Sources of new nitrogen in the Indian Ocean

Quantifying the different sources of nitrogen (N) within the N cycle is crucial to gain insights in oceanic phytoplankton production. To understand the controls of primary productivity and the associated capture of CO₂ through photosynthesis in the southeastern Indian Ocean, we compiled the physical and biogeochemical data from four voyages conducted in 2010, 2011, 2012, and 2013. Overall, higher NH4 + assimilation rates (-530 μmol m^-2 h^-1) relative to NO3 - assimilation rates (-375 μmol m^-2 h^-1) suggest that the assimilation dynamics of C are primarily regulated by microbial regeneration in our region. N₂ fixation rates did not decline when other source of dissolved inorganic nitrogen were available, although the assimilation of N₂ is a highly energetic process. Our data showed that the diazotrophic community assimilated -2 nmol N L^-1 h^-1 at relative elevated NH₄ + assimilation rates -12 nmol L^-1 h^-1 and NO₃ - assimilation rates -6 nmol L-^-1 h^-1. The small diffusive deep water NO₃ - fluxes could not support the measured NO₃ - assimilation rates and consequently point toward another source of dissolved inorganic NO₃ -. Highest NO₂ - values coincided consistently with shallow lower dissolved O₂ layers (100-200 m; 100-180 μmol L^-1). These results suggest that nitrification above the pycnocline could be a significant component of the N cycle in the eastern Indian Ocean. In our analysis we provide a conceptual understanding of how NO₃ - in the photic zone could be derived from new N through N₂ fixation. We conclude with the hypothesis that N injected through N₂ fixation can be recycled within the photic zone as NH₄ + and sequentially oxidized to NO₂ - and NO₃ - in shallow lower dissolved oxygen layers.