Reiner Giesler

Reindeer Rangifer tarandus , a large circumpolar herbivore, can influence whether nitrogen (N) or phosphorus (P) is the primary limiting nutrient in tundra plant communities. Specifically, findings from a site in northern Scandinavia suggest that under conditions where reindeer grazing stimulates inorganic N availability, grazing may drive ecosystems towards P limitation. However, the mechanisms driving such effects on tundra soil P biogeochemistry remain poorly understood. We conducted a study, where heavily grazed areas locate close to long‐term lightly grazed areas in both meadows and heaths in northern Norway, to assess how reindeer grazing moderates soil inorganic N and P concentrations, and particularly the relationship between P availability and soil P sorption capacity. We measured water extractable inorganic N (NH ₄ ‐N and NO ₃ ‐N) and water extractable P (PO ₄ ‐P) concentrations, P sorption index, and the P‐sorbing elements aluminium (Al) and iron (Fe). Our results demonstrate that high reindeer grazing pressure was related to higher extractable inorganic N concentrations in the organic soil layer in meadows, and a higher extractable inorganic N:P ratio in both vegetation types. Also, the higher reindeer grazing pressure significantly increased the soil P sorption index, which was positively related to soil Al + Fe concentrations and decreased extractable PO ₄ ‐P in the organic layer in the heath. In the meadow, extractable PO ₄ ‐P concentrations remained unchanged while the extractable N:P ratio increased from 21 to 30, thus decreasing the amount of extractable PO ₄ ‐P per unit of inorganic N. We conclude that herbivores shift both habitats in our study area in the direction of P limitation, but the mechanism seems to vary from increased P sorption in the heath to increased N availability in the meadow. This shift in mechanism could be important for understanding how herbivores can influence the relative importance of N and P limitation in ecosystems across the Arctic.

Permafrost degradation, driven by rising temperatures in high‐latitude regions, destabilizes previously sequestered soil organic carbon (OC), increasing greenhouse gas emissions and amplifying global warming. In these ecosystems, interactions with mineral surfaces and metal oxides, particularly iron (Fe), stabilize up to 80% of soil OC. This study investigates the mechanisms of Fe solubilization and OC release across a permafrost thaw gradient in Stordalen, Abisko, Sweden, including palsa, intermediate, and highly degraded permafrost stages. By integrating geophysical measurements—including relative elevation, thaw depth, soil water content, and soil temperature with redox potential and soil pore water chemistry, we identify the environmental conditions driving iron and organic carbon release into soil pore waters with permafrost degradation. Our results show that combining relative elevation, thaw depth, soil water content, soil pore water pH, and soil pore water conductivity with shifts in vegetation species enables very‐high‐resolution detection of permafrost degradation at submeter scales, distinguishing intact from degraded permafrost soils. We show that small‐scale changes in thaw depth and water content alter soil pH and redox conditions, driving the release of Fe and dissolved organic carbon (DOC) and promoting the formation of Fe‐DOC complexes in soil pore water. The amount of exported Fe‐DOC complexes from thawed soils varies with the stage of permafrost degradation, and the fate of Fe‐DOC complexes is likely to evolve along the soil–stream continuum. This study highlights how environmental conditions upon thaw control the type of Fe‐DOC association in soil pore waters, a parameter to consider when quantifying what DOC is available for microbial and photo‐degradation in aquatic systems which are significant sources of greenhouse gas emissions across Arctic landscapes.

The consequences of permafrost thaw for organic carbon release are mainly studied in summer, considering the frozen soil is inert in winter. Here, we show that biogeochemical processes also occur during early winter. We combine Si isotopes and Ge/Si with Fe and dissolved organic carbon (DOC) concentrations in soil porewater along a natural gradient of permafrost degradation (palsa, intermediate and degraded palsa sites) and in river water (Stordalen, Sweden) collected during late autumn and early winter. The data support: (i) the occurrence of early winter snowmelt water infiltration in soils diluting more extensively the soil porewater in dry well-drained palsa soils; (ii) the decrease of the redox potential (by 30 %) induced by snowmelt water infiltration and water table rise at the intermediate site, favouring Fe-oxides dissolution and the release of the associated DOC in soil porewater; (iii) the contribution of snowmelt water infiltration to the Fe and DOC lateral export from permafrost degrading soils to rivers.

The seasonality of gross primary production (GPP) in streams is driven by multiple physical and chemical factors, yet incident light is often thought to be most important. In Arctic tundra streams, however, light is available in saturating amounts throughout the summer, but sharp declines in nutrient supply during the terrestrial growing season may constrain aquatic productivity. Given the opposing seasonality of these drivers, we hypothesized that “shoulder seasons”—spring and autumn—represent critical time windows when light and nutrients align to optimize rates of stream productivity in the Arctic. To test this, we measured annual patterns of GPP and biofilm accumulation in eight streams in Arctic Sweden. We found that the aquatic growing season length differed by 4 months across streams and was determined largely by the timing of ice‐off in spring. During the growing season, temporal variability in GPP for nitrogen (N) poor streams was correlated with inorganic N concentration, while in more N‐rich streams GPP was instead linked to changes in phosphorus and light. Annual GPP varied ninefold among streams and was enhanced by N availability, the length of ice‐free period, and low flood frequency. Finally, network scale estimates of GPP highlight the overall significance of the shoulder seasons, which accounted for 48% of annual productivity. We suggest that the timing of ice off and nutrient supply from land interact to regulate the annual metabolic regimes of nutrient poor, Arctic streams, leading to unexpected peaks in productivity that are offset from the terrestrial growing season.

Arctic and subarctic ecosystems are experiencing substantial changes in hydrology, vegetation, permafrost conditions, and carbon cycling, in response to climatic change and other anthropogenic drivers, and these changes are likely to continue over this century. The total magnitude of these changes results from multiple interactions among these drivers. Field measurements can address the overall responses to different changing drivers, but are less capable of quantifying the interactions among them. Currently, a comprehensive assessment of the drivers of ecosystem changes, and the magnitude of their direct and indirect impacts on subarctic ecosystems, is missing. The Torneträsk area, in the Swedish subarctic, has an unrivalled history of environmental observation over 100 years, and is one of the most studied sites in the Arctic. In this study, we summarize and rank the drivers of ecosystem change in the Torneträsk area, and propose research priorities identified, by expert assessment, to improve predictions of ecosystem changes. The research priorities identified include understanding impacts on ecosystems brought on by altered frequency and intensity of winter warming events, evapotranspiration rates, rainfall, duration of snow cover and lake-ice, changed soil moisture, and droughts. This case study can help us understand the ongoing ecosystem changes occurring in the Torneträsk area, and contribute to improve predictions of future ecosystem changes at a larger scale. This understanding will provide the basis for the future mitigation and adaptation plans needed in a changing climate.

Global warming is enhancing the mobilization of organic carbon (C) from Arctic soils into streams, where it can be mineralized to CO₂ and released to the atmosphere. Abiotic photo‐oxidation might drive C mineralization, but this process has not been quantitatively integrated with biological processes that also influence CO₂ dynamics in aquatic ecosystems. We measured CO₂ concentrations and the isotopic composition of dissolved inorganic C (δ¹³C_DIC) at diel resolution in two Arctic streams, and coupled this with whole‐system metabolism estimates to assess the effect of biotic and abiotic processes on stream C dynamics. CO₂ concentrations consistently decreased from night to day, a pattern counter to the hypothesis that photo‐oxidation is the dominant source of CO₂. Instead, the observed decrease in CO₂ during daytime was explained by photosynthetic rates, which were strongly correlated with diurnal changes in δ¹³C_DIC values. However, on days when modeled photosynthetic rates were near zero, there was still a significant diel change in δ¹³C_DIC values, suggesting that metabolic estimates are partly masked by O₂ consumption from photo‐oxidation. Our results suggest that 6–12 mmol CO₂‐C m⁻² d⁻¹ may be generated from photo‐oxidation, a range that corresponds well to previous laboratory measurements. Moreover, ecosystem respiration rates were 10 times greater than published photo‐oxidation rates for these Arctic streams, and accounted for 33–80% of total CO₂ evasion. Our results suggest that metabolic activity is the dominant process for CO₂ production in Arctic streams. Thus, future aquatic CO₂ emissions may depend on how biotic processes respond to the ongoing environmental change.

Streams play an important role in the global carbon (C) cycle, accounting for a large portion of CO₂ evaded from inland waters despite their small areal coverage. However, the relative importance of different terrestrial and aquatic processes driving CO₂ production and evasion from streams remains poorly understood. In this study, we measured O₂ and CO₂ continuously in streams draining tundra‐dominated catchments in northern Sweden, during the summers of 2015 and 2016. From this, we estimated daily metabolic rates and CO₂ evasion simultaneously and thus provide insight into the role of stream metabolism as a driver of C dynamics in Arctic streams. Our results show that aquatic biological processes regulate CO₂ concentrations and evasion at multiple timescales. Photosynthesis caused CO₂ concentrations to decrease by as much as 900 ppm during the day, with the magnitude of this diel variation being strongest at the low‐turbulence streams. Diel patterns in CO₂ concentrations in turn influenced evasion, with up to 45% higher rates at night. Throughout the summer, CO₂ evasion was sustained by aquatic ecosystem respiration, which was one order of magnitude higher than gross primary production. Furthermore, in most cases, the contribution of stream respiration exceeded CO₂ evasion, suggesting that some stream reaches serve as net sources of CO₂, thus creating longitudinal heterogeneity in C production and loss within this stream network. Overall, our results provide the first link between stream metabolism and CO₂ evasion in the Arctic and demonstrate that stream metabolic processes are key drivers of the transformation and fate of terrestrial organic matter exported from these landscapes.

Streams are important emitters of CO₂ but extreme spatial variability in their physical properties can make upscaling very uncertain. Here, we determined critical drivers of stream CO₂ evasion at scales from 30 to 400 m across a 52.5 km² catchment in northern Sweden. We found that turbulent reaches never have elevated CO₂ concentrations, while less turbulent locations can potentially support a broad range of CO₂ concentrations, consistent with global observations. The predictability of stream pCO₂ is greatly improved when we include a proxy for soil‐stream connectivity. Catchment topography shapes network patterns of evasion by creating hydrologically linked “domains” characterized by high water‐atmosphere exchange and/or strong soil‐stream connection. This template generates spatial variability in the drivers of CO₂ evasion that can strongly bias regional and global estimates. To overcome this complexity, we provide the foundations of a mechanistic framework of CO₂ evasion by considering how landscape process domains regulate transfer and supply.

Climate change is rapidly reshaping Arctic landscapes through shifts in vegetation cover and productivity, soil resource mobilization, and hydrological regimes. The implications of these changes for stream ecosystems and food webs is unclear and will depend largely on microbial biofilm responses to concurrent shifts in temperature, light, and resource supply from land. To study those responses, we used nutrient diffusing substrates to manipulate resource supply to biofilm communities along regional gradients in stream temperature, riparian shading, and dissolved organic carbon (DOC) loading in Arctic Sweden. We found strong nitrogen (N) limitation across this gradient for gross primary production, community respiration and chlorophyll‐a accumulation. For unamended biofilms, activity and biomass accrual were not closely related to any single physical or chemical driver across this region. However, the magnitude of biofilm response to N addition was: in tundra streams, biofilm response was constrained by thermal regimes, whereas variation in light availability regulated this response in birch and coniferous forest streams. Furthermore, heterotrophic responses to experimental N addition increased across the region with greater stream water concentrations of DOC relative to inorganic N. Thus, future shifts in resource supply to these ecosystems are likely to interact with other concurrent environmental changes to regulate stream productivity. Indeed, our results suggest that in the absence of increased nutrient inputs, Arctic streams will be less sensitive to future changes in other habitat variables such as temperature and DOC loading.

CO₂evasion from freshwater lakes is an important component of the carbon cycle. However, the relative contribution from different lake sizes may vary, since several parameters underlying CO₂flux are size dependent. Here we estimated the annual lake CO₂evasion from a catchment in northern Sweden encompassing about 30,000 differently sized lakes. We show that areal CO₂fluxes decreased rapidly with lake size, but this was counteracted by the greater overall coverage of larger lakes. As a result, total efflux increased with lake size and the single largest lake in the catchment dominated the CO₂evasion (53% of all CO₂evaded). By contrast, the contribution from the smallest ponds (about 27,000) was minor (2flux rates and areal contribution of various sized lakes in assessments of CO₂evasion at the landscape scale.