ABSTRACT:

Stream confluences are ubiquitous interfaces in freshwater networks and serve as junctions of previously independent landscapes. However, few studies have investigated how confluences influence transport, mixing, and fate of organic matter and inorganic nutrients at the scale of river networks. To understand how network biogeochemical fluxes may be altered by confluences, we conducted two sampling campaigns at five confluences in summer and fall 2021 spanning the extent of a mixed land use stream network. We sampled the confluence mainstem and tributary reaches as well as throughout the mixing zone downstream. We predicted that biologically reactive solutes would mix non-conservatively downstream of confluences and that alterations to downstream biogeochemistry would be driven by differences in chemistry and size of the tributary and upstream reaches. In our study, confluences were geomorphically distinct downstream compared to reaches upstream of the confluence. Dissolved organic matter and nutrients mixed non-conservatively downstream of the five confluences. Biogeochemical patterns downstream of confluences were only partially explained by contributing reach chemistry and drainage area. We found that the relationship between geomorphic variability, water residence time, and microbial respiration differed between reaches upstream and downstream of confluences. The lack of explanatory power from network-scale drivers suggests that non-conservative mixing downstream of confluences may be driven by biogeochemical processes within the confluence mixing zone. The unique geomorphology, non-conservative biogeochemistry, and ubiquity of confluences highlights a need to account for the distinct functional role of confluences in water resource management in freshwater networks.

ABSTRACT:

Quantifying organic carbon (OC) removal in streams is needed to integrate the functional role of inland waters into landscape carbon budgets. To illustrate how in-stream OC removal measurements can be used to characterize ecosystem and landscape carbon fluxes, we compared two common methods: (1) bioassays measuring water column dissolved organic carbon (DOC) uptake and (2) daily rates of whole-stream metabolism and OC spiraling calculated from fluorescent dissolved organic matter, oxygen, and discharge measurements. We then assessed how OC removal rates from these two methods, measured in two low-productivity heterotrophic streams, affected estimates of terrestrial OC loading and export using a mass balance model. OC mineralization velocities calculated from whole-stream metabolism (0.06 ±0.03 m d-1 (mean±SD)) were greater than water column bioassay DOC uptake velocities (0.01 ±0.01 m d-1), which resulted in higher in-stream OC removal estimates (0.5-15.2% and 0.02-4.2% removal for whole-stream metabolism and bioassays, respectively). Furthermore, the terrestrial OC inputs needed to sustain in-stream OC concentrations differ among methods, with simulated inputs ranging from 79-1300 or 3-350 g OC d-1 for whole-stream metabolism or bioassays, respectively. We show how in-stream OC removal can be used to quantify terrestrial-aquatic linkages by estimating OC inputs needed to fuel whole-stream metabolism in low-productivity streams, and offer future directions to better link OC removal with whole-ecosystem OC budgets. Without appropriate conversions to whole-stream processes, bioassays systematically underestimate whole-stream carbon cycling. By integrating whole-stream metabolism with OC transport, we can better elucidate the role of running waters in landscape carbon budgets and the global carbon cycle.