Modélisation des cycles de l'azote et du phosphore dans les sédiments d'une lagune soumise à une exploitation conchylicole

A simulation of nitrogen and phosphorus exchanges for the sediment-water interface is presented, The aim of the sttudy is to understand the dynamic behaviour of an eutrophie coastal lagoon (Thau, France), ln situ experiments were performed to estimate the different pools of nitrogen and phosphorus in the water column and in the sediment. The diffusive fluxes between them were also estimated. A numerical mode1, simulating nitrogen and phosphorus cycles, was developed and calibrated with data recorded .at two seasons (winter and summer). This study leads to the conclusion that is necessary to know the different pools of nitrogen and phosphorus in order to estimate which fraction is bioavailable and the rate of nutrient exchange between sediment and water. It has been shown that temperature and oxygen concentration and redox potential al the interface are als ·import ant controlling factors in nutrient cycling. Finally, comparison with in situ data has pointed out the limits of the model for some of the ecosystem features. The simulations are reliable only for limited conditions of pH and oxygen and on a monthly scale. To predict nitrogen and phosphorus exhcanges in the sediment-water interface for an annual time scale of simulation, the model must consider the biological activities of the sediment and also include the refractory pools of nitrogen and phosphorus.