Distribution of sediment types in the north-eastern Irish Sea near Sellafield, Cumbria.

There have been very few surveys of the distribution of biotopes within this complex over large areas of sea floor with a gradient of sediment types. The most detailed is that of Hughes & Atkinson (1997), who used towed underwater video to plot the distribution of megafaunal burrowers in the north-eastern Irish Sea off the Cumbrian coast. Visual estimates of population densities were supplemented by data from box-core samples. Sediments were distributed in broad bands running roughly parallel to the coastline (see figure below), extending from an offshore muddy sand (10 - 30% silt-clay content) onto progressively finer muds (up to 75 - 85% silt-clay), before coarsening again close inshore (muddy sand, 20 - 30% silt-clay, underlain by a layer of shell gravel). The sea floor was heavily burrowed by megafauna, but community composition changed markedly over the gradient of sediment types. The offshore muddy sand was dominated by Callianassa subterranea (estimated density 88 individuals m^-2), occupying multi-opening burrows of the type previously described from the North Sea (Witbaard & Duineveld, 1989; Rowden & Jones, 1995). The large thalassinoidean Upogebia deltaura was also common (estimated 22 individuals m^-2). The crab Goneplax rhomboides was present at low density. No sea pens were seen here.

The central finer muds supported a more diverse megafaunal community, with sea floor topography dominated by burrow openings of Nephrops norvegicus and the large ejecta mounds of Maxmuelleria lankesteri. The visual survey gave an estimate of 0.6 - 1.3 Nephrops burrow systems m^-2, while box-core samples indicated a density of up to 10 Maxmuelleria individuals m^-2. The burrowing community also included Callianassa subterranea, Calocaris macandreae, Jaxea nocturna and Goneplax rhomboides. The sea pen Virgularia mirabilis was present but rare.

The inshore muddy sand had a much lower burrow density than the other two zones, with no large ejecta mounds or Nephrops burrows. The most common burrower was again Callianassa subterranea (estimated 23 individuals m^-2), with Upogebia deltaura present at much lower density. Goneplax rhomboides was fairly common (estimated 2 individuals m^-2). Virgularia mirabilis also occurred most commonly in this biotope (10 individuals m^-2). Although its characteristic ‘volcano’ mounds were not seen on the video recordings, box-coring showed that Maxmuelleria lankesteri was also sparsely present in this relatively coarse sediment.

The sediment characteristics and faunal composition of the biotopes observed are summarised in the table below. An assessment of their position in the MNCR biotope classification is also given.

* The locations of each station sampled by box-coring in June 1995 are given. These stations corresponded closely with areas surveyed by towed camera in September 1993.

A significant finding, obtained by comparing the results of towed camera and box-core surveys in the same area, was that visual estimates of megafaunal densities (derived from counts of burrow openings) consistently underestimated the number of animals present in the sediment. This issue will be discussed further in the section dealing with monitoring and surveillance options (Chapter VII).

Solid black circles: Offshore muddy sand with Callianassa subterranea and Upogebia deltaura. Solid grey circles: Central mud/sandy mud with Maxmuelleria lankesteri, Nephrops norvegicus and other burrowing megafauna. Solid square: Inshore muddy sand with Virgularia mirabilis, Callianassa subterranea and Upogebia deltaura.

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