In situ measurement of respiratory metabolism and nitrogen fluxes at the interface of oyster beds

Nitrogen and oxygen fluxes were measured in situ during monthly dark enclosure experiments on oyster beds. The incubations were performed on undisturbed sediment with its endofauna, on 10 oysters Crassostrea gigas isolated from the substratum, and on 10 oysters associated with the sediment, to assess the impact of oyster farming on the environment. Ammonia was released by the sediment following a seasonal pattern, &splaying low winter (51 pm01 m-' h-') and hlgh spring (237 pm01 m-' h-') and autumn (264 to 369 pm01 m-' h-') rates, in accordance with changes in temperature and oxygen consumption (22 and 163 mg O2 m-2 h-' in winter and spring respectively). Nitrate was mainly absorbed into the sediment, up to 120 pm01 m-2 h-' in winter, when the water column concentration was high. Organic nitrogen (urea and primary amines) contributed significantly to increase the rate of nitrogen release during summer and autumn. In spite of a summer deficit of ammonia exchange and of inorganic and organic nitrogen accumulation into the sediment, the coupling between oxygen uptake and nitrogen release was still significant. A similar seasonal pattern governs the metabolic rates of oysters; ammonia excretion increased in spring (3.21 pm01 g-' h-') and autumn (2.5 to 6.7 pm01 g-' h-'), compared to low winter values (0.28 pm01 g-' h-'), in accordance with changes in temperature and oxygen consumption (0.14 and 1.3 mg O2 g-' h-' in winter and summer respectively). Some nitrate production was observed, suggesting occasional nitrification. Primary amine exchanges were rather erratic, whereas urea seems to play an important role seasonally as a nitrogenous end-product, in relation to a lower condition index at the end of the winter A sirnp!e budge? sf sediment-water exchanges, calculated on a m-' basis (e. g. 2 kg oysters m-'), indicates that over the year the oysters' contribution to the fluxes averages 37 and 40 % for ammonia and urea release respectively, and 26 O/ O for oxygen uptake. However, the actual contribution of oysters to the exchanges at the water-sediment interface rarely fits the expected values from potential estimates. Oxygen consumption, ammonia release and nitrate absorption of the association of oysters with sediment are mainly depressed, except durlng winter and early spring when respiration is stimulated.