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The Arctic is warming and significant changes to the landscape, including increased vegetative cover ("greening"), are expected in the near future. These landscape changes may alter nitrogen (N) availability in terrestrial, stream, and coastal ecosystems, where production is often N limited, but the exact changes in nutrient cycling are uncertain. Here, we analyzed the relationship between vegetation greenness (i.e., NDVI) and dissolved inorganic (DIN) and organic (DON) concentrations in streams draining 14 headwater catchments (mean 3.6 km2, range 0.4–11 km2) across three samplings in the Zackenberg area, Northeast Greenland. We found large variation in DIN and DON concentrations across the sampled streams. We further show that this variation is correlated to water temperature and catchment NDVI, such that increased vegetation greenness and temperature correlated with lower DIN, and increased greenness also correlated with higher DON concentrations in streams. The results suggest that increased terrestrial vegetation due to rising air temperature could substantially alter dissolved N concentrations and form in streams, with potentially cascading impacts on coastal areas. Plain Language Summary: This study links terrestrial vegetation cover to stream water chemistry in High Arctic headwater catchments. We found significant spatial and seasonal variation in dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen (DON) availability in streams. Nitrate (NO3− ${{\text{NO}}_{3}}^{-}$) concentration in stream water decreases and DON increases with increasing spectral median normalized difference vegetation index (NDVI) in the stream catchment (i.e., proxy for terrestrial vegetation). This shift in the form of available nitrogen (N) has significant implications as terrestrial vegetation is changing quickly in High Arctic areas, which may then alter the amount and the dominant form of N exported from catchments to coastal areas. Key Points: High terrestrial vegetation greenness (NDVI) correlates negatively with dissolved inorganic nitrogen concentrations in High Arctic catchmentsDissolved inorganic nitrogen decreases and organic nitrogen increases with increased terrestrial vegetation greennessFuture climate changes in High Arctic areas may alter the amount and form of nitrogen exported from land to sea [ABSTRACT FROM AUTHOR]

Arctic terrestrial vegetation is responding to warming with higher biomass and productivity, but the effects for freshwater ecosystems are unclear. Biofilm extracellular enzymes are important mediators of organic matter processing in aquatic ecosystems, thus understanding how enzyme activities change with water temperature and organic matter availability is important to assess the effects of climate change. Here, we studied biofilm enzyme activities in six streams differing in their riparian vegetation cover and water thermal regime on Disko Island, Greenland. For all streams, biofilms enzymes showed a low activity to decompose plant material, as expected in the Arctic. However, in contrast to simple polysaccharides, there was a significant increase in the capacity for hemicellulose decomposition, in streams with high-vegetation cover. Moreover, biofilms in high-vegetation streams showed greater autotrophy (chlorophyll a to total organic matter content) and higher phosphatase activity than stream with low-vegetation cover ones. Enzyme temperature sensitivity (measured as the rate of change of enzyme activity by biofilms incubated at 3 and 22 °C) did not change across the streams, but differed among enzymes types. Phosphatase and phenol oxidase enzymes presented the highest sensitivity to water temperature, which suggests that the degradation of organic phosphorous compounds and lignin could be disproportionately enhanced under warming climate scenarios. [ABSTRACT FROM AUTHOR]

Climate change is causing drastic landscape changes in the Arctic, but how these changes modify stream biogeochemistry is not clear yet. We examined how catchment properties influence stream nitrogen (N) and dissolved organic carbon concentrations (DOC) in a high‐Arctic environment. We sampled two contrasting headwater streams (10–15 stations over 4.8 and 6.8 km, respectively) in Northeast Greenland (74°N). We characterized the geomorphology (i.e., bedrock, solifluction and alluvial types) and the vegetation (i.e., barren, fell field, grassland and tundra types) cover of each subcatchment area draining into each sampling station and collected water samples for hydrochemistry characterization. The two sampled streams differed in geomorphology and vegetation cover in the catchment. Aucellaelv catchment was mostly covered by a 'bedrock' geomorphology (71%) and 'fellfield' vegetation (51%), whereas Kæerelv was mostly covered by 'alluvial' geomorphology (65%) and 'grassland' and 'tundra' vegetation (42% and 41% respectively). Hydrochemistry also differed between the two study streams, with higher concentrations of inorganic N forms in Aucellaelv and lower DOC concentrations, compared to Kærelv. The results from the linear mixed model selection showed that vegetation and geomorphology had contrasting effects on stream hydrochemistry. Subcatchments with higher solifluction sheets and limited vegetation had higher nitrate concentrations but lower DOC concentrations. Interestingly, we also found high variability on the production and removal of nitrate across subcatchments. These results indicate landscape controls to nutrient and organic matter exports via flow paths, soil organic matter stocks and nutrient retention via terrestrial vegetation. Moreover, the results suggest that climate change induced alterations to vegetation cover and soil physical disturbance in high‐Arctic catchments will affect stream hydrochemistry, with potential effects in stream productivity, trophic relations as well as change of solute export to downstream coastal areas. [ABSTRACT FROM AUTHOR]

Global change is predicted to have a marked impact on freshwater ecosystems in the High Arctic, including temperature increase, enhanced precipitation, permafrost degradation and increased vegetation cover. These changes in river catchments can alter flow regime, solute transport to streams and substantially affect stream ecosystem functioning. The objective of this study was to evaluate changes in stream functioning in a high-Arctic stream in relation to changes in discharge, and runoff flow path. We measured environmental factors, biofilm structure, nutrient uptake rates and metabolism. We studied three reaches in a headwater stream in NE Greenland with different catchment characteristics in early and late summer (during two different years) to evaluate the potential influence of environmental change on Arctic stream ecosystem functioning. Highest nutrient uptake, primary production and ecosystem respiration was found in late summer showing that streams are more efficient at retaining nutrients and have higher autotrophic production, likely due to less impact of snowmelt, and lower discharge increasing the surface to volume ratio between streambed and water column. Nutrient uptake rates in late summer from high-Arctic tundra streams were comparable to uptake rates in temperate pristine streams, likely due to no shading by bank vegetation and longer days in the high-Arctic summer compared to temperate streams. Overall, the results of this study aids in the endeavor of predicting how climate-derived changes will affect in-stream nutrient uptake and metabolism in high-Arctic streams. The results suggest that their capacity to transport, cycle and retain carbon and nutrient may increase if the importance of soil water flow paths for streams also increase, thus with effect to stream trophic relations and solute export to coastal areas. [ABSTRACT FROM AUTHOR]

In the Arctic, climate changes contribute to enhanced mobilization of organic matter in streams. Microbial extracellular enzymes are important mediators of stream organic matter processing, but limited information is available on enzyme processes in this remote area. Here, we studied the variability of microbial extracellular enzyme activity in high-Arctic fluvial biofilms. We evaluated 12 stream reaches in Northeast Greenland draining areas exhibiting different geomorphological features with contrasting contents of soil organic matter to cover a wide range of environmental conditions. We determined stream nitrogen, phosphorus, and dissolved organic carbon concentrations, quantified algal biomass and bacterial density, and characterized the extracellular enzyme activities involved in catalyzing the cleavage of a range of organic matter compounds (e.g., β-glucosidase, phosphatase, β-xylosidase, cellobiohydrolase, and phenol oxidase). We found significant differences in microbial organic matter utilization among the study streams draining contrasting geomorphological features, indicating a strong coupling between terrestrial and stream ecosystems. Phosphatase and phenol oxidase activities were higher in solifluction areas than in alluvial areas. Besides dissolved organic carbon, nitrogen availability was the main driver controlling enzyme activities in the high-Arctic, which suggests enhanced organic matter mineralization at increased nutrient availability. Overall, our study provides novel information on the controls of organic matter usage by high-Arctic stream biofilms, which is of high relevance due to the predicted increase of nutrient availability in high-Arctic streams in global climate change scenarios.