Is in-stream processing an important control on spatial changes in carbon fluxes in headwater catchments?

Data on small-scale spatial variations in instantaneous fluxes and concentrations of dissolved organic carbon DOC., dissolved inorganic carbon DIC. and free carbon dioxide CO2. are presented for a small acidic headwater stream in NE Scotland. Chloride is used as a conservative element to estimate additional, diffuse inputs of water into the main stem of the stream, other than those from tributaries. Downstream changes in instantaneous carbon fluxes were calculated and then used to estimate losses and gains of carbon within the stream system. Dissolved organic carbon concentrations in the stream ranged from 1.19]6.06 mg ly1 at its source to a maximum of 10.0]25.3 mg ly1 as the stream passed through deep peats; DOC concentrations then declined in the lower part of the catchment. DIC concentrations were initially low, increased to 1.5]3.0 mg ly1 and then decreased to 0.1]1.65 mg ly1 at the lowest site. Free CO2 concentrations increased from 0.35 mg ly1 at the stream source to 3.30 mg ly1 as the stream passed through the peat dominated area. Continually high inputs of CO2-rich water )6.0 mg ly1. from tributaries maintained these high concentrations in the main stem, until approximately 1.74 km downstream, when there was a rapid decline in concentration. Significant changes in DOC, DIC and CO2 fluxes occur over a distance of 2.7 km downstream from the stream source to the catchment outlet. Between 5.64]41.5 mg C sy1 as DOC and 2.52]16.2 mg C sy1 as DIC are removed from the water column. Between 6.81 and 19.0 mg C sy1 as CO2 is lost along the stream length as progressive equilibration with the atmosphere occurs. We estimate that 11.6]17.6% of the total DOC flux is removed from streamwater by in-stream processes. Dissolved inorganic carbon HCO3y and free CO2. losses are in excess of nine times its measured flux at the outlet of the catchment. These results suggest that in-stream processing of DOC and DIC and outgassing of CO2 are important controls on the spatial variability of carbon fluxes within headwater streams in upland catchments dominated by organic-rich soils.