Nitrification in Mode Water Formation Regions and its Contribution to Low Latitude Nitrogen Supply

Subantarctic Mode Waters (SAMW) form in the Southern Ocean and transport nitrate northwards, a process that sustains productivity in the world’s ocean basins. These waters are deplete of silicate, attributed to high silicate demand during diatom blooms in the Subantarctic Zone. In this study, we take two contrasting approaches to quantify the extent of nitrification occurring on density layers that form SAMW. Firstly, we compile nitrate isotope data to assess how efficiently nitrate, following uptake by phytoplankton, is exported from, or remineralised within, the mixed layer. Secondly, we use the PISCES biogeochemical model to assess the spatial and temporal variability of nitrification and to quantify its significance to nitrate and Si* concentrations in SAMW. From the two approaches, we estimate that 15-35% of nitrate in SAMW has been recycled since upwelling in the Polar Frontal Zone. The model outputs imply that nitrification is responsible for pushing Si* to negative values in the SAMW, by enhancing nitrate concentrations. Our findings decrease the potential for silicic acid leakage in the glacial ocean decreasing atmospheric carbon dioxide concentrations and suggest that changes in biological production could alter the nitrate and Si* supply to the low latitude ocean.