Nutrient dynamics under different ocean acidification scenarios in a low nutrient low chlorophyll system: the Northwestern Mediterranean Sea

International audience; Two pelagic mesocosm experiments were conducted to study the impact of ocean acidification on Mediterranean plankton communities. A first experiment took place in summer 2012 in the Bay of Calvi (France) followed by an experiment in winter 2013 in the Bay of Villefranche (France) under pre-bloom conditions. Nine mesocosms were deployed: three served as controls and six were acidified in a targeted partial pressure of CO2 (pCO2) gradient from 450 to 1250 μatm. The evolution of dissolved organic and inorganic nutrient concentrations was observed using nanomolar techniques. The experiments were characterized by a large contribution of organic nutrients to nutrient pools and contrasting in situ conditions with an inorganic N/P ratio of 1.7 in summer and of 117 in winter. In the Bay of Calvi, initial conditions were representative of the summer oligotrophic Mediterranean Sea. While inorganic phosphate concentrations were depleted during both experiments, in situ inorganic nitrogen concentrations were higher in winter. However, nitrate was rapidly consumed in winter in all mesocosms during the acidification phase, leading to a decrease in N/P ratio to 13. During these first mesocosm experiments conducted in a low nutrient low chlorophyll area, nutrient dynamics were insensitive to CO2 enrichment, indicating that nutrient speciation and related biological processes were likely not impacted. During both experiments, nitrate and phosphate dynamics were controlled by the activity of small species that are favored in low nutrient conditions. In contrast to the theoretical knowledge, no increase in iron solubility at high pCO2 was observed.