Impacts Of Aquatic Organisms On Hydrochemical Characteristics And Karst Carbon Sink In Lijiang Basin

Global warming and climate change are becoming the most popular topics among scientists.In this century, the researches focusing on the process and mechanism of the carbon cycle,especially the research of reducing the concentration of atmospheric CO₂（carbon sink and carbon sequestration technology）, are the core issues of global change. Karst carbon sink was neglected due to the consideration that it was a geological process with long-time scale. Recently, studies have proved that the carbonate rock weathering is a rapid and sensitive process, and the Fifth Climate Change Assessment Report of IPCC has adjusted its time scale from ten thousand years to one thousand years. Carbonate rock dissolution is a reversible process, so karst geological effect is supposed to be a carbon shift process instead of carbon sink process by Curl. The carbonate rock weathering model, taking into account the interaction of water-rock-air-organism, has become the key to respond to Curl’s query. The aquatic plant utilizing HCO3- for photosynthesis is functioning as a biological carbon pump, the dissolved inorganic carbon is synthesized, and thus the geologic carbon sink（karst carbon sink especially） is fixed as organic carbon. This process has been verified in the laboratory and small watershed, but the study in large-scale karst river basin is still lacking. Therefore, Lijiang Basin, which is a typical large karst basin, was selected to verify the mentioned process in this study. By analyzing the hydrochemical and geochemical characteristics,the major impacts and seasonal effect of aquatic plant on hydrochemistry, inorganic and organic carbon content were characterized. Thus the contribution of aquatic plants to the karst carbon sink was evaluated for Lijiang Basin.Nine sample points were set from upstream to downstream in Lijiang basin, including HuaRiver, Ling Channel, Darong River, Guilin Hydrometrical Station, Chaotian River, Guan Rock,Xingping, Yulong River and Yangshuo Hydrometrical Station. Samples were collected every three months from April 2015 to January 2016, in order to study the seasonal variation of the aquatic photosynthesis. In 2015, diurnal variation during rainstorms from September 8th to 11 th was monitored in Yangshuo Hydrological Section. Flood-scale variation of inorganic and organic carbon and photosynthesis of aquatic organisms were analyzed.The results showed that Lijiang hydrochemical type was HCO3-Ca-type. During the non-flood period, the hydrochemical parameters were affected by precipitation and aquatic photosynthesis, and it showed an obvious seasonal feature. The concentrations of ions such as Ca2+ and HCO3- presented a high peak in autumn and a low peak in summer. The Rock weathering were the main source of Ca2+, Mg2+ and HCO3-, especially the carbonate rock weathering. The variation of K+ and Na+ concentrations were mainly affected by atmospheric precipitation, silicate weathering and human activities. Cl-, SO42- and NO3- concentrations were mainly influenced by atmospheric precipitation and human activities. The hydrochemical characteristics of Lijiang were primarily controlled by the dissolution of carbonate rock. Therefore, TDS, Ca2+, Mg2+ and HCO3-concentrations increased with the percentage of the carbonate rock distribution. For photosynthesis, in spring and autumn it influenced the hydrochemical characteristics significantly.Spatially, aquatic organism photosynthesis was more notable in karst tributary and mainstream from Guilin to Yangshuo, with high HCO3- concentration. This fact leaded to higher content of organic carbon, which was mostly dissolved organic carbon. On the other hand, during the flood period, primary hydrochemical parameters were affected by the fluctuation of discharge. The Ca2+,HCO3-,and DOC concentrations had a consistent trend. Their concentrations experienced a sharp decline due to the dilution effect of the precipitation, and then followed by a slowly rising process.Besides, these parameters had a clear hysteresis, the lowest value was shown 10 hours later after the highest discharge peak. However, the variation of POC and TSM had a similar pattern as the discharge, and they were higher than that under the non-flood conditions, which was correlated to soil erosion by strong surface runoff during high discharge period.The photosynthetic process uptaking DIC by aquatic organisms was ubiquitous and significant in Lijiang basin, which could result in higher δ13CDIC by isotopic fractionation. The influence of aquatic organisms on DIC showed strong seasonal variations. The high δ13CDIC values（3‰ isotopic fractionation） during spring and autumn would be consistent with the aquatic organisms flourished. While in winter there was hardly any effect on isotopic fractionation of DIC,because the low water temperature was not suitable for the growth of aquatic organisms. In summer, strong hydrodynamic conditions also caused strong air-water exchange, leaded to lowerutilization rates of HCO3-. In addition, the fraction of DIC directly coming from soil CO₂ can be neglected in summer. The variation of the δ13CDIC value during the flood is similar to that in summer. It was higher before the flood because of the aquatic organisms photosynthetic, and then the δ13CDIC value became lower during the flood process. This was caused by the reduction of HCO3- percentage in inorganic carbon, and the influence of the aquatic organisms was wakened.Furthermore, soil CO₂ with lighter δ13C was pulled into the river by the heavy rain, and made theδ13CDIC value lower. The variation of δ13CDIC value in the flood was relatively remarkable（more than 3 ‰）.There were significant differences between the organic carbon sources during the flood period and the non-flood period. In the non-flood period, DOC was the main form of organic carbon, and it came from the primary productivity of aquatic organisms. The contribution of the organic carbon formed by photosynthetic accounted for 60% of the allochthonous organic carbon.In the flood period, DOC had a similar concentration as POC（POC was slightly higher）, and the proportion of autochthonous organic matter got higher.HThe data collected in 2015 by Yangshuo Hydrologic Station was analyzed. The results showed that, the inorganic carbon came from the carbonate rock weathering was dominated（97.5%）. And the carbonate rock weathering could be divided in two types, one was the weathering of carbonate rock by H2CO3（79.2%）, the other was the weathering of carbonate rock by H2SO4（18.3%）. The carbon sink of the carbonate rock weathering, silicate rock and aquatic organism photosynthetic was 69.2, 4.8 and 14.5 tCO₂·a-1·km-2, respectively. It was significantly higher than in previous years, due to the sufficient amount of precipitation. Nevertheless, the relative proportion of part of the carbon sink was similar as that in previous years. Additionally,the contribution of the aquatic organism photosynthesis to the stability of karst carbon sink was respectable, which reached 20% of karst carbon sink.