Seasonal variations of P compounds and their concentrations in two coastal lagoons

The accumulation of nutrients, with its short or long term consequences (eutrophication, dystrophic crisis), is a major problem in coastal lagoons. During 1990 a study was carried out at three sampling sites in two coastal lagoons near Montpellier in order to estimate the quantities present and the exchanges between compartments: variables were measured simultaneously in the overlying water, the interstitial water and the sediments. Analysis of the interstitial water (ammonium, orthophosphate), carried out every three months, allows the determination of diffusion fluxes. A strong correlation between the ammonium and orthophosphate concentrations is observed. Analysis of the sediments, particularly phosphate fractionation, was also carried out at different depths in earth season. While total phosphate is essentially constant over time, considerable variations in the fractions appear, in particular in the iron bound phosphate fractions. A gradient from surface to bottom is observed throughout the year, and a distinct decrease in summer and autumn. Phosphate release appears to be important but is difficult to calculate. Diffusion into the sediments takes place throughout the year, but transition from the upper sediment layer to the overlying water seems to depend very much on the oxidation state of the iron. The quantity of iron bound phosphate dissolving from a 1 mm thick oxidized sediment layer appears to be roughly equivalent to 150 days' release by the upward diffusion.