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Atmospheric methane is a potent greenhouse gas that plays a major role in controlling the Earth's climate. The causes of the renewed increase of methane concentration since 2007 are uncertain given the multiple sources and complex biogeochemistry. Here, we present a meta-data analysis of methane fluxes from all major natural, impacted and human-made aquatic ecosystems. Our revised bottom-up global aquatic methane emissions combine diffusive, ebullitive and plant-mediated and/or fluxes from several sediment-water-air interfaces. We emphasize the high variability of methane fluxes within and between aquatic ecosystems and a positively skewed distribution of empirical data, making global estimates sensitive to statistical assumptions and sampling design. We find aquatic ecosystems contribute (median) 41% or (mean) 53% of total global methane emissions from anthropogenic and natural sources. We show that methane emissions increase from natural to impacted aquatic ecosystems, and from coastal to freshwater ecosystems. We argue that aquatic emissions will likely increase due to urbanization, eutrophication and positive climate-feedbacks, and suggest changes in land-use management as potential mitigation strategies to reduce aquatic methane emissions. Main text:Methane (CH4) is the second most important greenhouse gas after carbon dioxide (CO2), accounting for 16 to 25% of atmospheric warming to date 1,2 . Atmospheric methane nearly tripled since pre-industrial times with a steady rise between 1984 and 2000 (8.4 ± 0.6 ppb yr -1 ) 3 , little or no growth between 2000 and 2006 (0.5 ± 0.5 ppb yr -1 ) 3 , and a renewed growth to present day (2007 to 2020: 7.3 ± 0.6 ppb yr -1 ) 3-6 . Whether the renewed increase is caused by emissions from anthropogenic or natural sources, or by a decline in the oxidative capacity of the atmosphere, or a combination of all three factors remains unresolved [7][8][9] . Depending on the approach used, total Rivers (ice-corrected) 5.8 (1.8-21.0) 30.5 ± 17.1 This study Lakes (ice-cover, ice-melt corrected) < 0.001 km 2 21.2 (9.1-53.5) 54.5 ± 48.5 This study 0.001 -0.01 km 2 13.2 (5.6-33.1) 31.1 ± 23.7 This study 0.01 -0.1 km 2 4.4 (1.4-16.7) 22.4 ± 18.4 This study 0.1 -1 km 2 3.0 (1.1-8.0) 9.9 ± 7.0 This study > 1 km 2 14.0 (6.0-31.0) 33.0 ± 45.0 This study All lakes 55.8 (23.3-142.3) 150.9 ± 73.0 This study Reservoirs (ice-cover, ice-melt corrected) < 1 km 2 0.4 (0.1-1.3) 2.4 ± 4.7 This study > 1 km 2 14.7 (8.7-27.1) 22.0 ± 6.4 This study All reservoirs 15.1 (8.8-28.4) 24.3 ± 8.0 This study Freshwater wetlands 150.1 (138.3-164.6) 148.6 ± 15.2 Saunois et al. 11 (A) Freshwater aquaculture ponds 4.4 (0.4-7.9) 14.0 ± 18.8 This study Rice cultivation 29.9 (24.9-32.1) 29.8 ± 6.7 Saunois et al. 11 (B) Total inland waters 261.0 (197.5-396.2) 398.1 ± 79.4 This study Estuaries 0.23 (0.02-0.91) 0.90 ± 0.29 This study Coastal wetlands Saltmarshes 0.18 (0.02-0.89) 2.00 ± 1.51 This study Mangroves 0.21 (0.06-0.77) 1.46 ± 0.91 This study Seagrasses 0.13 (0.07-0.21) 0.18 ± 0.19 This study Tidal flats 0.17 (0.04...

Water-level fluctuations due to reservoir management could substantially affect the timing and magnitude of reservoir methane (CH) fluxes to the atmosphere. However, effects of such fluctuations on CH emissions have received limited attention. Here we examine CH emission dynamics in six Pacific Northwest U.S. reservoirs of varying trophic status, morphometry, and management regimes. In these systems, we show that water-level drawdowns can, at least temporarily, greatly increase per-area reservoir CH fluxes to the atmosphere, and can account for more than 90% of annual reservoir CH flux in a period of just a few weeks. Reservoirs with higher epilimnetic [chlorophyll a] experienced larger increases in CH emission in response to drawdown (R = 0.84, p < 0.01), suggesting that eutrophication magnifies the effect of drawdown on CH emission. We show that drawdowns as small as 0.5 m can stimulate ebullition events. Given that drawdown events of this magnitude are quite common in reservoirs, our results suggest that this process must be considered in sampling strategies designed to characterize total CH fluxes from reservoirs. The extent to which (and the mechanisms by which) drawdowns short-circuit connections between methanogenesis and methanotrophy, thereby increasing net CH fluxes to the atmosphere, should be a focus of future work.