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Headwater streams are important conduits for terrestrial carbon (C) and nutrients and are shaped and characterised by the landscape they drain. They evade significant amounts of carbon dioxide (CO₂) to the atmosphere relative to their surface area and receive C and nutrients from land, which affect stream functions such as metabolism from biofilms and CO₂ evasion. In boreal and Arctic zones, these processes are regarded to be largely controlled by factors such as temperature, discharge, and forestry. These factors vary in space and time, yet how they alter stream C and nutrient cycling is still poorly constrained. This thesis therefore asks how catchment variability affects nutrient limitation patterns and C fluxes, as well as how these effects are related to groundwater input and altered by forestry in Swedish boreal and Arctic headwater streams. 

I found that the nutrient limitation status of stream biofilms can vary from nitrogen to phosphorous limitation within an Arctic catchment. In a boreal headwater stream, I show that CO₂ evasion rates varied up to 12-fold along a relatively short distance. Using radon as tracer for groundwater in the same reach, I also show that the majority of CO₂ evaded to the atmosphere was supplied by groundwater input. Across the open water season, I show that 17% of stream C was lost through CO₂ evasion to the atmosphere compared to 83% transported downstream. When combining vertical and horizontal C fluxes, I found that the reach scale C evasion:export ratio varies from 0.03 to 0.37 and was mainly controlled by discharge. Further, clear-cutting parts of the forest around a boreal stream increased stream water CO₂ concentrations as well as CO₂ evasion in the first two years after clear-cutting. Moreover, I found that the diel difference between nighttime and daytime CO₂ evasion increased in the second year after clear-cutting in concert with increased nitrogen and phosphorous concentrations which suggest elevated CO₂ cycling by metabolic processes. Overall, this thesis provides important insights on the spatiotemporal catchment variation and drivers of nutrient limitation and C fluxes in streams, and how hydrology and land-use alter the magnitude and timing of important C fluxes in northern landscapes. Ultimately, the thesis highlights spatiotemporal variability of nutrient limitation within a single catchment and adds an important puzzle piece to estimates of C fluxes along boreal headwater systems, as well as how forestry can alter these fluxes.