Exports of organic carbon in two river systems in NE Scotland

Abstract

Riverine fluxes of dissolved organic carbon (DOC) and particulate organic carbon (POC) were estimated at 23 sites on two major river systems in NE Scotland, from fortnightly measurements of concentration and flow. The annual flux of DOC and POC increased cumulatively downstream in both the R. Dee and the R, Don. At the lowest, non-tidal site the estimated annual DOC export (95% confidence limits) was 4.1 (± 2.l) x 106 kg per year for the R. Dee and 2.3 (±1.3) x 106 kg per year for the R. Don. POC exports in the two rivers comprised about 10% of the total organic carbon flux in the R. Dee at 0.3 (±0.2) x 106 kg per year and 25% in the R. Don at 0.7 (±0.5) x 106 kg per year. When expressed on a unit area basis, DOC flux estimates were approximately 20 kg ha-1 per year and POC exports were approximately I-5 kg ha-1 per year at main stem sites on both rivers. However, there were considerable variations in fluxes between tributary and headwater sites on the R. Dee. DOC exports ranged from 13.2 to 115.0 kg ha-1 per year and POC exports were between 6.0 and 85.3 kg ha-1 per year. In a sample of 11 R. Dee sub-catchments, annual fluxes of DOC and POC both showed a strong positive correlation with the cover of hill peat in the catchment area, which indicates that areas of hill peat are a major source of organic carbon to the river. In the lower parts of the R. Dee and R. Don catchments, POC export rates during non-storm flow conditions also appeared to be related to the extent of arable land in the catchment.