Ocean acidification in the Mediterranean Sea: pelagic mesocosm experiments

Chair: Victoria Cole

Frédéric Gazeau(1,2), Laure Maugendre(1,2), Justine Louis(1,2), Jean-Pierre Gattuso(1,2), Cécile Guieu(1,2)
and the MedSeA mesocosm consortium

1 Sorbonne Universités, UPMC Univ Paris 06, UMR 7093, LOV, Observatoire océanologique, Villefranche-sur-mer, 06230, France
2 CNRS, UMR 7093, LOV, Observatoire océanologique, Villefranche-sur-mer, 06230, France

Background
As plankton plays a key role in the global carbon cycle, it is important to project the evolution of plankton community structure and function in a future high-CO2 world. The great majority of experiments conducted at the community level have been performed under high natural or nutrient-enriched conditions and very few data are available in areas with naturally low levels of nutrient and chlorophyll, although they represent a large and expanding part of the ocean surface.

Methods
In the frame of the MedSeA project, in situ mesocosms have been used to quantify the effects of CO2 enrichment in two Mediterranean coastal areas: the Bay of Calvi (Corsica, France) in summer 2012 and the Bay of Villefranche (France) in winter 2013. For both experiments, a pCO2 gradient approach has been chosen with 3 controls and 6 mesocosms with increasing pCO2 levels.

Findings
The two experiments were characterized by contrasted initial conditions in terms of inorganic nutrient concentrations and a large contribution of organic nutrients. While inorganic phosphate concentrations were depleted during both experiments, inorganic nitrogen concentrations were higher in winter although most of the nitrate was rapidly consumed inside all mesocosms during the acidification phase, leading to a decrease in N/P ratio to 13. Phytoplankton biomass in winter was much higher than in summer (~1 vs. 0.05 μg L-1) and both communities were dominated by small cells. Increasing pCO2 had no significant effect on community structure and metabolism.

Conclusions
While conditions in the Bay of Calvi were representative of summer oligotrophic conditions in the Mediterranean Sea, in winter, an experimental artifact led to an unnatural N/P co-limitation at this period of the year. These first mesocosm experiments focused on ocean acidification and conducted in an oligotrophic area showed that nutrient dynamics, community structure and metabolism were not dependent on seawater pH.