¹Key Laboratory of Tropical Marine Environmental Dynamics, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
²School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China

*Corresponding author: dxwang@scsio.ac.cn

Abstract

   The Pearl River Estuary empties into the South China Sea in southeastern China, and hypoxia is one of the major environmental problems here during summer. It can be caused by coastline, topography, hydrodynamics, tidal dynamics and biological processes (mainly by stratification and oxidation reaction). Since ecological processes are closely related to dynamic processes, we revise and link together both a physical and a biological model to establish a coupled hydrodynamic-ecological numerical model to study the summertime hypoxia. The model simulates physical and biological processes and reproduces the observed hypoxia in the bottom layer in two regions located in west shallow water and around Nei Lingding Island. A low-salinity plume forms after the freshwater drifts offshore. Strong stratification exists in the water body shoreward where the vertical mixing is weak. The salinity front and stratification cause a poor exchange in the vertical direction and greatly influence biological processes for nutrients, phytoplankton, and detritus, which induce the hypoxia occurrence in the bottom layer of the Pearl River Estuary. On the other hand, in two troughs the tide diminishes the strength of stratification and prohibits the large-scale growth of the hypoxia. The physical processes related to topography, run off and tidal mixing regulate the distribution of biological factors and the occurrence of hypoxia.