Carbon Processing Domains: Seasonal and Spatial Controls on Dissolved Organic Carbon in a Montane River

Abstract Lotic biogeochemical structure and function have been studied extensively, but rivers lack a functional classification system for assessing spatiotemporal variability in solute behavior. Dissolved organic carbon (DOC) is a basal resource for aquatic communities, is chemically complex, and has aquatic and terrestrial sources and sinks. DOC is a large carbon reservoir and quantifying its transport and processing contributes to understanding riverine and global carbon dynamics. Introduction of the Nutrient Processing Domain concept provides a framework for comparing spatiotemporal variation in the relative importance of nutrient sources and sinks along rivers. Here, we extended this approach for DOC by defining carbon processing domains (CPDs) using a reach‐scale mass balance based on upstream and downstream DOC concentrations, discharges, and reach areas. We applied the CPD framework along 216 km of a snowmelt‐dominated montane river in the northern Rocky Mountains, USA, using a 3‐year record comprising 656 DOC measurements. CPD classifications revealed distinct baseflow and snowmelt behavior as river segments transitioned among diluter, enhancer, conduit, compiler, and consumer domains. Breakpoint regressions yielded inferred allochthonous DOC (C tg ) estimates of 259 ± 56 μmol L −1 at low flow and 643 ± 128 μmol L −1 at high flow. A mixed‐effects comparison supported a change in slope near the 0.42 m d −1 operational breakpoint but not an abrupt intercept discontinuity; regime‐specific inferred biological DOC consumption ( F bio ) values are therefore interpreted as descriptive extrapolated intercepts rather than evidence of an instantaneous biological shift. Together, these results show how DOC transport and processing vary systematically with hydrologic regime.