地下河出口河流水化学昼夜动态变化——生物地球化学过程的控制

昼夜动态变化的研究有助于揭示水体中相对快速的生物地球化学过程,同时也有助于判别影响水化学变化的主导因子。本文选择由地下水补给且富含水生植物的典型河流,开展高分辨率水文地球化学监测和高频率水样取样工作,分析了水化学的昼夜动态变化特征并对比其沿流程的变化,探讨了水化学昼夜循环产生的生物地球化学控制机理。结果表明,河流水化学离子成分多呈现出昼夜动态变化的特点,Ca2+、DIC白天下降幅度达22.4%,昼夜循环主要受水生植物光合作用控制,但不同成分沿流程具有不同的变化规律,存在Ca2+、DIC和营养元素成分的流失。研究河段硝酸盐含量较高(农业活动产生的面源氮补给),沿流程有减小趋势,受生物同化作用控制,白天小幅升高夜间回落的昼夜动态变化,主要受硝化作用过程控制。DOC与TOC含量小时数据呈现快速波动特点,白天上升夜间下降,受生物代谢活动控制,DOC的日变化幅度可相差1倍。岩溶区地表河流水化学昼夜动态变化规律与生物地球化学过程研究,不仅能揭示水化学无机组分昼夜循环的控制因素,也有助于更好理解岩溶作用过程中无机碳与有机碳的快速转换特性,对提高岩溶碳汇的估算精度有重要意义。

Diel Aqueous Chemical Cycling in a Typical Karst Spring-fed Stream: Controls of Biogeochemical Processes

The study of diel variations can help to reveal biogeochemical processes that occur relatively rapidly in natural waters and also the main and important controlling factors that influence the changes of aqueous chemistry. Three-day monitoring with high resolution data logger and high frequency sampling with 1-hour interval were conducted in a typical karst spring-fed stream with abundant aquatic vegetation. Daily cycling of hydrochemistry and its changes along the stream flow were discussed and the influence of biogeochemical processes on hydrochemistry was analyzed. The results show that the changes in aqueous chemistry are closely associated with biogeochemical processes and featured as diel cycling. Concentrations of Ca2+ and DIC decrease during the daytime with an amplitude of 22.4%, which reflects the controls of photosynthesis of aquatic plants and suggests that the stream loses Ca2+, DIC and nutrients with downstream distance, but changes of various components along the stream flow are quite different. NO3- concentration also tends to decrease along the stream flow, which is consistent with plant assimilation and exhibits a slightly daytime increasing and nighttime decreasing cycling in a 24-hour time scale. The latter may be caused by nitrification in such a nutrient-rich stream. Data of DOC and TOC collected at hourly interval change quickly but still have a daytime increase and nighttime decrease cycling, with daily increases of as high as 100%. These diel DOC and TOC cycles are likely caused by metabolic processes. The study of diel hydrochemical cycling and biogeochemical processes in karst spring-fed streams will improve understanding of carbon conversion rate between DIC and DOC in karst aquatic environments and thus lead to better estimation of karst carbon sink.