Alder (Alnus spp.) and Pacific salmon (Oncorhynchus spp.) provide key nutrient subsidies to freshwater systems. In southwestern Alaska, alder-derived nutrients (ADNs) are increasing as alder cover expands in response to climate warming, while climate change and habitat degradation are reducing marine-derived nutrients (MDNs) in salmon-spawning habitats. To assess the relative influences of ADN and MDN on aquatic microbial community structure and function, we analyzed lake chemistry, bacterial community structure, and microbial metabolism in 13 lakes with varying alder cover and salmon abundance in southwestern Alaska. We conducted bioassays to determine microbial nutrient limitation and physical factors modulating microbial response to nutrient inputs (+N, +P and +NP treatments). Seasonal shifts in bacterial community structure (F = 7.47, P < 0.01) coincided with changes in lake nitrogen (N) and phosphorus (P) concentrations (r² = 0.19 and 0.16, both P < 0.05), and putrescine degradation (r² = 0.13, P = 0.06), suggesting the influx and microbial use of MDN. Higher microbial metabolism occurred in summer than spring, coinciding with salmon runs. Increased microbial metabolism occurred in lakes where more salmon spawned. Microbial metabolic activity was unrelated to alder cover, likely because ADN provides less resource diversity than MDN. When nutrients were added to spring samples, there was greater substrate use by microbial communities from lakes with elevated Chl a concentrations and large relative catchment areas (β estimates for all treatments > 0.56, all P < 0.07). Thus, physical watershed and lake features mediate the effects of nutrient subsidies on aquatic microbial metabolic activity.

桤木 ( Alnus spp.) 和太平洋鲑鱼 ( Oncorhynchusspp.) 为淡水系统提供关键的养分补贴。在阿拉斯加西南部，随着气候变暖导致桤木覆盖范围扩大，桤木衍生的营养物质 (ADN) 正在增加，而气候变化和栖息地退化正在减少鲑鱼产卵栖息地的海洋衍生营养物质 (MDN)。为了评估 ADN 和 MDN 对水生微生物群落结构和功能的相对影响，我们分析了阿拉斯加西南部 13 个桤木覆盖和鲑鱼丰度不同的湖泊中的湖泊化学、细菌群落结构和微生物代谢。我们进行了生物测定以确定微生物营养限制和调节微生物对营养输入（+N、+P 和 +NP 处理）反应的物理因素。细菌群落结构的季节性变化 ( F  = 7.47, P < 0.01) 与湖泊氮 (N) 和磷 (P) 浓度 ( r ²  = 0.19 和 0.16, P  < 0.05) 和腐胺降解 ( r ²  = 0.13, P  = 0.06) 的变化相吻合，表明流入和MDN的微生物用途。夏季发生的微生物代谢高于春季，与鲑鱼奔跑相吻合。在更多鲑鱼产卵的湖泊中，微生物代谢增加。微生物代谢活动与桤木覆盖无关，可能是因为 ADN 提供的资源多样性低于 MDN。当向春季样品中添加营养物质时，来自叶绿素a升高的湖泊的微生物群落会更多地使用底物浓度和较大的相对集水区（所有处理的 β 估计值 > 0.56，所有P  < 0.07）。因此，物理流域和湖泊特征调节养分补贴对水生微生物代谢活动的影响。