THE QUANTITATIVE DISTRIBUTION OF THE BENTHIC MACROFAUNA IN INTERTIDALSANDS OF ARCACHON BAY

The species composition and spatial distribution of macrofaunal assemb lages in intertidal sands of Arcachon Bay, on the southwestern Atlanti c coast of France, were investigated through a quantitative sampling o f twenty stations representative of the various sandy habitats found i n the bay, excluding the exposed beaches. Sampling was done on a singl e occasion, during the winter of 1988. A total of 150 macrobenthic spe cies (> 1 mm) were collected. The abundance of macrofauna ranged from 89 to 52000 ind.m-2 (with a high dominance of the gastropod Hydrobia u lvae in some sites) and the biomass ranged from 1.3 to 42.5 g.m-2 dry weight. Thirteen additional species (belonging to temporary meiofauna and small macrofauna) were also sampled in the sieving fraction compri sed between 1 mm and 100 mum. In this size fraction, the highest densi ties were those of H. ulvae juveniles in the inner bay and the small p olychaete Parapionosyllis gestans in the outer bay. Mean abundance of total fauna > 100 mum (excluding permanent meiofauna) was about 35000 ind.m-2 in both sheltered and semi-exposed stations. A correspondence analysis of the distribution of species among stations followed by an automatic hierarchical classification identified six benthic assemblag es, which were also characterized in terms of species dominance, const ancy and fidelity. Two assemblages, corresponding to the boreal Macoma community, were distinguished in the sands at mid-tide level in the i nner part of the bay: station-group A was characterized by Neomysis in teger and Streblospio shrubsolii, together with a low number of specie s occurring in low-salinity estuarine waters; station-group B, with Ce rastoderma edule, Arenicola marina, and Nereis diversicolor as charact eristic species, was a sandy facies of the Macoma community dominated by C. edule. Three other assemblages, each with a high species richnes s, included several species of the boreal Lusitanian Tellina community : station-group C: a Eunice vittata-Tubificoides benedeni assemblage a t lower tidal levels in the central part of the bay; station-group D: a Ruditapes decussatus-Goniada emerita assemblage at mid-tidal levels of the outer bay; station-group E: an Echinocardium cordatum-Magelona mirabilis-Aonides oxycephala assemblage at lower tidal levels of the o uter bay. A sixth assemblage (station-group F) was an Ophelia bicornis -Bathyporeia sarsi assemblage in fine dune sands of the outer bay. A d iscriminant analysis including five environmental variables (tidal lev el, salinity, sediment median, fine particles and organic contents) re vealed the first two variables to be the most important factors in the ordination of stations. With these five variables, only 65 % of the s tations were correctly classified in the station-groups identified by correspondence analysis. Among the other possible factors governing th e distribution of benthic assemblages, the importance of both exposure and circulation of water masses is stressed. Sheltered stations had a mean biomass of 22.8 g.m-2 dry weight and a mean species richness of 17.7 species; in semi-exposed stations, these values were 11.5 g.m-2 d ry weight and 36.1 species.