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

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 (N-15(2)) 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 N-2 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 N-2 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 N-2 fixation in the oligotrophic SCS. (C) 2020 Elsevier B.V. All rights reserved.