Oral Presentation Freshwater Sciences 2023

Unique thermal structures of canyon reservoirs regulate the timing and size of algal blooms - based on a study in Three Gorges Reservoir (#390)

Lei Zhang 1 2 3 , Xing Zhang 2 3 , Douglas G. Haffner 1 , William D. Taylor 4 , Lianghong Long 5 , Daobin ji 5
  1. Great Lakes Institute for Environmental Research, University of Windsor, Windsor, Ontario, Canada
  2. College of Resources and Environment, Southwest University , 1 Tiansheng Rd, Beibei, Chongqing
  3. National Base of International S&T Collaboration on Water Environmental Monitoring and Simulation in TGR Region , China
  4. University of Waterloo , Waterloo, Ontario, Canada
  5. Three Gorges University, Yichang, Hubei, China

This paper examines the factors regulating the timing, duration and size of harmful algal blooms in the Three Gorges Reservoir (China). We hypothesized that although the algal blooms are initiated with the onset of water column stratification that leads to the development of surface density layers (SDL) (surface metalimnion), the size and duration of the bloom is limited by nutrient diffusion into the SDL; When SDLs are broken or weakened by atmospheric events, nutrients can diffuse from the lower water column into the euphotic zone to support further development of algal blooms. We tested this hypothesis in Gaoyang Lake, in the Pengxi River of the Three Gorges Reservoir. The CE-QUAL-W2 model was applied to verify the lacustrine nature of Gaoyang Lake, where we sampled for 33 consecutive days from April 15th to May 17th in 2019. Surface 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 daily. Water temperature, dissolved oxygen, chlorophyll-a and water velocity were measured in situ by probes at the time of sample collection. The results revealed two interactive and interdependent processes regulating the timing, duration and size of algal blooms in Gaoyang Lake:

  1. During the initial stratification period, the formation of SDLs regulated the size of algal blooms as a result of minimal nutrient influx. Meanwhile, phosphorus from the sediment enriches the dissolved total phosphorus in the lower water column.
  2. Surface mixing events, however, lead to weakened SDL resulting in enhanced nutrient flux into the upper water column leading to the formation of subsequent algal blooms as the SDL is re-established.

Our results demonstrate how stratification processes can result in nutrient limitation even in hyper-eutrophic waters.

  1. Zhang, L., Xia, Z., Zhou, C., Fu, L., Yu, J., Taylor, W. D., et al., 2020. Unique Surface Density Layers Promote Formation of Harmful Algal Blooms in the Pengxi River, Three Gorges Reservoir. Freshwater Science, 39(4), 722-734. https://doi.org/10.1086/710332
  2. Tan, B., Hu, P., Niu, X., Zhang, X., Liu, J., Frenken, T., Hamilton, P.B., Haffner, G.D., Chaganti, S.R., Nwankwegu, A.S., Zhang, L., 2022. Microbial Community Day-to-Day Dynamics during a Spring Algal Bloom Event in a Tributary of Three Gorges Reservoir. Science of The Total Environment, p.156183.