A towed video survey of megafaunal bioturbation in the north- eastern Irish Sea

In September 1993 the Irish Sea floor near the Sellafield nuclear reprocessing plant was surveyed by towed video to assess the distribution, abundance and behaviour of large burrowing invertebrates believed to be important in the transport and mixing of radionuclide-contaminated sediments. Sixteen stations were surveyed, covering a range of sediment types. Burrow openings and mounds were counted and assigned to species where possible. Particular attention was paid to the similarities and differences between the features observed and those produced by the same species in Scottish sea lochs, the source of most relevant ecological data. Several different burrowing communities were identified from bottom topography. Offshore muddy sands supported dense populations of the thalassinidean crustacean Callianassa subterranea, in burrows similar to those described from the North Sea. Coarse sediments closer inshore had C. subterranea at lower density, with the thalassinidean Upogebia deltaura and the crab Goneplax rhomboides also present. Softer muds were dominated topographically by ejecta mounds of the echiuran Maxmuelleria lankesteri and burrows of the Norway lobster Nephrops norvegicus. The thalassinideans C. subterranea, Jaxea nocturna and Calocaris macandreae were also present at low density. Surface bioturbation features in this habitat were very similar to those described from sea lochs. population densities estimated from surface bioturbation features were always lower than counts of specimens from box-cores. The visual survey therefore gave minimum estimates of population density. Available data on rates of bioturbation by the species concerned are collated to give rough estimates of biogenic sediment transport in the habitats surveyed.