High Contribution of <i>Trichodesmium</i> to Nitrogen Fixation in the Bay of Bengal and Andaman Sea During Spring Intermonsoon

Abstract The Bay of Bengal (BoB) and adjacent seas receive substantial freshwater inputs, leading to strong stratification and nutrient limitation, along with frequent mesoscale eddies. These processes create significant physicochemical heterogeneity, which is presumed to regulate the diazotrophic community and nitrogen fixation rates (NFRs), though the underlying mechanisms remain unclear. This study examined diazotrophs and size‐fractionated (&gt;10 μm and &lt;10 μm) NFRs and primary production (PP) across BoB, Andaman Sea (AS), and equatorial Indian Ocean (EIO) during March–April 2021. Surface abundances of Trichodesmium and Richelia/Calothrix , as well as NFRs, were significantly higher in AS than in BoB and EIO. The southern AS, likely enriched by terrestrial iron and phosphorus, sustained Trichodesmium blooms (&gt;10,000 trichomes L −1 ) and elevated NFRs (up to 15.33 nmol N L −1 d −1 ). Based on the Redfield C/N ratio, NFRs were estimated to supply 1.3–21.3% of the nitrogen required for surface PP. nifH amplicon sequencing revealed Trichodesmium dominance in the iron‐replete AS and BoB, whereas Proteobacteria prevailed in the iron‐depleted EIO. Size‐fractionated NFRs experiments indicated that the &gt;10 μm fraction dominated in AS and BoB, whereas the &lt;10 μm fraction contributed more in EIO. Anti‐cyclonic eddies hosted higher Trichodesmium abundances and NFRs compared to a cyclonic eddy and the Wyrtki Jets. Redundancy analysis and generalized additive models demonstrated that diazotrophic composition and NFRs were closely associated with salinity, temperature, and phosphate availability. These findings highlight that spatial variability in diazotrophic communities and NFRs across the BoB and adjacent waters is largely regulated by terrestrial inputs and mesoscale eddy dynamics.