Nutrient accumulation in a lagoon sediment relating to hydrodynamic conditions.

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 homogene ous, the highest concentrations being found in the middle of the lagoon. Th e phosphate was principally bound onto the sediment in the west zone. The o bserved distribution was independent of grain size, the distribution of whi ch was rather homogeneous. In this lagoon water circulation is wind induced . Four different water flow structures were observed, as functions of the p revailing winds (NE, NW, SE, SW). The zones with the highest values of hydr odynamic variables, such as the velocity fields, corresponded to the highes t mineral carbon and total phosphate concentrations and with lower organic carbon and nitrogen concentrations. Accumulation of organic carbon and orga nic nitrogen was related to the sedimentation of macroalgae biomass. The di stribution of these two elements was dependent on the hydrodynamics, which stimulated their mineralization in the zones of turbulent water and their a ccumulation in the quieter ones. Phosphate distribution depended also on th e external input from the watershed. Phosphate bound onto the sediment accu mulated in the western part of the lagoon, as consequence of wind induced r esuspension. 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. (C) Elsev ier, Paris.