Distribution features of nutrients and nutrient structure in the East China Sea and the Yellow Sea in spring and autumn

The distribution features of nutrients and nutrient structure in the East China Sea and the Yellow Sea were discussed based on the data collected in October to November 2000 and in April 2001. The results showed that the high concentrations of nutrients in the surface water appeared mainly in the Changjiang diluted water area and the coastal water area off Jiangsu and Zhejiang, and the low values were presented in the continental shelf zone of the East China Sea and the northwest part of the Yellow Sea. The contents of nutrients in the middle area of the Yellow Sea varied significantly from spring to autumn due to strong and stabilizing thermocline in autumn and no obvious stratification in spring. The ratios of N/P, Si/N and Si/P in the surface water were much higher than Redfield ratios in the East China Sea and the Yellow Sea except for the northwest part of the Yellow Sea. The surface nutrient ratios in spring were higher in the East China Sea than those in the Yellow Sea, whereas the nutrient ratios in autumn were higher in the Yellow Sea than those in the East China Sea and the variation of nutrient ratios were also greater than those in spring. The surface nutrient ratios increased notably during phytoplankton bloom and the stratified period. Moreover, the surface nutrient ratios decreased evidently when the stratification lessened or eliminated and phytoplankton productivity weakened. Our investigation suggested that there was little possibility of N limitation for the photosynthesis of phytoplankton in the East China Sea and the Yellow Sea, but there mainly was P-limitation in most part of the studied area. The content of SiO_3-Si was generally high and it was not possible to be a limiting-factor for phytoplankton growth in the studied area. The excessive DIN and extra high N/P ratio in the Changjiang diluted water area and the coastal water area off Jiangsu and Zhejiang may be responsible for the enhancement of coastal eutrophication and the high occurrence frequency of red tide in that area over the last two decades.