新安江水库二氧化碳排放的时空变化特征

Spatial and Temporal Variability of CO_2 Emissions from the Xin'anjiang Reservoir

新安江水库是我国华东地区最大的水库,面积580 km2,平均深度30 m,水库水体处于中贫营养状态.为了研究新安江水库中CO_2排放的时空变化特征,2014年12月至2015年12月采用静态浮箱法收集水库表面以分子扩散方式排放的CO_2,使用气相色谱仪分析CO_2浓度.结果表明,新安江水库CO_2排放通量从上游入库河流[(120.39±135.41)mg·(m^2·h)^(-1)]至库区主体[(36.65~61.94)mg·(m^2·h)^(-1)]呈下降趋势,而大坝下游河流中CO_2排放通量[(1 535.00±1 447.46)mg·(m^2·h)^(-1)]显著增加,约分别是上游入库河流和库区主体的13倍和25~42倍.但随着与大坝距离增加,大坝下游河流中CO_2排放通量显著下降,如7 km处的CO_2排放通量仅为出库水体处的20%.在库区主体中,CO_2排放通量具有明显的季节变化:CO_2排放通量在秋、冬季时为正值,最大值出现在冬季(12月或1月),说明此时库区表层水体是CO_2排放源;而CO_2排放通量在春、夏季为负值,最小值出现在春季(3、4或5月),说明此时库区表层水体是CO_2吸收汇,这可能与春、夏季时水体中藻类繁殖有关.所以,在调查水库表面CO_2排放时,应对水库的上游入库河流、库区主体和坝下河流进行全面长期的观测,才能避免低估水库中CO_2排放总量.

Xin＇anjiang Reservoir is the largest reservoir in eastern China, with a surface area of 580 km2 and a mean depth of 30 m. It is in an oligotrophic or mesotrophic state at present. This study measured carbon dioxide （CO2 ） emissions from the upstream river, the reservoir＇s main body, and the river downstream of the Xin＇anjiang Reservoir to investigate the spatial and seasonal variability of CO2 emissions from the water surface using static floating chambers and gas chromatography. Results showed that the downstream river had, significantly, the highest CO2 emission flux [（1535. 00 ± 1447. 46） mg·（m2·h） - 1 ], followed by the upstream river [（120. 39 ± 135. 41） mg·（m2·h） - 1 ]. The reservoir＇s main body had the lowest flux [（36. 65-61. 94） mg·（m2·h） - 1 ]. The high CO2 emission flux in the downstream river was probably influenced by turbulence during the discharge periods, which would allow the dissolved CO2 in the hypolimnion before the dam to be released to the atmosphere in the watercourse of the downstream river. However, the CO2 emission flux decreased with distance to the dam, likely because of the drop in strength of the turbulence. Moreover, there was an obvious alternation between CO2 source and CO2 sink in the main body of the reservoir, with CO2 sources in autumn and winter and CO2 sinks in spring and summer. The maximum and minimum CO2 emission values occurred in winter and spring, respectively. Such variability in the CO2 emissions was probably influenced by the bloom of alga in spring and summer, because dissolved CO2 in the water was absorbed by the respiration of alga. However, hydrologic conditions were unstable in the upstream river due to a fast water flow, so alga was difficult to bloom there, and a CO2 source was observed throughout the year, except during April and August. The measurement of the flux from the upstream river, main body, and downstream river required a long period for the investigation of greenhouse gas emissions to avoid underestimating the total CO2 emission from a hydroelectric reservoir system.