Influence of aquatic photosynthesis on diel variations of hydrochemistry in karst river: A case study of the Lijiang River

The purpose of this work was to study the characteristics and influence factors of the biogeochemical diurnal variation process in the river section of non-karst and karst areas and discuss the effect of aquatic organisms on the river carbon sink in karst areas, especially the diel variations and influence factors of hydrochemistry in karst rivers. Taking the Lijiang River in Guilin City of Guangxi Zhuang Autonomous Region, China as an example, two monitoring sites were set up separately at Xiabei (non-karst) and Shengli (karst). Physical and hydrogeochemical parameters including pH, dissolved oxygen (DO), water temperature (T), electrical conductivity (EC), dissolved inorganic carbon (DIC), isotopes and other chemical parameters were examined by high-resolution on-line monitoring and high-frequency sampling conducted from 30 November to 1 October 2016. The results show that (1) Xiabei and Shengli sites are both of HCO3－Ca type water, however showing different diel variations of the hydrogeochemical process. The diel variations of physical and chemical parameters (EC, DO, pH, T, and major ions HCO-3, NO-3,Na+, Cl-,SO42-) at Shengli site are notable, showing regular diel variation in the monitoring, while those at Xiabei are relatively smaller, which is probably related to the special environment of the border between karst and the non-karst areas, where exist less aquatic plants. (2)The diurnal changes of nutrient elements (NO3-,SO2-4,Cl-,Na+) in the Shengli section are mainly controlled by assimilation of aquatic plants with a trend that increases in daylight and decreases at night. (3)The data of TOC and DOC show a daytime increase and nighttime decrease cycle, and their maximum daily changes are 79% and 61%, respectively. TOC is mainly derived from the primary productivity of aquatic organisms. The proportion of HCO-3 in inorganic carbon sources taken advantage by aquatic plants ranges from 67.42% to 99.75% and 57.76% to 69.78%, with average values 79.54% and 63.13%, respectively. (4)The concentration dissolved inorganic carbon (DIC) in Shengli section varies from 67.1 to 115.9 mg?L-1 with an average value of 96.5 mg?L-1. The variation of δ13CDIC ranges from -7.8‰ to -9.9‰ with an average value of -8.9‰. The dissolved inorganic carbon(DIC) andδ13CDIC at the Shengli site shows a reverse characteristic in diurnal fluctuations, where the dissolved inorganic carbon decreases in daylight and increases at night whileδ13CDIC increases in daylight and decreases at night. The dissolved inorganic carbon has a negative correlation with theδ13CDIC (with correlation coefficient -0.79). It was found that the circadian variation of dissolved inorganic carbon is controlled by photosynthesis and respiration of aquatic plants and calcium deposition. According to the diurnal amplitude of the dissolved inorganic carbon(DIC) and calcium concentrations, the mean conversion rate of the dissolved inorganic carbon(DIC) by aquatic vegetation photosynthesis at Shengli site was calculated to be 1.2×10-5 mmol·L-1·S-1 and the average Ca precipitation rate was estimated to be 0.18×10-5mmol·L-1·S-1. In conclusion, the photosynthesis and carbon sink capacity of aquatic organisms in the river segment of karst areas are significantly enhanced compared with that in non-karst areas.