L'eutrophisation en Baie de Somme : mortalités des peuplements benthiques par anoxie

Massive mortalities of the benthic fauna, notably the cockle Cerastoderma edule, a commercially exploited species, were observed in the Pale de Somme in 1982, 1983, 1985, 1989 and 1990. Measurements of oxygen saturation levels of the surface water provided evidence of oxygen depletion (65% in 1989 and 62% in 1990). Oxygen saturation was inversely related 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 low tide: intertidal pools, films of water on the sediment surface. From laboratory and field experiments, a simple model of the anoxia in intertidal water pools was constructed, based on the oxygen consumption of the water, the sediment and the four principal species present in high densities, namely Hydrobia ulvae, Nel-eis diversicolor, Macoma balthica and Cerastoderma edule. Two simulations, at oxygen saturation levels of 100% and 65%, of this simplified system, showed that oxygen was exhausted in two hours. This figure is to be compared with the average emergence time of eight hours of the zone represented in situ by the water pools where mortalities were observed and which were characterized by high biomass of benthic fauna 28-30 g.m(-2) (notably Cerastoderma edule 70%). Survival times may be expected to be variable, depending on the biomass, the oxygen saturation level and the temperature. The implication of these results is that two types of factors are involved: those which contribute to situations of anoxia, and those which tri er mortalities.