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, delta13C(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% per hundred, 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. delta(18)O of dissolved oxygen (DO) corroborates the delta(13)C(DIC) interpretation. A nested catchment sampling matrix reveals that similar processes affect the DIC pool and thus delta(13)C(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(13)C 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(13)C(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.