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The Thau lagoon is a shallow marine lagoon located on the French Mediterranean coast (43 24'N - 3 36'E) covering 75 km2, with a mean depth of 4 m, and a maximum depth of 10 m. The lagoon is connected to the sea by 3 narrow channels. Three oyster farming zones are located along the northwestern shore. The lagoon represents 10% of French oyster production and is the main oyster production center on the Mediterranean, with an estimated standing stock of 25,000 tons of oysters (Crassostrea gigas). Because of the weak tidal range (< l meters), the residence time of water masses (1-5 months) mainly depends on wind and barometric effects.
Systematic observations of physical (temperature, salinity), chemical (nutrient concentrations) properties have been made since 1971 and biological (phytoplankton > 5 µm) properties have been made since 1987. Picophytoplankton (picoeukaryotes and picocyanobacteria) has been counted by flow cytometry since 1991. The lagoon harbors Ostreococcus tauri, the smallest eukaryote in the world (Courties et al. 1994. Smallest eukaryotic organism. Nature 370: 255). Sampling frequency is twice per month, but can increase to once per week or more during periods favorable to harmful algae. Several stations are sampled but data presented here belong to station B. Over the last 30 years, a major perturbation has been a large decrease in soluble reactive phosphorus (summer values decreased from 10 µm to 1 µm, while winter values decreased from 3 µm to undetectable at present). The recent and almost simultaneous appearance of both picocyanobacteria (mostly Synechococcus) and the toxic Dinoflagellate Alexandrium catenella in Thau seems to be related to reduced nutrient loading and the increase in water temperature (Collos et al. 2009. Oligotrophication and emergence of picocyanobacteria and a toxic Dinoflagellate in Thau lagoon, southern France (J. Sea Research 61: 68-75).