A hydrobiological monitoring network has been in place since 1977 in the Bay of Marennes Oleron (France). Data collected for physical variables (seawater temperature, salinity, oxygen concentration), nutrients (ammonium, nitrates, phosphates and silicates) and chlorophyll a and pheophytin, from 5 representative stations in the bay, were examined by time-series analysis (Census II method) to study seasonal variability and trends. The seasonal changes were similar over the entire Marennes Oleron Bay for all variables and were chiefly influenced by fluxes in the Charente river. The seasonal range reached 180 units for nitrates and 70 and 100 units respectively for phosphates and silicates. These values were similarly mostly correlated with the Charente River fluxes. With regard to long-term trends, seawater temperature has shown a significantly increasing trend close to 2 °C over 18 years. At the same time, a 1 °C gradient was demonstrated from the northern to the southern part of the Bay. The salinity trend varied between 30 and 34‰for all stations. The trend for oxygen concentration, ranged from 90 to 100% but during a specific two year period (1980-1982) saturation decreased to 76% in the northern part of the Bay.
The trend analysis for nitrates showed a significant relationship with the water output level of the Charente. Phosphate inputs have been irregular during the two last decades which has affected primary productivity along the coastline (e.g., spring 1979-1983; 1990; 1993-1995).
Since 1988, a significant increasing trend for ammonium was observed at the mouth of the Seudre river (4 μmoles l-1 ) while other stations were well below this, ranging from 1 to 3 μmoles l-1. This should be considered as an indicator of seawater deterioration within the southern part of the Marennes Oleron Bay.