LONG-TERM CHANGES (1979-1994) IN 2 COASTAL BENTHIC COMMUNITIES (ENGLISH-CHANNEL) - ANALYSIS OF STRUCTURAL DEVELOPMENTS

The long-term variations of two coastal benthic communities (Eastern E nglish Channel, France) were studied between 1979 and 1994 by means of factor correspondence analysis (FCA), trophic structure and rank-freq uency diagrams (RFD). FCA identified periods of relative homogeneity a nd periods of strong variation in the structure of the two communities . The first community, established on heterogeneous sand, showed a mul ti-annual cycle Linked to grain-size variations in the sediment. As pa ssive supplies of organic matter were not important and suspension fee ders could hardly live in these conditions, the community remained poo r and stable and was dominated by deposit-feeders and carnivores. The other one, a rich and diverse mussel (Mytilus edulis) bed, showed grea t variations in the abundance of the main species which generated the enrichment of the community by its suspension-feeding behaviour (biode position). Beyond these variations, the community remained stable unti l 1990 as no continuous trend or permanent change could be seen. Since 1990, no recruitment of M. edulis occurred and the relative dominance of suspension feeders, as well as the number of species, strongly dec reased. During the following years, several species of tunicates (sess ile suspension feeders with gregarious recruitment) recruited and part ly took the ecological niche of M. edulis but they did not induce an i ncrease of biodeposition and the previous richness was not restored. A fter a strong and rapid shift in the structure, a new equilibrium has been established within the community. (C) Elsevier, Paris.