Stable isotope analysis reveals lower-order river dissolved inorganic carbon pools are highly dynamic

River systems draining peaty catchments are considered a source of atmospheric CO2,thus understanding the behavior of the dissolved inorganic carbon pool (DIC) is valuable. The carbon isotopic composition, delta C-13(DIC), and concentration, [DIC], of fluvial samples collected diurnally, over 14 months, reveal the DIC pools to be dynamic in range (-22 to -4.9 parts per thousand,0.012 to 0.468 mmol L-1 C), responding predictably to environmental influences such as changing hydrologic conditions or increased levels of primary production. 6180 of dissolved oxygen (DO) corroborates the delta C-13(DIC) interpretation. A nested catchment sampling matrix reveals that similar processes affect the DIC pool and thus delta C-13(DIC) across catchment sizes. Not so with [DIC]: at high flow, the DIC export converges across catchment size, but at low flow catchments diverge in their DIC load. Contextualizing delta C-13 with discharge reveals that organic soil-waters and groundwaters comprise end member sources, which in varying proportions constitute the fluvial DIC pool. Discharge and pH describe well [DIC] and delta C-13(DIC), allowing carbon to be apportioned to each end-member from continuous profiles, demonstrated here for the hydrological year 2003-2004. This approach is powerful for assessing whether the dynamic response exhibited here is ubiquitous in other fluvial systems at the terrestrial aquatic interface or in larger catchments.