Carbon processing domains: Seasonal and spatial controls on dissolved organic carbon in a montane river

The organization of lotic biogeochemical structure and function has been a major field of study for years, but it lacks a functional classification system to assess spatiotemporal variability of chemical species. Dissolved organic carbon (DOC) is one important solute that acts as a basal resource for aquatic communities, is chemically complex, and has various aquatic and terrestrial sources and sinks. DOC is also a large carbon reservoir and therefore quantifying its sources and sinks is a significant contribution to understanding global carbon dynamics. The recent introduction of the Nutrient Processing Domain (NPD) concept provides a framework for comparing spatiotemporal variation in the relative importance of sources and sinks of DOC along a river. We applied the NPD approach to define carbon processing domains (CPD) for DOC along 216 km of a snowmelt-dominated montane river in the northern Rocky Mountains (USA). Variation in CPD characterizations illustrate the spatiotemporal changes in the primary drivers of DOC abundance in a 3-year record (n=656). The CPD reveal that baseflow and snowmelt comprise distinct hydrologic regimes where DOC biogeochemistry markedly differs. River segments transitioned among several DOC processing domains (i.e., diluter, enhancer, conduit, compiler, and consumer) depending on seasonal changes in hydraulic conditions. During the low flow season, modeled allochthonous inputs were lower concentrations compared to snowmelt high flow season when concentrations were elevated, and biological consumption was significant across all modeled reaches. A conceptual model based on these observations helps visualize mechanisms for seasonal changes in hydrologic regime and associated changes in DOC biogeochemistry.