Global impact of benthic denitrification on diazotroph physiology and N2 fixation rates

<p>Nitrogen (N) is one of the crucial limiting nutrients for phytoplankton growth in the&#160;ocean.&#160;The main source of bioavailable N to the ocean is N₂-fixing diazotrophs in the surface layer. Since the global coverage of N₂ fixation observations is sparse on temporal and spatial scales, the fundamental processes and mechanisms controlling N₂ fixation are not well understood nor constrained. We implement benthic denitrification in the optimality-based plankton ecosystem model (OPEM), which is incorporated into an Earth System Model of intermediate complexity (UVic-ESCM 2.9). Benthic denitrification occurs mostly in coastal upwelling regions and shallow continental shelves, and affects significantly the marine fixed-N budget since it is the largest N-loss process in the global ocean. We carry out model calibration and parameter selection based on observations of Chl, NO₃^-, PO₄^3-, O₂ and N*=NO₃^--16PO₄^3-. Compared to considering water-column denitrification in suboxic zones as the only N-loss process in the ocean, our new model version simulates a more realistic distribution and higher global rates of N₂ fixation, which are supported by independent rate measurements. The optimized cellular N:P ratios of diazotrophs in the model version with benthic denitrification better correspond to independent culture estimates, and result in a closer reproduction of the particulate N:P ratios. Our model results indicate that benthic denitrification plays an important role shaping patterns, rates and even physiological aspects of N₂ fixation throughout the global ocean and should be accounted for when understanding and predicting changes to N₂ fixation.&#160;</p>