EUTROPHICATION IN THE BAIE DE SOMME - CON SEQUENCES AND IMPACTS ON THE BENTHIC POPULATION

Massive mortalities of the benthic fauna, notably the cockle Cerastode rma edule, a commercially exploited species, were observed in the Pale de Somme in 1982, 1983, 1985, 1989 and 1990. Measurements of oxygen s aturation levels of the surface water provided evidence of oxygen depl etion (65% in 1989 and 62% in 1990). Oxygen saturation was inversely r elated to phaeopigment a concentrations due to eutrophication, leading to the depletion of oxygen or anoxia and initiating the mortalities, which were important and localized in areas where water persisted at l ow tide: intertidal pools, films of water on the sediment surface. Fro m laboratory and field experiments, a simple model of the anoxia in in tertidal water pools was constructed, based on the oxygen consumption of the water, the sediment and the four principal species present in h igh densities, namely Hydrobia ulvae, Nel-eis diversicolor, Macoma bal thica and Cerastoderma edule. Two simulations, at oxygen saturation le vels of 100% and 65%, of this simplified system, showed that oxygen wa s exhausted in two hours. This figure is to be compared with the avera ge emergence time of eight hours of the zone represented in situ by th e water pools where mortalities were observed and which were character ized by high biomass of benthic fauna 28-30 g.m(-2) (notably Cerastode rma edule 70%). Survival times may be expected to be variable, dependi ng on the biomass, the oxygen saturation level and the temperature. Th e implication of these results is that two types of factors are involv ed: those which contribute to situations of anoxia, and those which tr i er mortalities.