Unique physical processes of canyon reservoirs regulate the timing and size of algal blooms - based on a study in Three Gorges Reservoir

The management and remediation of algal blooms on a global scale are primarily focused on controlling nutrient inputs, with an emphasis on limiting phosphorus inputs from point and non-point sources. It is well known, however, that physical factors such as water temperature, water column stability and advective flow also play critical roles in the timing and size of algal blooms. For nutrient control programs to be effective, it is essential to understand the relative importance of hydrological processes associated with nutrient dynamics and bioavailability. This study examines the relative importance of physical processes in hyper-eutrophic canyon reservoirs in regulating the development of algal blooms. We hypothesized that water column stability determines when algal blooms develop as well as the potential size of the bloom. To test the hypothesis, the study was implemented in Gaoyang Lake located in the middle of the Pengxi River, a 1st- order tributary in the Three Gorges Reservoir. The CE-QUAL-W2 model was applied to verify the lacustrine nature of Gaoyang Lake. We sampled the lake for 33 consecutive days from April 15th to May 17th in 2019. Algal composition and abundance, chlorophyll-a concentration, and different forms of P from the surface, middle and bottom of the water column, respectively, and available phosphorus in sediment were sampled and measured daily. Water temperature, dissolved oxygen, chlorophyll-a and water velocity were measured in situ daily. We demonstrate that: 1. During the initial stratification period, a surface density layer (SDL) was formed as a result of a lack of turbulent wind mixing. The formation of the SDL initiated the development of algal blooms as a result of surface optimum temperatures being achieved early in the year. Although the SDL maintained the algal community within the euphotic zone, it also limited nutrient influx to the surface waters resulting in a decline of bioavailable phosphorus. 2. Surface cooling events, however, weakened the SDL resulting in enhanced nutrient flux into the upper water column. The nutrient supply resulted in the formation of subsequent and more severe algal blooms when the SDL was re-established. Our results demonstrate that physical processes such as the formation and stability of the SDL can be used to predict the occurrence and size of algal blooms in canyon reservoirs. To control algal blooms, we suggest conducting further research to regulate the development and stability of SDL through interventions such as artificial mixing or flow modification during critical time periods. These approaches may provide rapid remediation options for managing harmful algal blooms.