| 其他摘要 | Based on numerous karst researches and new insight into earth’s systematic view of karst processes, studies involving water-rock-soil-gas-organism interactions are of special interest when searching effects of the karst processes on the global carbon cycle. Especially, in recent years, combined processes of carbonate dissolution and aquatic photosynthesis have been considered as a new direction in the global carbon sink and drawn great international attentions. Whether carbonate weathering could provide durable and stable carbon sink or not depends mainly on the efficiency of the utilization of dissolved inorganic carbon (DIC) by aquatic phototrophs. To understand above-mentioned aquatic carbon sequestration process, invesitigation of diurnal variations in hydrochemical characteristics and stable carbon isotope compositions of DIC (δ13CDIC) in surface water is shown to be an efficient way. In this work, four parameters, including water temperature, pH value, electrical conductivity (EC) and dissolved oxygen (DO), were synchronously and uninterruptedly monitored with high resolution (an interval of 15 min.) at spring, midstream pond and downstream pond in four seasons at Laqiao, Maolan, SW China, a typical karst area. In addition, water samples δ13CDIC were analyzed in laboratory. Moreover, a static floating chamber was placed on air-water interface to obtain CO2 exchange flux and to evaluate the biological carbon pump effect combining with hydrochemical data. Based on the mass balance, considering the input and output of DIC in spring fed pond, carbon sequestration capacity by “biological carbon pump” in different seasons could be obtained. Field campaigns have been made in 2013, including four times, spring (April 25-26, 2013), summer (July 25-26, 2013), autumn (October 24-25, 2013) and winter (January 27-28, 2013). According to in-situ monitoring, laboratory analysis and model calculation, we came to following conclusions: (1) In the spring and downstream pond where aquatic plants are less developed, diurnal hydrochemical variations are unconspicuous, while in midstream pond where aquatic plants (dominated by Chara Fragilis) flourished, DO, pH, SIC (saturation index of calcite) and δ13CDIC values presented the largest amplitude of fluctuation (increased during the daytime while decreased in nighttime), which is synchronous with the process of aquatic metabolism (photosynthesis dominated in daytime while respiration dominated in nighttime). However, EC, [HCO3-], [Ca2+] and pCO2 (CO2 partial pressure) showed the inverse variations, i.e., decreased during the daytime while increased in nighttime. (2) Seasonal hydrochemical variations were remarkable. Compared with spring and winter, the amplitudes of hydrochemical variations in summer and autumn were larger. This is mainly because that summer and autumn are the growing seasons while spring and winter are the dormant times. In storm periods, substances which came from groundwater and soil had great impact on the hydrochemical features in the spring-ponds water system, especially in the epikarst spring. (3) Diurnal variations of δ13CDIC were conspicuous in midstream pond where aquatic plants flourished. Photosynthesis resulted in a decrease in DIC concentrations and a simultaneous increase in δ13CDIC by preferential consumption of 12C. Conversely, respiration increased DIC concentration by producing CO2 and decreased δ13CDIC value by adding isotopically light biogenic CO2 with the δ13C signature which is similar to aquatic vegetation. (4) ?13CDIC values of epikarst spring and the spring-fed two ponds were more negative in spring and summer but more positive in autumn and winter. At spring vent, storm effect on hydrochemical features and ?13CDIC values was significant. For downstream pond, however, both tank effect and metabolism of aquatic plants affected hydrochemical characteristics and ?13CDIC, i.e., aquatic metabolism was dominant in growing season while tank effect was predo |
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