| Dissolved inorganic carbon (DIC) in karst water was deposited in the form of tufa, accompanying by the CO2 degas in the karst dynamic system which driven by hydrologic cycle. Therefore, karst carbon sink was questioned as carbon transfer process, the formation of carbon sink was not stable. However, the changes in water DIC was unitly influenced by the process of inorganic and organic that includes the stream degas, precipitation of calcium carbonate and aquatic photosynthetic organism physiological. Especially the process of photosynthesis in aquatic photosynthetic organism to form organic carbon by utilizing DIC, realize the transformation of inorganic carbon to organic carbon, and ultimately the formation of organic carbon burial in the stream, which can form stable carbon sink. Therefore, inconsideration of the biological effects, formed by karstification DIC can eventually by fixed, provides a scientific method to solved the problem of the stability of carbon sink. Hence, this paper chose the surface stream accepted typical karst underwater in south subtropical karst area in China as research area. Around the scientific problem of DIC mainly came from the solution of calcium carbonate change rule, transformation process and discussing carbon sink stability after underwater flow to surface under the influence of aquatic plant metabolism. In order to reveal the changes rate of stream degas, deposition rate and carbon sequestration capacity, and the change of DIC in the form of organic carbon settling down to for real carbon sink under the influence of air-water exchange, dissolve and precipitation of calcium carbonate and metabolism of aquatic plant.This paper mainly through the sample site G1 (outlet, distributed not obvious aquatic plant) and G2(downstream monitoring station, distributed a large amount aquatic plant of Hydrilla verticillata, Hornwort and so on) to do the routine monitoring sampling and high density monitoring of a combination of research on Guancun underground river surface stream, in order to analysis variation rule and influence factors of DIC around the biochemical process. The results of study show that:(1)The stream water chemistry type is Ca-HCO3-type, and pH value, SpC, SIc,pCO2 influenced by flow increased in dry season and decreased in rainy season from September 2013 to August 2014. CO2 degas due to the CO2 different value of air and water caused pCO2 decrease and SIc, pH decrease along the path. CO2degas flux showed a weak positive relation of flow (R2=0.24), indicated that stream flow was not only controlled factor of degas flux. When CO2degas flux increased, dissociation CO2 in water decreased, then H2CO3 transformed by dissociation CO2 decreased, ultimately caused the DIC at sample site G2 increase, further indicated CO2 degas caused DIC decrease along the path.The carbonate calcium deposition rate showed an obvious positive relationship with water temperature. Due to precipitation and flow, the deposition rate decreased with water temperation increasing. Its variation rule was the same with Ca2+and DIC, and showed a weak negative correlation with DO, indicated that aquatic plant through influencing the dissolved of calcium carbonate to influence the precipitation rate by respiration. Moreover, G2 annual average carbon isotope value of sediment is-11.01‰, indicated that the climate was warm and humid, the stream of good ecological environment.Metabolism flux of stream decreased in dry season and increased in rainy season, due to the low water temperature cause the weak organism activity and the organism number was smaller than the rainy season, so the metabolic capacity was weak during dry season. When during the rainy season, metabolism flux increased due to the high water temperature, density illumination and photosynthesis, but also influenced by rainfall, showed that the low metabolism flux during April to June. Moreover, the metabolism flux of stream had a positive correlation with stream flow (R2=0.62), indicated that aquatic plant also influenced by fow. And the same season variation trend between DIC and aquatic plant metabolism, further indicated that DIC decreased due to used by aquatic plant photosynthesis procee.(2)During the period of 8～14th, July,2013,18～20th, August,2013 and 22～23th, July,2014 diel monitoring, hydrochemistry at sample site G1 showed no obvious diel variation, but G2 showed significant diurnal variation. The influence factors of causing the diel variation at G2 were including physical factors (solar radiation, water temperature, rainfall) and biological process (photosynthesis and respiration). DIC and δ13CDIC showed the opposite diel variation, indicated that the effect factors of DIC concentration on diel variation were photosynthesis and respiration by aquatic plants and degas in the stream. By calculating the Henry constant, the influence of water temperature on the rate of pCO2was 27.48～54.88%. aquatic plants on water impact the pCO2was 45.12～72.52%. To show the effect of physiological function of aquatic plants in the main control effect on the diurnal variation of stream water chemistry characteristics. Gross primary productivity (GPP) refers to the unit time of organisms (mainly green plants) per unit area through photosynthesis channels fixed amount of organic carbon. Through the calculation of the stream of GPP, the result was 16.06 kg O2 d-1 on July 12,2013, and12.17 kg O2 d-1 on July 13,2013. During the day without rain weather, the enhancement of solar radiation and temperature in the stream would lead to a decline in GPP. During the rains, the stream water chemistry showed the dilution effect and leaching effect was obvious, and the rainfall resulted in metabolic rate slows down, thus affecting the GPP of the stream.(3)During the day without rain weather, degas process at sample site G1 was the physical process, due to CO2 concentration different between air and water. At the sample site G2 showed the diel variation of degas rate. The variation rule of degas rate at G2 showed that:night> morning> noon> afternoon. At night, degas rate was rising, and hydrochemistry mainly influenced by aquatic plants respiration, then pCO2, DIC and Ca2+ were rising and pH decreasing, indicated that aquatic plants respiration was the influence factor to degas at night. In the morning, photosynthesis enhanced and respiration homework the night fell. In the afternoon, because of the aquatic plants respiration rate and photosynthesis decreasing, weakened in stream GPP. Therefore, degas rate influenced by biological process. And the degas rate showed a positive correlation with respiration, indicated that organism respiration was main influence factor of degas rate diel variation. The CO2 degas flux at sample site G1 showed no diel variation rule, but at sample site G2 showed a diel variation rule. And the CO2 degas flux showed the sample change rule with degas rate. That was the CO2 degas flux change the same diel trend with pCO2, indicated that the CO2 degas flux also influenced by organism respiration. By calculating of organic carbon content fixed in stream, the result was about 10.34 kg d-1 during 22～23th, July,2014.The organic flux of stream was 2658.28 mg m-2 d-1.The carbonate calcium deposition was 38% of DIC loss, CO2 degas was 21%, and aquatic plant metabolism priocess was 41%. Thus, DIC could be used by aquatic plant to form stable carbon sink due to the CO2 degas was a small part of DIC loss. |
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