The carbon dioxide evasion cycle of an intermittent first-order stream: contrasting water–air and soil–air exchange

Ephemeral streams and wetlands are characterized by complex cycles of submersion and emersion, which influence the greenhouse gas flux rates. In this study we quantify the spatiotemporal variability in CO2 and CH4 concentrations and fluxes of an intermittent first-order stream over three consecutive wet and dry cycles spanning 56 days, to assess how hydrologic phase transitions influence greenhouse gas evasion. Water column excess CO2ranged from −11 to 1600 μM, and excess CH4 from 1 to 15 μM. After accounting for temporal changes in the ratio of wet versus dry streambed hydraulic radius, total CO2–C fluxes ranged from 12 to 156 mmol m−2 day−1, with an integrated daily mean of 61 ± 25 mmol m−2 day−1. Soil–air evasion rates were approximately equal to those of water–air evasion. Rainfall increased background water–air CO2–C fluxes by up to 780% due to an increase in gas transfer velocity in the otherwise still waters. CH4–C fluxes increased 19-fold over the duration of the initial, longer wet-cycle from 0.1 to 1.9 mmol m−2 day−1. Temporal shifts in water depth and site-specific ephemerality were key drivers of carbon dynamics in the upper Jamison Creek watercourse. Based on these findings, we hypothesise that the cyclic periodicity of fluxes of biogenic gases from frequently intermittent streams (wet and dry cycles ranging from days to weeks) and seasonally ephemeral watercourses (dry for months at a time) are likely to differ, and therefore these differences should be considered when integrating transient systems into regional carbon budgets and models of global change.