[Biogeochemical Characteristics in Shengli Site of Lijiang River Under the High Resolution Monitoring].

This study was done to understand the diel variation and factors influencing the hydrochemistry of the Lijing River in different seasons. This is a typical medium river located at Guilin City in the Guangxi Zhuang Autonomous Region, SW China. The Shengli site was selected for this study. Two-day monitoring work with a high resolution rate logger and high frequency sampling at 2 hour intervals was conducted at the Shengli site of the Lijiang River in summer and autumn separately. Physical and hydrogeochemical parameters including pH, dissolved oxygen (DO), water temperature (), electrical conductivity (EC), dissolved inorganic carbon (DIC), isotopes, and other chemical parameters were examined. The results show that:① the physical and hydrochemical parameters[, pH, DO, SIC, EC, (CO)] and major ions (HCO, Ca) at the Shengli site displayed regular diel variation during monitoring. The data for , pH, DO, and SIC increased in daylight and decreased at night, while the data for Ca, HCO, EC, and (CO) decreased in daylight and increased at night. ② The diurnal changes of nutrient elements (SO, NO, Cl, Na, Mg, and K) at the Shengli site were mainly controlled by photosynthesis and respiration of aquatic plants, and showed the trend of decrease in daylight and increase at night. Due to the influence of a flood in mid-August 2017, the amount of diurnal variation in the nutrient element levels in summer was less than that in autumn. ③ The C increased in daylight and decreased at night both in summer and autumn, reflecting the influences of photosynthesis and precipitation. Under the influence of different root systems, soil microbial respiration intensity, and seasonal variation of river hydrological factors, the C in summer was generally lighter than that in autumn, with average values of -10.08‰ and -8.90‰, respectively. ④ The daily average fixation amount of karst carbon sink caused by aquatic plants was calculated to be 2.12 mmol·L and 0.94 mmol·L for Autumn and Summer, respectively. To sum up, there is a higher efficiency of karst carbon sink caused by aquatic plants in Autumn than that in Summer.