Preliminary results on eelgrass regression and red seaweed dominance under increasing eutrophication (Thau Lagoon, France)

Abstract

    This study measures biomass, composition and specific diversity of marine macrophyte communities in three sites situated along eutrophic gradient, in the Mediterranean Thau Lagoon, subject to intensive shellfish farming activity (increase of siltation and shell fragments percentage in sediments to the detriment of sand). The study was conducted for one year at three different periods, at 1.50 m water depth. Along the increasing eutrophic gradient, the dominant communities were: (I) mixed eelgrass (Zostera noltii); (II) mixed eelgrass (Z. marina) invaded by seaweed (Rhodophyceae and Chlorophyceae); and (III) a red seaweed community dominated by three Gracilaria species. The algae diversity degree of the community was maximum in the second site, because the shell debris increase the settlement of algae. In the third site, due to the large quantity of mud, the sea-grass disappeared and the red algae were dominant in relation with the light intensity decrease. This increasing eutrophication gradient squares with the steps of the long-term historic eelgrass degradation. So, these results are important for the management of marine eutrophicated areas.