Hypolimnetic anoxia and sediment oxygen demand during stratification in a drinking water reservoir

Abstract

Purpose

Although hypolimnetic anoxia is a common occurrence in deep lakes and reservoirs, understanding the processes of oxygen depletion in water columns and sediment is challenging. This study investigates oxygen depletion in both the hypolimnion and sediments in a stratified reservoir. The areal hypolimnetic oxygen demand (AHOD) and sediment oxygen demand (SOD) were evaluated to determine key factors of hypolimnetic anoxia.

Materials and methods

To evaluate the characteristics of the AHOD in the Jinpen Reservoir (JPR), China, 4 years of field observations of dissolved oxygen (DO) and temperature datasets in the water column are analyzed. SOD was calculated as the sum of the sediment oxygen uptake (${\text{J}}_{{\text{O}}_{2}}$) and the flux of reduced substances from deeper sediment layers into the overlying water (F_red), where ${\text{J}}_{{\text{O}}_{2}}$ was obtained from DO profiles across the sediment–water interface (SWI) at a depth of approximately 94 m in the reservoir, and F_red was obtained from sediment cores.

Results

The AHOD in the JPR varied from 0.299 to 1.451 g m⁻² day⁻¹ during the summer stratification between 2014 and 2017. The SOD, containing ${\text{J}}_{{\text{O}}_{2}}$ and F_red, decreased from 0.534 to 0.386 g m⁻² day⁻¹ during stratification. Within the SOD, ${\text{J}}_{{\text{O}}_{2}}$ decreased from 0.415 to 0.168 g m⁻² day⁻¹, whereas F_red increased from 0.119 to 0.218 g m⁻² day⁻¹.

Conclusion

The increase in primary productivity (e.g., diatom blooms), inflows during rainfall, and oxygen concentrations can significantly increase AHOD. The relative importance of ${\text{J}}_{{\text{O}}_{2}}$ at 40–70% of the SOD is high in the JPR; thus, oxic respiration of organic carbon is the dominant pathway of DO consumption.