General Descriptions Of The Biotope Complexes

Distribution Of Biotope Complexes

In order to derive management plans and monitoring protocols for habitats designated under the EU Habitats Directive, it is necessary to define and describe these habitats. These sedimentary habitats cover large areas of the available shelf and thus are integral components of the other designated biotope complexes. Because of the nature of the wider marine and estuarine system encompassing these habitats, it is also necessary to link their features to other habitats (and/or biotope complexes) such as estuaries and large shallow inlets and bays.

In the context of Annex I of the Habitats Directive, this biotope complex is defined as ‘Mudflats and sandflats not covered by seawater at low tide’ (see Appendix I). The present report refers to this biotope complex as ‘intertidal mudflats and sandflats’ and considers both low and high energy flats. For consistency in the present report, the term ‘flats’ are used for intertidal features whereas ‘banks’ are usually reserved for subtidal features. In addition, and for completeness, sand beaches, which are predominantly coastal, are included in the term ‘sandflats’.

The community structure of intertidal flats is well studied and there are good background data for certain areas due to their importance and accessibility, especially their ease of study. This is particularly so for mudflats which contribute a large area of estuaries. There are well-defined communities such as Petersen's (1913) and Thorson’s (1957) Macoma community for muds and Tellina for sands (the linked tables A & B list the features and species common to the Biotopes within this Biotope Complex). It is of note that there are characteristically high abundances for muddy areas but relatively low for sandy areas. The abundances of those organisms are highly variable, with the common mudflat macrofaunal species being 10² - 10⁶ m² whereas species richness is relatively poor (up to 20 spp.). It is considered that the importance of these habitats centres on their role in the biological and physical functioning of the ecosystems. For example, mudflats for producing material for predators, such as birds, fishes and mobile epibenthic invertebrates, and mud and sandflats for coastal protection. The protection of this functioning relies on maintaining the size of area, the tidal elevation and substratum type plus maintaining an input of colonising organisms and the predator populations.

In the context of Annex I of the Habitats Directive classification, this biotope complex is included in the broad habitat of ‘Sandbanks which are slightly covered by sea water all the time’ (see Appendix). These areas include current-swept sands, maerl beds and mixed sediments and as such occur collectively around all coasts of the British Isles although each of these types, especially the maerl beds, has a less widespread distribution. Associated biotope complexes included in other Marine SAC reports are ‘Maerl beds’ and ‘Sea pens and burrowing megafauna’.

Subtidal mobile sandbanks are by definition highly dynamic and unstable with relatively coarse sediment, i.e. a high median sediment particle diameter and a low proportion of small particle material. In general, they support the Venus community of Petersen (1913) and Thorsen (1957) although elements of other classical communities will be present (see Appendix II for characteristic species). They are categorised by infaunal/epifaunal small crustaceans and molluscs which are adapted to the variable hydrography and mobile substratum. Where the proportion of silt increases slightly they are characterised by epifaunal and infaunal echinoderms and where light and hydrographic conditions permit, they will support macroalgae and maerl. The characteristic fauna of the faunal-dominated sandbanks contains, for example, magelonid polychaetes, the bivalve Fabulina, the sandeel Ammodytes and amphipods in very low abundances. They are usually dependent on an input of colonising organisms and have few species with benthic reproduction, thus any disruption to the water currents delivering colonising larvae will change the community. In addition, some sandbanks are likely to be sinks of materials at the centre of a gyre.

Mobile sandbanks are characterised by strong currents which may produce characteristic bed features such as mega-ripples (Pethick, 1984). These habitats are often important as fish nursery areas, e.g. for plaice (Lockwood, 1972). They may be characterised by low organic enrichment but there may be pockets of organic enrichment or it may receive anthropogenic waste, e.g. the Dogger Bank. The areas may be liable to severe substratum disturbance, e.g. 1 in 25 or 50 year storms which can turn over the sediment and disrupt the community. On a shorter time-scale and especially for sandbanks occurring in estuaries, there may be winter-summer erosion-deposition cycles and spring-neap erosion-deposition cycles, reflecting the periods of highest hydrodynamic energy (Dyer, 1998).

Candidate and possible Special Areas of Conservation for sandbanks which are slightly covered by sea water at all times (annotated S on the Figure) and mud and sandflats not covered by sea water at low tide (annotated M on the figure) and related habitats are shown in the following figure.

Figure - Candidate and possible Special Areas of Conservation for marine interest with regard to the present report

These occur predominantly in estuaries and the adjacent sedimentary coastal areas, in sheltered marine bays and semi-enclosed areas including lagoons. As such they are amongst the most dominant marine and estuarine habitats and cover areas from a few hectares to several square kilometres within a site and several times this within any geographical area. The linked figure above shows the distribution and size (ha) of intertidal flats on the (then) Nature Conservancy Council review sites.

These extend from a few hectares to a few km² in size although some areas have many contiguous sandbanks thus creating a large resource. The present report relates mainly to near-shore areas, i.e. those within the influence of coastal processes and hydrography and where SAC’s are designated. However, pertinent information relating to offshore features is also included.

The sandbanks are predominant in outer regions of estuaries, such as the Severn Estuary and Bristol Channel, and in the Scottish Firths such as the Solway and Tay (Davidson et al, 1991). They also occur in semi-enclosed marine bays where the hydrographic regime allows some accretion but of coarse material. There is no audit of these habitats such that the national resource, in area, is not known although Davidson et al (1991) describes their location and extent for estuarine sites (see linked figure above). The resource described by Davidson et al (1991) was termed estuaries but it included estuarine-type habitats such as fjords and semi-enclosed embayments. The latter include the Wash and Morecambe Bay which have extensive areas of intertidal and subtidal sedimentary areas (e.g. Hemingway & Cutts, in press).

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