A numerical study of the seasonal variations of nutrients in the Changjiang River estuary and its adjacent sea area

A lower trophic level nutrient-phytoplankton-zooplankton model is coupled to the Regional Ocean Modeling System (ROMS) in the Changjiang River estuary and its adjacent sea area with the aim of simulating the seasonal variations of three main nutrients (nitrate, phosphate, and silicate). The coupled model successfully reproduces the long-term variations of nutrients during 2004-2007, and the simulated nutrient distributions are also well consistent with that obtained by cruises. Based on the climatological simulation, the seasonal variations of nutrients in the study area are analyzed. In winter, the spatial distributions of nitrate and silicate show that they are mainly from terrestrial input. As for phosphate, the concentration is relatively high, but it is difficult to distinguish the main source merely on its spatial distribution. In summer, as the Changjiang River diluted water (CDW) flows eastward or northeastward, a great amount of nutrients are transported to the sea area on the east of 123°E via a thin layer in the surface. Despite all this, the available phosphate is still insufficient to meet the consumption by phytoplankton, which shows a strong phosphorus limitation in the surface. By contrast, the phosphate concentrations in lower layers are much higher, and they are mainly from two physical sources: one is the intrusion of the Taiwan Warm Current (TWC) from the south, and the other is a shoreward transport from the east. The spatial variations of N/P and N/Si ratios show that phosphate is the main limiting nutrient in the Changjiang River estuary and its adjacent sea area, although slight nitrogen limitation appears in the sea areas on the east of 122.5°E in winter. In addition, the seasonal variations of surface phytoplankton and zooplankton are briefly discussed.