Abstract
Sediment bypass tunnels (SBTs) are guiding structures used to reduce sediment accumulation in reservoirs during high flows by transporting sediments to downstream reaches during operation. Previous studies monitoring the ecological effects of SBT operations on downstream reaches suggest a positive influence of SBTs on riverbed sediment conditions and macroinvertebrate communities based on traditional morphology-based surveys. Morphology-based macroinvertebrate assessments are costly and time-consuming, and the large number of morphologically cryptic, small-sized and undescribed species usually results in coarse taxonomic identification. Here, we used DNA metabarcoding analysis to assess the influence of SBT operations on macroinvertebrates downstream of SBT outlets by estimating species diversity and pairwise community dissimilarity between upstream and downstream locations in dam-fragmented rivers with operational SBTs in comparison to dam-fragmented (i.e., no SBTs) and free-flowing rivers (i.e., no dam). We found that macroinvertebrate community dissimilarity decreases with increasing operation time and frequency of SBTs. These factors of SBT operation influence changes in riverbed features, e.g. sediment relations, that subsequently effect the recovery of downstream macroinvertebrate communities to their respective upstream communities. Macroinvertebrate abundance using morphologically-identified specimens was positively correlated to read abundance using metabarcoding. This supports and reinforces the use of quantitative estimates for diversity analysis with metabarcoding data.
Highlights
Catchment-based transformations from anthropogenic channel modifications such as reservoir and dam construction alter the physical, chemical and biological structure and function of rivers[1,2]
Auel et al.[17] observed macroinvertebrate community composition using the Bray-Curtis similarity index in upstream and downstream reaches at a dam with sediment bypass tunnels (SBTs) to increase in similarity over time since operation
We report a positive influence of SBT operation on macroinvertebrate assemblages downstream of SBT outlets at two rivers (Pfaffensprung and Egschi)
Summary
Catchment-based transformations from anthropogenic channel modifications such as reservoir and dam construction alter the physical, chemical and biological structure and function of rivers[1,2]. Traditional morphology-based macroinvertebrate assessments are constrained not just by time and survey costs, but more importantly by the large number of morphologically cryptic, relatively small-sized and undescribed species[18], resulting in coarse taxonomic level identifications[19] To mitigate these limitations, DNA metabarcoding has been used successfully and proven faster, less expensive, and more accurate for evaluating biotic diversities and distributions at higher taxonomic resolution than morphology-based assessments[20,21,22,23]. We hypothesized that SBT operations improve sediment conditions in residual reaches below dams, thereby positively effecting macroinvertebrate assemblages To test this hypothesis, we sampled three dam-fragmented rivers with SBTs in Switzerland with different operation time spans (2 to 92 years) and operation frequencies We used metabarcoding data from macroinvertebrate larval samples to identify species, estimate richness and diversity, and calculate community dissimilarities between upstream and downstream points in each river
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