Li Jianhong, Pu Junbing, Zhang Tao
Key Laboratory of Karst Dynamics, MNR & Guangxi, Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 541004, PR China; School of Geography, Nanjing Normal University, Nanjing 210023, PR China.
Chongqing Key Laboratory of Wetland Science Research of the Upper Reaches of the Yangtze River, Chongqing Key Laboratory of Surface Process and Environment Remote Sensing in the Three Gorges Reservoir Area & School of Geography and Tourism, Chongqing Normal University, Chongqing 401331, PR China.
Water Res. 2022 Feb 1;209:117905. doi: 10.1016/j.watres.2021.117905. Epub 2021 Nov 29.
Most reservoirs in subtropical areas experience periodic variations in the thermal structure of their water columns, with times of strong thermal stratification being succeeded by periods of mixing, over the course of the year. Understanding of the transport and transformation of dissolved inorganic carbon over such thermal cycles in artificial reservoirs remains poor. To address this problem, this study examined the spatiotemporal behavior of dissolved inorganic carbon (DIC), the partial pressure of CO (pCO), carbon isotope ratios (δC), and CO emission (FCO), from 2014 to 2018 in a subtropical, groundwater-fed reservoir in southern China. It was found that CO emissions during mixing periods are much higher than in thermally stratified periods (particularly during transition from stratified to mixing) as a result of upwelling and release of dissolved CO (CO) accumulated in the hypolimnion. CO emission fluxes at the water-gas interface accounted for only a small proportion of the DIC in the reservoir. The relationships between of DIC and δC displayed two distinct modes, due to spatial differences in water depths and to strong thermal stratification during warmer seasons: (1) DIC concentrations increase and δC values decrease from epilimnion to hypolimnion, and (2) δC values decrease with increasing DIC concentrations but δC is progressively enriched near the bottom during periods of thermal stratification. In addition, this study found three distinct processes of DIC accumulation and consumption in the reservoir: (1) DIC accumulated in the hypolimnion during thermal stratification periods, due to carbon retention but (2) DIC was substantially consumed in the epilimnion during such periods, and (3) average DIC concentrations and pCO increased significantly from upstream to downstream along the reservoir, while average δC values became lighter. These results highlight that carbon behavior in groundwater‑fed reservoirs is often controlled by a combination of biogeochemical processes and seasonal variations in thermal structure. Sampling and monitoring strategies should consider these factors in order to accurately estimate carbon budgets in reservoirs, lakes or ponds.
在亚热带地区，大多数水库水柱的热结构会经历周期性变化，在一年的时间里，强烈的热分层时期之后是混合时期。对于人工水库中溶解无机碳在这种热循环过程中的迁移和转化的理解仍然不足。为了解决这个问题，本研究调查了2014年至2018年中国南方一个亚热带、以地下水为水源的水库中溶解无机碳（DIC）、二氧化碳分压（pCO₂）、碳同位素比值（δ¹³C）和二氧化碳排放（FCO₂）的时空行为。研究发现，由于上升流以及在湖下层积累的溶解二氧化碳（CO₂）的释放，混合时期的二氧化碳排放量远高于热分层时期（特别是从分层向混合过渡期间）。水气界面的二氧化碳排放通量仅占水库中溶解无机碳的一小部分。由于水深的空间差异以及温暖季节强烈的热分层，溶解无机碳与δ¹³C之间的关系呈现出两种不同模式：（1）从表层水到湖下层，溶解无机碳浓度增加而δ¹³C值降低；（2）δ¹³C值随溶解无机碳浓度增加而降低，但在热分层时期，靠近底部时δ¹³C逐渐富集。此外，本研究发现水库中溶解无机碳存在三种不同的积累和消耗过程：（1）在热分层时期，溶解无机碳在湖下层积累，这是由于碳的滞留；（2）在此期间，溶解无机碳在表层水大量消耗；（3）沿水库从上游到下游，溶解无机碳平均浓度和pCO₂显著增加，而平均δ¹³C值变轻。这些结果突出表明，以地下水为水源的水库中的碳行为通常受生物地球化学过程和热结构季节性变化的共同控制。采样和监测策略应考虑这些因素，以便准确估算水库（湖泊或池塘）中的碳收支。
