Influence of spatial scales of observation on temporal change in diversityand 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 communiti es are restricted to bays and estuaries, but in the southern North Sea they stretch out to larger areas (mesoscale continuum). We studied three region s containing these communities, each subject to different hydrological cond itions and contrasting trophic structures of the water column. The Bay of M orlaix was strongly affected by the "Amoco Cadiz" oil spill of 1978 and rec overed slowly. The Bay of Seine is influenced by high levels of nutrient in put from the River Seine. A retention structure exists in the eastern part ensuring recruitment stability. The Gravelines area in the southern North S ea was invaded by the American jackknife clam (Ensis directus), which becam e a key species several years after its accidental introduction. These area s are important nursery grounds for demersal fish species. The distribution and evolution in trophic structure and diversity of macrofauna were analys ed in each region, permitting the identification of the roles of disturbanc e and natural factors in the organization and long-term evolution (includin g recovery after an event) of the communities. The effects of different spa tial scales of observation on the resulting image of macrobenthic community evolution are discussed. (C) 2000 international Council for the Exploratio n of the Sea.