Abstract
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 Nm, 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 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 100 m of the water column had a net consumption in both winter and spring but a net addition in fall.
Highlights
The major ocean basins at low latitudes are often nutrient depleted in their upper mixed layer because strong stratification in the pycnocline diminishes nutrient supplies from the depths through diapycnal mixing (Lewis et al, 1986)
At density levels of > 1026.2 kg m−3, no significant seasonal variations of θ-S distributions were observed in the central northern South China Sea (NSCS)
We focused on the nutrient dynamics in the upper 100 m of the central NSCS since biological alterations of nutrients are highest in the euphotic zone, which typically extends from the surface to ∼ 100 m water depth in the South China Sea (SCS) (Chen et al, 2008; Tseng et al, 2005) where any changes in nutrients would profoundly result in changes in primary productivity of the ecosystem
Summary
The major ocean basins at low latitudes are often nutrient depleted in their upper mixed layer because strong stratification in the pycnocline diminishes nutrient supplies from the depths through diapycnal mixing (Lewis et al, 1986). While coastal oceans are typically characterized by higher nutrient concentrations due to abundant riverine inputs at the surface (e.g., Cai et al, 2004; Chen and Chen, 2006; Han et al, 2012) and elevated supplies from the depths through processes such as strong upwelling and/or enhanced diapycnal mixing Some marginal seas are strongly influenced by the boundary currents of the adjacent open ocean (Chen, 2008; Gordon, 1967; Huertas et al, 2012; Matsuno et al, 2009; Qu et al, 2000) which are usually very oligotrophic.
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