The Arctic is warming rapidly as a result of global climate change. Permafrost - permanently frozen ground - plays a critical role in shaping arctic ecosystems and stores nearly one half of the global soil organic matter. Therefore, disturbance of permafrost will likely impact the carbon and related biogeochemical processes on local and global scales. In the Alaskan Arctic, fire and thermokarst (permafrost thaw) have become more common and have been hypothesized to accelerate the hydrological export of inorganic nutrients and sediment, as well as biodegradable dissolved organic carbon (BDOC), which may alter ecosystem processes of impacted streams. The biogeochemical characteristics of two tundra streams were quantified several years after the development of gully thermokarst features. The observed responses in sediment and nutrient loading four years after gully formation were more subtle than expected, likely due to the stabilization of the features and the dynamics controlling the hydrologic connectivity between the gully and the stream. The response of impacted streams may depend on the presence of water tracks, particularly their location in reference to the thermokarst and downslope aquatic ecosystem. We found evidence of altered ecosystem structure (benthic standing stocks, algal biomass, and macroinvertebrate composition) and function (stream metabolism and nutrient uptake), which may be attributable to the previous years' allochthonous gully inputs. The patterns between the reference and impacted reaches were different for both stream sites. Rates of ecosystem production and respiration and benthic chlorophyll-a in the impacted reaches of the alluvial and peat-lined streams were significantly lower and greater, respectively, compared to the reference reaches, even though minimal differences in sediment and nutrient loading were detected. Rates of ammonium and soluble reactive phosphorus uptake were consistently lower in the impacted reach at the alluvial site. The observed differences in metabolism, nutrient uptake and macroinvertebrate community composition suggest that even though the geochemical signal diminished, gully features may have long-lasting impacts on the biological aspects of downstream ecosystem function. In a separate study, a suite of streams impacted by thermokarst and fire were sampled seasonally and spatially. Regional differences in water chemistry and BDOC were more significant than the influences of fire or thermokarst, likely due to differences in glacial age and elevation of the landscape. The streams of the older (>700 ka), lower elevation landscape contained higher concentrations of dissolved nitrogen and phosphorus and DOC and lower BDOC compared to the streams of the younger (50-200 ka) landscapes that had lower dissolved nutrient and DOC quantity of higher biodegradability. The findings in this dissertation indicate that arctic stream ecosystems are more resilient than we expected to small-scale, rapidly stabilizing gully thermokarst features and disturbance caused by fire. Scaling the results of these types of studies should consider the size of thermokarst features in relation to the size of impacted rivers and streams. It remains to be determined how general permafrost thaw will affect the structure and function of arctic streams in the future.

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