Jncc coastal Directories Project Region 11 The Western Approaches

Coarse sediments of consolidated gravel, pebbles and
cobbles were dominated by the reef-building polychaete
Sabellaria (mainly S. alveolata, although some S. spinulosa was
also recorded).  These Sabellaria reefs may cover extensive
areas of the sea bed, particularly where there are tide-swept
hard substrata affected by turbid water - a feature rarely
found in other UK estuaries.  Indeed, the richest association
of species within the estuary (up to 25 species per sample)
was associated with these reefs.  Areas of medium/fine sand
in shallower water close to both north and south coasts were
characterised by a mix of bivalves, amphipods and
polychaetes.  Dives undertaken off Porlock Bay, Greenaleigh
Point and Watchet as part of the South-west Britain
Sublittoral Survey (Hiscock 1981) revealed a low species
richness compared with open sites further west.  Algae were
recorded only above chart datum level.  Stable hard substrata
were also found to have both species of Sabellaria, with the
sea squirt Dendrodoa grossularia, mussels Mytilus edulis and
the polychaete Polydora ciliata common at some sites.  The
Severn Estuary SMA has as its seaward boundary a line from
Brean Down to Lavernock Point (English Nature 1994a).
There is little difference between the intertidal and
subtidal habitats found along the northern shoreline of the
Severn Estuary and those on the southern shoreline: both
consist largely of mud and sand mixtures with varying
amounts of gravel and occasional exposures of rock.  A
survey of soft sediment macroinvertebrates between
Sudbrook and Lavernock Point on the Welsh coast revealed
three main invertebrate groupings from a total of 62 taxa
(Jones & Davies 1983, in Davies in prep. a).  These were
dominated by polychaete worms, bivalve molluscs and
amphipod crustaceans.  Rocky shores predominate between
Lavernock Point and Nash Point.  It appears that Nash Point
marks the eastern edge of a biogeographical transition zone,
which extends westwards into Swansea Bay.  For instance,
the gastropod Littorina neritoides and the kelp Laminaria
digitata are rarely found to the east of the headland (Davies
1995a).  
Offshore (defined as beyond 3 km or 50 m depth)
There is little offshore information available for the Cornish
coast of this region but the sublittoral habitats and
communities of the Bristol Channel and the Severn Estuary
have been well studied.  Here, grab and dredge sampling on
a grid of 155 stations in 1972/3 provided information on the
composition and distribution of communities within the
system (Warwick & Davies 1977).  These were divided into
five main community types (
Table 4.2.3
).  
4.2.3  Human activities
The effects of fisheries and mariculture on sea-bed habitats
and communities in the region are described in sections
5.5.3
, 
9.1.3 
and 
9.2.3
.  Other human activities that disturb the
sea bed include the extraction of marine aggregates and
navigational dredging (
section 9.4
) and maerl dredging
(section 5.5).  Water quality is affected by the discharge of
industrial and sewage effluent (
section 9.6
).
4.2.4  Information sources used
The information used is predominantly that gathered for the
JNCC’s Marine Nature Conservation Review (MNCR).  The
MNCR team (and their contractors) use a standard recording
methodology for both littoral and sublittoral surveys, which
includes descriptions of both habitats and their associated
communities (see Connor & Hiscock 1996).  Survey
information from other sources may vary considerably in its
methodology and coverage.  
Table 4.2.4
shows the number
of sites in the region with marine benthic (sea-bed and sea-
shore) habitat and species information held on the MNCR
database.  
Maps 4.2.2
and 
4.2.3
show the locations of all
littoral and all near-shore sublittoral surveys on the MNCR
database.  This information is not fully comprehensive.
Records additional to those cited here may exist in sources
that were not consulted.  The shores of Devon and Cornwall
were investigated by the Nature Conservancy Council-
commissioned Intertidal survey of Great Britain (Powell et al.
1978).  A considerable amount of published and
unpublished information on Cornish shores is held at the
Cornish Biological Records Unit, and on Devon’s shores by
the Devon Wildlife Trust.  The Institute for Marine and
Environmental Research, Plymouth, undertook a survey of
sea-bed habitats and communities in the Bristol Channel in
1972/73, taking grab and dredge samples; the distribution
of the main community types is mapped in Warwick &
Davies (1977).  Nearshore sublittoral habitats and
4.2  The sea bed
73
Table 4.2.3  
Sublittoral communities present in the Bristol Channel and the Severn Estuary
Community
Habitat description
Notes/associated species
I
Bivalve Venus
Sands, especially in the outer
A bivalve Tellina community on hard-packed sand, and a
community
Bristol Channel
bivalve Spisula community on loose sands.
II
Bivalve Abra
On silty or mixed bottoms in the outer 
The bivalve Abra alba, the polychaetes Scalibregma inflatum
community
Channel
and Lagis koreni, and others
III
Horse mussel
On hard substrate, mostly in the central 
The hermit crab Pagurus bernhardus, the scaleworm
Modiolus community
Channel
Lepidonotus squamatus, the brittlestar Ophiothrix fragilis,
and others
IV
Reduced species
On rocky substrate subjected to strong
The polychaetes Typosyllis armillaris, Eulalia tripunctata,
diversity hard
tidal scour, mostly in the inner part of
Sabellaria alveolata, Sabellaria spinulosa and others
substrate community
the Channel
V
Reduced species
In fluid muds of the inner Channel
The polychaetes Tharyx marioni, Nephtys hombergii and
diversity soft
Peloscolex spp.
substrate community
Source: after Warwick & Davies (1977)

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