Definition of biogenic reefs
Definition of Biogenic Reefs for this Report
Species Not Considered in Detail in this Report
Definition of Biogenic Reefs for this
The JNCC report on selection of Special Areas of Conservation in the UK
(Brown et al., 1997), discussed the background to selection on the basis of the presence
of reefs, and stated:
"Reefs are rocky marine habitats or biological concretions that
rise from the sea bed. They are generally subtidal but may extend as an unbroken
transition to the intertidal zone, where they are exposed to the air at low tide. Two main
types of reef can be recognised; those where structure is created by the animals
themselves (biogenic reefs) and those where animal and plant communities grow on raised or
protruding rock. Only a few invertebrate species are able to develop biogenic reefs, which
are therefore restricted in distribution and extent."
This is a definition with which most marine scientists would agree, but
not surprisingly there are difficulties in interpretation. There are obviously in many
cases continuous gradations between communities which are clearly not reefs (e.g.
scattered Modiolus within a gravel bed; a discontinuous bed of Mytilus;
scattered patches of Sabellaria spinulosa or S. alveolata crust;
moderate aggregations of serpulids upon rocks) and those which clearly are (e.g.
continuous, dense, raised Modiolus, Mytilus or S. spinulosa on mixed
substrata; massive aggregations of S. alveolata on more solid substrata; large,
discrete aggregations of Serpula vermicularis on muddy substrata). In this report
we have used the following criteria in defining biogenic reefs;
- the unit should be substantial in size (generally of the order of a metre or two across
as a minimum, and somewhat raised, mainly in order to disqualify nodule like aggregations
such as may be formed by S. spinulosa and scattered small aggregations such as
occurs with many of the species under consideration);
- and should create a substratum which is reasonably discrete and substantially different
to the underlying or surrounding substratum, usually with much more available hard
surfaces and crevices on and in which other flora and fauna can grow.
More importance has been attached to reefs which are reasonably stable
than to those which are more transient (e.g. Sabellaria spinulosa crusts which may
just about fit the above criteria but act as an annual feature, being destroyed by winter
storms and re-establishing each spring) but all have been discussed.
There are many cases where a community meets the two criteria we have
suggested above, except that it is not somewhat raised (i.e. it does not
rise from the seabed as in the JNCC definition). Although in many cases it is
probably more realistic to refer to these as beds, the ecology, biology, and sensitivity
of these areas are nevertheless likely to be very similar to those of protruding reefs,
and they are discussed here alongside true biogenic reef communities. There are also
cases, particularly with Mytilus, where widespread, dense aggregations are formed
on hard substrata and although they undoubtedly form a substratum which is
reasonably discrete and substantially different to the underlying or surrounding
substratum they are probably better regarded as part of the normal rocky shore
biota. In many such cases these aggregations are not substantially raised above the
surrounding area. Similar arguments may apply to some subtidal Modiolus, Sabellaria
alveolata, and S. spinulosa communities.
The definition of biogenic reefs as used in this report is therefore as
"Solid, massive structures which are created by accumulations
of organisms, usually rising from the seabed, or at least clearly forming a substantial,
discrete community or habitat which is very different from the surrounding seabed. The
structure of the reef may be composed almost entirely of the reef building organism and
its tubes or shells, or it may to some degree be composed of sediments, stones and shells
bound together by the organisms."
For the purposes of this report, biogenic reefs created by the
following species have been discussed in detail: Sabellaria alveolata, S. spinulosa,
Modiolus modiolus, Mytilus edulis and Serpula vermicularis.
Species Not Considered in Detail in this Report
There are other potential candidates for the title biogenic
reefs which are not considered in detail in this report; these are listed below with
Lophelia pertusa has not been included because it is probably
limited to very deep waters and there is little likelihood of reefs occurring in areas
which may be designated as SACs.
Oysters: on sheltered shores and estuaries in southern Europe dense
beds of oysters (Crassostrea gigas) occur. These tend to form sheets on rocky
shores and on artificial substrata such as piles or quaysides. This species is found in
southern parts of Britain but is restricted to occasional spatfalls rather than
established populations. Beds of European oysters (Ostrea edulis) used to occur
widely in the subtidal of northern Europe. To what extent these were natural is unknown,
as in some locations relaying of oyster beds is thought to date back to Roman times. Many
of these beds have been dredged out, or have been heavily impacted by disease. In many
locations they were almost certainly the remnants of culture activities, and it is
doubtful if natural beds would qualify as reefs in any case.
Limaria hians beds; although capable of binding sediment with their
byssus threads, these are probably best regarded as semi-infaunal bivalve beds.
Musculus discors; this is a small mussel which forms beds on
moderately exposed circalittoral rocks. However, these are not usually more than one
animal thick and do not form any significant raised reef area.
Ficopomatus enigmaticus (formerly Mercierella enigmatica);
this is an alien serpulid polychaete which can form extensive, well developed reefs in
scattered low salinity habitats within Britain. Due to the low salinity in which these
reefs occur few other species are found with them. It undoubtedly forms biogenic reefs,
but is not considered further here because of its alien status.
Lanice conchilega; During preparation of this report, the following
pers. comm. information was received from Paul Brazier (MNCR, JNCC) and Helen Vine (SAC
officer, the Wash and N. Norfolk Coast): during intertidal surveys of the North Norfolk
coast, unusual accretions formed by sand masons Lanice conchilega were found low on
the shore. These were up to 45 cm proud of the surrounding gravelly sediment, and
typically up to 1-2 m across (though occasional larger areas were found). They were
clearly stabilising the sediment. There was an associated faunal assemblage containing Sagartia
elegans, which was different to that in the surrounding, more gravelly, substratum. It
has been suggested that these might just about qualify as biogenic reefs. Lanice
conchilega is known to be capable of stabilising sediment and Larsonneur (1994)
reported that sand stabilised by sand masons is sufficiently stable to allow subsequent
colonisation by S. alveolata. However, it is not known how seasonal / stable these
features are, and it presently seems unlikely that they are sufficiently solid or altered
to qualify as biogenic reefs. They are not considered in this report.