The biogeochemistry of two eutrophic marine lagoons and its effect on microphytobenthic communities

This paper summarizes the results of a study on the biogeochemistry of two eutrophic marine lagoons. The lagoons investigated were the Bassin d'Arcachon situated on the Atlantic coast, and Étang du Prévost on the Mediterranean coast. The sites chosen for this study were characterized by the presence of dense communities of microphytobenthos. Both lagoons receive a large input of nutrients but they differ in several aspects. The Bassin d'Arcachon receives a large amount of iron. Iron is of great importance in reducing the effects of eutrophication. Ferric iron is an efficient scavenger of phosphate and it has been proposed that this is one of the mechanisms that controls primary productivity and algal growth in this lagoon. The mechanisms of phosphate mobilization were studied by using sediment slurries. These experiments demonstrated that not only ferric iron but presumably also calcium was responsible for phosphate binding. Another effect of the high iron content in the Bassin d'Arcachon was the precipitation of sulfide as iron sulfide or pyrite. In the Étang du Prévost sulfate reduction resulted in the accumulation of free sulfide. The relative low content of iron in Étang du Prévost not only allowed the formation of free sulfide but may also have limited the binding capacity of phosphate in the sediment. On the other hand sulfate reduction was not important for the release of phosphate from the sediment. In Étang du Prévost primary productivity is nitrogen rather than phosphorus limited. In contrast in the Bassin d'Arcachon primary productivity was presumably mostly phosphate limited. In Étang du Prévost the non-heterocystous cyanobacterium Oscillatoria sp. was the dominant nitrogen-fixing species. Heterocystous species were excluded from this lagoon as a result of the presence of free sulfide. It was demonstrated that heterocystous cyanobacteria are more sensitive towards sulfide than non-heterocystous species. The absence of free sulfide explained the presence of the heterocystous cyanobacterium Anabaena sp. in Bassin d'Arcachon.