MarLIN

information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

Maerl (Phymatolithon calcareum)

Distribution data supplied by the Ocean Biogeographic Information System (OBIS). To interrogate UK data visit the NBN Atlas.

Summary

Description

A fragile, coralline alga often confused with Lithothamnion corallioides. Its form is very variable but commonly resembles stag's horns of irregular diameter. Older specimens become somewhat erect with nodular branches and are reminiscent of red 'coral'. Unattached plants may reach about 7 cm in diameter with branches up to 6 mm in diameter and mauvish brown in colour. The surface can be smooth or flaky.

Recorded distribution in Britain and Ireland

Recorded around the Shetland Isles and Orkney, along the east coast of Scotland, and the south coast of England with isolated records at Bideford Bay, Pembrokeshire and Caernarfon Bay. Abundant around the west coasts of Ireland and Scotland.

Global distribution

Recorded from Norway down to northern Spain, including the western Baltic and the Mediterranean.

Habitat

Typically found in less than 20 m depth on sand, mud or gravel substrata in areas that are protected from strong wave action but have moderate to high water flow. The crustose form is very rare in the British Isles. Usually found as unattached plants forming beds of coralline algal gravel (maerl) in the sublittoral and occasionally lower littoral. Typically found together with Lithothamnion corallioides in the southern British Isles or Lithothamnion glaciale in the northern British Isles.

Depth range

1-30

Identifying features

  • Thallus comprising either attached crustose plant or unattached branching systems.
  • Unattached plants varying from single branches to subglobular or flattened branching systems, sparsely to densely branched.
  • Branches sometimes confluent, slender to robust, terete to flattened, to 6 mm diameter with apices rounded to flattened.
  • Surface mainly rather smooth or with white flaky areas.

Additional information

Maerl is a generic name for certain coralline algae that grow unattached on the sea bed. No crustose plants of Phymatolithon calcareum have been recorded from the British Isles.

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Biology review

Taxonomy

PhylumRhodophyta
ClassFlorideophyceae
OrderCorallinales
FamilyHapalidiaceae
GenusPhymatolithon
Authority(Pallas) W.H.Adey & D.L.McKibbin, 1970
Recent Synonyms

Biology

Typical abundanceHigh density
Male size rangeNot relevant
Male size at maturityNot relevant
Female size range3-10 cm
Female size at maturity3-10 cm
Growth formAlgal gravel
Growth rate1-2 mm/year
Body flexibilityNone (less than 10 degrees)
MobilitySessile
Characteristic feeding methodAutotroph
Diet/food sourceAutotroph
Typically feeds onNot relevant
SociabilityNot relevant
Environmental positionEpilithic
DependencyIndependent.
SupportsNone
Is the species harmful?No

Biology information

Phymatolithon calcareum grows as maerl nodules or medallions forming dense but relatively open beds of algal gravel.  Beds of these maerl form in coarse clean sediments of gravels and clean sands, and occur either on the open coast or in tide-swept channels of marine inlets (the latter are often stony).  In fully marine conditions, Phymatolithon calcareum is one of two dominant maerl species in England (Tyler-Walters, 2013). 

Phymatolithon calcareum does have a crustose permanently attached form but this has not been recorded in the British Isles. It is typically found as an unattached plant. Coralline algal thalli that form maerl beds have been found in densities of up to 22,000 thalli per square metre. The proportion of live to dead nodules varies considerably. As far as is known maerl continues to grow throughout its life but fragmentation limits the size of the nodules. There are no sexes and individual plants may reach up to 5 cm across.

Growth rates of European maerl species range between tenths of a millimetre to one millimetre per annum (Bosence & Wilson, 2003).  Recent studies suggested that 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 nodules may live for >100 years (Foster, 2001).

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 the 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 substratum, 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).  Lithothamnion coralloides occurs in the south-west of England and Ireland mixed in with Phymatolithon calcareum.

Maerl exhibits a complex three-dimensional structure with interlocking lattices providing a wide range of niches for infaunal and epifaunal invertebrates (Birkett et al., 1998a).  Pristine, un-impacted maerl grounds are more structurally complex than those which have been affected by dredging (Kamenos et al., 2003).  The interstitial space and open structure of provided by maerl beds allows 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 other coarse substrata.  Maerls are the pivotal, ecosystem engineer and biogenic reef species.  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).

Maerl beds are highly species rich with 150 macroalgal species and over five hundred faunal species (of which 120 are molluscs) recorded as living on or in maerl beds (Birkett et al., 1998a); see the maerl biotope SS.SMp.Mrl for further information. As far as is known, the maerl does not host any commensal or parasitic species. However, a few algae are almost entirely restricted to maerl communities e.g. the red algae Gelidiella calcicola, Gelidium maggsiae and the crustose Cruoria cruoriaeformis (Birkett et al., 1998a).

Habitat preferences

Physiographic preferencesEstuary, Open coast, Ria / Voe, Sea loch / Sea lough, Strait / sound
Biological zone preferencesLower infralittoral, Sublittoral fringe, Upper infralittoral
Substratum / habitat preferencesCoarse clean sand, Fine clean sand, Gravel / shingle, Maerl, Mixed, Mud, Muddy gravel, Muddy sand, Pebbles, Sandy mud
Tidal strength preferencesModerately Strong 1 to 3 knots (0.5-1.5 m/sec.), Strong 3 to 6 knots (1.5-3 m/sec.)
Wave exposure preferencesExposed, Moderately exposed, Sheltered
Salinity preferencesFull (30-40 psu)
Depth range1-30
Other preferences

No text entered

Migration PatternNot relevant

Habitat Information

Most frequent at depths between 1-10 m. May be found deeper in clear waters, e.g. found at depths of up to 30 m in outer Galway Bay.
Phymatolithon calcareum has been reported recently from 13-20m off Swange to Old Harry and off Bembridge, the Isle of Wight (Collins pers. comm.; Collins et al., 1990).

Life history

Adult characteristics

Reproductive typeVegetative
Reproductive frequency Data deficient
Fecundity (number of eggs)No information
Generation timeInsufficient information
Age at maturityNot relevant
SeasonInsufficient information
Life span20-100 years

Larval characteristics

Larval/propagule typeNot relevant
Larval/juvenile development Not relevant
Duration of larval stageNo information
Larval dispersal potential <10 m
Larval settlement periodInsufficient information

Life history information

Recruitment in France is believed to be primarily through fragmentation from crustose forms. No crustose forms are known from the British Isles so propagation must be virtually entirely vegetative. Maerl beds in the Sound of Iona are recorded as containing dead nodules up to 4,000 years old (Farrow, 1983, cited in Maggs et al., 1998). Insufficient information is available on reproductive frequency, fecundity and developmental mechanism. In the British Isles there are a few records of fertile plants but no records of the crustose forms that this reproduction would produce. Plants may be fertile and develop conceptacles throughout the year. Plants from Brittany are mostly fertile in winter. 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 corallioides and Phymatolithon calcareum nodules in maerl beds. Dominance cycles with periods of about thirty years have been recorded on some of the maerl beds of northern Brittany. Adey & McKibbon (1970) undertook growth studies of Phymatolithon calcareum in the field and under laboratory conditions. Their results for field studies in the Ria de Vigo, expressed as µ/day, 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 according to Maggs et al. (1998).

Sensitivity reviewHow is sensitivity assessed?

Resilience and recovery rates

All species of maerl, including Phymatolithon calcareum, 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 maerl deposits may take hundreds of years to develop, especially in high latitudes (BIOMAERL, 1998).  Growth rates of European maerl species range between tenths of a millimetre to 1 millimetre per annum (Bosence & Wilson, 2003).  Adey & McKibbin (1970) undertook growth studies of Phymatolithon calcareum in the field and under laboratory conditions.  Field studies in the Ria de Vigo, Spain, 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).  More recent studies suggested that the growth rate of Phymatolithon calcareum tips are 0.9 mm/per year under a wide range of field and laboratory conditions (Blake & Maggs, 2003).

Individual maerl nodules 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).  The maerl bed in the Sound of Iona was reported to be ˂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 is rarely produces reproductive spores.  Little information is available on reproductive frequency, fecundity and developmental mechanism.  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).

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 & Chamberlain, 1994).  Newly settled maerl thalli have never been found in the British Isles (Irvine and Chamberlain, 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 go through phases of reproduction with peaks every six years. This may account for observed changes in the relative proportions of live Lithothamnion coralloides and Phymatolithon calcareum nodules in maerl beds.  Dominance cycles with periods of about thirty years have been recorded on some of the maerl beds of northern Brittany.   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).

Resilience assessment.  The current evidence regarding the recovery of maerl, including Phymatolithon calcareum, 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'.

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.

Hydrological Pressures

 ResistanceResilienceSensitivity
High High Not sensitive
Q: High
A: Medium
C: Medium
Q: High
A: High
C: High
Q: High
A: Medium
C: Medium

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

Specimens of Phymatolithon calcareum from Stranford Lough, Northern Ireland, showed no significant difference on photosynthetic activity at 9°C (the control), 17°C or 25°C for 4-5 weeks but were judged to be dead after 90 minutes at 40°C (Wilson et al., 2004).  The temperatures used in thier study were typical of winter (9°C) and summer conditions (17°C) in the lough. Phymatolithon calcareum appear to have wide geographic range (from Norway to the Mediterranean) and are likely to be tolerant of higher temperatures than those experienced in the British Isles.  The geographic range of Phymatolithon calcareum may not accurately describe its ability to withstand localized changes in temperature, as it may be acclimatized to local conditions.  However, the range of Phymatolithon calcareum may to some extent display the limits of the species genetic ability to acclimatize to temperatures.

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

High High Not sensitive
Q: High
A: Medium
C: Medium
Q: High
A: High
C: High
Q: High
A: Medium
C: Medium

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

Phymatolithon calcareum appear to have wide geographic range (from Norway to the Mediterranean) and are likely to be tolerant of lower temperatures than those experienced in the British Isles. Species ranges may not accurately describe their ability to withstand localized changes in temperature, as species will be acclimatized to local conditions.  However, the range of Phymatolithon calcareum may to some extent display the limits of the species genetic ability to acclimatize to temperatures.

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

Low Very Low High
Q: Medium
A: Medium
C: Medium
Q: High
A: Medium
C: High
Q: Medium
A: Medium
C: Medium

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

Sensitivity assessment.  An increase in salinity above full is unlikely, except via the discharge of hypersaline effluents from desalination plants, none of which occur in the UK.  Where Phymatolithon calcareum is found in areas of reduced or variable salinity, an increase in salinity is very unlikely to have an effect.  Any increase in the salinity regime which results in hypersaline conditions is likely to have a significant negative impact on Phymatolithon calcareum.  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. Therefore, a resistance of ‘Low’ is suggested.  Resilience is probably ‘Very low’ so sensitivity is assessed as ‘High’.

Medium Very Low High
Q: Medium
A: Medium
C: Medium
Q: High
A: Medium
C: High
Q: Medium
A: Medium
C: Medium

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

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

High High Not sensitive
Q: Low
A: Low
C: Low
Q: High
A: Medium
C: High
Q: Low
A: Low
C: Low

Maerl species, including Phymatolithon calcareum, require a degree of shelter from wave action, to prevent burial and dispersal into deep water.  However, they also need enough water movement to prevent smothering with silt (Hall-Spencer, 1998).  Therefore, maerl is restricted to areas of strong tidal currents or wave oscillation (Birkett et al. 1998a).  For example, Birkett et al. (1998a) quote a flow rate of 0.1m / s across the maerl bed at spring tides in Greatman’s Bay, Galway, while the UK biotope classification (Connor et al., 2004) reports maerl occurring at sites with between moderately strong to very weak tidal streams.  As Birkett et al. (1998a) note, local topography and wave generated oscillation probably result in stronger local currents at the position of the bed.

Sensitivity assessment.  An increase in water flow to strong or very strong may result winnow away the surface of the bed and result in loss of the live layer of Phymatolithon calcareum.  However, beds in moderately strong water flow probably occur at greater depths than those in weaker flow.  Similarly, as a decrease in water flow may result increase siltation, smothering maerl and filling the bed with silt, causing death of maerl (see smothering/siltation below).  However, maerl beds in very week tidal streams probably receive wave mediated flow.  So the effect will depend on local hydrography and the wave climate.  However, a change of 0.1-0.2 m/s is unlikely to have significant effect. Both resistance and resilience are assessed as ‘High’, resulting in the species being ‘Not sensitive’ to this pressure at the benchmark.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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

High High Not sensitive
Q: High
A: Medium
C: Medium
Q: High
A: Medium
C: High
Q: High
A: Medium
C: Medium

Phymatolithon calcareum requires a degree of shelter from wave action, to prevent burial and dispersal into deep water.  However, enough water movement needed to prevent smothering with silt (Hall-Spencer, 1998).  Therefore, Phymatolithon calcareum develop in strong currents but are restricted to areas of low wave action.  For example, in Mannin Bay dense maerl beds were restricted to less wave exposed parts of the bay (Birkett et al., 1998a).  Areas of maerl subject to wave action often show mobile areas in the form of ripples or mega-ripples (Hall-Spencer & Atkinson, 1999, Keegan, 1974).  In Galway Bay, Keegan (1974) noted the formation of ripples due to wave action and storms, where the ripples were flattened over time by tidal currents.  Hall-Spencer & Atkinson (1999) noted that mega-ripples at their wave exposed site were relatively stable but underwent large shifts due to storms.  Similarly, Birkett et al. (1998a) noted that in areas where storms affected the maerl at a depth of 10 m, only the coarse upper layer of maerl was moved while the underlying layers were stable.  Deep beds are less likely to be affected by an increase in wave exposure.

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

Chemical Pressures

 ResistanceResilienceSensitivity
Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

‘Not sensitive’ - at the pressure benchmark of compliance with all relevant environmental protection standards.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

‘Not sensitive’ at the pressure benchmark of compliance with all relevant environmental protection standards.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

‘Not sensitive’ at the pressure benchmark of compliance with all relevant environmental protection standards.

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

‘No evidence’.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

‘Not sensitive’ at the pressure benchmark of compliance with all relevant environmental protection standards.

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

‘No evidence’. The toxic effect of hydrogen sulphide laden organic sediment (Wilson et al., 2004) is only indirectly linked to oxygen depletion. Therefore, there is insufficient evidence to support a sensitivity assessment on the effect of the pressure at the benchmark on this species.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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

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

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

None Very Low High
Q: High
A: High
C: High
Q: High
A: Medium
C: High
Q: High
A: Medium
C: High

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

Hall-Spencer et al. (2006) compared maerl beds under salmon farms with reference maerl beds.  It was found that maerl beds underneath salmon farms had visible signs of organic enrichment (feed pellets, fish faeces and/or Beggiatoa mats), and significantly lower biodiversity.  At the sites underneath the salmon nets, there were 10 – 100 times the number of scavenging species present compared to the reference sites.  Grall & Glémarec (1997) noted similar decreases in maerl bed biodiversity due to anthropogenic eutrophication in the Bay of Brest.  In Brittany, numerous maerl beds were affected by sewage outfalls and urban effluents, resulting in increases in contaminants, suspended solids, microbes and organic matter with resultant deoxygenation (Grall & Hall-Spencer, 2003).  This resulted in increased siltation, higher abundance and biomass of opportunistic species, loss of sensitive species and reduction in biodiversity.  Grall & Hall-Spencer (2003) note that two maerl beds directly under sewage outfalls were converted from dense deposits of live maerl in the 1950s to heterogeneous mud with maerl fragments buried under several centimetres of fine sediment with species poor communities.  These maerl beds were effectively lost. 

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

Physical Pressures

 ResistanceResilienceSensitivity
None Very Low High
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

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

None Very Low High
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

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

High High Not sensitive
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

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

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

None Very Low High
Q: High
A: High
C: High
Q: High
A: Medium
C: High
Q: High
A: Medium
C: High

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

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

None Very Low High
Q: High
A: High
C: High
Q: High
A: Medium
C: High
Q: High
A: Medium
C: High

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; Hauten et al., 2003; and others).  For example, in experimental studies, Hall-Spencer & Moore (2000a, c) reported that the passage of a single scallop dredge through a maerl bed could bury and kill 70% of living maerl in its path.  The passing dredge also re-suspended sand and silt that settled over a wide area (up to 15 m from the dredged track), and smothered the living maerl. 

Abrasion may break up Phymatolithon calcareum nodules into smaller pieces resulting in easier displacement by wave action, a reduced structural heterogeneity and reduced biodiversity of the bed (Kamenos et al., 2003).

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

None Very Low High
Q: High
A: High
C: High
Q: High
A: Medium
C: High
Q: High
A: Medium
C: High

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

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

Medium Very Low Medium
Q: Medium
A: Medium
C: Medium
Q: High
A: Medium
C: High
Q: Medium
A: Medium
C: Medium

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

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

None Very Low High
Q: High
A: High
C: High
Q: High
A: Medium
C: High
Q: High
A: Medium
C: High

Smothering results from the rapid deposition of sediment or spoil, which may occur after dredging (suction or scallop), capital dredging (channelization), extreme runoff, spoil dumping etc.  The effects depend on the nature of the smothering sediment.  For example, live maerl was found to survive burial in coarse sediment (Wilson et al., 2004) but to die in fine sediments.  Phymatolithon calcareum survived for four weeks buried under 4 and 8 cm of sand or gravel but died within 2 weeks under 2 cm of muddy sand.  Wilson et al. (2004) suggested that the hydrogen sulphide content of the muddy sand was the most detrimental aspect of burial since even those maerl nodules on the surface of the muddy sand died within two weeks. They also suggested that the high death rate of maerl observed after burial due to scallop dredging (Hall-Spencer & Moore, 2000a,c) was probably due to physical and chemical effects of burial rather than a lack of light (Wilson et al., 2004).

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

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

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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

Not Assessed (NA) Not assessed (NA) Not assessed (NA)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

‘Not assessed’.

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

‘No evidence’.

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

‘Not relevant’.

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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

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

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

‘Not relevant’.

Biological Pressures

 ResistanceResilienceSensitivity
No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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

None Very Low High
Q: High
A: High
C: High
Q: High
A: Medium
C: High
Q: High
A: Medium
C: High

No evidence of the effects of non- indigenous invasive species in the UK was found.  However, Grall & Hall-spencer (2003) note that beds of invasive slipper limpet Crepidula fornicata grew across maerl in Brittany.  As a result, the maerl thalli were killed, and the bed clogged with silt and pseudo-faeces, so that the associated community was drastically changed.  Bivalve fishing was also rendered impossible. 

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

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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

None Very Low High
Q: High
A: High
C: Medium
Q: High
A: Medium
C: High
Q: High
A: Medium
C: Medium

Phymatolithon calcareum can be sold dried as a soil additive but is also used in animal feed, water filtration systems, pharmaceuticals, cosmetics and bone surgery.  Maerl beds containing Phymatolithon calcareum are dredged for scallops (found in high densities compared with other scallop habitats) where extraction efficiency is very high.  This harvesting has serious detrimental effects on the diversity, species richness and abundance of maerl beds (BIOMAERL team, 1999). 

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

Kamenos et al. (2003) reported that maerl grounds impacted by towed demersal fishing gears are structurally less heterogeneous than pristine, un-impacted maerl grounds, diminishing the biodiversity potential of these habitats. Birkett et al. (1998a) noted that although maerl beds subject to extraction in the Fal estuary exhibit a diverse flora and fauna, they were less species-rich than those in Galway Bay, although direct correlation with dredging was unclear.  Grall & Glemarec (1997; cited in Birkett et al., 1998a) reported few differences in biological composition between exploited and control beds in Brittany.  Dyer & Worsfold (1998) showed differences in the communities present in exploited, previously exploited and unexploited areas of maerl bed in the Fal Estuary but it was unclear if the differences were due to extraction or the hydrography and depth of the maerl beds sampled.  In Brittany, many of the maerl beds are subject to extraction (Grall & Hall-Spencer, 2003).  For example, the clean maerl gravel of the Glenan maerl bank described in 1969 was degraded to muddy sand dominated by deposit feeders and omnivores within 30 years. Grall & Hall-Spencer (2003) noted that the bed would be completed removed within 50-100 years at the rates reported in their study. 

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

None Very Low High
Q: High
A: High
C: High
Q: High
A: Medium
C: High
Q: High
A: Medium
C: High

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

Sensitivity assessment.  The resistance to removal is ‘None’ due to the inability of Phymatolithon calcareum to evade collection. The resilience is ‘Very low’, with recovery only being able to begin when the harvesting pressure is removed altogether. This gives an overall sensitivity score of ‘High’.

Importance review

Policy/legislation

UK Biodiversity Action Plan Priority
Species of principal importance (England)
Species of principal importance (Wales)
Features of Conservation Importance (England & Wales)

Status

Non-native

Importance information

Maerl forms a very complex structure and provides a substratum for other species as well as crevices and shelter. Maerl beds are highly species rich with 150 macroalgal species and over five hundred faunal species (of which 120 are molluscs) recorded as living on or in maerl beds (Birkett et al., 1998a; Hall-spencer, 1998). Hall-Spencer et al. (2003) note that maerl beds are feeding areas for juvenile Atlantic cod, and host reserves of brood stock for razor shells Ensis spp., the great scallop Pecten maximus and the warty venus Venus verrucosa. and it may provide nursery areas for commercially important species of bivalves e.g. scallops (see the maerl biotope SS.SMp.Mrl for further information).

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).  The maerl bed in the Sound of Iona recorded to be ˂4000 years old (Hall-Spencer et al., 2003).  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).

Over 500,000 tonnes per annum of maerl are dredged up from the coast of Brittany. This is primarily sold dried as a soil additive. Maerl is frequently washed up in some locations in Scotland and Ireland in sufficient quantities to form white 'coral' beaches.

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Citation

This review can be cited as:

Perry, F. & Jackson, A. 2017. Phymatolithon calcareum Maerl. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from: http://www.marlin.ac.uk/species/detail/1210

Last Updated: 30/03/2017