Atmospheric nutrients in seawater under current and high pCO₂ conditions after Saharan dust deposition: Results from three minicosm experiments

The Mediterranean basin receives among the highest dust fluxes in the world ocean, and also appears to be one of the regions the most strongly impacted by ocean acidification. The aim of this study was to assess, on a short time scale (one-week), the effect of ocean acidification on the dissolution of nutrients (inorganic nitrogen, phosphate and iron) from Saharan dust. Three experiments were performed in three distinct seasons: in May, after the spring bloom with low autotrophic biomass, in September, at the end of the oligotrophic period, and in January, during the winter bloom. On each occasion, a dust flux of 10 g m⁻² was simulated at the surface of two minicosms (tanks of ∼0.3 m³) filled with filtered (<0.2 µm) seawater collected in the Bay of Villefranche (NW Mediterranean Sea). One minicosm served as a control and the other was acidified to reach a partial pressure of CO₂ (pCO₂) close to that projected for the end of this century (∼1250 μatm). Following a high-resolution sampling protocol, results showed that whatever the season and in situ biogeochemical conditions (1) all nitrogen from dust was soluble in seawater, allowing a large and stable increase in the stock of NO_x (nitrate + nitrite) under the two pCO₂ conditions (ambient and future), (2) transient increases in dissolved iron and phosphate concentrations were driven by scavenging processes, with a low dissolution percentage averaging 0.14 ± 0.08 and 4.7 ± 1.2%, respectively. While the absence of pCO₂ effects on the release of atmospheric nitrogen was confirmed in the present study, no clear conclusion could be drawn for phosphate and dissolved iron as a consequence of very low concentrations and rapid (within less than 1 h) dissolved-particulate exchanges. Nevertheless, as the lifetime of these elements in solution is limited to a few hours, whatever the pH conditions, our results suggest that ocean acidification would have only a minor impact on their bioavailability for surface phytoplankton communities in such Low Nutrient Low Chlorophyll areas.