Effects of physical and biochemical processes on the dissolved oxygen budget for the Pearl River Estuary during summer

Hypoxia in the Pearl River Estuary (PRE) during summer is one of the problems caused by increasing anthropogenic pollutant inputs due to population increase and economic development in recent years. The mechanism for hypoxia in the PRE has to be understood firstly before any management policy can be established. A three-dimensional water quality model was therefore developed to study the dissolved oxygen (DO) budget of the PRE in summer and to identify the roles that various physical and biochemical processes played in DO dynamics.Results show that above the pycnocline, horizontal transport of DO is mainly balanced by reaeration, and photosynthesis appears to play a more important role in the shelf area than inside the PRE due to reduced turbidity. Below the pycnocline, horizontal transport of DO is balanced by biochemical processes. Regarding the DO depletion processes in the PRE above the pycnocline, DOC oxidation is the largest consumer of DO, while nitrification and phytoplankton respiration rank second and third, respectively. Below the pycnocline, SOD dominates DO depletion processes, following by DOC oxidation, nitrification and phytoplankton respiration. The dominant role of SOD in DO depletion processes of the PRE is caused by the shallow topography and high deposition rate of POC. Vertical DO transport in the PRE shows remarkably spatial variability due to complicated hydrology and topography in the PRE. The vertical DO fluxes are dominated by advective fluxes as a result of intense tidal forcing and gravitational circulation in deep channels, while diffusive fluxes dominate in the vertical fluxes within the shoal area as a result of increasing DO vertical gradient due to hypoxia.