Contrasting Responses among Aquatic Organism Groups to Changes in Geomorphic Complexity Along a Gradient of Stream Habitat Restoration: Implications for Restoration Planning and Assessment

Eliza Hasselquist,Lisa Sandberg,Christer Nilsson,Maria Kahlert,Lina Polvi,Brendan McKie

doi:10.3390/w10101465

Eliza Hasselquist, Lisa Sandberg + Show 4 more

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https://doi.org/10.3390/w10101465

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Abstract

Many stream restoration projects aim to increase geomorphic complexity, assuming that this increases habitat heterogeneity and, thus, biodiversity. However, empirical data supporting these linkages remain scant. Previous assessments of stream restoration suffer from incomplete quantification of habitat complexity, or a narrow focus on only one organism group and/or one restoration measure, limiting learning. Based on a comprehensive quantification of geomorphic complexity in 20 stream reaches in northern Sweden, ranging from streams channelized for timber floating to restored and reference reaches, we investigated responses of macroinvertebrates, diatoms, and macrophytes to multiple geomorphic metrics. Sediment size heterogeneity, which was generally improved in restored sites, favored macroinvertebrate and diatom diversity and macroinvertebrate abundance. In contrast, macrophyte diversity responded to increased variation along the longitudinal stream profile (e.g., step-pools), which was not consistently improved by the restoration. Our analyses highlight the value of learning across multiple restoration projects, both in identifying which aspects of restoration have succeeded, and pinpointing other measures that might be targeted during adaptive management or future restoration. Given our results, a combination of restoration measures targeting not only sediment size heterogeneity, but also features such as step-pools and instream wood, is most likely to benefit benthic biota in streams.

Highlights

Researchers and restoration practitioners expect greater habitat complexity to favor higher species diversity and abundance [1,2,3,4,5]
PCA1 describes a gradient of increasing geomorphic complexity of the stream channel, while PCA2 is associated with large-scale, landscape factors that constrain the potential complexity of a local stream reach
These metrics were a sediment size heterogeneity metric, SedHetX; a planform metric, the coefficient of variation (CV) of channel widths (WidthCV); two longitudinal profile metrics, thalweg sinuosity (ThalSin) and longitudinal roughness (LongRuf) that describe the variation in the stream depth along the longitudinal profile; and the total volume of instream wood (WoodTot; Figure 2)

Summary

Introduction

Researchers and restoration practitioners expect greater habitat complexity to favor higher species diversity and abundance [1,2,3,4,5]. This expectation arises from basic ecological theory, which posits that habitats that are structurally more complex provide a greater range of niches and reduce the likelihood of competitive exclusion, together facilitating species co-existence [1,6]. The presence of more complex habitat structures in streams might support a greater accumulation of biofilm, by increasing the surface area of benthic substrates, and enhance retention of particulate matter [8,9,10,11,12]. The likely importance of complex habitat structures for benthic organisms in streams has long been recognized [13,14,15], especially for macroinvertebrates [16,17]

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