Spring phytoplankton community structure in the Thau lagoon, France (May, 2002).

Phytoplankton communities were investigated in the French Mediterranean Thau lagoon (75 km2, mean depth ca. 4.5 m), where there is important shellfish farming activity. Samples were collected during spring (May 2002), mainly at two sites (MPN and LP) characterised by different eutrophication levels and macrophyte populations, Zostera marina L. and Gracilaria bursa-pastoris (Gmelin) Silva. Taxonomy, cell abundance, carbon content and SHANNON-WEAVER diversity index were determined. The most common taxa were nanoflagellates (between 40 and 50% in terms of cells dm-3), diatoms (between 20% and 35%) and Cryptophyceae (around 12%). The sites differed in that Dinophyceae (<5%) were present at the oligotrophic station (MPN) and epiphytic and benthic diatoms at the eutrophicated station (LP). Chlorophyll a and abundance were higher at LP than at MPN. Carbon content was 8 times higher at MPN than at LP, because of the presence of big dinoflagellates, representing 90% of the biomass, at MPN. The authors discuss the species, which are indicators of the eutrophication level, and the important role of taxonomic analysis and compare the results with those of the Venice lagoon.