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

Abstract: Based on four cruises covering a seasonal cycle in 2009-2011, we examined the impact of the Kuroshio intrusion, featured by extremely oligotrophic waters, on the nutrient inventory in the central northern South China Sea (NSCS). The nutrient inventory in the upper 100m of the water column in the study area ranged from similar to 200 to similar to 290 mmol m(-2) for N+N (nitrate plus nitrite), from similar to 13 to similar to 24 mmol m(-2) for soluble reactive phosphate and from similar to 210 to similar to 430 mmol m(-2) for silicic acid. The nutrient inventory showed a clear seasonal pattern with the highest value appearing in summer, while the N+N inventory in spring and winter had a reduction of similar to 13 and similar to 30 %, respectively, relative to that in summer. To quantify the extent of the Kuroshio intrusion, an isopycnal mixing model was adopted to derive the proportional contribution of water masses from the SCS proper and the Kuroshio along individual isopycnal surfaces. The derived mixing ratio along the isopycnal plane was then employed to predict the genuine gradients of nutrients under the assumption of no biogeochemical alteration. These predicted nutrient concentrations, denoted as N-m, are solely determined by water mass mixing. Results showed that the nutrient inventory in the upper 100m of the NSCS was overall negatively correlated to the Kuroshio water fraction, suggesting that the Kuroshio intrusion significantly influenced the nutrient distribution in the SCS and its seasonal variation. The difference between the observed nutrient concentrations and their corresponding Nm allowed us to further quantify the nutrient removal/addition associated with the biogeochemical processes on top of the water mass mixing. We revealed that the nutrients in the upper 100m of the water column had a net consumption in both winter and spring but a net addition in fall.