Effect of thermal stratified flow on algal blooms in a tributary bay of the Three Gorges reservoir

After the impoundment of Three Gorges Reservoir (TGR), its tributaries have been frequented by algal blooms, especially the Xiangxi River (XXR). Stratified flow induced by temperature difference often occurs, which likely influences the water exchange and algae transport process between the tributary and TGR. To investigate the variation of flow field and algae concentration in XXR, a 2-dimensional hydrodynamic and water quality model is developed. The simulation results show good agreement with field observed data, reproducing the water exchange and algal bloom process. Notwithstanding the hydrodynamic environment of XXR is complicated, the thermal stratified flows are generalized into six typical patterns in this paper. Subsequently, algae transport processes under the influence of different thermal stratified flow patterns are examined. Thermal stratified flows from upstream and TGR intrude into the bay tributary through different layers with different combinations, resulting in water exchange of XXR is the major cause of algae transport difference. We show that when the mainstream intrusive flow and upstream inflow separately intrude into the XXR through the surface and bottom layer, hydrodynamic environments are favorable to algae transport, with the final chlorophyll-a (Chl-a) concentrations of entire XXR lower than algal bloom threshold value. The worst scenarios occur when the mainstream flow intrudes through middle or bottom layer while the upstream inflow enter through bottom layer, with the final algal bloom coverage rate (R-alg) of 82.3% and 78.1%, respectively. In terms of superimposed effect of different flow patterns, when the sequentially thermal stratified flows have different effect on algae transport direction, algae will concentrate in the reach rather than discharged which increase the risk of multiple algal blooms. These findings represent an important step towards understanding the multiple outbreaks of algal bloom in XXR during the spring and provide a new perspective for the research of algal bloom governance.

Automated summary

The study revealed that after the impoundment of Three Gorges Reservoir, algal blooms frequently occurred in its tributaries, especially the Xiangxi River. The complex hydrodynamic environment of the Xiangxi River was found to be influenced by thermal stratified flows, which affected the transport of algae. Different patterns of thermal stratified flows led to variations in the direction and concentration of algae transport.