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 100 m of the water column in the study area ranged from ∼200 to ∼290 mmol m^−2 for N + 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 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 ∼13% and ∼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 100 m 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 N_m 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 100 m of the water column had a net consumption in both winter and spring but a net addition in fall

Dissolved organic carbon in the South China Sea and its exchange with the Western Pacific Ocean

“…In winter, increased wind speed stimulates photosynthesis through strong mixing that entrains the nutrient-rich subsurface water to the upper layer; while in summer, the upper water column is highly stratified, which results in a reduction in biological productivity. Recently, Du et al (2013) quantify that the degree of the Kuroshio intrusion into the SCS also significantly impacts the upper SCS nutrient inventory.…”

“…Details of the application of the isopycnal mixing model are introduced by Du et al (2013). For any in situ observed water parcel shown in the T-S diagram in Fig.…”

“…The hydrology of the surface water ( o200 m) is introduced in Du et al (2013). Briefly, as shown in the Τ-S diagram (Fig.…”

“…Following Du et al (2013) salinity was chosen for water fraction estimation in the upper 50 m where θ was affected by solar radiation heating. However, at deeper layers, θ was used for the estimation, because of its higher sensitivity than salinity.…”

Controls on temporal and spatial variations of phytoplankton pigment distribution in the Northern South China Sea

“…The sources of the nutrients may come from the discharge of the Pearl River and other local rivers in Guandong province, the submarine ground discharge or the diffusion of nutrients generated from early diagenesis process. In addition, some other physical forcings, such as wind stress and direction, Kuroshio intrusion, and eddy formation and origins (Huang et al, 2010;Du et al, 2013), are also important parameters to influence the nutrient transport and mixing strength. Overall, nutrient supply appeared to be closely associated with the distribution of the coastal type pigments during summer seasons.…”