufficient evidence (IEv)
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Not relevant (NR)
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Insufficient evidence (IEv)
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Radionuclide contamination [Show more]

Radionuclide contamination

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

Evidence

Hernández et al. (2011) reported that Corallina elongata and Jania rubens accumulated plutonium (Pu) in granules but did not report any adverse effects on the species. Hence, the evidence does not support an assessment of resistance or sensitivity. 

No evidence (NEv)
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Not relevant (NR)
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No evidence (NEv)
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Introduction of other substances [Show more]

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

The results of the Rapid Evidence Assessment on the effects of contaminants on maerl-forming species are summarized below. The full Maerl evidence review should be consulted for details of the studies examined. 

Legrand et al. (2022) examined the effects of exposure to hydrogen peroxide (H202), used as an antiparasitic treatment in Norwegian salmon farms, on photosynthesis in the maerl-forming species Lithothamnion soriferum. Photosynthesis was significantly reduced at concentration >=200 mg/l H202 after 1-hour exposure but recovered after 48 hours or 28 days. However, they also showed significant bleaching (28% at 200 mg/l or 63% at 2,000 mg/l) after 28 days of recovery. McCoy & Kamenos (2015) noted that bleaching could indicate death in coralline red algae and could result in structural damage but was also reversible, and the long-term effects of bleaching required further study. The authors went on to suggest that prolonged or repeated exposure may have a greater impact. The authors also noted that the crustacean fauna of maerl beds may be particularly sensitive to H202 so the diversity of affected maerl beds may be threatened, although further study was required (Legrand et al., 2022).

Overall, the evidence suggests that the maerl-forming species Lithothamnion soriferum is adversely affected by hydrogen peroxide exposure >=200 mg/l, resulting in reduced photosynthesis, reduced growth, and significant bleaching. Therefore, resistance is assessed as ‘Medium’ as a precaution but with ‘Low’ confidence as mortality was not reported directly. Hence, resilience is assessed as ‘Very low’ and sensitivity as ‘Medium’

Medium
Low
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Very Low
High
Medium
High
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Medium
Low
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De-oxygenation [Show more]

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 is 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’.

Low
High
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Medium
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Very Low
High
Medium
High
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High
High
Medium
Medium
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Nutrient enrichment [Show more]

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 blooms (Bricker et al., 1999, 2008).

Johnston & Roberts (2009) undertook a review and meta-analysis of the effect of contaminants on species richness and evenness in the marine environment.  Of the 47 papers reviewed relating to nutrients as contaminants, over 75% found that it had a negative impact on species diversity, <5% found increased diversity, and the remaining papers finding no detectable effect.  None of the 47 papers considered the impact of nutrients on this biotope.  Yet this finding is still relevant as the meta-analysis revealed that the effects of marine pollutants on species diversity were ‘remarkably consistent’ between habitats (Johnston & Roberts, 2009).  It was found that any single pollutant reduced species richness by 30-50% within any of the marine habitats considered (Johnston & Roberts, 2009).  Throughout their investigation, there were only a few examples where species richness was increased due to the anthropogenic introduction of a contaminant.  These examples were almost entirely from the introduction of nutrients, either from aquaculture or sewage outfalls.  However research into the impacts of nutrient enrichment from these sources on intertidal rocky shores often lead to shores lacking species diversity and the domination by algae with fast growth rates (Abou-Aisha et al., 1995, Archambault et al., 2001, Arévalo et al., 2007, Diez et al., 2003, Littler & Murray, 1975).

Grall & Glemarec (1997) noted that increased turbidity and eutrophication due to agricultural runoff in Brittany prevented 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 bacterial mats of Beggiatoa.  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 (Hall-Spencer et al., 2006). 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. 

Sensitivity assessment.  The effect of eutrophication on maerl beds is difficult to disentangle from the effects of organic enrichment and sedimentation.  It is likely that nutrient enrichment could adversely affect the infauna and epiflora communities.  Nevertheless, the biotope is assessed as Not sensitive at the pressure benchmark of compliance with good status as defined by the WFD.

Not relevant (NR)
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Not relevant (NR)
NR
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Not sensitive
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Organic enrichment [Show more]

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) 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). 

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 of maerl biotopes.  However, the evidence strongly suggests that organic enrichment and resultant increased in organic content, hydrogen sulphide levels and sedimentation may result in loss of maerl beds.  Resistance has been assessed as ‘None’ and resilience has been assessed as ‘Very low’.  This gives an overall sensitivity score of ‘High’.

None
High
High
High
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Very Low
High
Medium
High
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High
High
Medium
High
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Physical Pressures

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ResistanceResilienceSensitivity
Physical loss (to land or freshwater habitat) [Show more]

Physical loss (to land or freshwater habitat)

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

Evidence

All marine habitats and 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.

None
High
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Very Low
High
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High
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High
High
High
High
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Physical change (to another seabed type) [Show more]

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

Maerl biotopes can contain a variety of sediment types including gravels, sand and mud.  However, maerl biotopes never contain bedrock.  Therefore, if rock or an artificial substratum was to replace the normal substratum within this biotope the physical conditions required for this biotope would be lost along with the biotope itself.  Therefore, resistance is likely to be 'None', resilience 'Very low' (permanent change) and sensitivity is assessed as 'High'

None
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Very Low
High
High
High
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High
High
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High
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Physical change (to another sediment type) [Show more]

Physical change (to another sediment type)

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

Evidence

The sediment associated with maerl biotopes varies from shell and maerl gravel through to sand and mud.  The characterizing maerl species is also not attached to the substratum, and instead, lies over the top of it.  Therefore, if the substratum were to change this wouldn’t have a negative effect on the characterizing species.  The other species within the associated community depend on different aspects of the sediment.  Those species that are found infaunally may be negatively affected.

Sensitivity assessment.  A change in this pressure at the benchmark will not affect the characterizing species yet may affect other species found infaunally within the biotope. The loss of an infaunal species will create a niche for another species to become established, therefore continuing the biological function and ensuring the character of the biotope remains. Resistance and resilience are assessed as ‘High’, resulting in an assessment of ‘Not Sensitive’.

High
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High
High
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Not sensitive
High
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Habitat structure changes - removal of substratum (extraction) [Show more]

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

As maerl need to photosynthesize only the top layer of the deposit which has access to light will be alive.  Live maerl also requires good water flow around it, a factor which is likely to be limited 30 cm into the bed.  Maerls requirement for both light and water flow suggest that the majority if not all of the live maerl will be found in the top 30 cm of a maerl bed.  This is also where many of the associated species will be found.  Although long-lived elements of maerl bed fauna are known to burrow up to 72 cm into the substrate (Hall-Spencer & Atkinson, 1999).  The extraction of the substratum to 30 cm within this biotope would remove the vast majority of the biological component of the biotope.

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. 

Sensitivity assessment.  The resistance of the biotope to this pressure at the benchmark is probably ‘None’, based on the above evidence, and the resilience is assessed as ‘Very low’, giving the biotope a ‘High’ sensitivity.

None
High
High
High
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Very Low
High
Medium
High
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High
High
Medium
High
Help
Abrasion / disturbance of the surface of the substratum or seabed [Show more]

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; and others).  For example, in experimental studies, Hall-Spencer & Moore (2000a, 2000c) 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.  The dredge left a 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. 

Abrasion may break up maerl thalli into smaller pieces reducing structural heterogeneity and lowering diversity of species (Kamenos et al., 2003).  Hall-Spencer et al. (2003) noted that certain maerl beds in the Bay of Brest have been dredged for 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).  Irish maerl is considered to be in generally good condition but some are deteriorating due to commercial extraction, mariculture, demersal fishing and the localized effects of boat mooring chains (Vize, 2005).

Sensitivity assessment.  Physical disturbance is likely to result in drastic changes in and loss of components of the community within the maerl bed.  Fragmentation of the maerl will not kill the maerl directly but 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 highly intolerant (see smothering and siltation (light) pressure).  The evidence from Hall-Spencer & Moore (2000a, 2000c) alone strongly suggests that resistance to physical disturbance and abrasion is ‘None’. Therefore, resilience is probably ‘Very low’, resulting in a sensitivity assessment of ‘High’. 

None
High
High
High
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Very Low
High
Medium
High
Help
High
High
Medium
High
Help
Penetration or disturbance of the substratum subsurface [Show more]

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

As maerl need to photosynthesise, only the top layer of the deposit which has access to light will be alive.  Live maerl also requires good water flow around it.  Maerl beds become less structurally complex if they have been affected by dredging (Kamenos et al., 2003).  A lack of structural complexity will restrict the niches for other species, reducing biodiversity and will also restrict water flow through the bed.  Penetration and disturbance both have the capacity to break up maerl into smaller fragments. The evidence provided within the abrasion and disturbance pressure (above) shows that maerl is it not resistant to physical disturbance.  Penetration of the maerl bed will exacerbate the negative effect by damaging more of the underlying maerl.

Sensitivity assessment.  Based on the evidence provided within the abrasion and disturbance assessment the resistance of the biotope to this pressure at the benchmark is considered ‘None’ and the resilience is assessed as ‘Very low’, giving the biotope a ‘High’ sensitivity.

None
High
High
High
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Very Low
High
Medium
High
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High
High
Medium
High
Help
Changes in suspended solids (water clarity) [Show more]

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

Maerl's requirement for light restricts live thalli to depths shallower than 32 m in the relatively turbid waters of northern Europe (Hall-Spencer, 1998).  Lithothamnion coralloides is restricted to depths shallower than 10 m in 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 Lithothamnion coralloides found towards the bottom of its depth limit in Europe (i.e. 10 m).  An increase in suspended solids within this biotope is likely to also increase scour, as there are characteristically high levels of water movement through maerl beds.  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).  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.

A decrease in suspended solids will increase light levels, which could benefit maerl.  However, a decrease in the suspended matter is likely to reduce the quantity of food available for filter feeders.  This could lead to a change in the species present within the community.

Sensitivity assessment.  Any factor which decreases the ability for the characterizing maerl species to photosynthesise will have a negative impact.  Examples of the biotope found at the very bottom depth limit may experience high levels of mortality of the characterizing species. The resistance of this biotope is assessed as ‘Medium’ and the resilience is ‘Very low’.  Hence, the sensitivity is assessed as ‘Medium’ to the pressure at the benchmark level.

Medium
Medium
Medium
Medium
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Very Low
High
Medium
High
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Medium
Medium
Medium
Medium
Help
Smothering and siltation rate changes (light) [Show more]

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 4 weeks 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 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 the 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.  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 these biotopes occur in areas of tidal or wave mediated water flow, the fine smothering material would penetrate the open matrix of the maerl bed rather than sit on top of the bed. At the pressure benchmark this biotopes resistance is assessed as ‘None’, and the resilience as ‘Very low’ so that sensitivity is assessed as ‘High’. 

None
High
High
High
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Very Low
High
Medium
High
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High
High
Medium
High
Help
Smothering and siltation rate changes (heavy) [Show more]

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 all species with a thick layer of fine materials.  The pressure is significantly higher than light smothering discussed above.  Therefore, resistance is assessed as assessed as ‘None’, the resilience as ‘Very low’, and sensitivity is assessed as ‘High’. 

None
High
High
High
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Very Low
High
Medium
High
Help
High
High
Medium
High
Help
Litter [Show more]

Litter

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

Evidence

Not assessed.

Not Assessed (NA)
NR
NR
NR
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Not assessed (NA)
NR
NR
NR
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Not assessed (NA)
NR
NR
NR
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Electromagnetic changes [Show more]

Electromagnetic changes

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

Evidence

No evidence was found

No evidence (NEv)
NR
NR
NR
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Not relevant (NR)
NR
NR
NR
Help
No evidence (NEv)
NR
NR
NR
Help
Underwater noise changes [Show more]

Underwater noise changes

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

Evidence

Species characterizing this habitat do not have hearing perception but vibrations may cause an effect, however, no studies exist to support an assessment.

Not relevant (NR)
NR
NR
NR
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Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Introduction of light or shading [Show more]

Introduction of light or shading

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

Evidence

Maerl forming species require light, which restricts them to depths shallower than 32 m in the relatively turbid waters of northern Europe, while Lithothamnion coralloides is restricted to depths shallower than 10 m in northern Europe (Hall-Spencer, 1998). This suggests that maerl is intolerant 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 the shallowest biotopes. There is a possibility that shading by artificial structures could result in a loss of live maerl in deep examples of the biotope, 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 biotope.  The sensitivity of this biotope is given as ‘No evidence’

No evidence (NEv)
NR
NR
NR
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Not relevant (NR)
NR
NR
NR
Help
No evidence (NEv)
NR
NR
NR
Help
Barrier to species movement [Show more]

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 seabed habitats. Physical and hydrographic barriers may limit propagule dispersal.  But propagule dispersal is not considered under the pressure definition and benchmark.

Not relevant (NR)
NR
NR
NR
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Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Death or injury by collision [Show more]

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 seabed habitats.  NB. Collision by grounding vessels is addressed under ‘surface abrasion’.

Not relevant (NR)
NR
NR
NR
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Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
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Visual disturbance [Show more]

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.

Not relevant (NR)
NR
NR
NR
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Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help

Biological Pressures

Use [show more] / [show less] to open/close text displayed

ResistanceResilienceSensitivity
Genetic modification & translocation of indigenous species [Show more]

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

This pressure is 'Not relevant' to the characterizing species within this biotope.  

Not relevant (NR)
NR
NR
NR
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Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Introduction or spread of invasive non-indigenous species [Show more]

Introduction or spread of invasive non-indigenous species

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

Evidence

Grall & Hall-Spencer (2003) reported that beds of the 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, faeces and pseudo-faeces, drastically changing the associated community. The high biodiversity of maerl beds is dependent on water flow through the bed (Hall-Spencer et al., 2003) so siltation is likely to drastically reduce biodiversity and well as kill the maerl. Densities of 400 individuals per m2 were found on maerl beds in the Bay of Saint-Brieuc by 1989 while a 4 km2 area of maerl bed had been smothered by Crepidula by 1994 (Thouzeau, 1989; Hamon & Blanchard, 1994; in French, cited from Grall & Hall-Spencer, 2003). Bivalve fishing was also rendered impossible because the dredges became clogged with Crepidula and scallop densities were lower on the Crepidula beds than on maerl beds (Thouzeau et al., 2000, in French, cited from Grall & Hall-Spencer, 2003). 

Crepidula fornicata was reported in maerl beds in Milford Haven and increased by 2005 (Tillin et al., 2020). It was reported to reach abundances of >1000 /m2 (max. 2,748 /m2) in the Milford Harbour Waterway (MHW) (Bohn et al., 2012). The increased siltation in the MHW has been attributed, in part, to Crepidula beds, and it is probably a threat to the maerl beds in Milford Haven (Tillin et al., 2020).  

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.  The evidence above suggests that smothering and siltation by Crepidula is likely to kill the resident maerl at the surface and dramatically alter the habitat resulting in a significant loss of biodiversity. The removal of the surface layer of Crepidula fornicata is possible but only with the removal of the surface layer of maerl itself, which would be highly destructive on live beds. Therefore, resistance is assessed as ‘None’ and resilience as ‘Very low’ because the Crepidula would need to be physically removed without further damaging the maerl bed. Hence, sensitivity is assessed as ‘High’.

None
High
High
High
Help
Very Low
High
High
High
Help
High
High
High
High
Help
Introduction of microbial pathogens [Show more]

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 found in the Pacific and could have devastating consequences for maerl beds in Europe. However, this disease was not known to be in Europe (Birkett et al., 1998a).  Many of the species that make up the biological community within this biotope will be susceptible to disease in the form of viruses or parasites.  However, ‘No evidence’ of the effects of diseases and pathogens on maerl beds was found.

No evidence (NEv)
NR
NR
NR
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Not relevant (NR)
NR
NR
NR
Help
No evidence (NEv)
NR
NR
NR
Help
Removal of target species [Show more]

Removal of target species

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

Evidence

Maerl is mainly sold dried as a soil additive but is also used in animal feed, water filtration systems, pharmaceuticals, cosmetics and bone surgery.  Maerl beds 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 which from which maerl has been collected are off the coast of Brittany, where 300,000 – 500,000 t / annum are dredged (Blunden, 1991), and off Falmouth Harbour in Cornwall where extraction is around 20,000 t / annum (Martin, 1994; Hall-Spencer, 1998).

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.  Hall-Spencer et al. (2010) note that maerl extraction was banned in the Fal in 2005.

The other species of commercial interest found within maerl beds are scallops, for which there are two methods of capture for these organisms.  Firstly, the use of a scallop dredge the effect of which is assessed under the abrasion and disturbance pressure.  The second method of removal is diver collection.  There is no evidence to suggest that there is a symbiotic relationship between maerl and scallops.  Consequently, the removal of this species is unlikely to have a significant effect on the health of the biotope.

Sensitivity assessment.  Maerl itself has historically been targeted for commercial collection.  The removal of this characterizing species is highly destructive for this biotope and the resistance is assessed as ‘None’, and the resilience is assessed as ‘Very low’, giving a sensitivity assessment of ‘High’.  However, the practice of removing maerl for industry is now banned in places such as the Fal.

None
High
High
Medium
Help
Very Low
High
Medium
High
Help
High
High
Medium
Medium
Help
Removal of non-target species [Show more]

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 above.  The extraction of maerl itself, scallop dredging for scallops and/or suction dredging for other commercially exploited shellfish would also damage other members of the community. For example, the red seaweed community is likely to be damaged or removed at the surface, and interstitial bivalves damaged within or removed from the bed.  The loss of these species and other associated species would decrease species richness and negatively impact on the ecosystem function.

Sensitivity assessment. Removal of a large percentage of the characterizing species would alter the character of the biotope. The resistance to removal is ‘Low’ due to the easy accessibility of the biotopes location and the inability of these species 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’.

Low
High
High
Medium
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Very Low
High
Medium
High
Help
High
High
Medium
Medium
Help

Bibliography

  1. Abou-Aisha, K.M., Kobbia, I., El Abyad, M., Shabana, E.F. & Schanz, F., 1995. Impact of phosphorus loadings on macro-algal communities in the Red Sea coast of Egypt. Water, Air, and Soil Pollution, 83 (3-4), 285-297.

  2. Adey, W.H. & McKibbin, D.L., 1970. Studies on the maerl species Phymatolithon calcareum (Pallas) nov. comb. and Lithothamnion corallioides (Crouan) in the Ria de Vigo. Botanica Marina, 13, 100-106.

  3. Arévalo, R., Pinedo, S. & Ballesteros, E. 2007. Changes in the composition and structure of Mediterranean rocky-shore communities following a gradient of nutrient enrichment: descriptive study and test of proposed methods to assess water quality regarding macroalgae. Marine Pollution Bulletin, 55(1), 104-113.

  4. Archambault, P., Banwell, K. & Underwood, A., 2001. Temporal variation in the structure of intertidal assemblages following the removal of sewage. Marine Ecology Progress Series, 222, 51-62.

  5. Barbera C., Bordehore C., Borg J.A., Glemarec M., Grall J., Hall-Spencer J.M., De la Huz C., Lanfranco E., Lastra M., Moore P.G., Mora J., Pita M.E., Ramos-Espla A.A., Rizzo M., Sanchez-Mata A., Seva A., Schembri P.J. and Valle C. 2003. Conservation and managment of northeast Atlantic and Mediterranean maerl beds. Aquatic Conservation: Marine and Freshwater Ecosystems, 13, S65-S76.

  6. BIOMAERL team, 1998. Maerl grounds: Habitats of high biodiversity in European seas. In Proceedings of the Third European Marine Science and Technology Conference, Lisbon 23-27 May 1998, Project Synopses, pp. 170-178.

  7. BIOMAERL team, 1999. Biomaerl: maerl biodiversity; functional structure and anthropogenic impacts. EC Contract no. MAS3-CT95-0020, 973 pp.

  8. Birkett, D.A., Maggs, C.A. & Dring, M.J., 1998a. Maerl. an overview of dynamic and sensitivity characteristics for conservation management of marine SACs. Natura 2000 report prepared by Scottish Association of Marine Science (SAMS) for the UK Marine SACs Project., Scottish Association for Marine Science. (UK Marine SACs Project, vol V.). Available from: http://ukmpa.marinebiodiversity.org/uk_sacs/publications.htm

  9. Blake, C. & Maggs, C.A., 2003. Comparative growth rates and internal banding periodicity of maerl species (Corallinales, Rhodophyta) from northern Europe. Phycologia42 (6), 606-612.

  10. Blake, C., Maggs, C. & Reimer, P., 2007. Use of radiocarbon dating to interpret past environments of maerl beds. Ciencias Marinas, 33 (4), 385-397.

  11. Blunden, G. & Wildgoose, P.B., 1977. The effects of aqueous seaweed extract and kinetin on potato yields. Journal of the Science of Food and Agriculture, 28 (2), 121-125.

  12. Blunden G., 1991. Agricultural uses of seaweeds and seaweed extracts. In Guiry M. D. & Blunden, G. Seaweed Resources in Europe: Uses and Potential. John Wiley & Sons, Chichester, pp. 65-81

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Citation

This review can be cited as:

Perry, F.,, Tyler-Walters, H.,, Garrard, S.L., & Watson, A., 2024. Lithothamnion corallioides maerl beds on infralittoral muddy gravel. In Tyler-Walters H. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 10-10-2024]. Available from: https://www.marlin.ac.uk/habitat/detail/219

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