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.

中文翻译：

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营养物质影响北极河流的季节性代谢模式和总生产力

流中的初级生产总值（GPP）的季节性是由多种物理和化学因素驱动的，但通常认为入射光是最重要的。然而，在北极苔原流中，整个夏季都有足够的光量，但是在陆地生长期营养物供应的急剧下降可能会限制水生生产力。考虑到这些驱动因素的季节性相反，我们假设春季和秋季的“肩膀季节”代表了关键时刻的时间窗口，当光线和养分对齐以优化北极河流的生产率时。为了测试这一点，我们测量了北极北极瑞典8条河流中GPP和生物膜积累的年度模式。我们发现水生生长期长度在溪流之间相差4个月，主要取决于春季的冰冻时间。在生长季节，GPP中氮（N）贫流的时间变异性与无机氮浓度相关，而在氮含量更高的流中，GPP则与磷和光的变化有关。年度GPP在河流之间变化了9倍，并且由于N的可用性，无冰期的长度和低洪水频率而得到提高。最后，GPP的网络规模估算突出了肩部季节的总体意义，其占年生产力的48％。我们建议，结冰的时间和来自陆地的养分供应相互影响，以调节养分贫乏，北极溪流，