Carbon (C) emissions from headwater streams are derived from both terrestrial inputs and in-stream metabolism of organic C (OC), but the role of metabolism in boreal stream C fluxes remains uncertain. Determining the factors that regulate OC metabolism will help predict how the C balance of boreal streams will respond to future environmental change. In this study, we addressed the question: what controls OC metabolism in boreal headwater streams draining catchments with discontinuous permafrost? We hypothesized that metabolism is collectively regulated by OC reactivity, phosphorus availability, and temperature, with discharge modulating each of these conditions. We tested these hypotheses using a combination of laboratory experiments and whole-stream ecosystem metabolism measurements throughout the Caribou-Poker Creeks Research Watershed (CPCRW) in Interior Alaska, USA. In the laboratory experiments, respiration and dissolved organic carbon (DOC) removal were both co-limited by the supply of reactive C and phosphorus, but temperature and residence time acted as stronger controls of DOC removal. Ecosystem respiration (ER) was largely predicted by discharge and site, with some variance explained by gross primary production (GPP) and temperature. Both ER and GPP varied inversely with watershed permafrost extent, with an inverse relationship between temperature and permafrost extent providing the most plausible explanation. Our results provide some of the first evidence of a functional response to permafrost thaw in stream ecosystems and suggest that the contribution of metabolism to stream C emissions may increase as climate change progresses.

In this paper we demonstrate that hyporheic exchange can be represented simply as a one-dimensional diffusion process, where the diffusivity decays exponentially with depth into the sediment bed. Based on a meta-analysis of 106 previously published laboratory measurements of hyporheic exchange (capturing a range of bed morphologies, hydraulic conditions, sediment properties, and experimental approaches) we find that the reference diffusivity and mixing length-scale are functions of the permeability Reynolds Number and Schmidt Number. These dimensionless numbers, in turn, can be estimated for a particular stream from the median grain size of the sediment bed and the stream’s depth, slope, and temperature. Application of these results to a seminal study of nitrate removal in 72 headwater streams across the United States, reveals: (1) streams draining urban and agricultural landscapes have a diminished capacity for in-stream and in-bed mixing along with smaller subsurface storage zones compared to streams draining reference landscapes; (2) under steady-state conditions nitrate uptake in the sediment bed is primarily biologically controlled; and (3) under transient conditions (e.g., spills or storms) physical transport processes are likely to influence nitrate removal rates as well. While further research is needed, the simplicity and extensibility of the framework described here should facilitate cross-disciplinary discussions and inform reach-scale studies of pollutant fate and transport and facilitate their scale-up to watersheds and beyond.

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.

Stream confluences are ubiquitous features in freshwater networks, have distinct hydrogeomorphic characteristics relative to upstream tributaries and downstream reaches, and serve as junctions of previously independent streams. Confluences may enhance or disrupt biological processes. How ecosystem functions (e.g., carbon metabolism, nutrient removal) change at confluences remains a knowledge gap in our understanding of the processes controlling water quality at the network-scale. To test how carbon and nutrient cycling may differ between confluences and their tributaries, we estimated dissolved organic carbon (DOC) and PO43- uptake in October 2018 and July 2019 in two tributary reaches as well as downstream of their confluence mixing zone using pulse injections of roasted barley leachate (a standardized, colored DOC source), K2HPO4, and NaCl (a non-bioreactive tracer). We hypothesized that biological processes would be enhanced at confluences due to the delivery and mixing of different microbial communities and/or carbon and nutrient sources. We calculated PO43- and DOC uptake velocities (vf-PO4, vf-DOC) and compared them across sites and season. In October 2018, vf-PO4 in each tributary was 10.2 and 4.9 mm/min while vf-DOC was 0.84 and 0.38 mm/min. vf-PO4 downstream (6.6 mm/min) was lower than vf-PO4 predicted from a mixing model of upstream vf-PO4 and proportional flow contributions of tributaries (10.1 mm/min), suggesting in-stream PO43- uptake was suppressed as a result of confluence mixing. Conversely, vf-DOC downstream (0.94 mm/min) was higher than vf-DOC predicted from a mixing model (0.75 mm/min). This difference in measured and predicted vf-DOC was supported by bioassay experiments, which found enhanced DOC uptake downstream of the mixing zone. DOC uptake within the confluence mixing zone was spatially heterogeneous (0.00 to 0.19 day-1) and varied more within mixing zone transects than among the two tributary reaches. Ongoing analyses are comparing uptake estimates among seasons. Our results suggest that DOC and PO43- uptake at confluences cannot be estimated from tributary DOC and PO43- uptake alone. A critical next step in this work is to identify the mechanisms behind confluence-derived changes in carbon metabolism and nutrient removal across freshwater networks.