Measurements of CO2 and CH4 evasion from UK peatland headwater streams

Peatland headwater streams are consistently supersaturated with respect to gaseous C and are known to degas CO2 and CH4 directly to the atmosphere. Using a combination of injection of a purposeful gas tracer (propane) and a soluble tracer (NaCl) we carried out 49 measurements of the gas transfer coefficient on 12 representative stream reaches to quantify the gas transfer rates of CO2 and CH4 in headwater (1st-3rd order) streams draining six UK peatlands. These were compared to measured stream reach physical variables, such as discharge and water travel time. Whilst we found that evasion rates were highly variable in space and time, (gas transfer coefficient of CO2) was positively related to discharge. Individual study sites showed a high degree of variability in gas transfer rates; at all 49 sites median/mean values for were 0.087/0.157 and 0.092/0.176 min(-1). Median/mean instantaneous CO2 and CH4 evasion rates were 133/367 and 0.22/1.45 mu g C m(-2) s(-1), respectively. Methane evasion rates were therefore more than two orders of magnitude lower than CO2, with CH4 invasion (rather than evasion) measured on 37 % of occasions. Our gas flux measurements from peatland headwater streams are higher than values previously used to estimate landscape scale fluxes and emphasise the importance of the evasion flux term in the overall carbon balance.