Contrasting methane emissions from upstream and downstream rivers and their associated subtropical reservoir in eastern China

Le Yang

doi:10.1038/s41598-019-44470-2

Abstract

Subtropical reservoirs are an important source of atmospheric methane (CH4). This study investigated the spatiotemporal variability of bubble and diffusive CH4 emissions from a subtropical reservoir, including its upstream and downstream rivers, in eastern China. There was no obvious seasonal variation in CH4 emissions from the main reservoir, which increased slightly from the first half year to the next half year. In the upstream river, CH4 emissions were low from February to June and fluctuated widely from July to January due to bubble activity. In the downstream river, CH4 emissions were lowest in February, which was possibly influenced by the low streamflow rate from the reservoir (275 m3 s−1) and a short period of mixing. There was spatial variability in CH4 emissions, where fluxes were highest from the upstream river (3.65 ± 3.24 mg CH4 m−2 h−1) and lowest from the main reservoir (0.082 ± 0.061 mg CH4 m−2 h−1), and emissions from the downstream river were 0.49 ± 0.20 mg CH4 m−2 h−1. Inflow rivers are hot spots in bubble CH4 emissions that should be examined using field-sampling strategies. This study will improve the accuracy of current and future estimations of CH4 emissions from hydroelectric systems and will help guide mitigation strategies for greenhouse gas emissions.

Highlights

Hydropower has historically been regarded as a clean energy source, the view is challenged by a growing body of research that considers hydroelectric reservoirs to be carbon sources
There were similar seasonal patterns of ebullition rate, bubble CH4 emission flux, and bubble CH4 concentration, all of which were lower in spring than in summer and autumn (Fig. 1)
On a monthly scale, mean diffusive CH4 fluxes during the sampling period were similar and generally constant over time among the three areas of the main reservoir; fluxes peaked in the southwest (SW) lake on 1 August (DOY: 213) and 8 February (DOY: 39; Fig. 2)

Summary

Introduction

Hydropower has historically been regarded as a clean energy source, the view is challenged by a growing body of research that considers hydroelectric reservoirs to be carbon sources. Emissions were greater in summer than in other seasons at the Three Gorges Reservoir and were regulated by temperature, DO, and water velocity[2], whereas they were only regulated by temperature at three lakes (Följesjön, Erssjön, and Skottenesjön) in southwest Sweden[20]. Analysis of these differences in effects of environmental factors on spatiotemporal variability in CH4 emissions from reservoirs may result in more accurate estimates of the total CH4 emissions than previously determined

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Results

Conclusion