Relationship between environment and resources: impact of shellfish farming on a Mediterranean lagoon (Thau, France).

Shellfish farming leaves its mark on the environment in which it has develo ped, and the men who depend upon it. These changes have altogether balanced the lagoon cycle and have caused disastrous episodic events. Increased wat er clarity caused by the uptake of particulate material by shellfish fannin g allows seagrass to grow in deeper areas of the lagoon (down to five metre s). Shellfish farming nutrient transformations increase ecosystem productiv ity, even if the filtration pressure keeps phytoplankton biomass at a low l evel. Storage of phosphorus and nitrogen in animal tissue limits eutrophica tion in this ecosystem. Transfer of oysters from growout facilities increas es animal and vegetal specific diversity. The presence of large amounts of shellfish allows for the development of a masive benthos, while organic enr ichment from biodeposition changes the specific composition of soft-bottom benthos. In the deeper areas, (less than six metres), where summer thermocl ines limit oxygen transfer from surface water, the organically enriched sub strate induces oxygen depletion and ammonium and nitrogen sulfide accumulat ion in the water column. This ecosystem dysfunction kills benthic populatio ns, and sometimes reaches pelagic populations and affects the shellfish far ming economy. (C) Elsevier, Paris.