Analyse spatio-temporelle des peuplements phytobenthiques de substrat meuble de l'etang de Thau (Herault, France)

Study of soft substrate macrophytobenthos from the mediterranean Thau lagoon has been carried out as a part of the ECOTHAU research program. All soft substrates have been sampled in autumn 1986. Data analysis is based on cartography, total and main species biomass assessment, as well as study of phytobenthos differentiation factors. Vegetation seasonality was analysed from october 1985 to october 1986 in the East area (shellfish breeding zone and "Eaux blanches" lagoon). The hydrologic and topographic characteristics give to vegetation a high specific richness (74 macrophytes on soft substrates and a total of 167 taxa for the lagoon). Compared to those of other mediterranean lagoons, the Thau phytobenthos appears to be original and diversified. This originality is emphasized by the occurence of exotic species as Sargassum muticum. In the autumn, total biomass is high and amounts 10075±2006 tPS i.e. on an average of 2.36±0.5 tPS/ha in the effectively covered area, dominated by Florideophyceae (67 %) and Zostera (22 %). Economic agarophytes (Gracilaria) biomass (30 %) is scattered in the lagoon and stay generally lower than those of production areas in the World. In the eastem lagoon area, seasonal variations of biomass are significant, with a minimum value in october and a maximum in june : the spring biomass is about twice the autumn one. The settlement is regulated by depth, hydrodynamism, catchment area inputs (fresh water, nutrients, pollutants) and sea water flows. These factors keep a balance level, variable according to sector : lack of phytobenthos in the deep central area, permanent populations of Halopitys incurvus and Zostera marina with high biomass (3.63 ±0.9 tPS/ha) in the west, heterogeneous and unstable populations with lower biomass (1.42±0.4 tPS/ha) in the east. On the whole, the phytobenthos bears witness to a rather good environment quality and the Ulvales (Ulva, Monostroma) with less than 10% of the autumn biomass, lie close to harbours and into the "Eaux blanches" lagoon. However, their increase from one year to the next, combined to the drop of the specific richness, may be interpreted as a local environment damage.