Nutrient and oxygen exchanges at the water-sediment interface in a shellfish farming lagoon (Thau, France)

The Etang de Thau (France) is a shallow lagoon characterised by the semi-intensive farming of oysters (Crassostrea gigas, Thunberg) cultured in suspension on frames. Analysis of the benthic fluxes of inorganic nutrients and oxygen over a period of a year has provided a basis for describing the dynamics of the water-sediment interface in the lagoon. Monthly measurements of fluxes at the water-sediment interface at two stations have been compared. One station (UC) is located under a culture table, and is subject to intensive accumulation of organic matter (biodeposition); the other (OC) is located outside the area directly under the impact of the culture activities. Oxygen consumption ranged from 288.24 to 1026.85 mu mol . m(-2). h(-1) according to the season and the station. Ammonium production was maximal at station UC in Summer (600 mu mol . m(-2). h(-1)) and minimal at station OC in the Autumn (30 mu mol . m(-2). h(-1)). In general, the fluxes recorded at station UC were 1.8-3 times higher than those recorded at station OC for oxygen and 1-5 times higher for ammonium. Nevertheless, the variability between stations was lower than the seasonal variability. Using a Multiple Correspondence Analysis (MCA), it was possible to point out the occurrence of an atypical event that was responsible for the disruption of the seasonal cycle. This event was a state of hypoxia known locally under the generic name of malaigue. The dystrophic crisis consists of a major perturbation of the ecosystem, responsible for a massive mortality affecting both the benthos and the reared stocks.