US Long-Term Ecological Research Network

graphs of stream CAlthough streams are now recognized as important components of regional and global carbon budgets, patterns and controls of stream CO2 and CH4 budgets are poorly documented for most landscapes. One major question is: Do streams support their CO2 and CH4 emissions through internal processing or do terrestrial sources and groundwater provide most of the gas supply? We assessed the sources, rates and landscape scale magnitude of stream gas emissions in the Northern Highlands Lake District using data collected from 2001-2012. In addition to stream CO2 emissions (~23 Gg C yr-1) that were nearly equivalent to lake emissions (27 Gg C yr-1), we documented some of the highest recorded CH4 fluxes from aquatic systems. Our data support the contention that both metabolism and groundwater control stream gas concentrations, whereas lakes did not affect stream gases through either surface or groundwater connections. Since dissolved gases are non-conservative and are predicted to have short residence times in the water column, we assessed downstream patterns in six streams with similar characteristics (Fig. 2). This seasonal sampling illustrated temporal coherence of CO2 supersaturation that diverges from the prediction of a single upstream CO2 source (i.e., a lake outlet or a groundwater seep) and points to discrete regions of high CO2 which we attribute to groundwater inflow. Assessments of stream metabolism using paired CO2 and O2 time series also indicate the strong influence of groundwater to stream CO2 pools, in addition to net heterotrophy. Our analysis was based solely on measurements of diffusive emissions, but recent work has suggested a dominant role of ebullitive (bubble induced) emissions, especially for CH4. This work further supports previous conclusions that small streams are an important piece of the global carbon cycle, and that more attention to CH4 cycling is warranted in these carbon rich ecosystems.

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