DOWNSTREAM CHANGES IN FREE CARBON-DIOXIDE IN AN UPLAND CATCHMENT FROM NORTHEASTERN SCOTLAND

Significant losses of free carbon dioxide (CO2) to the atmosphere are likely to occur when soil-water, supersaturated with free CO2, enters streams and equilibrates with atmospheric CO2. Spatial changes in dissolved free CO2 downstream from the river source should therefore demonstrate progressive equilibration with atmospheric CO2. Data on the spatial and diurnal variation in the concentration of dissolved free CO2 are described for a small headwater stream draining an acidic heather [Calluna vulgaris (L.) Hull] moorland catchment in northeastern Scotland. The degree to which free CO2 exceeded the concentration expected for atmospheric equilibration decreased rapidly downstream, from an excess partial pressure (epCO(2)) of >10 at the source of the stream to ca, 1.5 over a distance of 2 km downstream, suggesting that free CO2 was being lost from the water by outgassing as the water equilibrated with atmospheric CO2. Diurnal variation of +/-1.0 CO2 units was also measured at the lowest point in the stream, with levels of CO2 being highest during the early morning and late evening (measurements were not taken during times of darkness) and lowest in the period from late morning to midafternoon. An estimate of the flux of C as free CO2 suggests that it comprises ca. 10% of the combined fluxes of dissolved organic carbon (DOG) and particulate organic carbon (POC). These results suggest that outgassing from and transport within river systems of soil-derived CO2 forms an important component of the C flux from terrestrial ecosystems back to the atmosphere or to the ocean.