Spatial-temporal variations and influencing factors of carbon dioxide evasion from the Yellow River: An example of the Toudaoguai Gauging Station

Abstract

河流连接着海洋碳库和陆地碳库，河流碳逸出是全球碳收支的重要组成部分。本文以黄河上游和中游分界点-内蒙古段头道拐断面为研究对象，采用Li-7000静态箱法监测了断面4个采样点在2013-2015年期间四季的CO2逸出通量（FCO2），并分析了FCO2时空变化规律。基于研究断面主要水文和水化学指标的野外监测和室内分析结果，探讨了FCO2的主要影响因素。结果表明断面FCO2介于14-186 mol m-2 a-1，平均值为84 mol m-2 a-1；水体CO2分压（pCO2）介于467-2101 μatm，平均值为995 μatm；DOC浓度介于2.7-13 mg/L。FCO2季节性差异明显：夏季FCO2为全年最大456 mmol m-2 d-1，冬季最小33 mmol m-2 d-1。FCO2在4个采样点的空间差异显著：河道右岸S4点处最大为392 mmol m-2 d-1；河道中部S2和S3点基本相同；河道左岸S1点最小为86 mmol m-2 d-1。FCO2与河道流速呈现较好的正相关关系，与pCO2中等相关，与pH负相关，与风速的相关性不明显，说明对于该研究断面河道流速较pCO2对FCO2的贡献更大。本研究较为精细地探讨了头道拐断面的水体CO2逸出规律，表明即使在同一河道断面，FCO2也可能存在较大空间差异，流速较大处的FCO2较大，因此在野外监测FCO2时需要在河道断面选取具有代表性的采样点，特别是较大的河流。研究结果为黄河中上游CO2逸出量评价和河道断面FCO2监测点的布设提供了科学依据。The oceanic carbon pool and terrestrial carbon pool are connected by rivers. Carbon dioxide (CO2) evasion from rivers to the atmosphere represents a substantial flux in the global carbon cycle. The CO2 efflux (FCO2) and CO2 partial pressure (pCO2) in large rivers have been widely evaluated. Most studies concerning CO2 emission from the Yellow River, a typical river containing high sediment concentrations, focused on the lower reach and its estuary, but less is known about its upper and middle reaches. In this study, a river cross-section at the Toudaoguai Gauging Station in Inner Mongolia, the dividing point between the upper and middle reaches of the Yellow River was chosen as a study site. Evasion of CO2 was measured four times each year using Li-7000 static chamber method from 2013 to 2015 at four sampling points in a river cross-section. The spatial and temporal variations of FCO2 were analyzed. The relevant hydrological indexes, including water temperature, pH,and wind velocity as well as current velocity were measured at the four sampling points. The hydrochemical indicators, including ALK and DOC in water samples, were analyzed in the laboratory and pCO2 was estimated. The possible influential factors of FCO2 were further discussed using correlation analysis. The CO2 evasion from the river cross-section ranged from 14 to 186 mol m-2 a-1 and its average was 84 mol m-2 a-1. The pCO2 in the Yellow River at the Toudaoguai Gauging Station was within the range of 467-2101 μatm and the average value was 995 μatm. The concentration of DOC ranged from 2 to 13 mg/L. The FCO2 exhibited obvious seasonal variations, with the maximum FCO2 of 456 mmol m-2 d-1 occurring in summer and the minimum of 33 mmol m-2 d-1 occurring in winter. The FCO2 values were markedly different at sampling points, with the maximum value of 392 mmol m-2 d-1 at S4 near the right bank, similar values at S2 and S3 in the middle of the river section, and the minimum of 86 mmol m-2 d-1 at S1 near the left bank. The analysis of factors influencing FCO2 indicated that FCO2 was positively correlated with current velocity and pCO2, and negatively correlated with pH. There was no obvious correlation between FCO2 and wind speed. Results also showed that current velocity contributed more to FCO2 than to pCO2 in the river cross-section. In this study, evasion of CO2 from the Toudaoguai cross-section was determined on a relatively fine scale. The results suggested that a distinct spatial variation in FCO2 exists even at the level of river cross-section, with the maximum FCO2 found at the point with the highest current velocity. Thus, typical sampling points in a river cross-section should be chosen for FCO2 measurement. The study provided a scientific reference for both FCO2 evaluation in the upper and middle reaches of the Yellow River and FCO2 sampling in a river cross-section.