水生植物对岩溶区河水地球化学昼夜变化的影响——以漓江为例

为研究河流非岩溶区断面和岩溶区断面生物地球化学昼夜变化过程、特征及影响因素，探讨水生生物对岩溶区河水碳汇作用的影响，于2016年10月30日-2016年11月1日，在广西桂林漓江干流非岩溶区的峡背和岩溶区的省里设置两个监测点同时开展了为期48小时的高分辨率在线监测和高频率取样工作，研究其水文参数(电导率(EC)、水温(T)、pH以及Ca2+、HCO-3、NO-3等离子和溶解无机碳同位素(δ13CDIC)等水化学参数的昼夜变化规律，并分析其影响因素。发现:（1）峡背和省里两断面水化学类型为HCO3－Ca型，但水文地球化学昼夜变化过程不同:省里断面的物理化学参数昼夜变化显著，T、pH、DO、SIC白天上升夜间下降，Ca2+、HCO-3的质量浓度和EC、p (CO2)白天降低、夜晚上升；而峡背断面理化指标昼夜变幅小，这与峡背断面处于岩溶区与非岩溶区交界处，非岩溶河流汇入、生物量较小等环境特征有关。（2）省里断面营养元素(NO-3、SO42-、Cl-、Na+)昼夜变化过程主要受水生植物同化作用控制，呈现白天降低、夜间升高的变化规律。（3）峡背断面和省里断面TOC 与DOC白天上升、夜间下降，最高日变化幅度可达79%和61%，利用端元混合模型计算得出省里断面和峡背断面内源有机碳占总有机碳的比例分别为91.99%和88.39%，省里断面和峡背断面水生植物光合作用利用HCO-3作为无机碳源的比例为67.42%～99.75%和57.76%～69.78%，平均值分别为79.54%和63.13%。（4）省里断面溶解无机碳(DIC)变化范围为67.1～115.9 mg·L-1，平均值为96.5 mg·L-1，呈现白天下降夜间上升的变化。δ13CDIC变化范围-7.8‰～-9.9‰，平均值为-8.9‰，表现为白天偏重、晚上偏轻的动态变化，两者呈显著的负相关关系(相关系数为-0.79)。研究表明省里断面水生植物光合作用和呼吸作用以及钙沉降是控制DIC昼夜变化的主导因素。通过估算监测期间省里断面光合作用DIC转化速率平均值为1.2×10-5 mmol·L-1·S-1，Ca2+离子的沉积速率平均值为0.18×10-5mmol·L-1·S-1。因此，岩溶区河段水生生物光合作用及其固碳能力较非岩溶区河段明显增强。 

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.