Atmospheric deposits of dissolved inorganic nitrogen in the southwest of France

A 1-year study of bulk atmospheric dissolved inorganic nitrogen (DIN) deposits has identified a medium anthropogenic impact in the southwest of France (average NO3 concentration=26.2 μM; yearly NO3 flux=22.6±5 mmol/m2 per year; average NH4 concentration=17.8 μM; yearly NH4 flux=14.6±3 mmol/m2 per year). The air flow directions have shown that the contamination is derived from Spain and the south of France during the winter period and from northern Europe in the summer period. However, long range transport of ammonia is less probable because of its short residence time in the atmosphere due to its high solubility. Moreover, a seasonal variation has affected ammonium (2.4 times higher in the cultivation period), while nitrate remained statistically constant. Although no difference of ammonium concentration was observed under the influence of oceanic and continental winds, it must be influenced by regional emissions from agricultural areas. In the summer period, the atmospheric source makes up 26% of the total DIN inputs into the Arcachon Lagoon. Moreover, an exceptional storm event represented 96% of the total input in a single day. Thus, such a seasonal pattern suggests an important impact of atmospheric DIN deposits on primary production in coastal ecosystems of such a region less industrialized since they often show oligotrophic conditions in the summer period.