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Anthropogenic Perturbations Complicated the Downstream Greenhouse Gas Dynamics of a Large Subtropical Reservoir.

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Title: Anthropogenic Perturbations Complicated the Downstream Greenhouse Gas Dynamics of a Large Subtropical Reservoir.
Authors: Wan, Xiang1,2 (AUTHOR), Chen, Shuai3 (AUTHOR) shuaic@connect.hku.hk, Wang, Wanfa4,5 (AUTHOR), Dai, Mutan6 (AUTHOR), Shi, Wenhong4,5 (AUTHOR), Ran, Lishan3 (AUTHOR), Wu, Xiaoxu7 (AUTHOR), Tan, Wenfeng1 (AUTHOR)
Source: Journal of Geophysical Research. Biogeosciences. Dec2024, Vol. 129 Issue 12, p1-18. 18p.
Subject Terms: *Carbon cycle, *Bodies of water, *Gas reservoirs, *Anthropogenic effects on nature, *Greenhouse gases, Gas dynamics
Abstract: River damming can significantly alter the hydrology and nutrient levels of river water, resulting in substantial greenhouse gas (GHG) emissions to the atmosphere. However, the dynamics of greenhouse gases in the discharged water downstream of dams remain poorly understood, despite being recognized as a crucial source of GHG emissions in river‐reservoir systems. In this study, we conducted comprehensive measurements of GHG concentrations and water chemistry in a large subtropical reservoir and its upstream and downstream rivers to investigate the spatiotemporal patterns of GHG concentrations and fluxes and to identify their governing mechanisms, with a primary focus on downstream GHG dynamics. Our analysis revealed that the distribution of pCO2 among the reservoir and its upstream and downstream rivers was predominantly controlled by aquatic metabolism and atmospheric CO2 exchange. Conversely, the distribution of CH4 and N2O levels was largely influenced by anaerobic metabolism. Seasonal fluctuations in GHG dynamics were linked to hydroclimatic conditions, including water temperature, hydrologic connectivity between land and rivers, and reservoir thermal stratification. Anthropogenic activities (e.g., agricultural land use) were found to affect the downstream trend of GHG concentrations. Higher GHG fluxes in the downstream rivers compared to reservoir were attributed to the anaerobic production of CH4 in the reservoir and increased gas transfer velocity in the downstream rivers. These findings underscore the critical influence of anthropogenic activities on downstream GHG dynamics and emphasize the necessity of integrating anthropogenic impacts and seasonal variability in downstream GHG emissions to enhance our understanding of the carbon budget in river‐reservoir systems. Plain Language Summary: The discharged water from dams is a key source of greenhouse gases (GHGs) in river‐reservoir systems; however, we still lack a comprehensive understanding of the gases' behavior downstream. In our study, we measured GHG levels and water quality in a large subtropical reservoir and its surrounding rivers to investigate how these gases move over time and space. We focused on understanding how GHGs behave downstream. Our analysis showed that the distribution of CO2 in these water bodies is mostly influenced by how microbes and the atmosphere interact with the water, while the levels of CH4 and N2O are largely controlled by production in low‐oxygen conditions. Seasonal changes in GHG levels were tied to shifts in weather and water conditions. Human activities such as agriculture were seen to impact GHG concentrations downstream. We found that higher GHG levels in downstream rivers were due to the production of CH4 in the reservoir and the rate of gas emit to the air. Our study highlights how human actions affect GHG levels in downstream rivers and stresses the need to consider these impacts and seasonal variations when studying GHG emissions in river‐reservoir systems. Key Points: Aquatic metabolism and hydroclimatic conditions shape the spatiotemporal patterns of GHG dynamicsAnthropogenic activities affect the downstream trend of GHG concentrationsDownstream river water acts as a significant emission hotspot for CO2 and CH4 [ABSTRACT FROM AUTHOR]
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Database: GreenFILE
Description
Abstract:River damming can significantly alter the hydrology and nutrient levels of river water, resulting in substantial greenhouse gas (GHG) emissions to the atmosphere. However, the dynamics of greenhouse gases in the discharged water downstream of dams remain poorly understood, despite being recognized as a crucial source of GHG emissions in river‐reservoir systems. In this study, we conducted comprehensive measurements of GHG concentrations and water chemistry in a large subtropical reservoir and its upstream and downstream rivers to investigate the spatiotemporal patterns of GHG concentrations and fluxes and to identify their governing mechanisms, with a primary focus on downstream GHG dynamics. Our analysis revealed that the distribution of pCO2 among the reservoir and its upstream and downstream rivers was predominantly controlled by aquatic metabolism and atmospheric CO2 exchange. Conversely, the distribution of CH4 and N2O levels was largely influenced by anaerobic metabolism. Seasonal fluctuations in GHG dynamics were linked to hydroclimatic conditions, including water temperature, hydrologic connectivity between land and rivers, and reservoir thermal stratification. Anthropogenic activities (e.g., agricultural land use) were found to affect the downstream trend of GHG concentrations. Higher GHG fluxes in the downstream rivers compared to reservoir were attributed to the anaerobic production of CH4 in the reservoir and increased gas transfer velocity in the downstream rivers. These findings underscore the critical influence of anthropogenic activities on downstream GHG dynamics and emphasize the necessity of integrating anthropogenic impacts and seasonal variability in downstream GHG emissions to enhance our understanding of the carbon budget in river‐reservoir systems. Plain Language Summary: The discharged water from dams is a key source of greenhouse gases (GHGs) in river‐reservoir systems; however, we still lack a comprehensive understanding of the gases' behavior downstream. In our study, we measured GHG levels and water quality in a large subtropical reservoir and its surrounding rivers to investigate how these gases move over time and space. We focused on understanding how GHGs behave downstream. Our analysis showed that the distribution of CO2 in these water bodies is mostly influenced by how microbes and the atmosphere interact with the water, while the levels of CH4 and N2O are largely controlled by production in low‐oxygen conditions. Seasonal changes in GHG levels were tied to shifts in weather and water conditions. Human activities such as agriculture were seen to impact GHG concentrations downstream. We found that higher GHG levels in downstream rivers were due to the production of CH4 in the reservoir and the rate of gas emit to the air. Our study highlights how human actions affect GHG levels in downstream rivers and stresses the need to consider these impacts and seasonal variations when studying GHG emissions in river‐reservoir systems. Key Points: Aquatic metabolism and hydroclimatic conditions shape the spatiotemporal patterns of GHG dynamicsAnthropogenic activities affect the downstream trend of GHG concentrationsDownstream river water acts as a significant emission hotspot for CO2 and CH4 [ABSTRACT FROM AUTHOR]
ISSN:21698953
DOI:10.1029/2024JG008408