Physical optima for nitrogen fixation in cyclonic eddies in the Subtropical Northwestern Pacific

Nitrogen fixation is a vital new nitrogen source in the oligotrophic ocean. Although our knowledge of the controlling factors of marine nitrogen fixation have increased rapidly, the physical controls, particularly eddies-induced upwelling and light intensity, remain elusive. In this study, conducted in the Subtropical Northwestern Pacific, we measured nitrogen fixation rates (NFR) in two cyclonic eddies (CEs). Our observations in one CE revealed that depth-integrated NFR (INFR) in core stations were significantly higher than in edge stations, indicating that CEs-induced upwelling might enhance nitrogen fixation. However, more intense upwelling in another CE resulted in lower INFR in core stations compared to edge stations. The INFR distributions in CEs were driven by the upwelling intensity, showing a unimodal response, i.e., the maximum INFR appeared at optimal upwelling intensity. This finding reconciles the debate about whether CEs inhibit nitrogen fixation. Additionally, results from light manipulation incubations proved that light intensity is a key driver for the vertically unimodal pattern of NFR, i.e., peaks at the subsurface layer with an optimum light intensity of 20% to 50% of surface PAR. Furthermore, molecular evidence showed that UCYN-A dominated in the upwelling area, while UCYN-B dominated in the non-upwelling area, indicating that CEs-induced physical perturbation regulates the niches of diazotrophs. Taken together, these results suggest that physical dynamics exert profound controls on the spatial heterogeneity of diazotrophic distribution and activity in the Subtropical Northwestern Pacific, providing new insights into the physical drivers of nitrogen fixation on mesoscale hydrodynamics..