SEASONAL-CHANGES OF NUTRIENTS IN WATER AND SEDIMENT IN A MEDITERRANEAN LAGOON WITH SHELLFISH FARMING ACTIVITY (THAN LAGOON, FRANCE)

The French Mediterranean lagoon of Thau is characterized by an importa nt eutrophication dominated by shellfish farming(ca. 15 times the terr estrial inputs). On the basis of increasing eutrophication; three area s were identified and monitored for one year (overlying and sediment p ore water nutrients, macrophytic biomass and water column chlorophyll a). Though some parameters show similar changes in the three areas (sa linity, temperature), others are elevated in eutrophicated sites; e.g. organic content and siltation of the sediments, dissolved inorganic n itrogen (136.1 mu mol l(-1) overlying water, and 1185 mu mol l(-1) por e water), dissolved reactive phosphorus (DRP) (20.9 mu mol l(-1) overl ying water), the summer oxygen depletion (1.1 mg l(-1)), the peak of m acrophytic biomass (8 kg w . wt m(-2)) and phytoplanktonic bloom (14 m u g l(-1)). Differences in DRP levels arise from sediment release duri ng the summer anoxia: DRP appeared to play a key role as a limiting fa ctor, and regulates competition between macrophytes and phytoplankton in spring. The macrophytes (seagrass Zostera and seaweeds Gracilaria a nd Ulva) may sustain the environment they are living in, acting as add itional eutrophication sources. (C) 1997 International Council for the Exploration of the Sea.