Ocean acidification state in the Ross Sea surface waters: physical and biological forcing.

The Ross Sea is vulnerable to Ocean Acidification (OA) due to its relatively low total alkalinity and because of increased CO2 solubility in cold water. OA induced decreases in the saturation state (Ω) for calcite and aragonite have potentially serious consequences for Antarctic food webs. Throughout the ocean, mesoscale processes (on spatial scales of 10-100 km and temporal ranges from hours to days) have first-order impacts on phytoplankton physiochemical controls and are critical in determining growth patterns and distribution. The circulation of the surface waters in the Ross Sea is affected by the presence of small-scale structures such as eddies, fronts and filaments, which can penetrate deep below the surface layer and hence influence the intensity of the bloom by supplying nutrients and trace elements, such as iron. Little is known about the effects of mesoscale structures on the carbonate system , but predicting future surface OA state and estimating future CO2 fluxes on a regional scale require understanding of the mesoscale processes controlling the carbonate system. To this purpose, water samples were collected in January 2014 in the framework of Ross Sea Mesoscale experiment (RoME) Project to evaluate the physical and biological forcing on the carbonate system at distance between stations of 5-10 km. Remote sensing supported the determination of the sampling strategy and helped positioning each sampling station. Total alkalinity, pH, dissolved oxygen, phytoplankton pigments and composition were investigated in combination with measurements of temperature, salinity and current speed. Total inorganic carbon, sea water CO2 partial pressure and Ω for calcite and aragonite were calculated from the measured total alkalinity and pH. In addition, continuous measurements of atmospheric CO2 concentration were completed. Different mesoscale physical features, such as fronts and eddies were observed in the investigated areas, which influenced the distribution of chemical parameters and of phytoplankton community in terms of biomass concentration (Chl-a) and species composition. The carbonate system properties in surface waters exhibited mesoscale variability with a horizontal length scale of about 10 km. Our results document substantial spatial heterogeneity and complexity in surface water carbonate system properties and the magnitude of the CO2 flux at a horizontal length scale of about 10 km, emphasizing the importance of mesoscale events to regional biogeochemistry. We believe that the resolution of these short length scale distributions provides insight into the biogeochemical dynamics which drive surface and subsurface variability in the Ross Sea.