Seasonal changes of nutrients in water and sediment in a Mediterranean lagoon with shellfish farming activity (Thau Lagoon, France)

Abstract

The French Mediterranean lagoon of Thau is characterized by an important eutrophication dominated by shellfish farming (ca. 15 times the terrestrial inputs). On the basis of increasing eutrophication, three areas were identified and monitored for one year (overlying and sediment pore water nutrients, macrophytic biomass and water column chlorophyll a). Though some parameters show similar changes in the three areas (salinity, temperature), others are elevated in eutrophicated sites, e.g. organic content and siltation of the sediments, dissolved inorganic nitrogen (136.1 μmol l−1 overlying water, and 1185 μmol l−1pore water), dissolved reactive phosphorus (DRP) (20.9 μmol l−1overlying water), the summer oxygen depletion (1.1 mg l−1), the peak of macrophytic biomass (8 kg w · wt m−2) and phytoplanktonic bloom (14 μg l−1). Differences in DRP levels arise from sediment release during the summer anoxia; DRP appeared to play a key role as a limiting factor, and regulates competition between macrophytes and phytoplankton in spring. The macrophytes (seagrass Zostera and seaweeds Gracilaria and Ulva) may sustain the environment they are living in, acting as additional eutrophication sources.