Responses of Nutrient Biogeochemistry to a Mesoscale Eddy Detached From the Kuroshio Loop Current

Mesoscale eddies play important roles in ocean biogeochemical cycles by enhancing lateral and vertical transport of materials and energy. We investigated the nutrient biogeochemistry, including the nitrogen cycle, in the northern South China Sea (NSCS) under the influence of an anticyclonic mesoscale eddy detached from the Kuroshio loop current (KLC). The detached eddy transported substantial amounts of water and nutrients that originated from the Kuroshio to the southwest hinterland of the NSCS. Using a nitrogen mass and isotope balance model, we estimated that the enhanced particulate nitrogen flux exported from the euphotic zone at the eddy center area was 2.4 ± 1.7 times that in background area. In addition, the nitrogen fixation and atmospheric depositions (NFAD) nitrogen flux was estimated to be 0.18 ± 0.15 mmol m⁻² d⁻¹. The average proportion of new production sustained by NFAD nitrogen relative to dissolved inorganic nitrogen (DIN) supplied from below was 12%, and the turnover time of DIN in the euphotic zone within the eddy was estimated to be ∼9 days. The eddy-trapped Kuroshio water influenced the distribution of nutrients and nitrate isotope composition above 400-m depth in the NSCS. The remineralization of newly fixed nitrogen originating from NFAD and the subsequent nitrification regulated nitrate isotope composition above 800 m, whereas assimilation also played a role within the euphotic zone. Deep-water communication with the North Pacific and the input of the North Pacific Intermediate Water introduced the denitrification signal that originated from the eastern tropical North Pacific into the NSCS.