Impact of the Kuroshio intrusion on the nutrient inventory in the upper northern South China Sea: insights from an isopycnal mixing model

[摘要] Based on four cruises covering a seasonal cycle in 2009–2011, we examinedthe impact of the Kuroshio intrusion, featured by extremely oligotrophicwaters, on the nutrient inventory in the central northern South China Sea(NSCS). The nutrient inventory in the upper 100 m of the water column in thestudy area ranged from ∼200 to ∼290 mmol m⁻² forN + N (nitrate plus nitrite), from ∼13 to∼24 mmol m⁻² for soluble reactive phosphate and from∼210 to ∼430 mmol m⁻² for silicic acid. The nutrientinventory showed a clear seasonal pattern with the highest value appearing insummer, while the N + N inventory in spring and winter had a reduction of∼13 and ∼30%, respectively, relative to that insummer. To quantify the extent of the Kuroshio intrusion, an isopycnal mixingmodel was adopted to derive the proportional contribution of water massesfrom the SCS proper and the Kuroshio along individual isopycnal surfaces. Thederived mixing ratio along the isopycnal plane was then employed to predictthe genuine gradients of nutrients under the assumption of no biogeochemicalalteration. These predicted nutrient concentrations, denoted asN_m, are solely determined by water mass mixing. Results showedthat the nutrient inventory in the upper 100 m of the NSCS was overallnegatively correlated to the Kuroshio water fraction, suggesting that theKuroshio intrusion significantly influenced the nutrient distribution in theSCS and its seasonal variation. The difference between the observed nutrientconcentrations and their corresponding N_m allowed us to furtherquantify the nutrient removal/addition associated with the biogeochemicalprocesses on top of the water mass mixing. We revealed that the nutrients inthe upper 100 m of the water column had a net consumption in both winter andspring but a net addition in fall.

[学科分类] 地球化学与岩石