BENTHIC MICROPHYTIC ACTIVITY AT 2 MEDITERRANEAN SHELLFISH CULTIVATIONSITES WITH REFERENCE TO BENTHIC FLUXES

Microphytobenthos activity in coastal waters has been studied at two s hellfish fanning sites: an oyster farm in Thau lagoon and a mussel far m in Carteau bay (NW Mediterranean coast). In both cases, shellfish ar e cultivated on ropes hanging from rigid structures (tables) in shallo w waters (- 5 m). An ''under table'' station and an ''outside table'' (reference) station were chosen at each site. Oxygen and ammonia fluxe s at the water-sediment interface were measured in dark and transparen t benthic chambers. Chlorophyll a concentrations were two- to threefol d higher in ''under table'' sediments than outside (260 mg Chl a m(-2) under the oyster table, 436 mg Chl a m(-2) under the mussel table in winter). Net oxygen fluxes were negative at all stations, except at th e Carteau reference station and under the oyster table at the beginnin g of the summer (+ 20 mg O-2 m(-2) h(-1)). Maximum negative fluxes (- 70 mg O-2 m(-2) h(-1) under a mussel table) were recorded during sprin g at all stations, when mollusc trophic activity was maximal as reflec ted in high phaeopigment content under the tables. Gross primary produ ction was lowest in spring at the Thau stations as well as at the Cart eau reference station. Under the mussel table, primary production incr eased steadily from autumn to summer, from 0 to 100 mg O-2 m(-2) h(-1) . Ammonia release was higher under shellfish farming structures (200 m u mol N-NH4+ m(-2) h(-1)) than at the reference stations (70 mu mol N- NH4+ m(-2) h(-1)). The discrepancy between O-2 production and microphy tic biomass suggests that mechanisms other than photosynthesis prevail under shellfish structures. Mussel cultures were shown to have a stro nger impact on benthic fluxes below the tables than oysters. In locali ties with shellfish farms, oxygen produced by microphytobenthos activi ty cannot always meet the total oxygen demand of the sediment, increas ed by accumulation of mollusc biodeposits and, sometimes drastically i n the lagoon, by macroalgae decay.