Carbon dioxide partial pressure and its diffusion flux in karst surface aquatic ecosystems: a review

Carbon dioxide (CO 2 ) emissions from aquatic ecosystems are an important component of the karst carbon cycle process and also a key indicator for assessing the effect of karst carbon sinks. This paper reviewed the CO 2 partial pressure (pCO 2 ) and its diffusion flux (FCO 2 ) in karst surface aquatic ecosystems, mainly rivers, lakes, and reservoirs, and their influencing factors summarized the methods for monitoring CO 2 emissions in karst aquatic ecosystems and discussed their adaptation conditions in karst areas. The pCO 2 and FCO 2 decreased in the order of rivers > reservoirs > lakes, and the values in karst lakes were eventually significantly lower than those in global lakes. The pCO 2 and FCO 2 of karst aquatic ecosystems had patterns of variation with diurnal, seasonal, water depth and hydrological cycles, and spatial and temporal heterogeneity. The sources of CO 2 in karst waters are influenced by both internal and external sources, and the key spatial and temporal factors affecting the CO 2 emissions from karst rivers, lakes, and reservoirs were determined in terms of physicochemical indicators, biological factors, and biogenic elements; additionally, the process of human activity interference on CO 2 emissions was discussed. Finally, a conceptual model illustrating the impacts of urban development, agriculture, mining, and dam construction on the CO 2 emissions at the karst surface aquatic ecosystem is presented. Meanwhile, based on the disadvantages existing in current research, we proposed several important research fields related to CO 2 emissions from karst surface aquatic ecosystems.