Study of Annual and Seasonal Dissolved Inorganic Nitrogen Inputs into the Arcachon Lagoon, Atlantic Coast (France)

The aim of this study is to determine, by applying a direct method, dissolved inorganic nitrogen (DIN) inputs which reach a coastal lagoon: the Arcachon Bay (France). Yearly monitoring of the different sources (groundwater, surficial water and atmospheric water) allows the annual DIN discharge and the variations of DIN discharges throughout the year to be quantified. The Arcachon Bay received 870 t-N in the year 1996. Study of the two main hydrologic seasons (high and low water-table periods) has shown that 78% of this annual discharge occurs during the wet period (from December to April). River DIN inputs account for more than 90% of the total annual discharge and during the wet period they account for 95% of total DIN inputs. Rivers are therefore the most important source of DIN for the Arcachon Bay and thus the principal factor in the Bay's eutrophication. The reasons for this are an important water discharge (more than 74% of the total freshwater discharge) and relatively high DIN concentrations (95μM on average) in comparison with groundwater and atmospheric water (33 and 43μM, respectively). As for the atmospheric source, it accounts for 9% of the annual DIN discharge with wide variations throughout the year since it accounts for 5% in the wet period and 26% in the dry period (May to November). This fluctuation is due to a rise in the DIN atmospheric flux which increases from 32 to 49 t-N. Further, this study proves that, nowadays, the groundwater which flows into the Arcachon Bay is not DIN contaminated. Combined with the values mentioned above, the low groundwater discharge (less than 3% of the total annual discharge) yields a slight contribution (less than 1%).