| Summary: | Arctic and sub-Arctic freshwaters are currently experiencing substantial ecosystem changes due to the effects of global warming. Global warming effects on these freshwaters include increasing water temperatures, altered hydrological patterns, shifts in riparian vegetation and changes in the export of nutrients and carbon from soils. How these alterations to the physical and chemical hab-itat will affect stream ecosystem functioning largely depends on the responses by autotrophic pro-ducers and heterotrophic primary consumers. In this thesis, I explore how key stream ecosystem processes such as metabolic rates and nutrient cycling vary as a function of climate and landscape drivers, particularly light, temperature, and nutrient and carbon availability. To do this I leveraged natural gradients in vegetation, altitude, disturbance, and precipitation throughout the year in northern Sweden, as well as long- and short-term manipulations of nutrient availability. I also synthesized nutrient limitation data from lakes and streams to more holistically assess the re-sponses of boreal to Arctic freshwaters to changes in nutrients and climate variables. I found that nutrient availability, and especially nitrogen (N), is a main driver of spatial and temporal patterns of biofilm productivity, whole system metabolic rates, and short term N uptake in Arctic and sub-Arctic streams. I also show the importance of light and temperature constraints during early spring and late autumn, which set the limit for the aquatic growing season and annual productivity pat-terns. I present a first comparison of combined drivers of lake and stream responses to nutrient addition, which points to a shared importance of N and phosphorus (P) rather than light or tem-perature in driving the magnitude of nutrient limitation across these systems. Ultimately, I pro-pose that across large ranges in habitat variables, widespread nutrient limitation of Arctic fresh-waters constrain other climate change effects on ecosystem functions. The results presented ...
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