Download Maerl Beds - Scottish Natural Heritage

Please quote as: Lancaster, J. (Ed.), McCallum, S., Lowe A.C., Taylor, E., Chapman A. & Pomfret, J. (2014).
Development of detailed ecological guidance to support the application of the Scottish MPA selection guidelines in
Scotland’s seas. Scottish Natural Heritage Commissioned Report No.491. Maerl Beds – supplementary document.
Maerl Beds
Biotopes:
SS.SMp.Mrl.Lcor
Lithothamnion corallioides maerl beds on infralittoral muddy gravel.
SS.SMp.Mrl.Lgla
Lithothamnion glaciale maerl beds in tide-swept variable salinity
infralittoral gravel.
`
SS.SMp.Mrl.Pcal
Phymatolithon calcareum maerl beds in infralittoral clean gravel or
coarse sand.
Sub-biotopes:
SS.SMp.Mrl.Pcal.R
Phymatolithon calcareum maerl beds with red seaweeds in shallow
infralittoral clean gravel or coarse sand.
SS.SMp.Mrl.Pcal.Nmix
Phymatolithon calcareum maerl beds with Neopentadactyla mixta
and other echinoderms in deeper infralittoral clean gravel or coarse
sand.
Territorial/Offshore waters: Territorial
Maerl is calcareous red algae that grow as nodules (occasionally crusts) forming
dense, but relatively open beds of algal gravel. Maerl beds, including dead maerl,
have a complex open structure formed by interlocking maerl thalli allowing water to
circulate, providing perfect conditions for a diverse community of organisms to exist.
The maerl provides an attachment site for animals such as feather stars, hydroids
and bryozoans. The loose structure provides shelter for small gastropods,
crustaceans, bivalves and juvenile fish, and the infauna includes many bivalves such
as Mya truncata and Dosinia exoleta. Epifauna include small Crustacea (Farnham &
Bishop, 1985; Jones et al., 2000). Red seaweeds, sea firs and scallops may also
colonise the surface.
Many species have a high specificity to maerl beds, such as species of polychaetes
(e.g. Glycera lapidum, Sphaerodorum gracilis and Polygordius lacteus) and
amphipods (e.g. Parametaphoxus fultoni, Atylus vedlomensis and Animoceradocus
semiserratus) (BIOMAERL team, 1999). Several species of algae are almost entirely
restricted to calcareous habitats and are characteristically found in maerl beds (e.g.
Halymenia latifolia, Scinaia turgida, Gelidiella calcicola, Gelidium maggsiae and
Cruoria cruoriaeformis) (Birkett et al., 1998). Studies of the faunal composition from
within maerl beds have shown that they often exist as isolated islands of high benthic
diversity and biomass (BIOMAERL team, 1999). Maerl beds add considerable
biological diversity to an area as well as adding value to other ecosystem functions.
For example Hauton et al. (2003), commented that undamaged maerl grounds can
be of long-term benefit to fisheries, acting as reproductive reservoirs for future
generations of commercially important species, e.g. cod, edible crabs and scallops.
Functional Links
Functional links and associations with Priority Marine Features

Flame shell beds: Nests of the flame shell Limaria hians are often found in
conjunction with maerl (Hall-Spencer et al., 2003). L. hians binds maerl
together with its byssal threads, helping to stabilise the maerl bed (Birkett et
al., 1998).

Maerl or coarse shell gravel with burrowing sea cucumbers: Areas of
predominantly dead maerl and other coarse gravels with burrowing sea
cucumbers often occur on the edge of maerl beds.

Native oyster (Ostrea edulis): Maerl beds have been suggested as important
nursery areas for native oysters, Ostrea edulis (Hall-Spencer et al., 2003).

Horse mussel beds: Modiolus modiolus binds maerl together with its byssal
threads, helping to stabilise the maerl bed (Birkett et al., 1998).
Functional links and associations with wider Scottish marine ecosystem
Maerl beds provide feeding areas for juvenile gadoid fish such as Atlantic cod (HallSpencer et al., 2003; Kamenos et al., 2004a).
Maerl beds act as important nursery areas for commercially valuable molluscs such
as scallops (Pecten maximus and Aequipecten opercularis) and razor shells (Ensis
spp.) (Birkett et al., 1998; Hall-Spencer et al., 2003; Kamenos et al., 2004b,c) and
also crustacea (Cancer pagurus).
Maerl is important in calcium carbonate cycling, providing a source for other coastal
habitats, e.g. beaches and dunes (Birkett et al., 1998).
Deep burrowers and tube dwellers (e.g. Cerianthus sp., Sabella sp., Chaetopterus
sp. and Upogebia sp.) can stabilise the underlying sediment upon which maerl beds
develop (Birkett et al., 1998).
Biological Diversity
Habitat/Biotope description for Scottish waters
There are four biotopes (and two sub-biotopes) described for maerl beds. Three
biotopes (and two sub-biotopes) occur in Scottish waters, SS.SMp.Mrl.Pcal (and the
sub biotopes SS.SMp.Mrl.Pcal.R and SS.SMp.Mrl.Pcal.Nmix), SS.SMp.Mrl.Lcor and
SS.SMp.Mrl.Lgla The following biotope descriptions are taken directly from the
Marine Habitat Classification Hierarchy (Connor et al., 2004). Where needed and
possible, below each box, they are supplemented with additional information specific
to Scotland from the scientific literature.
Phymatolithon calcareum maerl beds in infralittoral clean gravel or coarse
sand (SS.SMp.Mrl.Pcal)
“Maerl beds characterised by P. calcareum in gravels and sands. Associated
epiphytes may include red algae such as Dictyota dichotoma, Halarachnion
ligulatum, Callophyllis laciniata, Cryptopleura ramosa, B. byssoides and Plocamium
cartilagineum. Algal species may be anchored to the maerl or to dead bivalve shells
2
amongst the maerl. Polychaetes, such as Chaetopterus variopedatus, Lanice
conchilega,
Kefersteinia
cirrata,
Mediomastus
fragilis,
Chone
duneri,
Parametaphoxus fultoni and Grania sp. may be present. Gastropods such as Gibbula
cineraria, Gibbula magus, Calyptraea chinensis Dikoleps pusilla and Onoba aculeus
may also be present. Liocarcinus depurator and Liocarcinus corrugatus are often
present, although they may be under-recorded; it would seem likely that robust
infaunal bivalves such as Circomphalus casina, M. truncata, D. exoleta and other
venerid bivalves are more widespread than available data currently suggests. It
seems likely that stable wave-sheltered maerl beds with low currents may be
separable from this biotope, having a generally thinner layer of maerl overlying a
sandy /muddy substratum with a diverse cover of epiphytes but insufficient data
currently exists on a national scale. Wave and current-exposed maerl beds, where
thicker depths of maerl accumulate, frequently occur as waves and ridge / furrows
arrangements. At some sites where P. calcareum occurs, there may be significant
patches of maerl gravel containing the rare burrowing anemone Halcampoides
elongatus; this may be a separate biotope, but insufficient data exists at present.
Northern maerl beds in the UK do not appear to contain Lithothamnion corallioides
but in south-west England and Ireland L. corallioides may occur to some extent in
this biotope as well as Lcor1, where it dominates” (Connor et al., 2004).
Whilst Connor et al. (2004) indicates that northern maerl beds do not appear to
contain L. corallioides, there are records of this species in Scottish territorial waters
(Jackson, 2007a).
There are two sub-biotopes to SS.SMp.Mrl.Pcal which are described below:
Phymatolithon calcareum maerl beds with red seaweeds in shallow infralittoral
clean gravel or coarse sand (SS.SMp.Mrl.Pcal.R)
“Upper infralittoral maerl beds characterised by P. calcareum in gravels and sand
with a wide variety of associated red seaweeds. These algae typically include
D. dichotoma, P. cartilagineum, Phycodrys rubens, Chondrus crispus, H. ligulatum,
Chylocladia verticillata, Hypoglossum hypoglossoides and Nitophyllum punctum.
These species are not restricted to maerl beds but their abundance on maerl beds
differentiates this biotope from Pcal.Nmix2. Anthozoans and echinoderms are much
less common in this biotope than in Pcal.Nmix2, which typically occurs deeper than
this biotope” (Connor et al., 2004).
Phymatolithon calcareum maerl beds with Neopentadactyla mixta and other
echinoderms in deeper infralittoral clean gravel or coarse sand
(SS.SMp.Mrl.Pcal.Nmix)
“Lower infralittoral maerl beds characterised by P. calcareum in gravels and sand
with a variety of associated echinoderms. The echinoderm N. mixta is frequently
observed in this biotope. Other echinoderms such as Echinus esculentus, Ophiura
1
Lithothamnion corallioides maerl beds on infralittoral muddy gravel
Phymatolithon calcareum maerl beds with Neopentadactylla mixta and other echinoderms in
deeper infralittoral clean gravel or coarse sand
2
3
albida and rarely Luidia ciliaris may also be present. Red seaweed such as P.
cartilagineum may be present but at a much lower abundance than in Pcal.R3 and
with fewer species present. Other, more ubiquitous echinoderms such as Asterias
rubens may also be found in low numbers throughout P. calcareum biotopes”
(Connor et al., 2004).
Lithothamnion corallioides
(SS.SMp.Mrl.Lcor)
maerl
beds
on
infralittoral
muddy
gravel
“Live maerl beds in sheltered, silty conditions which are dominated by L. 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 P. calcareum and Phymatolithon purpureum, may also
occur as a less abundant component. Species of seaweed such as D. dichotoma,
H. 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 this biotope whereas E. esculentus
tends to occur more in other types of maerl. The seaweeds Saccharina latissima and
Chorda filum may also be present in some habitats. This biotope has a southwestern distribution in Britain and Ireland. Sheltered, stable, fully saline maerl beds
in the north of Great Britain (where L. corallioides has not been confirmed to occur)
may need to be described as an analogous biotope to this one (see Pcal4)” (Connor
et al., 2004).
Whilst Connor et al. (2004) indicate in their biotope description for Pcal4 that northern
maerl beds do not appear to contain L. corallioides, it is important to highlight that
there are records of this species in Scottish territorial waters (Jackson, 2007a).
Lithothamnion glaciale maerl beds in tide-swept variable salinity infralittoral
gravel (SS.SMp.Mrl.Lgla)
“Upper infralittoral tide-swept channels of coarse sediment in full or variable salinity
conditions support distinctive beds of L. glaciale maerl 'rhodoliths'. P. calcareum may
also be present as a more minor maerl component. Associated fauna and flora may
include species found in other types of maerl beds (and elsewhere), e.g.
Pomatoceros triqueter, C. lloydii, Sabella pavonina, C. variopedatus, L. conchilega,
M. truncata, P. cartilagineum and P. rubens. Lgla5, however, also has a fauna that
reflects the slightly reduced salinity conditions, e.g. Psammechinus miliaris is often
present in high numbers along with other grazers such as chitons and Tectura spp.
Hyas araneus, Ophiothrix fragilis, Ophiocomina nigra and the brown seaweed D.
dichotoma are also typically present at sites. In Scotland, this biotope may show
considerable variation but the community falls within the broad description defined
here. This biotope can often be found at the upper end of Scottish sea lochs where
the variable salinity of the habitat may not be immediately obvious” (Connor et al.,
2004).
3
Phymatolithon calcareum maerl beds with red seaweeds in shallow infralittoral clean gravel
or coarse sand
4
Phymatolithon calcareum maerl beds in infralittoral clean gravel or coarse sand
5
Lithothamnion glaciale maerl beds in tide-swept variable salinity infralittoral gravel
4
Species diversity
Some of the most diverse examples of this search feature are described from the
Sound of Arisaig on the west coast, where three maerl species were recorded along
with a rich associated infauna (236 taxa) and epifauna (185 taxa) in five survey sites
(Moore et al., 2004). One transect of 100m2 was conducted for epibiota in each of the
five maerl sites surveyed. The most sheltered maerl sites were found in the inner
region of Loch Ailort (biotope .Lgla5) and in the south channel of Loch Moidart
(biotope .Pcal.R3) on silty sediments. These sites had the highest number of
epibionts of the maerl habitats surveyed with 92 epibionts per 100m2 in both sites.
Other sites containing the biotope .Pcal.R3 in the outer region of Loch Ailort and Loch
Ceann Traigh had 77 and 67 associated epibionts respectively per 100m2. In Loch
Moidart North Channel (biotope .Pcal4) there were 40 species of epibiont per 100m2
(Moore et al., 2004). In addition to this, core samples were taken for the infauna
communities and details of the species richness and diversity by biotope are
provided in Table 1.
Moore et al. (2006) also conducted work in maerl beds in Loch nam Madadh SAC,
where the number of epibiota taxa ranged from 46 to 100 per transect (100 m 2) for
biotopes .Pcal4 and .Pcal.R3. The mean infauna abundance ranged from 1884 to
2880 individuals/0.1 m2. The mean number of infauna taxa ranged from 30 to 46 per
0.1 m2.
Table 1. Species richness data for infauna for biotopes listed within this search feature
(calculated from data in Moore et al., 2004)
Biotope
Mean abundance
Mean no. of Mean
Mean Shannonof infauna
species
per Pielou
Wiener
individuals per
core
evenness
diversity (log2)
3
3
core (0.00167m )
(0.00167m )
SS.SMp.Mrl.Pcal
89
36
0.9
4.63
(site O3)
SS.SMp.Mrl.Pcal.R
331
53
0.82
4.65
(sites C6, S6 and Z59)
SS.SMp.Mrl.Lgla
123
52
0.91
5.21
(site Y10)
5
Key and characterising species
These have been taken from JNCC’s biotope descriptions (Connor et al., 2004).
Biotope type
Key species for
identification
Phymatolithon
calcareum
Additional characterising species
SS.SMp.Mrl.Pcal.R
Phymatolithon
calcareum
Cerianthus lloydii, Chaetopterus variopedatus, Lanice
conchilega, Pomatoceros triqueter, Pagurus bernhardus,
Liocarcinus depurator, Gibbula cineraria, Asterias rubens,
Echinus
esculentus,
Bonnemaisonia
asparagoides,
Cystoclonium purpureum, Plocamium cartilagineum,
Lomentaria clavellosa, Nitophyllum punctatum, Phycodrys
rubens, Brongniartella byssoides, Trailliella intricata,
Dictyota dichotoma, Desmarestia aculeata, Desmarestia
viridis, Chorda filum, Saccharina latissima, Ulva spp.
SS.SMp.Mrl.Pcal.Nmix
Phymatolithon
calcareum and
N. mixta
Cerianthus lloydii, Chaetopterus variopedatus, Lanice
conchilega, Pomatoceros triqueter, Pagurus bernhardus,
Liocarcinus depurator, Gibbula magus, Tectura virginea,
Pecten maximus, Ensis sp., Luidia ciliaris, Asterias rubens,
Ophiura albida, Echinus esculentus, Pomatoschistus spp.,
Plocamium cartilagineum, Desmarestia viridis, Saccharina
latissima.
SS.SMp.Mrl.Lcor
Lithothamnion
corallioides
Cerianthus lloydii, Anemonia viridis, Nematoda, Pisione
remota, Harmothoe impar, Pholoe inornata, Glycera
lapidum, Kefersteinia cirrata, Sphaerosyllis bulbosa,
Lumbrineris gracilis, Protodorvillea kefersteini, Aonides
paucibranchiata, Caulleriella alata, Mediomastus fragilis,
Notomastus latericeus, Terebellidae, Lanice conchilega,
Pista cristata, Nannonyx goesii, Microdeutopus versiculatus,
Caprella
acanthifera,
Janira
maculosa,
Pagurus
bernhardus, L. corrugatus,
Liocarcinus depurator,
Polyplacophora, Gibbula magus, Pecten maximus, Mysella
bidentata, Parvicardium scabrum, Gari tellinella, Hiatella
arctica,
Asterias
rubens,
Marthasterias
glacialis,
Amphipholis
squamata,
Phymatolithon
calcareum,
Dudresnaya verticillata, Halarachnion, Dictyota dichotoma,
Chorda filum, Saccharina latissima, Ulva spp.
SS.SMp.Mrl.Lgla
Lithothamnion
glaciale
Cerianthus lloydii, Pomatoceros triqueter, Pagurus
bernhardus, Carcinus maenas, Gibbula. cineraria,
Buccinum undatum, Ostrea edulis, Asterias rubens,
Ophiothrix fragilis, Ophiocomina nigra, Psammechinus
miliaris,
Echinus
esculentus,
Corallina
officinalis,
Phymatolithon calcareum, Chondrus crispus, Trailliella
intricata, Dictyota dichotoma, Chorda filum, Saccharina
latissima.
SS.SMp.Mrl.Pcal
Porifera, Porifera indet crusts, Obelia dichotoma, Nemertea,
Nematoda, Harmothoe impar, Kefersteinia cirrata, Eurysyllis
tuberculata, Trypanosyllis coeliaca, Sphaerosyllis taylori,
Aonides paucibranchiata, Mediomastus fragilis, Polycirrus,
Chone duneri, Grania, Parametaphoxus fultoni, Socarnes
erythrophthalmus, Austrosyrrhoe fimbriatus, Ceradocus
semiserratus,
Gammaropsis
cornuta,
Leptocheirus
hirsutimanus, Leptocheirus pectinatus, Cymodoce truncata,
Vaunthompsonia cristata, Cumella pygmaea, Nannastacus
unguiculatus, Liocarcinus depurator, Gibbula cineraria,
Dikoleps pusilla, Onoba aculeus, Crisia, Alcyonidium
diaphanum, Scrupocellaria reptans, Escharoides coccinea,
Microporella ciliata, Cellepora pumicosa, Asterias rubens,
Amphipholis squamata, Clavelina lepadiformis, Didemnidae,
Callionymus lyra, Hildenbrandia rubra, Corallinaceae,
Corallina officinalis, Lithothamnion glaciale, Saccharina
latissima.
6
Coherence
Typicalness
Maerl is a collective term for several species of calcified red seaweed which grow as
unattached nodules, or rhodoliths, on sediments (OSPAR, 2008). They have a hard
chalky skeleton and form small rounded nodules or short branched twig-like shapes
called thalli (BIOMAERL team, 1999). In favourable conditions, these species can
form extensive beds, typically 30% cover or more, mostly in coarse clean sediments
of gravels and clean sands or muddy mixed sediments (OSPAR, 2008). Living,
photosynthetically active maerl occupies the surface layer, while deeper layers of
maerl are dead (but still an important habitat). The branching nodules of maerl form
an interlocking but relatively open structure that provides a considerable surface area
for attachment of other organisms. Maerl beds can be mobilised through high water
movement, such as winter storms, as the thalli are not attached. However, some
bivalves and burrowing species (e.g. Modiolus modiolus, Limaria hians, Cerianthus
spp. & Sabella spp.) and seaweed (e.g. Saccharina latissima) can act to stabilise the
underlying sediment and maerl bed (Birkett et al., 1998).
Maerl beds typically develop where there is some tidal flow, such as in the narrows
and rapids of sea lochs, or the straits and sounds between islands (BIOMAERL
team, 1999). Beds may also develop in more open areas where wave action is
sufficient to remove fine sediments, but not strong enough to break the brittle maerl
branches. Maerl beds have been recorded from a variety of depths, ranging from the
lower shore to 30m depth (OSPAR, 2008). As maerl requires light to
photosynthesize, the depth at which it grows is determined by water turbidity. As a
result of this, it is mainly found on coarse clean sands and gravels either on the open
coast or in tide swept channels to a depth of about 20m, although occasional records
from muddier sediments exist, e.g. in 1-5m of water at the head of Loch Torridon
(Moore & Atkinson, 2012).
Ecological variation across Scottish waters
The maerl species present varies depending on sediment type, water movement and
salinity. In fully marine conditions the dominant maerl is typically Phymatolithon
calcareum, which is found on coarse sand and gravel, such as in Wyre and Rousay
Sounds (Hirst et al. 2013). Under variable salinity conditions, for example in some
sea lochs such as Loch Sween and Loch Ewe, beds of Lithothamnion glaciale have
developed (Moore et al. 2011; Moore & Harries 2013) . Lithothamnion coralloides is
generally found in sheltered conditions on silty sediments (Jackson, 2007a).
The associated communities of maerl beds also vary with sediment type, water
movement, depth of bed and salinity (Davies & Hall-Spencer, 1996; Connor et al.,
2004; Moore et al., 2004). On coarse sediments, in shallow conditions, .Pcal maerl
beds are generally characterised by epiphytic red algae (SMp.Mrl.Pcal.R) while
deeper examples have more echinoderms, e.g. Neopentadactyla mixta
(SMp.Mrl.Pcal.Nmix). Thicker maerl beds occur in areas of high water movement
(from waves or currents), while sheltered beds tend to be thinner with more
epiphytes. Within the North MPA region at Sullom Voe, Shetland, small patches of
maerl (Phymatolithon calcareum) have been recorded in clean deep infralittoral
gravel and coarse sand with an associated community dominated by hydroids and
echinoderms (SS.SMp.Mrl.Pcal.Nmix; Mair et al., 2010). In contrast, in the West MPA
region, in Loch na Droghniche, the Sound of Handa and Tob Valasay (Outer
Hebrides), shallow maerl beds were found, associated with a significant cover of a
variety of algal species (up to 65% cover in the Sound of Handa; James, 2004; BMT
7
Cordah Ltd, 2004). Species included Dictyota dichotoma, Enteromorpha sp.,
Ectocarpus sp., Halidrys siliquosa, Saccharina latissima, Laminaria hyperborea and
other filamentous and foliose red algae, and coralline crusts. Associated fauna
included Asterias rubens, Echinus esculentus, Cerianthus lloydii, and Luidia ciliaris
(James, 2004; BMT Cordah Ltd, 2004). Further south in Loch Sunart, there are areas
of maerl and maerl gravel. Here, beds were found associated with scattered algae
and large numbers the anemone Cerianthus lloydii bordering stands of dense S.
latissima (Mercer et al., 2007)
Moore et al., (2004) found variation in the maerl species and associated communities
in a study of the Sound of Arisaig SAC on the west coast. Lithothamnion corallioides
and L. glaciale were present in the inner region of Loch Ailort on silty sediments,
along with the echinoderm Psammechinus miliaris. This is possibly a reflection of the
reduced salinity in this area. However, in the outer region of Loch Ailort, where there
are stronger currents, P. calcareum was the dominant species and beds here had the
highest number of infaunal taxa, and densest algal cover of all the maerl habitats in
the study. In Loch Moidart P. calcareum was the only maerl species found. All three
of these aforementioned sites also contained the anemones C. lloydii, A. viridis,
Metridium senile and S. elegans. However, the most exposed sites in Loch Ceann
Traigh and the mouth of the Loch Moidart North Channel had lower levels of fauna, a
lack of anemones and contained P. calcareum and L. glaciale. More L. glaciale was
present in the more exposed area, i.e. in the mouth of the Loch Moidart North
Channel. In Loch Ceann Traigh the community was dominated by the algae
Desmarestia aculeate, whilst in the Loch Moidart North Channel tubeworms and
barnacles were the most dominant taxa, with no hydroids or echinoderms recorded
(Moore et al., 2004). Therefore exposure does seem to be an important variable in
maerl bed community structure. Findings of Moore et al., (2004) confirm previous
work by Davies and Hall-Spencer (1996) who found lower infaunal and epifaunal
species richness at the most exposed sites in the same area. A similar pattern was
found in Loch nam Madadh where Moore et al., (2006) found the most exposed
maerl bed site also had the lowest number of epifaunal and infaunal taxa and there
was a significant difference in the species composition between sheltered and
exposed maerl beds.
Viability
The dispersal potential of maerl is thought to be limited from current knowledge of its
reproduction and propagation methods. L. glaciale has reproductive conceptacles
that produce dispersive spores allowing sexual and asexual reproduction (Jackson,
2007a). The dispersal potential of sexual propagules is unknown because these have
not been observed in UK waters and there have been no published studies of the
genetic exchange between populations (Hill et al., 2010). L. glaciale can also
propagate by vegetative (somatic) growth (Jackson, 2007a). However, for P.
calcareum and L. corallioides, vegetative growth is the main form of propagation
(Irvine & Chamberlain, 1994; Jackson, 2007b). As the reproduction of Scottish maerl
is assumed to be mainly vegetative, the dispersal potential is limited and is probably
less than 1 km, based on the dispersal of adults by water movements (Hill et al.,
2010).
Whilst many of the mapped maerl beds in Scottish waters are less than or around 1
km2 some are larger, up to around 9 km2. Due to the vegetative propagation methods
and low dispersal distances of adult plants individual maerl beds could be largely
8
self-sustaining, especially beds of P. calcareum and L. corallioides. In light of the
uncertainty around reproduction, viable area size and dispersal of maerl, it is
recommended that a precautionary approach should be taken to protect maerl beds
(Hill et al., 2010). Therefore, regardless of the size of the maerl bed it is
recommended that the whole bed is protected.
Longevity
Maerl is extremely slow-growing and its growth rates have been recorded in data
from Ireland, England, France, Norway, Scotland and Spain. These are of the order
of tenths of millimetres to two millimetre per year (Bosence & Wilson, 2003, Jackson
2008; 2007a, b). Adey (1970) estimated the life span of individual plants of L. glaciale
to be from 10 to 50 years. However, Jackson (2008; 2007a, b) indicates that the life
span of P. calcareum and L. glaciale may be slightly longer from 20 to 100 years. It is
thought that maerl beds are able to persist in one location for over 5000 years (Grall
& Hall-Spencer, 2003). The various maerl species within a single bed can show
periodic dominance and decline over time scales of between 3 and 30 years (Birkett
et al., 1998).
Fragmentation
The proportion of live maerl and level of fragmentation can vary greatly with water
movement, depth (light penetration), sediment and salinity. In the Sound of Arisaig a
considerable degree of variation was found in the proportion of live maerl to other
substrates. From the south channel of Loch Moidart to Rubha Ghead a Leighe,
waves of maerl gravel and sand were recorded mosaiced with rocky reefs and
coarse sediments with little or no live maerl (Moore et al. 2004).
9
Indicators of Least Damaged/More Natural
Up to date information on the sensitivity of Maerl beds to pressures associated with
human activities are included in the Feature Activity Sensitivity Tool (FeAST; Marine
Scotland, 2013). Below, information on indicators of naturalness and damage are
taken from primary literature (where referenced) or from MarLIN sensitivity data
(Jackson, 2007b; 2008) (where reference is absent).
Table 1. Indicators of damage and naturalness
Indicators of Naturalness
Indicators of Damage
Potential Sources of
damage
Presence of maerl bed with a lack
of smothering sediment
Layer of sediment over maerl
thalli (which may smother the
maerl)
Resuspended sediment,
altered hydrography,
extraction, eutrophication (e.g.
from aquaculture (Hall-Spencer
et al. 2006)
Presence of maerl bed with a low
cover of ephemeral algae
Increase in ephemeral algae
cover
Eutrophication (Haskoning UK
Ltd, 2006)
Intact maerl thalli with few broken.
Presence of typical assemblage of
associated biota
Broken/damaged maerl thalli.
Broken fragments of other
fragile animals e.g. tests of
Psammechinus miliaris and the
arms of Ophiothrix fragilis.
Reduction in number of sessile
epifauna such as Modiolus
modiolus or Limaria hians,
sponges and the anemone
Metridium senile (Hall-Spencer
& Moore, 2000a).
Physical disturbance increased
wave action, extraction (HallSpencer & Moore, 2000a, b;
De Grave & Whitaker, 1999)
Presence of intact, unburied maerl
bed, with no mobile fishing gear
tracks or creel/anchor damage in
bed
Mobile fishing gear tracks,
creels/pots or anchor damage in
bed, broken thalli, or thalli (often
dead) buried under sediment
(resulting in increased mortality)
Physical disturbance (HallSpencer & Moore, 2000a, b).
Maintenance of maerl bed area
Loss of maerl bed area
Extraction (De Grave &
Whitaker, 1999)
Full assemblage of associated
biota
Loss of associated high levels of
biological diversity
Changes in nutrient levels and
oxygenation (Willson et al.
2004)
Community composition
comprising of typical species (see
biological diversity section). Few
‘southern species’ present.
Increases in abundance of
‘southern species’ and/or
declines in characteristic
species within the community.
Increased ambient water
temperature possibly as a
result of climate change.
(Hiscock et al., 2004)
Risk Assessment
The details of the assessment of risk for each MPA search feature is addressed in a
separate report (Chaniotis et al., 2014).
10
Recovery Potential
The impacts of any damage to maerl beds are long lasting because the key habitat
structuring species has a very poor regenerative ability due to its extremely slow
growth rates (Hall-Spencer & Moore, 2000a; Bosence & Wilson, 2003). Maerl is
considered a non-renewable resource (Bosence & Wilson, 2003; OSPAR, 2008).
Smothering of maerl thalli
Smothering of the maerl thalli can result in a reduction in photosynthesis, with burial
under just 2.5 mm of muddy sand for two weeks leading to death (Willson et al.
2004). In a study of the impact of fish-farm deposition on maerl beds, Hall-Spencer
et al. (2006) highlighted that even in areas of relatively high currents there was likely
to be a build-up of organic material which could lead to death of the maerl thalli.
MarLIN’s sensitivity assessment for maerl beds states the recoverability of maerl to
smothering as being ‘very low’ or ‘no recovery’ (Jackson, 2007b; 2008).
Increase in ephemeral algae
Nutrient enrichment can lead to increased suspended sediments and smothering,
including by ephemeral epiphytic algae. This may impact the photosynthetic capacity
of the maerl. No information was found on recovery of maerl beds from an increase
in ephemeral algae.
Broken thalli
No specific information was found on the recovery potential from broken thalli
however, as successful continued growth of thalli after fragmentation is very sporadic
and growth rates are very slow (Bosence & Wilson, 2003), recovery rates are also
likely to be slow.
Physical disturbance
Physical damage to the surface layer of a bed can result in living maerl nodules
being buried and disrupt the physical integrity of accreted maerl beds (Jackson
2008). Hall-Spencer & Moore (2000a, b) reported that a single pass of a scallop
dredge could bury and kill 70% of the living maerl, redistribute coarse sediment and
affect the associated community. Recovery was slow, with dredge tracks remaining
visible for over two and a half years (Hall-Spencer & Moore, 2000a, b). Hall-Spencer
& Moore (2000a, b) suggested that repeated anchorage could create impacts similar
to towed fishing gear. Additionally, creels/pots can also cause surface impacts to
which maerl beds are sensitive (Tillin et al. 2010). MarLIN’s sensitivity assessment of
maerl beds states that this habitat has ‘very low’ or ‘no recovery’ ability from physical
disturbance (Jackson, 2007b; 2008).
Loss of maerl bed area
As maerl is considered a non-renewable resource (OSPAR, 2008) it is unlikely to
recover from any extraction activity. Partial removal of a bed and recovery of maerl
thalli is very slow and likely to take in the order of several decades to recover, if
recovery occurs at all (Jackson, 2008).
11
Loss of associated high levels of biological diversity
No information was found on recoverability of loss of biological diversity associated
with a maerl bed.
Climate change
L. glaciale is a northern species and most UK records are in Scotland, especially in
locations with variable or reduced salinity. A decline in this species and therefore the
biotope .Lgla in response to warming is likely (Hiscock et al. 2004). L. coralloides,
however, is a southern species and seawater warming may enable it to expand its
distribution north (Hiscock et al. 2004). P. calcareum is widely distributed from
Scotland to France and it is suggested that these maerl beds would persist.
Geographic Variation
The distribution of live maerl is determined by the physical conditions that favour
maerl growth; it cannot withstand desiccation or low light so is restricted to the low
intertidal and subtidal, to depths of around 30 m, in the relatively turbid waters of
Scotland. Maerl also requires a degree of shelter from wave action, to prevent
dispersal into deeper water, but requires sufficient water movement to prevent
smothering with silt and provide sufficient gaseous exchange (Hall-Spencer, 1998).
Therefore, maerl is usually restricted to places such as the sills of fjords and fjards
(sea lochs in Scotland), together with the shores to the leeward of headlands and
island archipelagos (Hall-Spencer, 1998; 2001a, b). Maerl beds are found all along
the west coasts of the UK, from Shetland to south-west England, but the vast
majority of beds are in Scotland (30% of the north-west European resource; HallSpencer, 2001a). Within Scotland, maerl beds are found in the west, north and
southwest MPA regions, with the west and north regions having the most records.
In the west region all three maerl biotopes (and the two .Pcal sub biotopes) have
been recorded. Biotopes .Pcal, .Pcal.R and .Pcal.Nmix are widespread along the
region’s coastline, concentrated within sea lochs and inlets on the mainland such as
the Sound of Arisaig and Loch Laxford and areas such as Loch nam Madadh and the
Sound of Barra in the Outer Hebrides. .Lcor and .Lgla are less widespread in the
region and are only recorded in a few locations on the west coast and Outer
Hebrides.
In the north region there are widespread records of maerl beds in tide-swept areas,
especially in Orkney and Shetland (Howson et al. 2009). .Pcal and .Pcal.R are
widespread in Shetland, Orkney and Loch Eriboll on the north coast of the Scottish
mainland. .Pcal.Nmix has also been recorded in Orkney and Shetland (Scapa Flow,
Lunna Ness and Out Skerries) but .Lgla has only been recorded on the west coast of
Shetland.
In the south-west region there is a single .Lcor record with .Lgla records in the region.
.Lgla has been recorded in areas including Loch Fyne, at the northern tip of the Isle
of Bute and in the Clyde Sea around Inchmarnock.
12
Geographic context
Elsewhere in the UK this habitat is recorded in Southern England at the Fal Estuary
(Jackson, 2007a; 2007b), South Wales, the Isle of Man and Lundy. Sparse records
exist from north and south-western Ireland (Howson et al., 2009). The abundance of
suitable physical habitat conditions for maerl growth in Scotland makes it an
important place in Europe for maerl, with the vast majority of maerl beds in the UK
situated in Scotland. Maerl is rare in the English Channel, Irish Sea, and North Sea
(Jackson, 2007a; 2007b).
Outside of the UK, maerl beds are found world-wide. In the northeast Atlantic maerl
occurs in discrete areas from the Canaries and Madeira (McMaster & Conover 1966;
Cabioch, 1974), north-west Spain (Adey & McKibbin, 1970), Brittany (Cabioch 1970;
Grall & Hall-Spencer, 2003), Denmark (King & Schramm, 1982), Russia, the Faeroe
Isles, Iceland, the western Baltic (although it is rare in these waters), along the
Norwegian shelf (Freiwald & Henrich, 1994) to the Arctic (Kjellmann, 1883; Adey &
Adey,1973). Although maerl beds are found in many places across the north-east
Atlantic, Scottish maerl beds are of particular importance with 30% of the north-west
European maerl resource thought to be in Scotland (Hall-Spencer, 2001a). In the
north-west Atlantic, maerl beds are recorded from Cape Cod, north to Arctic Canada
and into Greenland and it has also been found in northern Japan and China in the
western Pacific (Howson et al., 2009).
13
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