Fluvial organic carbon composition and concentration variability within a peatland catchment—Implications for carbon cycling and water treatment

Fluvial organic carbon (OC) transformations are an important component of carbon cycling and greenhouse gas production in inland waters resulting in considerable recent interest in the fate of fluvial OC exported from carbon rich soils such as peatlands. Additionally, peatland catchments are important drinking water collection areas, where high OC concentrations in runoff have water treatment implications. This analysis presents the results from a year‐round intensive study within a water treatment catchment draining an area of peatland, considering carbon transformations along a continuum from headwater river, through a storage reservoir and pipe, to a water treatment works. The study uses a unique combination of methods (colourmetric, ultrafiltration, and ¹⁴C radiocarbon dating) to assess catchment wide changes in fluvial carbon composition (colour, size, and age) alongside concentration measures. The results indicate clear patterns of carbon transformations in the river and reservoir and dissolved low molecular weight coloured carbon to be most subject to change, with both loss and replacement within the catchment residence time. Although the evidence suggests dissolved OC (DOC) gains are from particulate OC breakdown, the mechanisms of DOC loss are less certain and may represent greenhouse gas losses or conversions to particulate OC. The transformations presented here appear to have minimal impact on the amount of harder to treat (<10 kDa) dissolved carbon, although they do have implications for total DOC loading to water treatment works. This paper shows that peatland fluvial systems are not passive receptors of particulate and dissolved organic carbon but locations where carbon is actively cycled, with implications for the understanding of carbon cycling and water treatment in peatland catchments.