Ebullitive methane emissions from oxygenated wetland streams JOHN T. CRAWFORD 1,2 , EMILY H. STANLEY 2 , SETH A. SPAWN 3 , JACQUES C. FINLAY 4 , LUKE C. LOKEN 1 andROBERT G. STRIEGL 1 1 U.S. Geological Survey, National Research Program, Boulder, CO 80303, USA, 2 Center for Limnology, University of Wisconsin-Madison, 680 N. Park St., Madison, WI 53706, USA, 3 St. Olaf College, Northfield, MN 55057, USA, 4 Department of Ecology, Evolution and Behavior, University of Minnesota, St. Paul, MN 55108, USA Abstract Stream and river carbon dioxide emissions are an important component of the global carbon cycle. Methane emis- sions from streams could also contribute to regional or global greenhouse gas cycling, but there are relatively few data regarding stream and river methane emissions. Furthermore, the available data do not typically include the ebullitive (bubble-mediated) pathway, instead focusing on emission of dissolved methane by diffusion or convection. Here, we show the importance of ebullitive methane emissions from small streams in the regional greenhouse gas balance of a lake and wetland-dominated landscape in temperate North America and identify the origin of the meth- ane emitted from these well-oxygenated streams. Stream methane flux densities from this landscape tended to exceed those of nearby wetland diffusive fluxes as well as average global wetland ebullitive fluxes. Total stream ebullitive methane flux at the regional scale (103 Mg C yr À1 ; over 6400 km 2 ) was of the same magnitude as diffusive methane flux previously documented at the same scale. Organic-rich stream sediments had the highest rates of bubble release and higher enrichment of methane in bubbles, but glacial sand sediments also exhibited high bubble emissions rela- tive to other studied environments. Our results from a database of groundwater chemistry support the hypothesis that methane in bubbles is produced in anoxic near-stream sediment porewaters, and not in deeper, oxygenated groundwaters. Methane interacts with other key elemental cycles such as nitrogen, oxygen, and sulfur, which has implications for ecosystem changes such as drought and increased nutrient loading. Our results support the conten- tion that streams, particularly those draining wetland landscapes of the northern hemisphere, are an important com- ponent of the global methane cycle. Keywords: carbon dioxide, ebullition, methane, rivers, upscaling, wetlands Received 30 October 2013 and accepted 8 April 2014 Introduction Stream and river carbon dioxide (CO 2 ) emissions are recognized as an important component of the global carbon (C) cycle (Cole et al., 2007; Butman & Raymond, 2011; Raymond et al., 2013). There is also evidence that streams and other freshwaters could be important natu- ral sources of atmospheric methane (CH 4 ) (Bastviken et al., 2011), but there remains large uncertainty due to a paucity of studies reporting CH 4 fluxes, and a lack of data for many parts of the globe. Although a significant freshwater CH 4 source is possible, wetlands are the dominant natural global source of atmospheric CH 4 (Walter et al., 2001; Dlugokencky et al., 2011). The glo- bal atmospheric CH 4 budget is reasonably well con- strained (Dlugokencky et al., 2011), yet large discrepancies have been shown between bottom-up (field scale) and top-down (atmospheric inverse technique) estimates of emissions, particularly in wet- lands, with a potential ‘missing’ wetland CH 4 source of ~87 Tg C yr À1 (Walter et al., 2001). However, a more recent assessment revealed potential overestimates from bottom-up approaches (Kirshcke et al., 2013), highlighting the continued uncertainty of the global CH 4 budget. One potential explanation for the large discrepancies between budgeting approaches, particu- larly for the earlier bottom-up underestimates, is that additional CH 4 sources such as freshwater ecosystems, and/or source pathways such as ebullition (bubble- mediated), are not accounted for. Recent inventories of freshwater CH 4 emissions may even require a reduc- tion in top-down wetland emission estimates globally (Bridgham et al., 2013). Despite a general omission of aquatic accounting in the global CH 4 budget, recent work suggests that global lake CH 4 emissions (includ- ing both diffusive and ebullitive pathways) range from 8 to 48 Tg C yr À1 (Bastviken et al., 2004), with a poten- tial additional source of 3.8 Tg C yr À1 from permafrost thaw lakes in North Siberia alone (Walter et al., 2006). Correspondence: John T. Crawford, tel. 608 262 3014, fax 608 265 2340, e-mail: jtcrawford@wisc.edu 1 © 2014 John Wiley & Sons Ltd Global Change Biology (2014), doi: 10.1111/gcb.12614 Global Change Biology