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

Two complementary surveys of the benthos around the United Kingdom coastlin e and offshore are described. The first sampled the macroinfauna by day gra b at several stations around the England and Wales coastline as part of a w ider interdisciplinary assessment of environmental quality by the regulator y authorities. The second sampled the epifauna with a small beam trawl at t he 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 weste rn UK coasts in comparable environmental conditions. Tidal current velocity and sediment characteristics accounted for a significant amount of the obs erved variability in species richness and densities. There was no evidence of any adverse effects on these measures of assemblage structure arising fr om trace metal contamination of sediments. Coastal influences (proximity to large estuaries), depth, tidal current vel ocity, and temperature all helped to explain the distribution of epifaunal assemblages. However, sediment type appeared to be the main structuring for ce, with a coarser component to samples collected from the north and west o f the survey area, i.e. especially around the UK coastline, supporting a mu ch wider variety of sessile taxa. Grab samples provide unambiguously quantitative data which can be easily li nked with sediment type within the small unit area of the sample. Trawls pr ovide integrated samples of the fauna over a much larger area. However, bot h the design of the trawl, and inherent uncertainties over its sampling eff iciency, determine that the survey results are "operationally defined". and consistency in sampling procedures is essential, especially for the analys is of temporal trends. There is a need to provide better working descriptions of the environment a long trawl tows where sediments are variable; a combination of acoustic met hods and underwater photograpy may be most suitable.