IMPACT OF SHELLFISH FARMING EUTROPHICATION ON BENTHIC MACROPHYTE COMMUNITIES IN THE THAU LAGOON, FRANCE

In a large marine lagoon (Thau lagoon, southern France) with a shellfi sh farming dominant eutrophication, the macrophyte communities were sa mpled by six transects of three depths (1.5, 2.5 and 5 m) and their ch aracteristics (species composition, diversity and biomass) were descri bed in relation to environmental and sediment parameters. With increas ing eutrophication (total inorganic nitrogen, 0.140-0.295 mg 1(-1); di ssolved reactive phosphorus, 0.045-0.110 mg 1(-1); and N/P atomic rati o, 3-22), silt fraction and shell fragments in sediments increased (12 -93 and 0-65% dry wt respectively). Different types of macrophytic com munities could be defined in the shallow zone (1.5-2.5 m) correspondin g to four main and successive stages of degradation. A pure eelgrass s tand (Zostera marina and Z. noltii) and an eelgrass community colonize d by macroalgae weve observed in SW sites and could be distinguished b y their sedimentary features. In sites (NE) move affected by eutrophic ation (fine-textured sediment), available incident light determined tw o main seaweed communities: an Ulva rigida community, outside the shel lfish tables, and a Gracilaria bursa-pastoris community in the shellfi sh tables (lower incident light).