Nutrient Dynamics In The Upwelling Area Of The Changjiang Estuary

In recent years, frequent outbreaks of red tides in East China Sea became a growing concern, of which 74.7% took places in the area of 122°15′–123°10′E/30°30′–32°00′N in the Changjiang (Yangtze River) estuary, where several researches have been made on coastal upwelling in eutrophication in the region. However, available knowledge on the nutrient dynamic and its influence on chlorophyll-a distribution in the estuary are not systematic and comprehensive so far.Dynamics of nutrient movement and its influence on chlorophyll-a distribution in upwelling area of Changjiang estuary were investigated in February, May, August and November 2004.In spring, the upwelling was in low temperature, high salinity, and low dissolved oxygen in the region of 122°20′–123°00′E/31°00′–32°00′N. It is found for the first time that, besides bringing up high concentration of phosphate, the upwelling also brought up relatively low concentrations of nitrate and silicate to the upper layers above 10m. Nutrients transported by upwelling modified the nutrient ratios that close to the Redfield Ratios, and enhanced the nutrient structure in the upper layers. Also, the spring upwelling improved the sunlight transparency in the euphotic zone in favor of phytoplankton growth, and the chlorophyll-a distribution changed correspondingly. Affected by strong influence of Changjiang diluted water (CDW), the summer upwelling was weaker than the spring one, and could hardly affect the upper layers.In summer, the upwelling covered mainly a narrower latitudinal belt of 122°20′–123°00′E/31°15′–31°50′N and affected the nutrient distribution in the layers below 10m. The upper layers were darkened by CDW bearing thick suspended particulates, which became one of the limiting factors to the growth of phytoplankton. Chlorophyll-a content in the summer upwelling area was much lower than that in spring, indicating that the dynamics of nutrients and natural environment in spring are more beneficial to the regional phytoplankton growth.During autumn and winter, the nutrient distribution was affected by the shelf-intrusion of the Kuroshio surface water. Nutrient concentration and nutrient ratio between the bottom and the surface layers were close for the waters transportation which eliminated the seawater stratification. In the region of 122°30′–123°05′E, there existed an area with low chlorophyll-a contents and the correlation between nutrient ratios and chlorophyll-a distribution was discussed.The calculated estimation of nutrient flux indicates that the phosphate flux in spring upwelling was two times higher than that of the Changjiang river water. In contrast, the fluxes of nitrate and silicate were much lower than the inputs from Changjiang river water.