Net Fluxes of Dissolved Arsenic, Cadmium, Copper, Zinc, Nitrogen and Phosphorus from the Gironde Estuary (France): Seasonal Variations and Trends

The Gironde Estuary (south-western France), because of high contamination with metals and the proximity of large oysterfarming areas, has been a reference study site for 20 years. Research performed to date suggests a trend toward decontamination of the estuary, but a lack of information on seasonal variations has made difficult the evaluation of interannual changes. This study, which examines data obtained during five cruises over a 1-year period, takes into account various seasonal and hydrodynamic conditions. The fluctuations in dissolved net fluxes at the outlet of the upper estuary and the As, Cd, Cu, Zn, N and P concentrations are related to the season (January, February, May and October) or freshwater flow (400 to 1600 m3 s-1). Large seasonal variations are observed for As, Cu, N and P, and fluxes differ 3- to 4-fold according to the sampling month. Seasonal variations in net Cd and Zn fluxes are slight and relatively insignificant according to the confidence interval. For the other elements studied, the relationship with the river flow accounts for most of the seasonal variations. In the long-term, an increase in fluxes is found for nutrients, despite large seasonal differences. For Cd, the trend toward long-term decontamination is more clearly established, confirming the data of the French Mussel Watch, whereas the amplitude of seasonal variations masks any long-term trend for Cu. This study makes clear that when estuaries are subjected to marked fluctuations in freshwater flows and climatic conditions, an analysis of the concentrations or instantaneous fluxes of the various elements must be interpreted only as a part of the annual geochemical cycle.