Macrophytobenthos de substrat meuble de l'étang de Thau (France, Méditerranée) et facteurs environnementaux associés

The study of soft-bottom macrophytobenthos of the Mediterranean Thau lagoon 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 characteristic species: Solieria chordalis in the deep zone; Ulva rigida, Monostroma obscurum and Gracilaria verrucosa to the northeast; Gracilaria spp. with Sargassum muticum in shellfish breeding zones; Halopitys 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 eastern part of the lagoon, unstable environmental conditions and organic pollution induce the development of heterogeneous euryhaline and nitrophilous communities. Conversely, to the west, shallow bottoms, moderate nutrient concentrations and relative short-term hydrologic stability permit the differentiation of permanent Florideophyceae communities.