| 摘要: |
| 河流碳是全球碳循环中的重要组成部分，也是流域内发生的各种自然过程和人类活动的记录载体。梯级水库建设严重改变河流的自然状态和物质输送，但其对高寒地区河流碳迁移转化过程的影响尚未获得充分研究。本研究选择澜沧江自然河道及其下游梯级水库河水断面，调查分析了河水基本理化参数、各形式河流碳及其稳定碳同位素的昼夜变化特征，并通过不同断面间的对比研究，揭示了河流碳在自然河道-梯级水库连续体内的迁移转化过程及梯级水库对其带来的影响。结果表明，库区颗粒有机碳含量较自然河道大幅降低，反映了梯级水库对颗粒态物质的巨大拦截作用（98%悬浮物被拦截）。溶解无机碳与溶解有机碳在自然河道与库区断面的浓度和通量均未发生明显变化，说明梯级水库对河流溶解态物质影响较小。水体二氧化碳分压(pCO2)则显示出由自然河道（平均689 μatm）向库区（平均值为587μatm）降低趋势，且在冬季下降幅度更大。两个河水断面的各水体物化参数、碳浓度及其碳同位素组成的均无显著昼夜差异，且溶解氧与水体pCO2未出现协同变化，说明河流内部水生光合作用较弱。结合稳定碳同位素数据指示的河流碳来源组成变化及河流碳通量变化，可以推断河流有机碳被大量埋藏在水库底部从而削弱了水体内部有机碳氧化分解过程，并导致库区表层水体pCO2值下降。水体营养元素含量低、水库建成时间短、流量调节频繁等因素是库区水生生物光合作用弱、对河水化学和碳循环影响小的重要原因。目前状态下，澜沧江水库建设对于流域碳循环为净碳汇效应，但随着库龄增加及流域环境演化，梯级水库的影响机制可能会发生改变，需对此开展深入研究和动态评估。 |
| 关键词: 河流碳 稳定碳同位素 梯级水库 昼夜变化 澜沧江 |
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| 基金项目:国家重点研发计划项目(2020YFA0607700), 国家自然科学基金项目(41877402)和中国科学院青年创新促进会联合资助 |
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| Diurnal variation of riverine carbon transport and transformation processes in the natural channel-cascade reservoirs of the Lancang River |
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Wu Enpeng, Sun Huiguo, Liu Wenjing, Xu Zhifang
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Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences
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| Abstract: |
| Riverine carbon is an important component of the global carbon cycle, and it is also a record carrier of various natural processes and human activities in the basin. The cascade reservoirs seriously alter the natural state and material cycle of rivers, but its impact on the carbon migration and transformation process of rivers in alpine regions has not been fully studied. In this study, the natural river section of Lancang River and its downstream cascade reservoirs were selected to investigate and analyze the basic physical and chemical parameters of river water, the diurnal variation characteristics of carbon and its stable carbon isotope in various forms of rivers. Through the comparative study between different sections, the migration and transformation process of river carbon in the natural river-cascade reservoirs continuum and the impact of cascade reservoirs on it were revealed. The results indicate that the particulate organic carbon content in the reservoir area is significantly lower compared to the natural river channel, reflecting the substantial trapping effect of the cascade reservoirs on particulate matter (98% of suspended solids are intercepted). The concentrations and fluxes of dissolved inorganic carbon and dissolved organic carbon showed no significant changes between the natural river channel and the reservoir sections, suggesting that the cascade reservoirs have minimal impact on dissolved substances in the river. However, the partial pressure of carbon dioxide (pCO2) in the water exhibited a decreasing trend from the natural river channel (average 689 μatm) to the reservoir area (average 587 μatm), with a more pronounced decline during the winter season. No significant diurnal variations were observed in the physicochemical parameters, carbon concentrations, or carbon isotope compositions of the water bodies at both river sections. Furthermore, there was no synergistic change between dissolved oxygen and pCO2, indicating weak aquatic photosynthesis within the river. Based on the changes in the carbon source composition and carbon flux indicated by stable carbon isotope data, it can be inferred that a large portion of riverine organic carbon is buried in the reservoir"s bottom, thereby weakening the internal oxidation and decomposition processes of organic carbon and leading to a decrease in pCO2 in the surface waters of the reservoir area. The low nutrient content, short reservoir age, and frequent flow regulation are key factors contributing to the weak aquatic photosynthesis in the reservoir and its limited impact on river chemistry and the carbon cycle. Under the current conditions, the construction of reservoirs in the Lancang River basin acts as a net carbon sink in the carbon cycle of the watershed. However, with the aging of the reservoirs and the evolution of the basin"s environment, the impact mechanisms of the cascade reservoirs may change, necessitating further research and dynamic assessments. |
| Key words: riverine carbon stable carbon isotope cascade reservoir diurnal variation Lancang River |
