Accumulation des sels nutritifs dans un sédiment lagunaire et environnement hydrodynamique

The Mejean-Perols Lagoon is situated on the French Mediterranean coast. It is a small (747 ha) and shallow (70 cm) lagoon. Some treated effluent from Montpellier flows into this hypereutrophic lagoon. The cartography of C, N and P concentrations in the sediment was studied based on 35 bottom samples. The nitrogen and organic carbon concentrations were more or less homogeneous, the highest concentrations being found in the middle of the lagoon. The phosphate was principally bound onto the sediment in the west zone. The observed distribution was independent of grain size, the distribution of which was rather homogeneous. In this lagoon water circulation is wind induced. Four different water flow structures were observed, as functions of the prevailing winds (NE, NW, SE, SW). The zones with the highest values of hydrodynamic variables, such as the velocity fields, corresponded to the highest mineral carbon and total phosphate concentrations and with lower organic carbon and nitrogen concentrations. Accumulation of organic carbon and organic nitrogen was related to the sedimentation of macroalgae biomass. The distribution of these two elements was dependent on the hydrodynamics, which stimulated their mineralization in the zones of turbulent water and their accumulation in the quieter ones. Phosphate distribution depended also on the external input from the watershed. Phosphate bound onto the sediment accumulated in the western part of the lagoon, as consequence of wind induced resuspension. It was principally present in inorganic form (70 % of total P). The concentration of phosphate extracted with Na-EDTA (calcium-bound) was higher than that of the phosphate extracted with Ca-NTA (iron-bound). The organic fractions represented 29 % and dissolved phosphate 1 % of the total P. The phosphate mobility accounted for 30 % of total phosphate.