水生植物与水气界面气体交换对岩溶泉水化学及碳循环影响

Effect on Hydrochemistry of Karst Spring Water and Karst Carbon Cycle from Aquatic Plants and Gas Exchange Across Water-air Interface

为揭示水生植物光合作用及水气之间气体交换对岩溶水化学及碳循环的影响,在姜家泉沿程选取5个监测点,于2017年12月至2018年11月对其水化学性质进行监测。结果显示:(1)在降水较少的月份,JJQ3的Ca^(2+)浓度和HCO_(3)^(-)浓度明显小于JJQ1和JJQ2,并且溶解氧(DO)与电导率(EC)、Ca^(2+)浓度、HCO_(3)^(-)浓度之间均呈负相关关系,说明池水处水生植物光合作用将碳酸盐岩溶蚀产生的HCO_(3)^(-)转化成了稳定的有机碳,从而增强了岩溶碳汇效应。(2)JJQ4的Ca^(2+)和HCO_(3)^(-)浓度与JJQ1、JJQ2没有明显差异,但pH和溶解氧(DO)却明显高于JJQ1和JJQ2,并且JJQ4处溶解无机碳的稳定碳同位素(δ^(13)C_(DIC))偏正于JJQ1和JJQ2,这主要是跌水导致水-气之间发生了气体交换。大气中的氧气通过曝气作用进入水中,而水中二氧化碳分压(pCO_(2))远高于大气,导致水中溶解的CO_(2)脱气。(3)姜家泉水化学受到水生植物光合作用和水-气间气体交换的共同影响,并且在强酸参与下,CO_(2)脱气将进一步减弱岩溶碳汇效应。因此,为准确估算岩溶碳通量,有必要对两种作用方式进行区分。

The hydrochemical properties of five points were monthly monitored in Jiangjia Spring from from December 2017 to November 2018,in order to reveal the effect on hydrochemistry of karst spring water and karst carbon cycle from photosynthesis of aquatic plants and gas exchange.The results showed that the Ca^(2+)concentration and HCO_(3)^(-)concentration at JJQ3 were significantly low in the months with less precipitation,comparing with those at JJQ1 and JJQ2.Meanwhile the dissolved oxygen(DO)negatively correlated with conductivity(EC),Ca^(2+)and HCO_(3)^(-)concentrations.It indicated that HCO_(3)^(-)generated from carbonate dissolution was partly converted into stable organic carbon through photosynthesis of aquatic plants,which enhanced the karst carbon sink.However,the high flow rate could limit the photosynthesis of aquatic plants,resulting in little difference of chemical properties among monitoring points in the month with more precipitation.Despite that HCO_(3)^(-)concentrations at JJQ4 were similar to those at JJQ1 and JJQ2,while the pH and DO at JJQ4 were significantly higher than that at JJQ1 and JJQ2,and the stable isotopes of dissolved inorganic(δ^(13)C_(DIC))at JJQ4 were heavier than those at JJQ1 and JJQ2.The above phenomenon may attribute to the gas exchange between water and atmosphere because of head fall,which caused that the oxygen in the atmosphere dissolved into the water and the CO_(2) in the spring water was released into the atmosphere due to high pCO_(2).The hydrochemistry of Jiangjiaquan were controlled by photosynthesis of aquatic plants as well as gas exchange between water and atmosphere.Although the two factors had similar effects on hydrochemistry,they brought different effects on karst carbon cycle.Especially,CO_(2) degassing would further weaken the effect of karst carbon sink,at the presence of strong acid such as H2SO4.Therefore,it was necessary to distinguish the two factors,in order to accurately estimate the karst carbon flux.