Influence of spatial scales of observation on temporal change in diversity and trophic structure of fine-sand communities from the English Channel and the southern North Sea

Along the coast of France in the English Channel, muddy fine-sand communities are restricted to bays and estuaries, but in the southern North Sea they stretch out to larger areas (mesoscale continuum). We studied three regions containing these communities, each subject to different hydrological conditions and contrasting trophic structures of the water column. The Bay of Morlaix was strongly affected by the ''Amoco Cadiz'' oil spill of 1978 and recovered slowly. The Bay of Seine is influenced by high levels of nutrient input from the River Seine. A retention structure exists in the eastern part ensuring recruitment stability. The Gravelines area in the southern North Sea was invaded by the American jackknife clam (Ensis directus), which became a key species several years after its accidental introduction. These areas are important nursery grounds for demersal fish species. The distribution and evolution in trophic structure and diversity of macrofauna were analysed in each region, permitting the identification of the roles of disturbance and natural factors in the organization and long-term evolution (including recovery after an event) of the communities. The effects of different spatial scales of observation on the resulting image of macrobenthic community evolution are discussed. 2000 International Council for the Exploration of the Sea