The effects of Asian summer monsoons on algal blooms in reservoirs

An important characteristic of lakes and reservoirs in the East Asian summer monsoon region is the dramatic seasonal difference in hydrologic inputs, with annual rainfall commonly concentrated in a few heavy rain events. In this study, we surveyed the monthly variations of phytoplankton density in 3 large deep reservoirs and 7 small shallow reservoirs and analyzed the effect of large precipitation events on phytoplankton. During heavy rains, stream phosphorus concentrations increased sharply, and phosphorus loadings into reservoirs were not continuous but episodic shock loadings. In deep stratified reservoirs, however, the concentrations of phosphorus and chlorophyll a were much lower than expected from the high total phosphorus levels in the storm runoff. Inflowing storm waters laden with phosphorus flowed into metalimnetic layers because deep reservoirs had strong thermal stratification and the storm water was cooler than the epilimnion. The result was the formation of an ecosystem resilient to phosphorus shock loadings during monsoon. Nutrients in the metalimnion seemed to be dispersed gradually toward the epilimnion, and phytoplankton reached maximum densities, called “monsoon blooms,” after the monsoon. By contrast, shallow reservoirs with short hydraulic residence times had lower chlorophyll a concentrations during the monsoon season because the high flushing rate was the major limiting factor of phytoplankton growth. In conclusion, summer monsoon is the major determinant of phytoplankton density in reservoirs of the East Asian region, but their responses can vary widely depending on hydrologic characteristics.