Zooplankton Size Structure in Relation to Environmental Factors in the Xiangxi Bay of Three Gorges Reservoir, China

Body size is sensitive to environmental changes and recognized as one of the fundamental traits linking ecological functions. Recently, size structure has been suggested as a useful indicator for environmental monitoring and assessment in aquatic ecosystems. However, the organisms’ size structure and the relationship with environmental factors remain seldom addressed in reservoir ecosystems. In this study, firstly, we investigated the zooplankton community composition and size structure, including the slope of the normalized biomass size spectrum (NBSS) and size diversity in the Xiangxi Bay of Three Gorges Reservoir, China. Then we analyzed how the environmental factors affect the zooplankton abundance and size structure (NBSS slope and size diversity) by using the structural equation model (SEM). In terms of the community composition, rotifers were the predominant zooplankton group in the Xiangxi bay during the whole research period, which abundance is significantly higher than protozoan, cladoceran, and copepod (Mann-Whitney U tests, P < 0.001). For the size structure, both the slope of NBSS and size diversity showed high spatiotemporal dynamics. The slope of NBSS ranged from − 2.201 to -0.097, and the size diversity ranged from 0.631 to 3.291. And lowest values of NBSS slope and size diversity were observed in the upstream areas of Xiangxi Bay. Further analyses based on SEMs found a clear pathway revealing how nutrient variables affect the zooplankton abundance and size structure. That is, dissolved inorganic nitrogen had an indirect effect on the zooplankton abundance, NBSS slope, and size diversity by affecting the phytoplankton biomass. In addition, SEM found that water temperature had a significant negative effect on the size diversity but had nonsignificant effects on zooplankton abundance and NBSS slope. This finding suggests that size diversity is a useful index in measuring the zooplankton size structure. Our study highlights that size diversity is a robust indicator for environmental monitoring and assessment, especially in complex and dynamic reservoir ecosystems.

Application of 2D Hydrodynamic Model for the Assessment of the Influencing Factor in Xiangxi Bay of the Three Gorges Reservoir, China

A two-dimensional (2D), laterally averaged, hydrodynamic and water quality model, CE-QUAL-W2, was used to simulate the hydrodynamics and water temperatures of Xiangxi Bay (XXB) of the Three Gorges Reservoir (TGR). Algal blooms occurred seasonally in XXB which is influenced by density currents. The CE-QUAL-W2 model was calibrated using data collected in XXB from January to December 2010. The model was then used to examine the environmental factors that affect the density currents. The model predicted density currents such as overflows, interflows and underflows of the XXB. The results suggest that more than 25%, 58.33% and 16.67% of the density currents that occurred in the XXB were overflow, interflow and underflow, respectively. Field observations were conducted including travel distance and thickness of intrusion layer for 24 days in 2010. The average travel distance of overflow, interflow and underflow in XXB was 15.50 km, 8.78 km, and 9 km based on observations and 16 km, 9.40 km, 9.60 km based on the model results. Therefore, numerical simulations of density currents can be predicted to better understand the hydrodynamics and aquatic ecosystems in XXB of TGR.

Application of a 2D hydrodynamic model for the assessment of the influencing factor in Xiangxi Bay of the Three Gorges Reservoir

A two-dimensional (2D), laterally averaged, hydrodynamic and water quality model, CE-QUAL-W2, was used to simulate the hydrodynamics and water temperatures of Xiangxi Bay (XXB) of the Three Gorges Reservoir (TGR). Algal blooms occurred seasonally in XXB which is influenced by density currents. The CE-QUAL-W2 model was calibrated using data collected in XXB from January to December 2010. The model was then used to examine the environmental factors that affect the density currents. The model predicted density currents such as overflows, interflows and underflows of the XXB. The results suggest that more than 25%, 58.33% and 16.67% of the density currents that occurred in the XXB were overflow, interflow and underflow, respectively. Field observations were conducted including travel distance and thickness of intrusion layer for 24 days in 2010. The average travel distance of overflow, interflow and underflow in XXB was 15.50 km, 8.78 km, and 9 km based on observations and 16 km, 9.40 km, 9.60 km based on the model results. Therefore, numerical simulations of density currents can be predicted to better understand the hydrodynamics and aquatic ecosystems in XXB of TGR.

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

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.