Seasonal variation of CO2 diffusion flux from a large subtropical reservoir in East China

•We report the CO2 emission from a subtropical large reservoir.•Reservoir surface can absorb large amount of atmospheric CO2 in warm seasons.•Downstream the dam has the similar contribution of CO2 emission to reservoir surface.

The Xinanjiang Reservoir (E118°42′-E118°59′, N29°28′-N29°58′), has a surface area of 567 km2, with a mean water depth of 34 m, and is in an oligotrophic state at present. In this study, five cruises were carried out in April, June, August and November 2010, and January 2011, to understand the seasonal variation of CO2 emission and to estimate the annual diffusion flux of CO2 from this reservoir. The partial pressure of CO2 (pCO2) in surface water was determined using a continuous measurement system (equilibrator-infrared system) from upstream to central reservoir, and pCO2 along the water column in central reservoir and in downstream of the dam was also determined. Results showed that pCO2 in reservoir surface water varied significantly in different seasons, ranging from 5 μatm in August to 1700 μatm in January; while in downstream of the dam, it ranged from 1400 μatm in April to 3800 μatm in August. Along the water column in central reservoir, pCO2 also showed significantly variations seasonally, and a water mass with quite high pCO2 (>4000 μatm) formed below water depth of 20 m in warm seasons. The highest diffusion flux of CO2 (F-CO2) appeared in January, with values of 20.9 mmol m−2 d−1 at river reach and 25.2 mmol m−2 d−1 at central reservoir, while the lowest values of F-CO2 were −10.0 at river reach and −10.8 mmol m−2 d−1 at central reservoir in August. Downstream of the dam had quite high positive F-CO2 during the whole year, with a range of 129.1 in April to 381.5 mmol m−2 d−1 in August. In total, about 88.9 kton a−1 CO2 was emitted to the atmosphere from the Xinanjiang Reservoir surface, while 39% of it (35.1 kton a−1) was absorbed by surface water in warm seasons. Downstream and turbine had a comparable CO2 outgas flux of 61.5 kton a−1. When taking the whole reservoir surface, turbine and downstream into account, the Xinanjiang Reservoir system had a net CO2 emission flux of 115.3 kton a−1.