Carbon dioxide transport across the hillslope-riparian-stream continuum in a boreal headwater catchment

Abstract

Headwater streams export CO 2 as lateral downstream export and vertical evasion from the stream surface. CO 2 in boreal headwater streams generally originates from adjacent terrestrial areas, so determining the sources and rate of CO 2 transport along the hillslope-riparian-stream continuum could improve estimates of CO 2 export via the aquatic pathway, especially by quantifying evasion at higher temporal resolutions. Continuous measurements of dissolved CO 2 concentrations and water table were made along the hillslope-riparian-stream continuum in the Västrabäcken sub-catchment of the Krycklan catchment, Sweden. Daily water and CO 2 export from the hillslope and riparian zone were estimated over one hydrological year (October 2012-September 2013) using a flow-concentration model and compared with measured lateral downstream CO 2 export. Total water export over the hydrological year from the hillslope was 230 mm yr -1 compared with 270 mm yr -1 from the riparian zone. This corresponds well (proportional to the relative upslope contributing area) to the annual catchment runoff of 265 mm yr -1. Total CO 2 export from the riparian zone to the stream was 3.0 g CO 2 -C m -2 yr -1. A hotspot for riparian CO 2 export was observed at 30-50 cm depth (accounting for 71 % of total riparian export). Seasonal variability was high with export peaks during the spring flood and autumn storm events. Downstream lateral CO 2 export (determined from stream water dissolved CO 2 concentrations and discharge) was 1.2 g CO 2 -C m -2 yr -1. Subtracting downstream lateral export from riparian export (3.0 g CO 2 -C m -2 yr -1) gives 1.8 g CO 2 -C m -2 yr -1 which can be attributed to evasion losses (accounting for 60 % of export via the aquatic pathway). The results highlight the importance of terrestrial CO 2 export, especially from the riparian zone, for determining catchment aquatic CO 2 losses and the importance of the CO 2 evasion component to carbon export via the aquatic conduit.