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

Nitrogen (N) is one of the crucial limiting nutrients for phytoplankton growth in the ocean. 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.