1. Although most lotic ecosystems experience frequent and sometimes large disturbances, opportunities are uncommon to study primary succession in streams. Exceptions include new stream channels arising from events such as glacial retreat, volcanism, and catastrophic landslides. In 1980, the eruption and massive landslide at Mount St. Helens (WA, U.S.A.) created an entire landscape with five new catchments undergoing primary succession. We asked if riparian and lotic assemblages at early successional stages (36 years after the eruption) showed predictable change along longitudinal gradients within catchments, and whether assemblages were similar among five replicate catchments.
2. In July 2016, we collected environmental data and characterised riparian, algal, and benthic macroinvertebrate assemblages at 21 stream reaches distributed within and among five neighbouring catchments. We evaluated patterns of richness, abundance, biomass, multivariate taxonomic community structure, and functional traits both longitudinally and among catchments.
3. We found minimal evidence that longitudinal gradients had developed within catchments at 36 years post‐eruption. Increases in diatom and macroinvertebrate richness with downstream distance were the only biological responses with longitudinal trends. Conversely, we documented substantial variation in community structure of riparian plants, soft‐bodied algae, diatoms, and macroinvertebrates at the among‐catchment scale. Among‐catchment differences consistently separated two eastern catchments from three western catchments, and these two groups also differed in stream water chemistry, water temperature, and geomorphology.
4. Overall, we documented greater diversity in the young catchments than predicted by ecologists in the years immediately following the eruption, yet functional traits indicate that these catchments are still in relatively early stages of succession. Variation at the among‐catchment scale is likely to be driven in part by hydrological source variation, with the two eastern catchments showing environmental signatures associated with glacial ice‐melt and the three western catchments probably fed primarily by springs from groundwater aquifers. Contemporary flow disturbance regimes also varied among catchments and successional trajectories were probably reset repeatedly in streams experiencing more frequent disturbance.
5. Similar to new stream channels formed following glacial retreat, our results support a tolerance model of succession in streams. However, contrasting abiotic templates among Mount St. Helens catchments appear to be driving different successional trajectories of riparian plant, algal, and macroinvertebrate assemblages among neighbouring small catchments sharing the same catastrophic disturbance history.

中文翻译：

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美国圣海伦斯山主要演替景观中河岸和溪流组合的变化

1. 尽管大多数抽水生态系统经常遭受干扰，有时甚至遭受大的干扰，但研究河流中主要演替的机会却很少。例外情况包括冰川退缩，火山活动和灾难性山体滑坡等事件引起的新河道。1980年，圣海伦斯山（美国华盛顿州）的喷发和大规模滑坡创造了一个完整的景观，五个新集水区正在进行一次演替。我们询问了在演替初期（喷发后36年）的河岸和抽水组合是否在集水区内沿纵向梯度显示出可预测的变化，以及五个重复集水区的组合是否相似。
2. 2016年7月，我们收集了环境数据，并对分布在五个相邻流域内和之间的21个河段的河岸，藻类和底栖大型无脊椎动物集合进行了特征分析。我们评估了纵向，集水区的丰富度，丰度，生物量，多元生物分类群落结构和功能性状的模式。
3. 我们发现极少的证据表明，喷发后36年流域内出现了纵向梯度。硅藻和大型无脊椎动物的丰富性随着下游距离的增加而增加，是唯一具有纵向趋势的生物学反应。相反，我们记录了沿流域规模的河岸植物，软体藻，硅藻和大型无脊椎动物的群落结构有很大变化。集水区之间的差异始终将两个东部集水区与三个西部集水区区分开，并且这两组在溪流水化学，水温和地貌方面也有所不同。
4. 总体而言，我们记录到，在爆发后的几年中，年轻集水区的多样性比生态学家所预测的要大，但是功能性状表明这些集水区仍处于相对早期的演替阶段。流域间尺度的变化可能部分是由水文资源变化引起的，其中两个东部流域显示出与冰川融冰有关的环境特征，而三个西部流域则可能主要由地下水含水层的泉水喂养。流域之间的当代水流扰动机制也各不相同，在遭受更频繁扰动的河流中，演替轨迹可能会反复重置。
5. 与冰川退缩后形成的新河道相似，我们的结果支持了河道演替的耐受模型。然而，在圣海伦斯山流域中非生物模板的对比似乎正在驱动沿河地区的植物，藻类和大型无脊椎动物组合的不同演替轨迹，这些流域在具有相同灾难性骚扰历史的相邻小流域之间。