Global impact of benthic denitrification on marine N₂ fixation and primary production simulated by a variable-stoichiometry Earth system model

Abstract. Nitrogen (N) is a crucial limiting nutrient for phytoplankton growth in the ocean. The main source of bioavailable N in the ocean is delivered by N₂-fixing diazotrophs in the surface layer. Since field observation of N₂ fixation are spatially and temporally sparse, the fundamental processes and mechanisms controlling N₂ fixation are not well understood and constrained. Here, we implement benthic denitrification in an Earth System Model of intermediate complexity (UVic-ESCM 2.9) coupled to an optimality-based plankton ecosystem model (OPEM v1.1). Benthic denitrification occurs mostly in coastal upwelling regions and on shallow continental shelves, and is the largest N-loss process in the global ocean. We calibrate our model against three different combinations of observed Chl, NO₃^-, PO₄^3-, O₂ and N* = NO₃^- −16PO₄^3- +2.9. The inclusion of N* provides a powerful constraint on biogeochemical model behavior. Our new model version including benthic denitrification simulates higher global rates of N₂ fixation with a more realistic distribution extending to higher latitudes that are supported by independent estimates based on geochemical data. Oxygen deficient zone volume and water column denitrification rates are reduced in the new version, indicating that including benthic denitrification may improve global biogeochemical models that commonly overestimate anoxic zones. With the improved representation of the ocean N cycle, our new model configuration also yields better global net primary production (NPP) when compared to the independent datasets not included in the calibration. Benthic denitrification plays an important role shaping N₂ fixation and NPP throughout the global ocean in our model, and should be considered when evaluating and predicting their response to environmental change.