Hydrologie du bassin de Marennes- Oléron. Analyse de la base de données "RAZLEC" 1977- 1995

An hydrobiological monitoring survey has been carried out in the Bay of Marennes-Oléron since 1977. Five stations, distributed over the Bay, were sam pied twice a month. Spatial and temporal trends of the physical characteristics (i.e., seawater, temperature, salinity, dissolved oxygen and suspended matter), nutrients (azote, nitrite, nitrate, silicate) and organic variables (chlorophyll a and pheopigments) were assessed by using a statistical time-series analysis mode\. It was used to distinguish seasonal variability from long term trends. Pluviometric data as weil as the Charente River fluxes were similary analyzed by the model. The seasonal variability showed an homogeneous pattern over the five stations for all parameters, but the site-dependent ammoniac. Nitrates, phosphates and chlorophyll a concentrations showed highly variable cycles. The trend analysis allowed to specifY the characteristics of the various bay sites, and emphasized the critical role of the Charente River in nitrate-phosphate nutrients enrichment. However, sediment nutrients were likely mobilized with concomitant Charente fluxes decrease. The decreasing phosphate concentration pattern should be emphasized as a likely limitant factor for phytoplanktonic blooms, from a quantitative and qualitative point of view. The azote (N-NH4+) concentration in Seudre River has steadily increased over the last 10 years, therefore demonstrating il significant seawater quality decrease in the southern part of the Bay of Marennes-Oléron. Moreover, the seawater temperature analysis showed a significant 1°C increase., concomittant with a north-south thermal gradient.