Greenhouse gas flux from headwater streams in New Hampshire, USA: Patterns and drivers

Abstract Freshwater ecosystems can be considerable sources of greenhouse gases (GHG), however, much uncertainty remains in global estimates and understanding of drivers of these emissions. Furthermore, headwater streams have received insufficient attention and may contribute disproportionately to global GHG flux. Our objective was to quantify carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O) flux and assess the impact of changes in dissolved organic carbon (DOC) and NO 3 concentrations on GHG flux in three streams in the Lamprey River watershed in New Hampshire, U.S.A., that contrast in surface water DOC: NO 3 . We measured DOC, NO 3 and dissolved gas partial pressures and estimated gas flux in surface waters monthly from May 2011 to April 2012. We found higher GHG partial pressures and fluxes in the two streams with high DOC concentrations. The stream with high DOC and high NO 3 showed high N 2 O and low CH 4 flux, while the high DOC, low NO 3 stream showed high CH 4 and low N 2 O flux. Our results support a model in which C inputs drive total GHG production, while NO 3 input regulates the relative importance of CH 4 and N 2 O, likely by suppressing methanogenesis and stimulating denitrification. Results suggest streams in this region are small sources of CO 2 , but potentially important sources of CH 4 and N 2 O. Since CH 4 and N 2 O are more powerful than CO 2 at trapping heat in the atmosphere, freshwater emissions of these gases have the potential to partially offset climate benefits of terrestrial carbon sinks, a possibility that has not been sufficiently incorporated into climate models.