MODELING PHOSPHORUS AND NITROGEN CYCLING IN THE SEDIMENT OF A MEDITERRANEAN SHELLFISH LAGOON

A simulation of nitrogen and phosphorus exchanges for the sediment-wat er interface is presented. The aim of the study is to understand the d ynamic behaviour of an eutrophic coastal lagoon (Thau, France). In sit u experiments were performed to estimate the different pools of nitrog en and phosphorus in the water column and in the sediment. The diffusi ve fluxes between them were also estimated. A numerical model, simulat ing nitrogen and phosphorus cycles, was developed and calibrated with data recorded at two seasons (winter and summer). This study leads to the conclusion that it is necessary to know the different pools of nit rogen and phosphorus in order to estimate which fraction is bioavailab leand the rate of nutrient exchange between sediment and water. It has been shown that temperature and oxygen concentration and redox potent ial at the interface are also important controlling factors in nutrien t cycling. Finally, comparison with in situ data has pointed out the l imits of the model for some of the ecosystem features. The simulations are reliable only for limited conditions of pH and oxygen and on a mo nthly scale. To predict nitrogen and phosphorus exchanges in the sedim ent-water interface for an annual time scale of simulation, the model must consider the biological activities of the sediment and also inclu de the refractory pools of nitrogen and phosphorus.