Modeling the Effects of Hydrodynamics on Thermal Stratification and Algal Blooms in the Xiangxi Bay of Three Gorges Reservoir

Abstract

Algal blooms have been reported in some tributary bays since the initial impoundment of Three Gorges Reservoir, which has seriously affected the water ecology and drinking water safety. Hydrodynamics plays a crucial role in algae growth. The recent numerical models of hydrodynamics and water quality are effective to identify the effects of hydrodynamics on phytoplankton and find potential strategies for controlling algal blooms. In this study, the CE-QUAL-W2 model was applied to simulate the hydrodynamics and algal blooms in the Xiangxi Bay (XXB) of the Three Gorges Reservoir. The model performed well in simulating flow patterns, water temperature profile, and algal blooms. The results indicated that the hydrodynamics showed the spatial and temporal differences in the XXB. In the mouth area, the intensity and plunge depth of density currents were dynamic and characterized by a typical seasonal pattern. The transformation of density currents from interflow to overflow will provide more opportunities for vertical mixing, resulting in un-stratification and reducing of algal blooms. However, in the middle and upper areas, strong stratification and low velocity at upstream provide enough favorable conditions for the growth of algae and increase algal blooms. The simulation results revealed that the variation of mixing depth explains the spatial and temporal differences of Chl.a. It played a vital role in seasonal stratification and the dynamics of phytoplankton succession in XXB.

Effects of dimensionality on the performance of hydrodynamic models

“…A large variety of different numerical models have been used for simulating temperature and hydrodynamics in lakes and reservoirs, as well as the biogeochemical and ecological processes that depend on it (e.g. Dissanayake et al, 2019;Guseva et al, 2020;Wang et al, 2020;Xu et al, 2021). While the mechanistic description of underlying physical processes is identical or at least similar in all models, they differ in their dimensionality, i.e.…”

Vertical Boundary Mixing Events during Stratification Govern Heat and Nutrient Dynamics in Windy Tropical Lakes with High Water-Level Fluctuations: A Long-Term (2001-2018) Study

“…Furthermore, the changes in metabolism found in VB [13] associated to the variation of vertical mixing, have been found to alter both the metabolic balance and the role of the lake as a source or sink of carbon to the atmosphere [14], and all this would depend largely on the variation of WLF. As observed before, changes in water level and therefore mixing events can also drive changes in the planktonic composition, both for phytoplankton [17,50] and for zooplankton [51]. In particular, increased mixing can favor diatoms and decrease cyanobacterial blooms [17].…”

“…A cascade of ecosystemic processes are potentially affected by boundary mixing and vertical nutrient transport, as has been addressed broadly in the literature [45][46][47][48][49][50]. Understanding the relationships between temperature and vertical transport is key to model emergent ecosystem functions in a warmer world [18].…”

Vertical Boundary Mixing Events during Stratification Govern Heat and Nutrient Dynamics in Windy Tropical Lakes with High Water-Level Fluctuations: A Long-Term (2001-2018) Study

“…A cascade of ecosystemic processes can be affected by boundary mixing and vertical nutrient transport, as has been addressed broadly in the literature [53][54][55][56][57][58]. At the same time, the temperature and nutrient signals of these processes are rapidly blurred in the surface layer, both by surface heat transport and by the multiple processes that make nutrients non-conservative, including competitive assimilation and transformation processes (e.g., [19][20][21][22]).…”

“…Furthermore, important changes in phytoplankton groups were also observed in the same study, in which low water levels were characterized by increasing biomass of planktonic diatoms and a significant decrease in harmful cyanobacteria (filamentous Nostocales and coccoids, e.g., Microcystis). The effects of these changes in phytoplankton [17,58] and zooplankton [60] in the rest of the ecosystem are still to be studied. In particular, changes in zooplankton composition could favor fisheries availability [13].…”