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

Abstract

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.