Spatiotemporal persistence of the Abra alba - Pectinaria koreni muddy-fine sand community of the eastern Bay of Seine

The spatial distribution and temporal variations of the Abra alba-Pectinaria koreni community from the eastern part of the Bay of Seine were investigated through four winter surveys (1986, 1987, 1988 and 1991). A grid of 40-67 stations was sampled each year using a Hamon grab (0.25 m(-2)) for macrofauna collection and sediment analysis. The fauna was dominated by polychaetes and in a lower part by molluscs and echinoderms. Although species richness with similar to 100 species collected during each survey was high, total density and biomass (+/-SE) resulted principally from about ten dominant species, and varied between 846.9 +/- 163.5 and 1135.3 +/- 186.7 ind m(-2) and 23.52 +/- 4.41 and 27.48 +/- 4.45 g m(-2) respectively The dominant species were patchily distributed and exhibited weak spatio-temporal fluctuations except the two bivalves A. alba and Cultellus pellucidus. Factorial correspondence analysis (FCA) of the distribution of main species among stations and automatic hierarchical classification identified five faunal assemblages which were relatively stable in space and time. The temporal stability of the community could be generated by processes of larval retention near parental population, and sediment stabilization induced by the low abundance of deposit feeders and the high densities of the tube-dwelling polychaete Owenia fusiformis. While the sediment variables were a poor indicator of the spatial structure of the community, the salinity gradient off the Seine Estuary and post-settlement processes (e.g. food limitation and postlarval drifting) have been proposed to explain it. Comparison with an earlier survey (i.e. 1971) suggested a degree of long-term persistence of the community structure in qualitative and quantitative terms.