Carbon Dioxide and Methane Dynamics in a Peatland Headwater Stream: Origins, Processes and Implications

First-order streams flowing through peatlands receive, carry and transform large amounts of organic carbon, methane (CH₄) and carbon dioxide (CO₂) but remain poorly documented. The objectives of this study were to (a) identify the origins of CO₂ and CH₄ (thereafter C-GHG for carbon greenhouse gases) in a peatland headwater stream, (b) determine the environmental factors driving C-GHG export and emissions, and (c) quantify C-GHG losses from this system and discuss its implications. Data were collected from eight sampling sites along a 3 km boreal peatland headwater stream (Eastern Canada) over the growing seasons 2019 and 2020. The studied stream was oversaturated in pCO₂ [min: 2,044; max: 23,306 μatm] and pCH₄ [14; 17,614 μatm]. A mass balance model showed that ∼81% of in-stream CO₂ originated from porewater seepage while the remaining 17% and 2% originated from in-stream productivity and methane oxidation, respectively. Porewater seepage was concluded to be the primary source of CH₄. Seasonal dissolved C-GHG concentrations were negatively correlated with the peatland water table depth, suggesting an active release of carbon-rich peat porewater during the base flow. Nevertheless, greater C-GHG losses occurred during stormflow periods which acted as pulses with most of the C-GHG being shunted downstream. The sum of C-GHG export and emissions at our site was 8.08 gC m⁻² y⁻¹ with 86% being released to the atmosphere and 14% being exported downstream. Our study demonstrates that peatland headwater streams act as large sources of C-GHG and that precipitation events and topography control the magnitude of the fluxes.