Carbon dioxide dynamics in an agricultural headwater stream driven by hydrologyand primary production

Abstract. Headwater streams are known to be hotspots for carbon dioxide (CO2) emissions to the atmosphere and are hence important components in landscape carbon balances. However, surprisingly little is known about stream CO2 dynamics and emissions in agricultural settings, a land-use type that globally cover ca 40 % of the continental area. Here we present continuously measured in-situ CO2 concentration data from a temperate agricultural headwater stream covering more than one year of open-water season. The stream CO2 concentrations during the entire study period were generally high (median 3.44 mg C L−1, corresponding to partial pressures (pCO2) of 4778 µatm) but were also highly variable (IQR = 3.26 mg C L−1). The CO2 concentration dynamics covered a variety of different time-scales from seasonal to hourly, and with an interplay of hydrological and biological controls. The hydrological control was strong (although with both positive as well as negative influences dependent on season) and CO2 concentrations changed rapidly in response to rainfall and snowmelt events. However, during growing-season baseflow and receding flow conditions, aquatic primary production seemed to control the stream CO2 dynamics resulting in elevated diel patterns. Given the observed high levels of CO2 and its temporally variable nature, agricultural streams clearly need more attention in order to understand and incorporate these considerable dynamics in large scale extrapolations.