[Spatial and Temporal Variability of CO₂ Emissions from the Xin'anjiang Reservoir]

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.