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 CO 2 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 NH 4 + assimilation rates (~530 µmol m −2  h −1 ) relative to NO 3 − assimilation rates (~375 µmol m −2  h −1 ) suggest that the assimilation dynamics of C are primarily regulated by microbial regeneration in our region. N 2 fixation rates did not decline when other source of dissolved inorganic nitrogen were available, although the assimilation of N 2 is a highly energetic process. Our data showed that the diazotrophic community assimilated ~2 nmol N L −1  h −1 at relative elevated NH 4 + assimilation rates ~12 nmol L −1  h −1 and NO 3 − assimilation rates ~6 nmol L −1  h −1 . The small diffusive deep water NO 3 − fluxes could not support the measured NO 3 − assimilation rates and consequently point toward another source of dissolved inorganic NO 3 − . Highest NO 2 − values coincided consistently with shallow lower dissolved O 2 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 NO 3 − in the photic zone could be derived from new N through N 2 fixation. We conclude with the hypothesis that N injected through N 2 fixation can be recycled within the photic zone as NH 4 + and sequentially oxidized to NO 2 − and NO 3 − in shallow lower dissolved oxygen layers.