Sediment Trapping by Dams Creates Methane Emission Hot Spots Andreas Maeck,* ,| Tonya DelSontro, † Daniel F. McGinnis, ‡,∥,& Helmut Fischer, § Sabine Flury, ¶,& Mark Schmidt, ‡ Peer Fietzek, ‡,⊥ and Andreas Lorke | | Institute for Environmental Sciences, University of Koblenz-Landau, 76829 Landau, Germany † Swiss Federal Institute of Aquatic Science and Technology, Eawag, 6047 Kastanienbaum, Switzerland and Institute of Biogeochemistry and Pollutant Dynamics, ETH, 8092 Zurich, Switzerland ‡ GEOMAR Helmholtz Centre for Ocean Research, RD2 Marine Biogeochemistry, 24148 Kiel, Germany ∥ Nordic Center for Earth Evolution (NordCEE), Institute of Biology, University of Southern Denmark, 5230 Odense M, Denmark § Federal Institute of Hydrology (BfG), 56068 Koblenz, Germany ¶ Department of Bioscience, Center for Geomicrobiology, Aarhus University, 8000 Aarhus C, Denmark ⊥ CONTROS Systems and Solutions GmbH, 24148 Kiel, Germany ABSTRACT: Inland waters transport and transform sub- stantial amounts of carbon and account for ∼18% of global methane emissions. Large reservoirs with higher areal methane release rates than natural waters contribute significantly to freshwater emissions. However, there are millions of small dams worldwide that receive and trap high loads of organic carbon and can therefore potentially emit significant amounts of methane to the atmosphere. We evaluated the effect of damming on methane emissions in a central European impounded river. Direct comparison of riverine and reservoir reaches, where sedimentation in the latter is increased due to trapping by dams, revealed that the reservoir reaches are the major source of methane emissions (∼0.23 mmol CH 4 m −2 d −1 vs ∼19.7 mmol CH 4 m −2 d −1 , respectively) and that areal emission rates far exceed previous estimates for temperate reservoirs or rivers. We show that sediment accumulation correlates with methane production and subsequent ebullitive release rates and may therefore be an excellent proxy for estimating methane emissions from small reservoirs. Our results suggest that sedimentation- driven methane emissions from dammed river hot spot sites can potentially increase global freshwater emissions by up to 7%. ■ INTRODUCTION Inland waters are significant sources of the atmospheric greenhouse gases carbon dioxide (CO 2 ) and methane (CH 4 ). 1,2 While microbial degradation of organic matter in oxic sediments mainly produces CO 2 , anaerobic pathways, e.g. in freshwater sediments, also produce CH 4 . Methane released to the atmosphere has a 25 times higher global warming potential than CO 2 per mass on a 100 year time scale; 3 therefore, a shift in the degradation pathway in sediments from aerobic to anaerobic increases the climatic impact of the aquatic system. River segmentation and disruption by dams changes the suspended particle and bedload transport and leads to the accumulation of sediments in the basins upstream of dams. 4,5 Since settling particles build up cohesive sediment layers, the sediments at high deposition zones (i.e., forebays of dams, sidebays) are frequently anoxic. 4 Worldwide, over 50,000 large dams (storage height > 15 m) and millions of smaller impoundments exist, which has resulted in a reduction of terrestrial organic carbon flux to the ocean by 26% and a storage of 83−250 Tmol (1−3 Pg) of carbon in these reservoirs. 5 Ultimately, the combination of two important factors - 1) the continuous trapping of both allochthonous and autochthonous organic material in reservoirs, and 2) increased CH 4 production via anaerobic degradation of organic carbon in reservoir sediments - leads to the hypothesis that reservoirs emit significant amounts of CH 4 to the atmosphere. 6 Quantitative estimates of methane emissions from reservoirs have mainly been obtained for large systems. In the initial phase after construction of the reservoirs, freshly inundated biomass is the major source of methanogensis, while during the aging of the reservoirs, deposited sediment containing organic carbon becomes more important. 7 However, the zones of sedimenta- tion of allochthonous material that enter the reservoir via river inflows are relatively small compared to the large surface area. In small reservoirs, e.g. impounded rivers, the zones of sediment deposition cover a larger fraction of the reservoir’s surface area and sediment accumulation rates can be very high. Received: January 27, 2013 Revised: May 16, 2013 Accepted: June 25, 2013 Published: June 25, 2013 Article pubs.acs.org/est © 2013 American Chemical Society 8130 dx.doi.org/10.1021/es4003907 | Environ. Sci. Technol. 2013, 47, 8130−8137