Abstract
We examined riparian system responses to an extreme rainfall event on 1–4 December 2007, in eleven small watersheds (mean area—13.2 km2) from 2008–2016 at debris flow, high flood, and low flood reaches (all extended overbank flows). Macroinvertebrate responses followed expected outcomes after extreme disturbance including increasing chironomids and other multi-voltine species. A core assemblage of twenty abundant and common species-maintained populations even after debris flow (likely by recolonizing quickly) with total richness during project of 253 including 183 rare species (<0.01 total abundance) supporting an annual turnover of species from 22 to 33%. Primary disturbance changes to habitat were declines in shade and in-channel wood at all reaches, more strongly at debris flow reaches. Macroinvertebrate communities across disturbance intensities became increasingly similar after the storm. Combined effects of the flood reducing channel complexity and previous logging decreasing in-channel wood recruitment from riparian systems, limits habitat complexity. Until this feature of forested watershed streams returns, there appears to be a ceiling on reach scale aquatic biological diversity.
Highlights
Disturbance processes are an integral component in the maintenance of habitat complexity and biological diversity in stream ecosystems
We investigated the influence of disturbance intensity on stream habitat and aquatic macroinvertebrate communities in a set of stream reaches located on managed forest land in western Washington, USA
We focused on physical variables—canopy closure, channel dimensions, and instream features—that could characterize the sample reach and capture the channel attributes most likely to be altered by hydrologic disturbance
Summary
Disturbance processes are an integral component in the maintenance of habitat complexity and biological diversity in stream ecosystems. The opposite is true; frequent, severe disturbance can reduce availability of habitat that develops under conditions of infrequent disturbances and leads to reduced representation of species that rely on those conditions [27] This process of disturbance and recovery is a consistent characteristic of streams in forest ecosystems [2,21,28]. We investigated the influence of disturbance intensity on stream habitat and aquatic macroinvertebrate communities in a set of stream reaches located on managed forest land in western Washington, USA. At these streams, we contrasted patterns of disturbance impact and recovery after a major storm in December 2007. Determining how disturbance severity influences the degree of system alteration (thermal, fine sediment, instream habitat regimes) and rates of biotic recovery were primary objectives of the investigation
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