A comparison of benthic biodiversity in the North Sea, English Channel, and Celtic Seas

Two complementary surveys of the benthos around the United Kingdom coastline and offshore are described. The first sampled the macroinfauna by day grab at several stations around the England and Wales coastline as part of a wider interdisciplinary assessment of environmental quality by the regulatory authorities. The second sampled the epifauna with a small beam trawl at the grab stations, and at several additional stations, most of which were in the central and southern North Sea. Similar infaunal assemblages were encountered on both the eastern and western UK coasts in comparable environmental conditions. Tidal current velocity and sediment characteristics accounted for a significant amount of the observed variability in species richness and densities. There was no evidence of any adverse effects on these measures of assemblage structure arising from trace metal contamination of sediments. Coastal influences (proximity to large estuaries), depth, tidal current velocity, and temperature all helped to explain the distribution of epifaunal assemblages. However, sediment type appeared to be the main structuring force, with a coarser component to samples collected from the north and west of the survey area, i.e. especially around the UK coastline, supporting a much wider variety of sessile taxa. Grab samples provide unambiguously quantitative data which can be easily linked with sediment type within the small unit area of the sample. Trawls provide integrated samples of the fauna over a much larger area. However, both the design of the trawl, and inherent uncertainties over its sampling efficiency, determine that the survey results are "operationally defined". and consistency in sampling procedures is essential, especially for the analysis of temporal trends. There is a need to provide better working descriptions of the environment along trawl tows where sediments are variable; a combination of acoustic methods and underwater photograpy may be most suitable.