Estuaries

Large Shallow Inlets and Bays

The extensive and dominant nature of the sedimentary biotope complexes dictates that the present report also mentions related habitats such as Estuaries (Habitats Directive Annex I and Large Shallow Inlets And Bays (see Appendix I). In addition, an additional habitat, Lagoons will support these sedimentary Biotope Complexes under appropriate conditions.

Depending on the precise definition used, there are more than 100 estuaries in the British Isles which gives the UK the largest part of the European estuarine resource (27% by area) compared to other north-eastern Atlantic countries. The status, number and type of these habitats is described fully in Davidson et al (1991) and they range from shallow coastal embayments which receive a freshwater input to deep fjordic systems. The predominant habitat types within estuaries, and often the reason for their biological and physical importance, are the habitats considered here - intertidal sand and mudflats and subtidal sandbanks. The mosaic of biotopes which constitutes an estuary dictates that each sub-area may require consideration and management as well as taking an overall view (D. Connor pers. comm. 1998).

Estuaries are primarily controlled by salinity combined with structuring due to their topographical features which create the hydrophysical regime (McLusky, 1989). The modification of salinity by changes to the hydroclimate regime is likely to lead to changes in species' distributions, especially the degree of landward penetration of marine organisms. In addition, the input of organic materials, from both allochthonous and autochthonous sources are required to support these primarily detritus-driven systems. Decreases in these inputs can lead to an oligotrophic condition whereas increases can produce hypernutrification and possibly eutrophication. In addition, the importance of estuaries is not only for those species resident during much of the year but also for migrant fish and bird populations (Elliott et al, 1990). The movement of predators to estuaries for feeding, reproduction, nursery and refuge is well described, as is their role as migration routes for fishes passing between sea and freshwaters.

There is no readily available inventory of the habitat as a whole although the component biotopes have been recorded (Connor et al, 1997). Most coasts of the UK have some shallow inlets and bays although many are relatively small. By definition many will have a soft sedimentary substratum in the body of the inlet or bay and will be bounded by hard substratum. The bays and inlets are created primarily by the underlying geological features and then infilled with the prevailing mobile substrata and modified by the hydrographic regime. The size and shape of the area together with these characteristics thus creates the available niches for colonisation.

The areas may be heterogeneous where there is a complicated coastline and where different niches are created. Although the shape of these habitats are relatively easy to define, the seaward boundary and thus the area is rather more difficult to specify. The areas are primarily marine and thus fully saline with little dilution by river runoff although they may have diffuse inputs of freshwater from land-runoff.

The sedimentary regime will be dictated by the hydrographic regime and thus may have high and low energy areas depending on the tidal, wind-induced and residual current patterns. Where the habitats are open features, such as coastal embayments, then the direction of the prevailing currents and the length of fetch will be of importance in structuring the habitat. These features will dictate the relative composition of the component habitats, especially the subtidal sandbanks, the intertidal sand and mudflats, lagoons and reefs. The depth of the area will be defined by the tidal regime and underlying geology and bathymetry and will be variable. The coastal waters will allow good light penetration and thus support an extensive

infralittoral area of algal and angiosperm growth. Each of the above conditions will create a mosaic of habitats which will include intertidal, infralittoral and circalittoral biotopes covering both hard and soft substratum.

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