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

Hui Xu,Daobin Ji,Jun Ma,Meng Yan,Defu Liu,Zhengjian Yang,Lianghong Long

doi:10.3389/fevo.2020.610622

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.

Highlights

The CE-QUAL-W2 model was selected for this study because it can well-reproduce the dominant density currents and water temperature profiles in the tributaries of the Three Gorges Reservoir (TGR) (Ma et al, 2015; Long et al, 2016b, 2019; Ji et al, 2017)
The results show the spatiotemporal difference of hydrodynamics and stratification in the tributary bay of the Three Gorges Reservoir (Figures 5, 6)
The two-dimensional hydrodynamic and algal model of Xiangxi Bay (XXB) based on CE-QUAL-W2 was able to adequately simulate the shift in density current, seasonal stratification, and algal blooms

Summary

Introduction

The act of damming and impounding a river imposes fundamental physical changes upon the river continuum, which in turn fragments habitats (Grill et al, 2014; Winemiller et al, 2016; Anderson et al, 2018), disrupts the hydrological cycle of floodplain (Hu et al, 2008; Zeilhofer and De Moura, 2009; Guo et al, 2012), releases large amounts of methane (Lima et al, 2008; Fearnside and Pueyo, 2012; DelSontro et al, 2016), and causes eutrophication and even harmfulHydrodynamics-Stratification-Algal Model algal blooms (Wang et al, 2013; Rafiee and Jahangirirad, 2015). A large number of studies show that algal blooms in the tributaries of TGR are mostly affected by hydrodynamics (Liu et al, 2012; Xiong et al, 2013; Yang et al, 2018a), thermal stratification (Cui et al, 2012; Yang et al, 2012; Zhou et al, 2016), and nutrient loads (Zheng et al, 2009a; Yang et al, 2018b; Nwankwegu et al, 2020). It is a consensus that the significant change in hydrodynamic condition is a key factor to control algal blooms by affecting the thermal stratification, nutrient transport, and the relationship between the mixing depth and euphotic depth (Ji et al, 2017; Yang et al, 2018a; Chuo et al, 2019)

Methods

Results

Discussion

Conclusion