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Climate change is rapidly reshaping Arctic landscapes through shifts in vegetation cover and productivity, soil resource mobilization, and hydrological regimes. The implications of these changes for stream ecosystems and food webs is unclear and will depend largely on microbial biofilm responses to concurrent shifts in temperature, light, and resource supply from land. To study those responses, we used nutrient diffusing substrates to manipulate resource supply to biofilm communities along regional gradients in stream temperature, riparian shading, and dissolved organic carbon (DOC) loading in Arctic Sweden. We found strong nitrogen (N) limitation across this gradient for gross primary production, community respiration and chlorophyll-a accumulation. For unamended biofilms, activity and biomass accrual were not closely related to any single physical or chemical driver across this region. However, the magnitude of biofilm response to N addition was: in tundra streams, biofilm response was constrained by thermal regimes, whereas variation in light availability regulated this response in birch and coniferous forest streams. Furthermore, heterotrophic responses to experimental N addition increased across the region with greater stream water concentrations of DOC relative to inorganic N. Thus, future shifts in resource supply to these ecosystems are likely to interact with other concurrent environmental changes to regulate stream productivity. Indeed, our results suggest that in the absence of increased nutrient inputs, Arctic streams will be less sensitive to future changes in other habitat variables such as temperature and DOC loading.

The seasonality of gross primary production (GPP) in streams is driven by multiple physical and chemical factors, yet incident light is often thought to be most important. In Arctic tundra streams, however, light is available in saturating amounts throughout the summer, but sharp declines in nutrient supply during the terrestrial growing season may constrain aquatic productivity. Given the opposing seasonality of these drivers, we hypothesized that "shoulder seasons"-spring and autumn-represent critical time windows when light and nutrients align to optimize rates of stream productivity in the Arctic. To test this, we measured annual patterns of GPP and biofilm accumulation in eight streams in Arctic Sweden. We found that the aquatic growing season length differed by 4 months across streams and was determined largely by the timing of ice-off in spring. During the growing season, temporal variability in GPP for nitrogen (N) poor streams was correlated with inorganic N concentration, while in more N-rich streams GPP was instead linked to changes in phosphorus and light. Annual GPP varied ninefold among streams and was enhanced by N availability, the length of ice-free period, and low flood frequency. Finally, network scale estimates of GPP highlight the overall significance of the shoulder seasons, which accounted for 48% of annual productivity. We suggest that the timing of ice off and nutrient supply from land interact to regulate the annual metabolic regimes of nutrient poor, Arctic streams, leading to unexpected peaks in productivity that are offset from the terrestrial growing season.

Climate change in the Arctic is altering the delivery of nutrients from terrestrial to aquatic ecosystems. The impact of these changes on downstream lakes and rivers is influenced by the capacity of small streams to retain such inputs. Given the potential for nutrient limitation in oligotrophic Arctic streams, biotic demand should be high, unless harsh environmental conditions maintain low biomass standing stocks that limit nutrient uptake capacity.

We assessed the drivers of nutrient uptake in two contrasting headwater environments in Arctic Sweden: one stream draining upland tundra and the other draining an alluvial valley with birch forest. At both sites, we measured nitrate (NO₃⁻) uptake biweekly using short-term slug releases and estimated rates of gross primary production (GPP) and ecosystem respiration from continuous dissolved oxygen measurements.

Catchment characteristics were associated with distinct stream chemical and biological properties. For example, the tundra stream maintained relatively low NO₃⁻ concentrations (average: 46 µg N/L) and rates of GPP (0.2 g O₂ m⁻² day⁻¹). By comparison, the birch forest stream was more NO₃⁻ rich (88 µg N/L) and productive (GPP: 1.7 g O₂ m⁻² day⁻¹). These differences corresponded to greater areal NO₃⁻ uptake rate and increased NO₃⁻ use efficiency (as uptake velocity) in the birch forest stream (max 192 µg N m⁻² min⁻¹ and 96 mm/hr) compared to its tundra counterpart (max 52 µg N m⁻² min⁻¹ and 49 mm/hr) during 2017. Further, different sets of environmental drivers predicted temporal patterns of nutrient uptake at these sites: abiotic factors (e.g. NO₃⁻ concentration and discharge) were associated with changes in uptake in the tundra stream, while metabolic activity was more important in the birch forest stream.

Between sites, variation in uptake metrics suggests that the ability to retain pulses of nutrients is linked to nutrient supply regimes controlled at larger spatial and temporal scales and habitat properties that promote biomass accrual and thus biotic demand.

Overall, constraints on biotic potential imposed by the habitat template determined the capacity of these high latitude streams to respond to future changes in nutrient inputs arising from climate warming or human land use.