Umeå universitets logga

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.