Characteristics of the zooplankton community and impactfactors in the Yangtze estuary coastal eara after third stage impoundment of the Three Gorges Dam

The Three Gorges Dam( TGD) is the largest hydropower project ever built in China and worldwide. In 2010,the TGD reached the designed water level of 175 m,storing approximately 39.3 billion cubic meters of water. Impoundment of the TGD has changed the hydrological and sedimental regimes downstream, which inevitably affected the estuarine ecosystem. Therefore,a comprehensive survey and assessment after the impoundment at 175 m would be helpful for establishing benchmark values for future analyses. Community variations in zooplankton,which are at the middle level of the food chain in aquatic ecosystems, can reflect environmental changes. Therefore, this study investigated the spatial distribution of the zooplankton community and the relationships between zooplankton and environmental factors through biota-environment matching( BIOENV) based on a hydro-environmental and biological survey of the Yangtze estuary in August and November of 2010 and May of 2011. Noctiluca scientillans and Calanus sinicus were the predominant species inspring,and Acartia pacifica steuer and Paracalanus aculeatus were the predominant species in summer and autumn,respectively. The spatially averaged seasonal biomass of zooplankton was highest in summer( 970.6 mg / m~3),followed by autumn( 613.8 mg / m~3) and spring( 571.5 mg/m~3). Similarly,the spatially averaged seasonal abundance of zooplankton was highest in summer( 783. 5 individuals / m~3),followed by spring( 691. 3 individuals/m~3) and autumn( 399. 5 individuals / m~3). Interestingly,the spatial distribution patterns of zooplankton biomass and abundance were consistent in the summer,but not the spring and autumn. The major environmental factors explaining the observed variations in the zooplankton community were temperature in the middle layer,salinity,and silicate( Si) concentrations of the bottom layer in the spring; salinity,Si,and total dissolved nitrogen of the middle layer in the summer; and temperature of bottom layer,suspended solids in the surface layer,and Si of the middle layer in autumn. The primary environmental factor affecting the zooplankton community was salinity of the bottom layer throughout the year; temperature was the second most important factor. In summary,the zooplankton community in the Yangtze estuary appeared to be changing due to variations in the hydrological regime and water quality. Long-term observations of zooplankton are required to determine species adaptability and time-lag responses to envir