The Upper Clark Fork River (UCFR), Montana, a mid-order well-lit system with contemporary anthropogenic nitrogen (N) enrichment and natural geogenic sources of phosphorus (P), experiences annual algal blooms that influence ecosystem structure and function. This study was designed to assess the occurrence of riverine algal blooms (RABs) in the UCFR by characterizing the succession of periphyton and biogeochemical conditions following annual snowmelt runoff through autumnal baseflow conditions, and to provide a framework for assessing RAB progression in montane mid-order rivers more broadly. Using a 21-year database (2000–2020) collected over the growing season at three sites, historical assessment of the persistent and recurrent character of RABs in the UCFR showed that the magnitude of the summer bloom was, in part, moderated by snowmelt disturbance. Abundance and growth forms of benthic algae, along with river physicochemistry (e.g., temperature) and water chemistry (N and P concentration), were measured over the course of snowmelt recession for three years (2018–2020) at the same three sites. Results documented the onset of major blooms of the filamentous green algae Cladophora across all sites, commensurate with declines in dissolved inorganic N. Atomic N:P ratios of river water suggest successional transitions from P- to N-limitation associated with mid-season senescence of Cladophora and development of a secondary bloom of N-fixing cyanobacteria, dominated by Nostoc cf. pruniforme. Rates of N-fixation, addressed at one of the sites during the 2020 snowmelt recession, increased upon Cladophora senescence to a maximal value among the highest reported for lotic systems (5.80 mg N/m²/h) before decreasing again to background levels at the end of the growing season. Based on these data, a heuristic model for mid-order rivers responding to snowmelt disturbance suggests progression from phases of physical stress (snowmelt) to optimal growth conditions, to conditions of biotic stress later in the growing season. Optimal growth is observed as green algal blooms that form shortly after peak snowmelt, then transition to stages dominated by cyanobacteria and autochthonous N production later in the growing season. Accordingly, interactions among algal composition, reactive N abundance, and autochthonous N production, suggest successional progression from reliance on external nutrient sources to increased importance of autochthony, including N-fixation that sustains riverine productivity during late stages of snowmelt recession.

蒙大拿州克拉克福克河上游 (UCFR) 是一个光照充足的中级系统，具有当代人为氮 (N) 富集和磷 (P) 天然地源来源，每年都会经历藻华，影响生态系统的结构和功能。本研究旨在通过表征每年融雪径流至秋季基流条件下附生植物和生物地球化学条件的连续性来评估 UCFR 中河流藻华 (RAB) 的发生，并为评估山地中阶 RAB 进展提供框架更广泛的河流。使用在三个地点的生长季节收集的 21 年数据库（2000 年至 2020 年），对 UCFR 中 RAB 的持续性和经常性特征的历史评估表明，夏季水华的程度在一定程度上是，受融雪干扰的影响。在三年（2018-2020）融雪衰退过程中，在同一三个地点测量了底栖藻类的丰度和生长形式，以及河流物理化学（例如温度）和水化学（氮和磷浓度）。结果记录了丝状绿藻大量繁殖的开始所有地点的刚毛藻，与溶解无机氮的下降相称。河水的原子 N:P 比率表明，从 P 限制到 N 限制的连续转变与刚毛藻的季中衰老和固氮蓝藻二次繁殖的发展相关。 ，以发菜为主cf。梅花目。在 2020 年融雪衰退期间，其中一个地点的固氮率在刚毛藻衰老时增加到了据报道的流水系统最高值中的最大值（5.80 mg N/m ²/h)，然后在生长季节结束时再次降低至背景水平。基于这些数据，中级河流响应融雪扰动的启发式模型表明，从物理胁迫（融雪）阶段到最佳生长条件，再到生长季节后期的生物胁迫条件。观察到的最佳生长是在融雪高峰后不久形成绿藻大量繁殖，然后在生长季节后期过渡到以蓝藻和本土氮产生为主的阶段。因此，藻类组成、活性氮丰度和本土氮生产之间的相互作用表明，从依赖外部营养源到本土氮的重要性不断增加，包括在融雪衰退后期维持河流生产力的固氮。