The overlooked contribution of a fish bypass channel to the density and diversity of macroinvertebrate drift in a heavily modified river system

Abstract

The freshwater biodiversity decline affects all trophic levels, prompting restoration efforts. While habitat restoration for fish is well-studied, habitat restoration to benefit other sensitive taxa such as Ephemeroptera, Plecoptera, and Trichoptera (EPT) remains poor. Consequently, this study investigated the habitat function of a man-made naturelike bypass system for EPT taxa in a highly modified alpine river with several drift feeding fishes. Spatially and temporally distributed drift samples were used to test the hypotheses that the bypass (a) enhances the density of drifting individuals compared to the main river and (b) diversifies drift assemblages compared to the main river. Mean drift density of EPT taxa was lowest in the main stem of the river close to the inlet of the bypass (38.24 Ind./100m 3 ) and significantly increased toward the downstream direction of the bypass, near the outflow (61.49 Ind./100m 3 ). In addition, drift assemblages significantly changed from the main stem to the bypass and the number of EPT taxa detected more than doubled (main stem: 16 genera; last site in the bypass: 33 genera), indicating that the bypass can provide a macroinvertebrate source to the main stem. Seasonal and diel drift patterns were similar to those observed in natural streams, underpinning the finding that nature-like bypass systems can feature an EPT community similar to natural rivers.This study aligns with a growing body of evidence on the habitat function of nature-like bypass systems for aquatic taxa and highlights the importance of this restoration measure beyond the aspect of solely restoring habitat connectivity.