Nutrient Regulation of Diazotroph Communities and Nitrogen Fixation in the Northeastern Indian Ocean

Biological nitrogen fixation in the Indian Ocean exhibits pronounced spatial variability, yet its regulatory mechanisms remain poorly understood. In this study, rates of dinitrogen (N 2 ) fixation and nitrogenase gene ( nifH ) abundances of major diazotrophs were investigated in the northeastern Indian Ocean during the pre‐monsoon period (March‐May). Observed N 2 fixation rates ranged from below detection to 5.17 nmol N L −1 d −1 , with elevated rates (approximately 2–5 nmol N L −1 d −1 ) in surface waters at equatorial stations, whereas lower rates (<1 nmol N L −1 d −1 ) were recorded in the Bay of Bengal (BoB). N 2 fixation rates also displayed strong spatial heterogeneity within the BoB: no detectable rates were observed at southern stations (2.5–7°N), while measurable rates (0.25–0.59 nmol N L −1 d −1 ) occurred in surface waters at northern stations (7–14°N). Quantitative polymerase chain reaction (qPCR) and correlation analyses identified Trichodesmium spp. as the dominant diazotrophs and the primary contributors to N 2 fixation. In addition, relatively high N 2 fixation rates detected at depths dominated by Crocosphaera watsonii (UCYN‐B) suggest a substantial contribution from this group. Overall, elevated N 2 fixation rates were observed in equatorial surface waters, which exhibited elevated iron‐to‐nitrogen ratios (Fe:N) and lower dissolved inorganic nitrogen‐to‐total dissolved phosphorus (DIN:TDP) ratios than those in the BoB, suggesting more favorable conditions for N 2 fixation. We further propose that the supply of Fe, P (in both inorganic and organic forms), and N—potentially derived from freshwater inputs, upper‐ocean mixing, and atmospheric deposition—collectively exerts a key control on diazotroph community structure and rates of N 2 fixation in this region.