Bubble trap construction, deployment, and sampling
Bubble traps were installed in each stream to estimate CH4 emissions via ebullition. The traps consisted of a 25 cm diameter plastic funnel fitted with a 60 mL plastic syringe and three-way stopcock, all sealed with water-tight sealant. To install a trap in the stream channel, a 1 m long steel stake was hammered into the stream bottom and a trap was affixed to the stake by plastic zip-ties. Locations for bubble trap deployment, hereafter referred to as patches, were selected for each stream. An initial patch near a long-term monitoring location for water quality was established at each stream, and subsequent patches were distributed approximately every 10 to 15 m upstream or downstream. Four patches were located at Cart Creek, Dube Brook, and Sawmill Brook, and three patches were chosen at College Brook due to limited access to this stream. We assume patches are independent of one another and the distribution of patches is therefore the representative of the stream reach apart from avoidance of rocky substrates. Two seasons of monitoring were performed at Cart Creek and Sawmill Brook, and one season at College Brook and Dube Brook. Bubble traps were visited 1-2 times per week throughout the observation periods. The volume of displaced water at each trap was recorded, indicating total ebullition volume, and syringe volumes larger than 5 mL were collected via an additional syringe and stored for analysis of methane concentration
Gas concentration analysis
All gas samples were analyzed in the Trace Gas Biogeochemistry Laboratory at the University of New Hampshire. Ebullitive gas samples were always analyzed for CH4, and when enough sample was available, for CO2 and nitrous oxide (N2O) as well. The concentration in parts per million (ppmv) of CH4 was determined by analysis with a Shimadzu Gas Chromatograph Flame Ionization Detector (GC-FID; Treat et al. 2007), CO2 (ppmv) using an infrared gas analyzer (LI-6252 CO2 InfraRed Gas Analyzer [IRGA]), and N2O using a Shimadzu GC-8A with an electron capture detector (GC-ECD). Methane was standardized using the average area response of 10 injections of a standard gas mixture (Northeast Airgas, 2.006 ppmv or Maine Oxy, 1000 ppmv) to determine an instrument precision of analysis (Frolking & Crill, 1994). For CO2, an instrumentation response factor for the IRGA was identified by first using a linear regression analysis to determine the slope and y-intercept of the standards (Northeast Airgas, 980.9 ppmv). Triplicate standards were run by injecting incremental volumes of CO2 standard gas (1, 3, 4, 5, 10 mL; Treat et al. 2014). Finally, for N2O (ppmv) triplicate injections of three standard gases (0.267, 0.638, and 1.98 ppmv) were used to develop a standard curve response.
Flux calculations
Not all measured ebullitive gas volumes were analyzed for gas concentration (Table S1). To estimate the gas flux of these samples, we implemented bootstrap resampling to assign concentrations to these unanalyzed samples (Treat et al. 2018). We randomly sampled from the population of analyzed ebullitive methane concentrations at a stream with replacement to assign a concentration to any non-measured sample volume. This sampling was repeated 1,000 times and the resulting median concentration calculated for each missing ebullitive gas sample was used in analysis. The ebullitive flux was then calculated as the mass of methane emitted per sampling area per unit time:
Ebullitive flux =(Gas concentration × Volume captured)/(Area of bubble trap × Time since last measurement) (1)
A flux was calculated for each trap for each observation period. The mean, standard deviation, median, minimum, and maximum of all traps and all measurements at a stream over a 15-day period is presented here.
