Shellfish farming leaves its mark on the environment in which it has developed, and the men who depend upon it. These changes have altogether balanced the lagoon cycle and have caused disastrous episodic events. Increased water clarity caused by the uptake of particulate material by shellfish farming allows seagrass to grow in deeper areas of the lagoon (down to five metres). Shellfish farming nutrient transformations increase ecosystem productivity, even if the filtration pressure keeps phytoplankton biomass at a low level. Storage of phosphorus and nitrogen in animal tissue limits eutrophication in this ecosystem. Transfer of oysters from growout facilities increases animal and vegetal specific diversity. The presence of large amounts of shellfish allows for the development of a masive benthos, while organic enrichment from biodeposition changes the specific composition of soft-bottom benthos. In the deeper areas, (less than six metres), where summer thermoclines limit oxygen transfer from surface water, the organically enriched substrate induces oxygen depletion and ammonium and nitrogen sulfide accumulation in the water column. This ecosystem dysfunction kills benthic populations, and sometimes reaches pelagic populations and affects the shellfish farming economy.L'impact de la conchyliculture dans les lagunes mediterraneennes (etang de Thau) se manifeste aussi bien dans les mentalites des populations qui y vivent que dans le fonctionnement de la lagune. Les transferts d'huitres entre bassins ont contribue a l'augmentation de la diversite specifique des organismes vegetaux et animaux. Le developpement des supports colonisables du a la conchyliculture a permis une installation massive de benthos de substrat dur, alors que l'enrichissement organique du a la biodeposition a transforme le benthos de substrat meuble. Fonctionnellement, les transformations se font sentir aussi bien sur la masse d'eau (frein aux courants, enrichissement en sels nutritifs, impact sur le phytoplancton et la production primaire) que sur le systeme benthique (enrichissement en matiere organique, developpement des herbiers a zosteres, changement faunistique). La conchyliculture joue aussi un role dans la limitation de l'eutrophisation de l'ecosysteme soumis aux apports du bassin versant.