The distribution of phosphate in sediments and its relation with eutrophication of a Mediterranean coastal lagoon

A major problem of the Mediterranean coastal lagoons is an excessive input of nutrients (i.e. N and P), causing eutrophic conditions in summer. The sediments of these lagoons can serve as a reservoir by fixing phosphate, or as a source when this phosphate is released under certain conditions. Knowledge of nutrient sources and fluxes is needed if coastal lagoons are to be protected against eutrophication. Therefore, we have evaluated the total pool of phosphate in the lagoon sediments, and the quantity of phosphate which may be released.Sediment profiles have been analysed at two stations of the Lagune de Thau both in and outside the oyster-bank zone. A sequential fractionation, using chelating agents was performed to extract the inorganic (iron and calcium bound phosphate) and the organic phosphate fractions. A statistical analysis of the data set has revealed several significant factors which explain the fluctuations of the concentrations of each phosphate fraction. These factors are: the time of year (seasons), the depth (5 cm layers of sediment), and the site (station).A spatial and a temporal variation of the concentration of Tot-P was found. The largest variation between the two different zones appeared only in the first five cm of sediment. There is only a slight seasonal variation in the amount of phosphate at other depths at the two different zones. Season and station are the factors which control the variations in distribution of phosphate fractions. The spatial and temporal variations of the iron and calcium bound phosphate are explained by the redox potential and pH in the top layer of the sediment.Abbreviations In principle the general abbreviations are used (see page vi). Futhermore Fe(OOH)-P= Ferric hydroxide bound phosphate - CaCO₃-P= Calcium carbonate bound phosphate - ASOP= Acid soluble organic phosphate - ROP= Residual organic phosphate