Valérie Mesnage

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.

Filling of a wetland (Seine estuary, France): natural eutrophication or anthropogenic process? A sedimentological and geochemical study of wetland organic sediments

For over a century the Seine estuary has been highly affected by human activities, resulting in a reduction of the surface of wetland habitat. Several ponds of the Vernier Marsh, one of the main wetlands of the estuary, are being filled in by sediment. The St. Aubin canal, the only inlet from the Seine to the wetland, has regularly been opened in summer to maintain a sufficiently high water level in the system. The purpose of this study was to identify the origin of filling: natural evolution of eutrophication or human intervention by the opening of the St. Aubin canal? Sediment cores were sampled at 19 stations randomly distributed in the ponds of the wetland. Granulo-metric, micro-granulometric and geochemical analyses (ignition loss, C/N/P, chemical fractionation of particulate phosphate) were performed on the top 10 cm of sediment. Moreover nutrient concentrations (ammonium and phosphate) were measured in the interstitial waters using diffusion samplers. The results revealed that sediments sampled from the north are weakly organic (ignition loss: 19%, 15% C, 0.9% N, 40% organic P of total P) whereas those sampled from the southern stations are highly organic (ignition loss: 50–93%, 35% C, 1% N: 59–81% organic P of total P). In summary, sediments accumulating in the north of the ecosystem mainly consist of minerogenic particles from the Seine transported by the St. Aubin canal and thus are a result of the human interference. Interestingly the accumulation of sediments in the southern part is the result of a natural evolution of eutrophication: a peat bog. These sediments are highly organic, enriched with organic particulate phosphate, specifically a humic-acid-bound-phosphorus, a refractory form of phosphorus.