DOWNSTREAM CHANGES IN FREE CARBON-DIOXIDE IN AN UPLAND CATCHMENT FROMNORTHEASTERN 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 diss olved free CO2 downstream from the river source should therefore demon strate progressive equilibration with atmospheric CO2. Data on the spa tial 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 Scotla nd. The degree to which free CO2 exceeded the concentration expected f or atmospheric equilibration decreased rapidly downstream, from an exc ess partial pressure (epCO(2)) of >10 at the source of the stream to c a, 1.5 over a distance of 2 km downstream, suggesting that free CO2 wa s being lost from the water by outgassing as the water equilibrated wi th atmospheric CO2. Diurnal variation of +/-1.0 CO2 units was also mea sured at the lowest point in the stream, with levels of CO2 being high est during the early morning and late evening (measurements were not t aken during times of darkness) and lowest in the period from late morn ing 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 sugg est that outgassing from and transport within river systems of soil-de rived CO2 forms an important component of the C flux from terrestrial ecosystems back to the atmosphere or to the ocean.