Seasonality of biological and physical controls

Seasonality of biological and physical controls on surface ocean CO2 from hourly observations at the Southern Ocean Time Series site south of Australia

Abstract

The Subantarctic Zone (SAZ) is important for air-sea CO2 exchange, ventilation of the lower thermocline, and nutrient supply to global ocean productivity. Here we present the first high-resolution autonomous observations of mixed layer CO2 partial pressure (pCO2) and hydrographic properties covering a full annual cycle in the SAZ. The amplitude of the seasonal cycle in pCO2 (60 uatm), from near atmospheric equilibrium in late winter to 330 uatm in mid summer, results from opposing physical and biological drivers. Decomposing these contributions demonstrates that the biological control on pCO2 (up to 100 uatm), is four times larger than the thermal component, and driven by annual net community production of 2.45+-1.47 mol C/m^2/yr^1. After the summer biological pCO2 depletion, the return to near atmospheric equilibrium proceeds  slowly, driven in part by autumn entrainment into a deepening mixed layer, and achieving full equilibration in late winter and early spring as respiration completes the annual cycle. The shutdown of winter convection and associated mixed layer shoaling proceeds intermittently, appearing to frustrate the initiation of production. Horizontal processes, identifed from salinity anomalies, are associated with biological pCO2 signatures, but with diering impacts in winter (when they reflect far-field variations in dissolved inorganic carbon and/or biomass) and summer (when they suggest promotion of local production by the relief of nutrient limitation). These results provide clarity on SAZ seasonal carbon cycling and demonstrate that the magnitude of the annual pCO2 cycle is twice as large as that in the subarctic high-nutrient, low-chlorophyll waters, which can inform the selection of optimal global models in this region.