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Arctic streams are highly sensitive to climate change and their biological communities are subjected to considerable changes. Benthic diatom communities are an important basal resource for higher trophic levels in these oligotrophic streams. However, our knowledge on how they respond to stream physicochemical characteristics in the Arctic is limited. The objective of this study was to describe diatom communities’ characteristics and assess how they are related to physicochemical parameters in 15 stream reaches in Northeast Greenland. Overall, we found 36 species across the streams from which Planothidium frequentissimumwas the most abundant followed by Nitzschia palea. Based on physicochemical variables, including water nitrogen concentration, dissolved organic carbon concentrations and conductivity, the sampling sites were separated into three stream types: headwater, valleyand glacial and snowpack runoff streams. Trait analyses showed that the majority of the species belonged to the ‘macro’ cell size category, strongly correlated with higher concentration of nitrogen. Diatom diversity was negatively affected by water velocity with glacial and snowpack runoff streams to be characterized by the lower number of species. Very small variation was explained solely by the physicochemical variables indicating that the distribution of diatom communities is influenced significantly by catchment characteristics. Overall, our study underlines the need for more comprehensive assessments in order to understand how diatom community composition is controlled by larger scale environmental variables in Arctic streams.

Dissolved organic matter (DOM) composition exerts a direct control on its degradation and subsequent persistence in aquatic ecosystems. Yet, under certain conditions, the degradation patterns of DOM cannot be solely explained by its composition, highlighting the relevance of environmental conditions for DOM degradation. Here, we experimentally assessed the relative influence of composition vs. environment on DOM degradation by performing degradation bioassays using three contrasting DOM sources inoculated with a standardized bacterial inoculum under five distinct environments. The DOM degradation kinetics modeled using reactivity continuum models showed that composition was more important than environment in determining the bulk DOM decay patterns. Changes in DOM composition resulted from the interaction between DOM source and environment. The role of environment was stronger on shaping the bacterial community composition, but the intrinsic nature of the DOM source exerted stronger control on the DOM degradation function.

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. 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. 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 High terrestrial vegetation greenness (NDVI) correlates negatively with dissolved inorganic nitrogen concentrations in High Arctic catchments Dissolved inorganic nitrogen decreases and organic nitrogen increases with increased terrestrial vegetation greenness Future climate changes in High Arctic areas may alter the amount and form of nitrogen exported from land to sea