SOFT-BOTTOM MACROPHYTOBENTHOS AND ASSOCIA TED ENVIRONMENTAL-FACTORS IN THAU-LAGOON (FRANCE, MEDITERRANEAN)

The study of soft-bottom macrophytobenthos of the Mediterranean Thau l agoon was carried out in autumn 1986. The total biomass is high (10073 +/- 2006 tDW) and is dominated by Florideophyceae (67 %) and Zostera (22 %). Seven communities have been identified, with the following cha racteristic species: Solieria chordalis in the deep zone; Ulva rigida, Monostroma obscurum and Gracilaria verrucosa to the northeast; Gracil aria spp. with Sargassum muticum in shellfish breeding zones; Halopity s incurvus to the southwest; and Zostera spp. along the shallow edge. The differentiation of these communities depends essentially on depth, catchment area inputs (fresh water, nutrients, pollutants), sea water flows, hydrodynamism and shellfish breeding activities. In the easter n part of the lagoon, unstable environmental conditions and organic po llution induce the development of heterogeneous euryhaline and nitroph ilous communities. Conversely, to the west, shallow bottoms, moderate nutrient concentrations and relative short-term hydrologic stability p ermit the differentiation of permanent Florideophyceae communities.