Physical and biological forcing of mesoscale variability in the carbonate system of the Ross Sea (Antarctica) during summer 2014

Water samples (0–200m) were collected in a coastal area of the Ross Sea in January 2014 to evaluate the physical and biological forcing on the carbonate system at the mesoscale (distance between stations of 5–10km). 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 the saturation state (Ω) for calcite and aragonite were calculated from the measured total alkalinity and pH. In addition, continuous measurements of atmospheric CO2 concentration were completed. LADCP measurements revealed the presence of a significant change in current speed and direction that corresponded to a clearly defined front characterized by gradients in both temperature and salinity. Phytoplankton biomass was relatively high at all stations and the highest values of chlorophyll-a were found between 20 to 50m, with the dominant taxonomic group being haptophyceae. The carbonate system properties in surface waters exhibited mesoscale variability with a horizontal length scale of about 10km. Sea-ice melt, through the input of low salinity water, results in a dilution of the total alkalinity and inorganic carbon, but our observations suggest that phytoplankton activity was the major forcing of the distribution of the carbonate system variables. Higher CO3−, Ω and pH in the surface layer were found where the highest values of chlorophyll-a were observed. The calculated ΔpCO2 pattern follows both MODIS data and in situ chlorophyll-a measurements, and the estimated CO2 fluxes ranged from –0.5±0.4 to –31.0±6.4mmolm−2d−1. The large range observed in the fluxes is due to both the spatial variability of sea water pCO2 and to the episodic winds experienced.