Coastal upwelling brings nutrient-rich deep water to the euphotic zone, stimulating the growth of phytoplankton. Although coastal upwelling occupies only 1% of the ocean surface area, it contributes to over 10% of the ocean's primary productivity. The Qiongdong upwelling is one of the important upwelling system in the South China Sea. The spatiotemporal distribution of nutrients and its transport are not well studied in the Qiongdong upwelling. Here, based on a high-resolution physical-biogeochemical coupled developed in the northern South China Sea in 2010. The model can well simulate spatiotemporal variations of the temperature, salinity, current, nutrients, and chlorophyll.
The model results show that the upwelling starts at the later May and ends in early October, reaching its peak intensity in June. The upwelling brings bottom cold and high-salinity water by up to ~50 m. In the upwelling core (18.75°N-19°N), the temperature decrease reaches to 4°C, and the salinity increase reaches 0.6 psu as compared to the non-upwelling region. The influence areas extends from coastal to 100km offshore. The simulated nitrate show high values in the upwelling core, mostly found about 20km offshore. Following the onset of upwelling, the surface nitrate concentration in the core area rapidly increases from 0.1 mmol m^-3 to 3.0 mmol m^-3 and then maintains at a relatively high level. However, a decline is found in surface water from August 21 to September 8, corresponding with the weakening of the southwest monsoon and the subsequent reduction in upwelling. We further calculated the nutrient transport in the Qiongdong Upwelling system. The results show that the nutrients in the Qiongdong upwelling area are primarily sourced from local upwelling instead of lateral transport and riverine inputs.
 

Marine biogeochemistry,nitrogen cycle,upwelling