Reconstructing NCP from O2 and Ar concentrations in winter sea ice of the Ross Sea, Antarctica

Measuring the net community production (NCP) – the balance between O2 production by primary producers and the respiration of the entire community – in sea ice is very challenging due to its heterogeneous nature (mixture of pure ice, brines, gas bubbles and salts). NCP measurements are also scarce in sea ice, especially in winter. Here we present a reconstruction of the NCP levels by measuring the concentrations of O2 and Ar at high resolution using gas chromatography in sea-ice cores collected during the PIPERS (polynyas, ice production and seasonal evolution in the Ross Sea) field project. This is one of the rare projects to be conducted in the Ross Sea during the austral winter. Two main conclusions can be drawn from this study: it has been possible to dissociate the biotic and abiotic controls on O2 concentrations in sea ice and therefore to reconstruct the levels of NCP during this winter period. The discrimination of abiotic and biotic controls is based on the use of O2/Ar ratios because O2 concentration is modified by physical processes (temperature and salinity changes, brine convection) and by biological activity (photosynthesis and respiration), whereas Ar is only influenced by physical processes. A dominance of the physical processes was highlighted in most of the stations analyzed, with a contrast between the polynya stations, where negligible traces of biological activity could be identified, and non-negligible biological activity observed in the center of the Ross Sea and along the coast. To reconstruct the levels of NCP, a multidisciplinary approach was used because of the spatial and temporal variability of the cores sampled during the mission. Estimates of the age of the cores (and thus of the period of biological activity) required the use of satellite data and of a thermodynamic model. The concentrations measured in the ice made it possible to calculate the deviation from the saturation ratio (Δ(O2/Ar)), the equilibrium O2 concentration in the brines and, eventually, the O2 concentration due to biological activity. From the time evolution of the latter, the NCP allowed us to determine the general regime (auto- or heterotrophic) in place. These NCP values are compared to the literature for the winter period and the good correspondence confers reliability to our results. Finally, we compare and contrast NCPs calculated in cores located along the coast and in the marginal ice zone and in cores from the central Ross Sea.