Diurnal Variation Of River Biogeochemistry

Based on numerous karst researches and new insight into earth’s systematic view of karst processes,studies involving water-rock-soil-gas-organism interactions are of special interest when searching effects of the karst processes on the global carbon cycle.Especially,in recent years,combined processes of carbonate dissolution and aquatic photosynthesis have been considered as a new direction in the global carbon sink and drawn great international attentions.Whether carbonate weathering could provide durable and stable carbon sink or not depends mainly on the efficiency of the utilization of dissolved inorganic carbon（DIC）by aquatic phototrophs.To understand above-mentioned aquatic carbon sequestration process,invesitigation of diurnal variations in hydrochemical characteristics,stable carbon isotope compositions of dissolved inorganic carbon(δ¹³C_DIC)and DOC（dissolved organic carbon）in surface water is shown to be an efficient way.We chosed 3 sampling points from upstream to downstream of Lijiang including Xiabei,Shengli and Yangshuo Hydrometrical Station.Samples were obtained every month from July of 2017 to June of 2018.2-day monitoring work with high resolution date logger and high frequency sampling at2-hour interval was conducted at Xiabei site,Shengli site and Yangshuo site of the Lijiang River in summer and autumn separately to understand the diel variation and influencing factors of hydrochemistry in different seasons and sections.Physical and hydrogeochemical parameters including pH,dissolved oxygen（DO）,water temperature（T）,electrical conductivity（Ec）,dissolved inorganic carbon（DIC）,isotope and other chemical parameters were examined.In addition,the seasonal variations of dissolved organic carbon（DOC）and particulate organic carbon（POC）at different sections were measured to reveal the control mechanism of organic matter formation in different karst aquatic ecosystems.Finally,the net carbon sequestration of aquatic plants in different sections and seasons due to the"aquatic carbon pump"was estimated by calculating the net production of dissolved oxygen in water.Field campaigns have been made in 2016-2017 including two times,summer（September 14-16,2016）,autumn（October 30 to November 1,2017）.According to in-situ monitoring,laboratory analysis and model calculation,we came to following conclusions:（1）The hydrochemical type of Lijiang River was HCO₃-Ca-type, Ca²⁺ was the main cation, HCO₃^-was main anion.The concentration of SO₄^2–and NO₃^–were inferior to HCO₃^–in the anion composition.Controlled by karstification and intensity of human activities,the concentrations of Ca²⁺,Mg²⁺,HCO₃^–,SO₄^2–and NO₃^–in the monitoring sections gradually increased from upstream to downstream.Influenced by precipitation dilution in rainy season and the duration of water-rock interaction,the concentrations of HCO₃^–and Ca²⁺showed seasonal variation characteristics of lower in rainy season and higher in dry season.The ratios of Cl^–/Na⁺in the Xiabei section,Shengli section and Yangshuo section were 1.10,1.12 and 1.46,respectively,which indicated that the influence of human activities on the river increases with the increase of the process.The hydrochemical parameters of Lijiang River were mainly affected by water-rock interaction,seasonal variation of precipitation and physiological activities of aquatic plants,showing obvious seasonal and spatial variations.The average values of pH,Ec,TDS and DO increased gradually from upstream to downstream.Influenced by rainfall dilution effect,Ec and TDS was lower in the rainy season than that in the dry season.The DO concentration controlled by the intensity of physiological activities of aquatic plants was higher in spring and autumn than that in summer and winter.The DOC concentration increased spatially from the upstream（non-karst area）to the downstream（karst area）,with the highest in autumn and the lowest in summer.POC concentration from upstream to downstream did not change significantly with the process,showing seasonal variation of the highest in summer and the lowest in winter.（2）Influenced by lithology and aquatic biomass,the diurnal variation characteristics of biogeochemistry in the Xiabei site were different from those in the Shengli section and Yangshuo section.The Xiabei site in the non-karst area did not show obvious diurnal variation,while Shengli section and Yangshuo section in the karst area showed obvious diurnal variation:water temperature,DO,pH,SI_C increased during the day and decreased at night,while pCO₂,Ec,DIC and Ca²⁺decreased during the day and increased at night.Photosynthesis of aquatic plants stored DIC in the form of organic carbon and reduced CO₂ emission at the water-air interface.While nocturnal respiration produced CO₂,which led to rapid increase of pCO₂ and decrease of SIC,and the water body was unsaturated.Potential degassing may occur.The pCO₂ in Yangshuo section was always near the critical line of atmospheric equilibrium,which means that the possibility of degassing in water decreased with the increase of process.It suggested that the upstream aquatic plants utilize DIC through photosynthesis.On the one hand,DIC was converted into organic carbon to form carbon sink.On the other hand,it was conducive to the balance of carbonate in water body and restrain the escape of CO₂ from water-air interface,thus improving the stability of carbon sinks.（3）The hourly data of TOC and DOC in river showed the characteristic of rapid change,showing the increase during the day and decrease at night.The proportion of endogenous organic carbon in the Xiabei site,Shengli section and Yangshuo section was 88.39%,91.99%and 93.33%,the proportion of aquatic plants using HCO₃^–as inorganic carbon source was 63.13%,79.54%,and68.48%.The concentration of TOC and DOC,the proportion of endogenous organic carbon in total organic carbon,and the proportion of organic carbon produced by aquatic plants using HCO₃^–as inorganic carbon source in the water body of the Shengli section and Yangshuo sections were higher than those in the Xiabei section,indicating that the karst section had a higher carbon sink effect.（4）In summer and autumn,the NO₃^–concentration at the Shengli and Yangshuo sections decreased during daytime and increased at night,indicating that the diurnal variation of NO₃^–concentration at the Shengli and Yangshuo sections in different seasons was controlled by the process of biological metabolism（assimilation）.Influenced by floods and water turbidity,the consumption of NO₃^–in summer was significantly less than that in autumn.Influenced by the aquatic plant biomass in the monitoring sections,the NO₃^–consumption in Yangshuo section was lower than that in Shengli section both in summer and autumn.（5）The diurnal variation ofδ¹³C_DIC increased in daylight and decreased at night both in summer and autumn reflecting the influence of photosynthesis and precipitation.Compared with autumn,the value ofδ¹³C_DIC was lighter in summer due to the destruction of aquatic plant communities by summer floods,the weakening of photosynthesis by higher water turbidity and flow rate,and the strong influence of soil root system and soil microbial respiration.Through comparison,it was found that theδ¹³C_DIC of Yangshuo section in autumn and summer was heavier than that in the Shengli section,which indicated that theδ¹³C_DIC of Lijiang River may had a gradually heavier trend with the increase of flow.On the one hand,it implied that the upstream aquatic plants used the DIC of water through photosynthesis to convert the DIC into organic carbon sink,giving priority to the use of ¹²C with lighter isotopes,so that theδ¹³C_DIC value was gradually heavy.On the other hand,the isotope balance between DIC and atmospheric CO₂ was established because the flow velocity in the middle and downstream slowed down,which also made the value ofδ¹³C_DIC gradually heavier along the flow.（6）By calculation,the daily average fixation amount of karst carbon sinks caused by the aquatic plants in autumn was 2.13 mmol L^-1 and 0.59 mmol L^-1,and in summer was 0.95 mmol L^-11 and 0.32mmol L^-1,respectively.In summer and autumn,the aquatic ecosystems of Yangshuo and Shengli sections become carbon sink.To sum up,there was a higher efficiency of karst carbon sink caused by aquatic plants in Autumn than that in Summer.Moreover,the daily average fixation amount of karst carbon sinks caused by the aquatic plants in Shengli site was higher than that in Yangshuo site.