Kristin L. Olson

Headwater streams are tightly linked to the terrestrial environments and receive large amounts of terrestrially‐derived carbon. However, in permafrost environments, terrestrial‐aquatic linkages are likely to shift as ongoing permafrost thaw progresses and extent decreases. As solutes exist in varying quantities throughout soil profiles, changing dissolved solutes in streams can be used to identify shifts in terrestrial flow paths. This research aimed to identify terrestrial sources that contribute carbon to the headwaters using a Bayesian end‐member mixing model and 𝛿 ¹³ C‐DIC (stable carbon isotope of dissolved inorganic carbon) to compare source contributions with permafrost distribution. In a boreal catchment underlain by discontinuous permafrost, we found that deep and shallow terrestrial sources have distinct solute concentrations and isotopic signatures compared to the stream surface water. Further, there were differences in source contribution among the sub‐catchments with varying permafrost extents. The stream with the highest catchment‐wide permafrost had the largest amount of shallow soil water contribution to streamflow and experienced the flashiest flow regime. As expected, other headwater streams in catchments with little to no permafrost received higher amounts of deep groundwater to support baseflow than in the high permafrost catchment. Further, we identified multiple deep groundwater sources, which appear to be spatially distinct and contribute to baseflow in different sub‐catchments. We found evidence that permafrost influences terrestrial flow paths and the proportional contribution of terrestrial sources, thus dictating stream chemistry. As permafrost thaw progresses, there will likely be shifts in solute delivery to high‐latitude headwater streams.

The consequences of climate change on boreal ecosystems are evident in declining permafrost extent, amplifying positive climate feedback loops, and altering the timing and intensity of hydrologic events. Thawing permafrost in the discontinuous permafrost zone could affect carbon and nutrient cycling in stream ecosystems. We examined stream chemistry and climate trends over a 20+‐year period across catchments in the Caribou Poker Creeks Research Watershed underlain with varying extents of permafrost (4%–53%). The study aimed to evaluate patterns in dissolved inorganic carbon (DIC, p CO ₂ ), dissolved organic carbon (DOC), nitrogen (Dissolved organic nitrogen, and NO ₃ ⁻ ), geochemical solutes (Ca ²⁺ , Mg ²⁺ , SO ₄ ²⁻ ), and discharge to determine how altered terrestrial flowpaths and climate change‐related trends in temperature and precipitation have transformed solute transport in high‐latitude watersheds during the ice‐free season. We analyzed long‐term trends in stream chemistry using Thiel‐Sen analysis and a mixed effects model to quantify the influence of abiotic factors on solute concentrations. Results indicate significant declines in DOC (−109.0 to −169.9 μg L ⁻¹ yr ⁻¹ ) and p CO ₂ (−24.1 ppmv yr ⁻¹ ) in higher permafrost extent sub‐catchments. The highest permafrost catchment is experiencing the greatest amount of change, contrasting sharply with opposite to fewer trends in the catchments with lower permafrost extent. Model results indicate the importance of moisture conditions and discharge ( p < 0.05), especially for changes in organic solutes. As climate change progresses, the role of these abiotic factors and permafrost thaw will remain important for solute transport dynamics in boreal headwater streams, with consequences for in‐stream communities and downstream solute yields.

Metabolism in stream ecosystems alters the fate of organic carbon (OC) received from surrounding landscapes, but our understanding of in‐stream metabolic processes in boreal ecosystems remains limited. Determining the factors that regulate OC metabolism will help predict how the C balance of boreal streams may 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 OC (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 one plausible explanation. Our results provide some of the first evidence of a functional response to permafrost thaw in stream ecosystems and suggest the role of metabolism in landscape C cycling may increase as climate change progresses.

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.