Title:

Coupled carbon and nitrogen dynamics in Arctic stream ecosystems

Warming and permafrost thaw in the Arctic will continue to change the structure and function of stream ecosystems. Permafrost thaw can release previously stored carbon and nutrients to receiving waters, which may alter aquatic metabolism and biogeochemical cycling. To assess how small Arctic streams may respond to increased carbon (C) and nitrogen (N) availability, we conducted a series of experimental nutrient additions in two streams in northern Sweden that had contrasting light, thermal, and background nutrient regimes (tundra vs. birch forest). At each stream, we conducted four short-term constant rate additions adding ammonium and nitrate individually, with and without a co-release of labile C (acetate). From these additions, we determined nutrient uptake metrics and assessed the response of greenhouse gas (GHG) concentrations. To assess how heterotrophic respiration changed across treatments, we also performed constant rate additions of resazurin. Results from these experiments showed a fast metabolic and biogeochemical response to manipulated C and N availability, especially when both ammonium and labile C were added to the stream. However, a lack of a clear response in GHG concentrations suggest that short-term enrichments may not result in detectable increases in gas production. Altogether, our findings provide insight into the biogeochemical response of Arctic streams to varying stoichiometry of organic C and dissolved inorganic N that may result from permafrost thaw.

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