Nutrient Dynamics in the Upwelling Area of Changjiang (Yangtze River) Estuary

Abstract

Nutrient dynamics and its influence on the distribution of chlorophyll-a in the upwelling area of the Changjiang (Yangtze) River estuary were investigated in the spring (May) and summer (August) of 2004. In the spring, upwelling was apparent in the region of 122°20′–123°00′ E, 31°00′–32°00′ N and was associated with low temperature (16–21°C), high salinity (24–33 practical salinity units [psu]), and low dissolved oxygen (2.5–6.0 mg L−1) in the upper 10 m of the water column. The spring upwelling increased the mixed-layer phosphate, nitrate, and silicate concentrations to roughly 1, 15, and 15 μmol L−1, respectively, and improved the light transparency in the euphotic zone. This improvement in phytoplankton growing conditions was followed by an increase in chlorophyll-a concentrations. The summer upwelling was weaker and occurred over a smaller geographical area (122°20′–123°00′ E, 31°15′–31°50′ N). Strongly influenced by turbid Changjiang diluted water (CDW), it had little impact on the upper 10 m of the water column but instead increased nutrient concentrations at greater depths. The high concentration of particulates in the CDW reduced light transmission in the upper 10 m and, hence, limited phytoplankton growth throughout the water column. Chlorophyll-a concentrations in the summer upwelling area were roughly an order of magnitude lower than in the spring. Water clarity, as influenced by the CDW, appears to be the principal factor limiting the impact of upwelling on phytoplankton biomass in this area.