Effect of mesoscale eddies on diazotroph community structure and nitrogen fixation rates in the South China Sea

Abstract Mesoscale eddies play a key role in ocean biogeochemistry, yet their effects on dinitrogen ( N 2 ) fixation are not well understood. Using quantitative polymerase chain reaction (qPCR) and dinitrogen-15 (15N2) tracer methods, we examined the abundance of nifH genes in N 2 -fixing microorganisms and N 2 fixation rates in the upper water column (0–100 m) of mesoscale eddies in the northeastern South China Sea (SCS). High N2 fixation rates affected by an anticyclonic eddy were associated with high abundances of unicellular nitrogen-fixing cyanobacteria in groups A and B (UCYN-A, UCYN-B), while low nitrogen fixation was found in both cyclonic eddy and reference stations. The estimated N2 fixation rates met 9% ± 5% of the nitrogen demand for primary production affected by the anticyclonic eddy. Our results indicate that mesoscale eddies can affect nitrogen fixation through affecting nutrient availability and eddy transport, and play a crucial role in regulating N2 fixation in the oligotrophic SCS.