Potential Contribution of Unicellular Cyanobacteria and Non‐Cyanobacterial Diazotrophs to Biological Nitrogen Fixation on the Continental Shelf of the East China Sea During Spring and Autumn

Marine N 2 fixation fuels primary production and carbon sequestration in the global ocean. However, the distribution patterns of N 2 fixation rates (NFRs), their main contributors, and controlling factors on the continental shelf of the East China Sea (ECS) remain poorly understood. Filamentous cyanobacteria ( Trichodesmium and Richelia / Calothrix ) have been historically considered as the dominant diazotrophs in the ECS, but their observed distribution could not fully explain the NFR variations across the ECS, suggesting potential contributions from unicellular cyanobacteria and non‐cyanobacterial diazotrophs (NCDs). Here, for the first time, size‐fractionated incubations (>10 and <10 μm fractions) combined with the 15 N 2 dissolution method were used to measure NFRs throughout the euphotic zone on the ECS shelf during spring and autumn. The depth‐integrated NFRs were 4.34 ± 4.35 μmol N/m 2 /d in spring and 89.81 ± 118.87 μmol N/m 2 /d in autumn. Filamentous cyanobacteria were mainly confined to the southern ECS, which was inconsistent with the spatial distribution of bulk NFRs. The diazotrophs of <10 μm fractions contributed 63.1% and 77.9% to total depth‐integrated NFRs in spring and autumn, respectively. Notably, elevated NFRs were detected near the bottom of the euphotic zone in autumn, accounting for 52.7% of the entire water‐column N 2 fixation. Spearman correlation analysis showed strong associations between size‐fractionated NFRs and nutrient concentrations as well as water‐column stratification. These findings highlight the substantial contribution of small‐sized diazotrophs to N 2 fixation on the ECS shelf. Importantly, adopting more accurate measurement methods and extending sampling depths are crucial for improving estimates of the regional N 2 fixation budget.