STANDING STOCK AND DYNAMICS OF PICOPHYTOPLANKTON IN THE THAU LAGOON (NORTHWEST MEDITERRANEAN COAST)

Eucaryotic picophytoplankton (PEUC), picocyanobacteria, and larger phy toplanktonic cells from the Thau Lagoon (northwest Mediterranean coast ) were numbered by flow cytometry from November 1991 to February 1994. PEUC cells dominated the phytoplanktonic assemblage and exhibited sea sonal dynamics. Monthly mean abundances of larger phytoplanktonic cell and PEUC were significantly correlated with temperature and solar irr adiance values, with the highest correlations having been obtained for PEUC abundances. The mean abundance of total picophytoplankton (3.5 x 10(4) cells ml(-1)) was among the highest recorded in marine waters. The dominance of picophytoplankton over larger phytoplanktonic cells ( average abundance, 5 x 10(3) cells ml(-1)) in the nutrient-rich waters of the lagoon was related to grazing from large-scale shellfish breed ing (oyster's biomass approximate to 35,000 t), which seems to act pre ferentially on the largest cells. Several hypotheses, including the po tential selective effect of copper, were proposed to explain the domin ance of the PEUC form (average abundance, 3.4 x 10(4) cells ml(-1)).