Characteristics And Control Factors Of Water Chemistry And CO₂ Flux At Water-gas Interface In Lijiang River Basin

The river is one of the most significant components of global carbon cycle and water cycle research.On one hand,river water chemistry and its distribution could reflect the water-rock interaction in the river basin,the environmental quality,regional environmental chemical characteristics,the migration and transformation of water elements to some extent.On the other hand,the degassing phenomenon of river water is common.CO₂ escaping from the surface water actively participates in carbon cycle,making the river become an important channel connecting the terrestrial and marine ecosystems as two carbon pools,meanwhile it is a key link of biogeochemical cycle.CO₂ flux at the river water-air interface is the key to accurately evaluate the contribution of surface water to global CO₂ emission.At present,there are relatively few studies on CO₂ exchange flux and hydrochemistry at karst,non-karst areas and underground river outlets.In this paper,Lijiang river basin is selected as the study area,nine monitoring sites are determined from the upstream to the downstream,and two day and night monitoring are carried out at the outlet of underground river in Maocun.The characteristics and influencing factors of CO₂ exchange flux at water-air interface at different time scales are revealed.Meanwhile,the temporal and spatial variation characteristics of water chemistry and its controlling factors are analyzed.The study results show that:The hydrochemical type of Lijiang river basin is HCO₃^Ca,and the concentration sequence of anion and cation is HCO₃^->SO₄^2->NO₃^->Cl^-,Ca²⁺>Mg²⁺>Na⁺>K⁺.Ca²⁺?Mg²⁺and HCO₃^-are generally high in the middle reaches and low in the upper and lower reaches due to the influence of carbonate rock distribution,and Cl^-?Na⁺?K⁺are affected by atmospheric rainfall.While the concentration in the middle and lower reaches is higher than that in the upper reaches;Rainfall is larger in rainy season and ion concentration is less than that in dry season due to dilution.Ion in river water are mainly affected by weathering and dissolution of rocks,Ca²⁺?Mg²⁺and HCO₃^-are mainly derived from weathering dissolution of carbonate rocks,NO₃^-and SO₄^2-are not only from weathering and dissolution of rocks,but also from rainfall and human activities in a small amount.Meanwhile Cl^-?Na⁺?K⁺are mainly derived from weathering of rocks,and cyclic salts also contribute a little to them.The seasonal and diurnal monitoring of CO₂ flux at the outlet of underground river in Maocun shows obvious variation characteristics,and the main performance is the rainy season is greater than the dry season,less than night during the day.This is because the temperature difference between day and night is large,and the temperature can affect the growth of phytoplankton and the activity of microorganisms in the water,thereby affecting the emission of CO₂ gas by controlling the photosynthetic respiration of aquatic organisms,During the day,the light intensity is larger,and the photosynthesis of phytoplankton in water body is stronger,which is favorable for CO₂ in the air to enter the water,while at night,the respiration of phytoplankton is relatively enhanced,which is favorable for CO₂ in water to enter the atmosphere.On the seasonal scale,pH?Ca²⁺?HCO₃^-?DOC?rainfall and flux have some influence on CO₂ flux except temperature.On the seasonal scale of other monitoring sites in this basin,the flux value of CO₂ exchange mainly shows that the rainy season is larger than the dry season.Pearson correlation analysis and principal component analysis were performed on 21 possible factors affecting CO₂ flux,it is found that there are many and complex factors affecting CO₂ flux,which can be divided into three categories:one is the influence of carbonic acid equilibrium system on CO₂ flux,the second is the influence of meteorological environment factors on CO₂ flux,and the third is the influence of organic carbon and inorganic carbon on CO₂ flux.By comparing the variance contribution rates of these three factors,it is found that the carbonic acid equilibrium system is the most important factor affecting CO₂ exchange flux.