NUTRIENT DISTRIBUTIONS IN THE EAST CHINA SEA AND CHANGES OVER THE LAST 25 YEARS

A survey of seasonal variations in the distribution of nutrients along a standard section of the East China Sea was carried out between 1991 and 2015. Here, interannual variations of five key nutrients are analyzed and discussed in the context of possible controlling factors, with special attention given to seasonal variations recorded over the year 2015. Results indicate that concentrations of dissolved inorganic nitrogen (DIN), phosphate (DIP) and silicate (DSi) exhibit gradual increases over this period. Measured concentrations of DIN, DIP and DSi were 5.34-11.87 mu mol/L, 0.31-0.53 mu mol/L and 11.12-23.15 mu mol/L, respectively. Since 1996, concentrations of DIN, DIP and DSi have increased significantly. The annual average concentration of DIN, DIP and DSi in the East China Sea were -2 times greater in 2015 than in 1991, respectively. The N/P ratio exhibits an increasing trend followed by steady-state asymmetric M-type fluctuations. Conversely, the Si/N ratio is generally stable and exhibits an asymmetric W-type fluctuation. The distribution of nutrients in the East China Sea exhibits clear seasonal variations, with phosphate and inorganic nitrogen contents generally higher in autumn and winter than in spring and summer Silicate content is higher in summer and winter than in autumn and spring. The main source of nutrients into the East China Sea is the Yangtze River plume, with the Taiwan Warm Current providing an additional, secondary source. The combined effects of the Yangtze River plume and the Taiwan Warm Current enforce a decreasing trend in nutrient concentrations from west to east.