By establishing in situ experimental ecosystems, it was possible to conduct pollution research under actual conditions. The experiments were carried out in an oyster culture basin of a lagoon in the Sables d'Olonne region by using devices to section off the water into a number of similar natural environmental compartments to be used for pollutant treatments and controls. The product Corexit produced only weak and temporary changes. Sefoil acted as a fertilizer, provoking a rapid increase in photosynthesis and chlorophyll a concentration; this was followed by considerable growth in the zooplankton, particularly the copepods, which resulted in the disappearance of the phytoplankton by the end of the experiment. The surface-active fraction of the third product used, Gamosol, caused an almost complete inhibition of photosynthesis during the entire 15 days of the experiment.