Reshaping of New Nitrogen Regime by Anticyclonic Eddies in the Northern South China Sea

Mesoscale anticyclonic eddies (ACEs) act as an important physical disturbance for marine biogeochemical cycle, but our knowledge of the dynamics of critical new nitrogen (N) sources in such environments remains ambiguous. Here, we report concurrent data on two major sources of new N, that is, the N-2 fixation rate (via N-15(2) bubble release method) and vertical diffusive nitrate flux (F-diff-NO3-), for the euphotic zone (EZ) in the northern South China Sea ACEs during summer 2020. Depth-integrated N-2 fixation rates (INF) were moderately elevated (similar to 30%) in the center of the ACEs compared with those in the outside stations, suggesting that ACEs generally provide a more favorable environment for N-2 fixation. In contrast, the upward F-diff-NO3- into the EZ were greatly lowered by an order of magnitude in the ACE center (center: 26.0 +/- 8.2 mu mol N m(-2) d(-1); outside: 124.0 +/- 127.6 mu mol N m(-2) d(-1)), thus making N-2 fixation a much more significant contributor to new production under ACE influence. Such contribution is further demonstrated in the nutrient depleted layer where substantial carbon export may be taking place. Interestingly, a significant positive correlation for the ratio of the INF to the upward F-diff-NO3- versus sea level anomaly was observed. ACE will likely leave an imprint on the isotopic composition of exported N, implying that there is possibly a need to take mesoscale forcings into account when interpreting isotopic signals from sinking particles. These findings will improve our understanding of N-2 fixation dynamics in response to mesoscale ACEs in tropical/subtropical oceanic regions, also helping better constrain biogeochemical models.