Water depth and productivity regulate methane (CH4) emissions from temperate cascade reservoirs in northern China

Abstract

Reservoirs are important sources of emissions of the greenhouse gas methane, but the temporal and spatial variability of methane emissions and the underlying mechanisms driving methane flux in cascade reservoirs remain poorly understood. In this study, we used floating chambers to study methane emission fluxes and their temporal and spatial variations in three temperate cascade reservoirs with different nutrient levels and morphologies in northern China. We measured the methane production rate, sediment properties and water quality to identify key factors driving methane emissions. The results showed that the three cascade reservoirs were persistent sources of atmospheric methane during the ice-free period, and the methane flux ranged from 1.01 to 107.17 µmol m-2h−1, with a mean of 11.17 µmol m-2h−1. The mean methane flux of the eutrophic reservoir was more than 2 and 4 times that of the two mesotrophic reservoirs. Overall, methane flux showed high spatial and temporal variability within and among reservoirs, and the three reservoirs showed different patterns of spatial and temporal variability. Statistical analysis showed that the three reservoirs had different key drivers, indicating that the methane emissions of the three reservoirs were regulated by different underlying mechanisms. When all three reservoirs were pooled, we found that Chl-a was the most important driver of methane emissions. Our results suggest that to obtain an accurate estimate of whole-reservoir methane flux, different reservoir zones and seasons must be considered. Our results also highlight that regional and global methane budget upscaling should include considerations of reservoir diversity (different climatic zones, morphology and trophic levels).