1. 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.
2. 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.
3. 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.
4. 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.
5. 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.

中文翻译：

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氮供应和物理干扰通过对代谢活动的影响影响北极河流氮的吸收

1. 北极的气候变化正在改变营养物质从陆地到水生生态系统的输送。这些变化对下游湖泊和河流的影响受小溪流保留此类输入的能力的影响。鉴于贫营养北极溪流中养分限制的可能性，生物需求应该很高，除非恶劣的环境条件保持低生物量常备库存，限制养分吸收能力。
2. 我们评估了瑞典北极地区两种截然不同的源头环境中养分吸收的驱动因素：一条河流排出高地苔原，另一条河流排出白桦林冲积谷。在这两个地点，我们使用短期段塞释放和估计的总初级生产速率 (GPP) 和来自连续溶解氧测量的生态系统呼吸，每两周测量一次硝酸盐 (NO ₃ ^- ) 吸收。
3. 流域特征与不同的河流化学和生物特性有关。例如，苔原流保持相对较低的NO ₃ ^-浓度（平均：46 µg N/L）和GPP 速率（0.2 g O ₂  m ^-2 天^-1）。相比之下，桦树林溪流更富含NO ₃ ^- (88 µg N/L) 和生产力更高（GPP：1.7 g O ₂  m ^-2 天^-1）。这些差异对应于更大的面积 NO ₃ ⁻吸收率和增加的 NO ₃ ⁻使用效率（作为吸收速度）在桦树林溪流（最大 192 µg N m ^-2 min ^-1和 96 mm/hr) 与其苔原对应物（最大 52 µg N m ^-2  min ^-1和 49 mm/hr）在 2017 年相比。此外，不同的环境驱动因素预测了这些地点养分吸收的时间模式：非生物因素（例如NO ₃ ^-浓度和排放）与苔原溪流的吸收变化有关，而白桦林溪流中的代谢活动更为重要。
4. 在不同地点之间，吸收指标的变化表明，保留养分脉冲的能力与在更大的空间和时间尺度上控制的养分供应制度以及促进生物量积累和生物需求的栖息地特性有关。
5. 总体而言，栖息地模板对生物潜力的限制决定了这些高纬度河流对未来因气候变暖或人类土地利用引起的养分输入变化做出反应的能力。