INFLUENCE OF SHELLFISH FARMING ACTIVITIES ON NITRIFICATION, NITRATE REDUCTION TO AMMONIUM AND DENITRIFICATION AT THE WATER-SEDIMENT INTERFACE OF THE THAU LAGOON, FRANCE

The seasonal patterns of nitrification, denitrification and dissimilat ory ammonium production (DAP) rates were studied in the sediment of 2 stations in the Thau lagoon (south of France). The station ZA was loca ted within the shellfish farming zone and the station B was the refere nce site. A marked effect of shellfish fanning on bacterial activities was observed. Spatial differences were associated with discrepancies in the organic content and the reduction state of sediments, i.e. high est reductive processes (denitrification and DAP) were noted in shellf ish farming area, whereas the oxidative process (nitrification) was pr edominant outside the farming zone. At both stations, the DAP activity increased in September (autumn) concomitant with an increase of the C /N ratio in the sediment due to the sedimentation of the summer phytop lanktonic production. Nitrification and denitrification rates exhibite d maxima in November (winter) corresponding to dissolved inorganic nit rogen inputs from the surrounding land. In the shellfish farming site, 98 % of nitrate was reduced to NH4+ and 2 % to N2O showing that the m ost of the NO3- was reduced to ammonium and remained available for the ecosystem.