1. Nitrogen Cycling in Reservoir Drawdown Areas and the Impacts on Water Quality.
- Author
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Liu, Dongsheng, Chen, Qiuwen, Maavara, Taylor, Zhang, Jianyun, and Chen, Yuchen
- Subjects
RESERVOIR drawdown ,NITROGEN cycle ,WATER quality management ,WATER quality ,WATER levels ,OIL field flooding ,RESERVOIRS ,ATMOSPHERIC nitrogen - Abstract
Reservoir drawdown areas (DAs) can be both important nitrogen (N) sources to river networks and hot spots for N removal from freshwater ecosystems. The net effect of DAs on the N availability in reservoirs within a full hydrological cycle remains unclear. In this paper, the N dynamics in the DA of the Three Gorges Reservoir, Yangtze River, China, are investigated through a combination of discrete and continuous in situ observations and sampling over a span of 2 years, complemented by numerical modeling. We show that the DA is a net source of N to the water column, and that about 30% of the total annual N load released from the DA is mitigated by the sediment through denitrification and capture. The annual net load of the total N from the DA to the reservoir is ca. 0.59 kg per meter along the river, which is on the same order of magnitude as the input load from the density current of the Yangtze River to its tributaries, generally considered to be the primary driver of eutrophication in tributaries. N release in the DA mainly occurs during the drying period, whereas denitrification in the sediment mostly takes place during the flooding period when the oxido‐reducing potential is low. Our findings quantify and therefore clarify the N source/sink dynamics from the DA to the reservoir, offering a new perspective on the importance of DA nutrient loading in decision‐making related to integrated management of inundated lands to alleviate reservoir eutrophication by river damming. Plain Language Summary: Nitrogen (N) is an essential nutrient element, but it can be harmful to water quality in excess amounts. Drawdown areas (DAs) are regions at the edges of reservoirs that are alternately flooded and dried as water levels rise and fall. In some conditions, DAs can cause N pollution by adding N to the reservoirs, while in other conditions, DAs can alleviate pollution by removing N from the reservoir by turning it into a gas through the process of denitrification. The annual balance for these pollution‐driving versus pollution‐alleviating processes across the entire year remains unclear. Based on 2 years of continuous field observations and modeling of N cycling in the DA of the Three Gorges Reservoir, China, this paper finds that annually sediment denitrification and retention in the DA eliminates 30% of the N load released from the sediment itself. N release in the DA mainly occurs when reservoir water level falls as the weather warms up. Denitrification, which turns nitrate into a gas that can leave the reservoir and enter the atmosphere, in sediment under high water levels is the main mechanism that removes N from the DAs. We find that the amount of N released to the reservoir from the DAs is similar in size to the N load received from the upstream river, and thus needs to be considered in reservoir water quality management decisions. Key Points: The drawdown area (DA) is a net source of nitrogen (N) to the Three Gorges Reservoir water column within a full hydrological cycleAbout 30% of the total N load to the water column from the reservoir DA is mitigated by the sediment through denitrification and captureN release from the reservoir DA mainly occurs during the drying period, whereas N removal from the water column mainly occurs during flooding [ABSTRACT FROM AUTHOR]
- Published
- 2024
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