The dynamics of the phytoplankton community were investigated in a marine coastal lagoon (Thau, NW Mediterranean) from February 1999 to January 2000. Dilution experiments, chlorophyll a (Chl a) size-fractionation and primary production measurements were conducted monthly. Maximum growth and microzooplankton grazing rates were estimated from Chl a biomass fractions to separate pico-from nano-and microphytoplankton and by flow cytometry to distinguish between picoeukaryotes and picocyanobacteria. In spring, the phytoplankton community was dominated by Chaetoceros sp. and Skeletonema costatum, which represented most of biomass (B) and primary production (P). Nano-and microphytoplankton growth was controlled by nutrient availability and exceeded losses due to microzooplankton grazing (g). Picoeukaryote and cyanobacteria growth was positively correlated with water temperature and/or irradiance, reaching maximum values in the summer (2.38 and 1.44 day-1 for picoeukaryotes and cyanobacteria, respectively). Picophytoplankton accounted for 57% of the biomass-specific primary productivity (P/B). Picophytoplankton was strongly controlled by protist grazers (g = 0.09-1.66 day-1 for picoeukaryotes, g = 0.25-1.17 day-1 for cyanobacteria), and microzooplankton consumption removed 71% of the daily picoplanktonic growth. Picoeukaryotes, which numerically dominate the picoplankton community, are an important source of organic carbon for the protistan community and contribute to the carbon flow to higher trophic levels.

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A cross-ecosystem comparison of data obtained from 20 French Mediterranean lagoons with contrasting eutrophication status provided the basis for investigating the variables that best predict chlorophyll a (Chl a) concentrations and nutrient limitation of phytoplankton biomass along a strong nutrient enrichment gradient. Summer concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) comprised only a small fraction of total nitrogen (TN) and total phosphorus (TP). On the basis of inorganic nutrient concentrations, the most oligotrophic lagoons appeared to be phosphorus-limited, with a tendency towards the development of nitrogen limitation as eutrophication increased, as evidenced by decreasing DIN:DIP ratios. A weak but significantly positive relationship was found between dissolved silicate (DSi) and Chl a, reflecting DSi accumulation in the water column along the trophic state gradient and implying a progressive shift away from potential Si limitation of phytoplankton growth. Observed concentrations of Chl a were far better explained by TN and TP than by DIN and DIP concentrations, suggesting that a total nutrient based approach is likely to be the most appropriate for managing eutrophication in Mediterranean lagoons and other coastal waters. These results give credence to the idea that marine and freshwater environments respond in a similar fashion to nutrient enrichment.