Hydrographic conditions

Sediment particle size and organic enrichment

In British and Irish waters, the ‘Sea pens and burrowing megafauna’ biotope complex extends from the shallow subtidal (< 10 m depth) to over 100 m in the northern North Sea (Dyer et al., 1982), the western Irish Sea (Hensley, 1996), and the deepest sea lochs (Howson & Davies, 1991). Thalassinidean mud-shrimps occur in the low intertidal zone in many parts of the world but are not known to do so here, with the exception of the rarely-recorded Axius stirhynchus. The biotope characterized by Virgularia mirabilis and Philine aperta (IMU.PhiVir) can be found in water less than 10 m deep in the sheltered inner basins of some sea lochs (Howson et al., 1994). In the Caol Scotnish basin, Loch Sween, Maxmuelleria lankesteri, Callianassa subterranea and Jaxea nocturna occur in very fine, stable muds in as little as 8 m depth (Nickell et al., 1995b), but sea pens are absent here. The burrowing megafauna is typically more abundant in slightly deeper water (> 15 m), while the large sea pen Funiculina quadrangularis occurs at depths > 20 m.

Low-energy conditions are a prerequisite for the existence of fine sedimentary substrata. Areas supporting this biotope complex are usually highly sheltered from wave exposure and with weak or negligible tidal streams. The most extremely sheltered conditions occur in the almost landlocked inner basins of many sea lochs. The biotope coded CMS.AfilEcor, which includes Callianassa subterranea and Virgularia mirabilis, exists in slightly more energetic conditions, with consequently coarser sediments than are found in the inner sea lochs.

Biotopes within this complex occur in conditions of fully marine salinity and do not extend into estuaries (the sole exception seems again to be Axius stirhynchus, which has been recorded on estuarine shores). Shallow coastal habitats in the British Isles clearly experience wide seasonal temperature changes. Species characteristic of these biotopes may respond with seasonal patterns of behaviour, but within the British Isles most do not appear to be temperature-limited in their distribution.

The size distribution of particles in a marine sediment is an extremely important ecological parameter and can have a major influence on the composition of the biological community. Grain size distribution can be expressed in a number of ways, but a simple three-way division can be made between sand (particles 62 - 2000 m m in diameter), silt (4 - 62 m m) and clay (< 4 m m) fractions (Buchanan, 1984). A further simplification is often made by pooling the two smaller size fractions into a silt-clay category. Sediments with silt-clay fractions > 80% are generally classified as ‘muds’, those with fractions in the range 30 - 80% as ‘sandy muds’, and those with 10 - 30% silt-clay as ‘muddy sands’. The finest sediments at the heads of sheltered sea lochs may have a silt-clay content exceeding 95% (of sediment dry weight). Generally, finer sediments (higher silt-clay component) are found in conditions of lower exposure to waves and currents, and have a higher organic content than sandier substrata. This is because low-energy conditions favour the accumulation of settling plankton and detritus from the water column, and because the smaller sediment grains provide a larger total surface area for the growth of bacteria and other microorganisms. Organic content of unpolluted coastal sediments can range from < 1% in clean sands to 7 - 8% in sea loch muds (much higher figures can exist in situations of gross organic pollution by sewage, alginate waste or fish farm effluent)

Sea pens are anchored within the sediment but do not depend upon it for food. Of the three British species, Virgularia mirabilis has the broadest environmental tolerances, occurring both in extremely fine inner sea loch muds (biotope IMU.PhiVir) and in much sandier substrata containing large numbers of small stones and shell fragments (biotope CMS.VirOph.HAs).

Megafauna which burrow within the sediments, and in many cases feed from it, have a more obvious interaction with the substratum, and some species are characteristically found in a particular set of conditions. Buchanan (1963) found Calocaris macandreae only in areas where silt-clay formed more than 20% of the sediment. Highest densities occurred where the silt-clay content exceeded 60%. Jaxea nocturna is also associated with fine, organic-rich sediments. Callianassa subterranea is able to inhabit a wider range of sediment types, including fine sea loch muds and the much sandier sediments of the central North Sea. The animal constructs a distinctively different burrow in each of these two environments (Atkinson & Nash, 1990; Rowden & Jones 1995).

The burrowing megafauna are generally absent from very coarse sands. The limiting factors may be the low organic content (inadequate food supply) and low cohesion (burrows will tend to collapse easily) of sandier sediments. In coarser sediments Callianassa subterranea will line its burrow with mucus to prevent collapse, a precaution unnecessary in highly cohesive muds (Atkinson & Nash, 1990). To date, most studies of burrowing megafauna have been carried out in sea lochs with very fine muddy substrata, and the communities in coarser sedments have received less attention. At Lundy, Cepola rubsecens, Callianassa subterranea and Upogebia spp. are found burrowing in muddy gravel (Hoare & Wilson, 1976). In the Clyde Sea, Cepola is also commonest on relatively coarse sediments (R.J.A. Atkinson, personal communication).

The larger burrowers also appear to be excluded from areas of very high organic enrichment, for example the centres of sewage sludge disposal grounds (Pearson & Rosenberg, 1978; Smith 1988). This phenomenon will be discussed further in the section of this report dealing with the impact of human activities on the biotope complex.

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