Inorganic carbon removal and isotopic enrichment in Antarctic sea ice gap layers during early austral summer

The biogeochemical composition of 2 spatially separate surface gap layers on a single Antarctic sea ice floe during early austral summer was predominantly controlled by the growth of diatoms and, especially, Phaeocystis. These algal communities in and near the gap layers imposed large geochemical changes in the chemical and isotopic composition of the gap waters, typical of intense autotrophic activity. These included a large deficit in all major dissolved inorganic nutrients (dissolved inorganic carbon [CT], nitrate, soluble reactive phosphorus, silicic acid), O2 accumulation above air saturation, large pH shifts into the alkaline spectrum, and a large, closely coupled 13C enrichment of the CT pool and the accumulated particulate organic carbon, in all cases relative to the composition of surface oceanic water. The amount of inorganic carbon removed from the gap water exceeded that which can be predicted from the deficits of dissolved inorganic nitrogen or phosphorus and the elemental composition of the biogenic matter suspended in it or the mean elemental composition of oceanic phytoplankton. This stoichiometric deviation suggests either (1) the operation of the inorganic carbon overconsumption mechanism via the biological production of particular classes of intraor extracellular carbon rich compounds, or (2) substantial utilisation of ammonium and urea as autotrophic nitrogen sources in addition to nitrate, or (3) both.