Reshaping of New Nitrogen Regime by Anticyclonic Eddies in the Northern South China Sea
Journal
Journal of Geophysical Research: Oceans
Journal Volume
130
Journal Issue
1
ISSN
2169-9275
2169-9291
Date Issued
2025-01
Author(s)
Tian, Juan
Zhang, Yue
Jing, Hongmei
Mao, Huabin
Zhou, Hantao
Zheng, Minfang
Chen, Mengya
Zhang, Run
Chen, Min
Abstract
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 N2 fixation rate (via 15N2 bubble release method) and vertical diffusive nitrate flux (Fdiff- (Formula presented.)), for the euphotic zone (EZ) in the northern South China Sea ACEs during summer 2020. Depth-integrated N2 fixation rates (INF) were moderately elevated (∼30%) in the center of the ACEs compared with those in the outside stations, suggesting that ACEs generally provide a more favorable environment for N2 fixation. In contrast, the upward Fdiff- (Formula presented.) into the EZ were greatly lowered by an order of magnitude in the ACE center (center: 26.0 ± 8.2 μmol N m−2 d−1; outside: 124.0 ± 127.6 μmol N m−2 d−1), thus making N2 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 Fdiff- (Formula presented.) 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 N2 fixation dynamics in response to mesoscale ACEs in tropical/subtropical oceanic regions, also helping better constrain biogeochemical models.
Subjects
15N isotope
mesoscale anticyclonic eddies
N2 fixation
northern South China Sea
Publisher
American Geophysical Union (AGU)
Type
journal article