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 Report

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 follows:

"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 brief reasons:

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.

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