Carbon Fluxes Across Water-air Interface And Its Controlling Factors In A Karst Groundwater-fed Subtropical Reservoir, SW China

CO₂ degassing in karst water is a universal phenomenon which lead to many scientists query stability of carbon sink in karst processes. Therefore, it is very important to deeply understanding the CO₂ degassing processes, fluxes and controlling factors in karst system for exactly calculating carbon sink. This study aimed to understand space and time differences of CO₂ exchange fluxes across watergas interface and its controlling mechanism in a karst groundwater-fed reservoir, which could benefit exact assessment of karst carbon sink.This study chose the typical karst groundwater-fed reservoir, Dalongdong reservoir which is located in Shanglin county, Guangxi Province, SW China, as the main research area. Field monitoring works were carried out respectively in July, November and August, 2014, and in March and June, 2015, which covered different seasons, events and reservoir operation mode. A comparative experiment was conducted in a semi-karst reservoir, Wulixia reservoir and a non-karst reservoir, Si’anjiang reservoir, in August 2014.Water temperature（T）, p H, electrical conductivity（EC）, dissolved oxygen（DO）, transparency, turbidity（Tb）, chlorophyll-a（chla）, HCO3-, Ca2+, wind speed, air temperature and light intensity were test in situ. Some parameters including anion, cation, TOC, DOC, δ13CDIC of water samples have been tested in the laboratory. Two common approaches, floating chamber（FC） and thin boundary layer models（TBL）, were employed to research and contrast greenhouse gas exchange across water-air interface from the three reservoirs. Results were listed as follows.Experimental results show that:1. Hydrochemical characteristics and stratification effect of Dalongdong reservoirHydrochemistry of Dalongdong reservoir is HCO3-Ca type, which controlled by carbonate balance system. In the five monitoring courses, obviously stratification effects in Dalongdong reservoir was found in July and August, 2014 and March and June, 2015, while mixing effects was found in Novermber, 2014. During the stratification periods, HCO3-、Ca2+、p H、DO、Chl-a、p CO₂ and other hydrochemical indicators also showed stratifying phenomenon.2. Variations of CO₂, CH₄ fluxes across water-air interface and its controlling factors（1） CO₂ fluxes across water-air interface decreased during the daytime and increased during the nighttime in Dalongdong reservoir in summer sunny days while diurnal variations of CO₂ fluxes is obscure in rainy day or fall. There is no significant diurnal variations of CH₄ fluxes because CH₄ evasion is main in the form of bubbles.（2） The ranking of magnitude of CO₂ degassing fluxes is November> July> June> March> August while the ranking of CH₄ degassing fluxes is November > August> July> June>. All indicates that CO₂ and CH₄ degassing fluxes in Dalongdong reservoir is much higher in the mixing period than in the stratification period.（3） In spatial scale, CO₂ degassing fluxes is higher in midstream than in upstream and downstream.Principal component analysis indicated that carbonate balance system, biological effects, organic carbon and atmospheric environmental factors are the factors affecting CO₂ and CH₄ effluxes across water-air interface. The impact of DIC on CO₂ and CH₄ effluxes is greater than DOC. Day-night vertical movement, photosynthesis and respiration of planktonic algae is the main cause of the diurnal variation of CO₂ effluxes. The large rainfall in June and August could lead to dilution effect on the surface water, which result in the ranking of magnitude of CO₂ fluxes is July, 2014 > June, 2015> August, 2014. CO₂ and CH₄ effluxes in midstream were the highest that may be caused by the high wind speed in island vicinity. CO₂ effluxes in downstream was higher than in upstream mainly because planktonic algae gradually decrease from upstream to downstream, which result in the photosynthesis intensity became smaller, and the water depth becomes deeper, which lead to hypoxia degree increased in the reservoir bottom.3. Thermal stratification cans temporary storage CO₂ in hypolimnion.During the thermal stratification period, hypolimnion has poor water exchange with epilimnion and thermocline, which could lead to CO₂ was accumulated continuously in hypolimnion. Partly CO₂ in the process of upward diffusion will be absorbed by the algae. During the mixing period, water with high p CO₂ in hypolimnionn will migrate to epilimnion, so CO₂ fluxes is much higher in the mixing period than in the stratification period of.4. Geological settings could play a fundamental role in CO₂ fluxes across water-air interface.In the absence of submerged soil organic matters and plants, CO₂ efflux in karst groundwater-fed reservoir was much higher than in non-karst area reservoir due to groundwater with rich DIC recharge from karst aquifer and thermal stratification. CO₂ and CH₄ effluxes of discharging water under dam were much higher than that in surface-layer water in a reservoir regardless of karst reservoir or non karst reservoir. Accordingly, more attention should be paid on the CO₂ and CH₄ emission from discharging water under dam.