Long-lasting effects of experimental flow intermittency on alpine stream macroinvertebrates (Val Roseg, Switzerland)

Abstract

Changing weather patterns and receding glaciers are predicted to increase flow intermittency in alpine streams. If aquatic macroinvertebrate communities largely comprise taxa adapted to perennial flows, an increase in flow intermittency substantially reduces biodiversity and affects functional processes. We conducted a before-after-control-impact field experiment to examine how macroinvertebrate communities in an alpine headwater stream responded to and recovered from a repeated experimental increase in flow intermittency. Flow in one channel was manipulated to simulate increased summer intermittency (June–September) over two consecutive years, whilst an adjacent channel served as a control. We monitored the density of benthic macroinvertebrates, periphyton and organic matter at approximately monthly intervals over three years during the snow-free period. Before manipulation, both channels had similar ecological properties. The flow manipulation reduced the overall macroinvertebrate density, and especially the proportional rheophile density, across both years. Recovery of the macroinvertebrate community following experimental flow intermittency took more than a year, and longer than our study period. This could be due to long aquatic life stages, dispersal limitation and biotic interactions. We conclude that climate-induced changes in alpine stream flow regimes can lead to a fundamental shift in macroinvertebrate assemblages through local extinctions, mostly of rheophilic species.

Flow intermittency affects structural and functional properties of macroinvertebrate communities in alpine streams

“…This implies that recolonisation rates were high in this catchment, probably as a result of high ecological connectivity (Leys et al, 2016;Siebers et al, 2019). However, previous studies have also shown that when summer drying events occur in alpine streams that used to flow throughout the season, macroinvertebrate densities and richness are greatly reduced and assemblages are slow to recover (Drost et al, 2022;Piano et al, 2019;Siebers et al, 2020).…”

“…This implies that recolonisation rates were high in this catchment, probably as a result of high ecological connectivity (Leys et al, 2016; Siebers et al, 2019). However, previous studies have also shown that when summer drying events occur in alpine streams that used to flow throughout the season, macroinvertebrate densities and richness are greatly reduced and assemblages are slow to recover (Drost et al, 2022; Piano et al, 2019; Siebers et al, 2020). Assemblages in Val Roseg may therefore be adapted to the current prevalence of flow intermittency at a network scale given that flow intermittency has been widespread across the valley for—at the very least—over a decade (Robinson, Thompson, et al, 2016).…”

“…A commonly reported pattern of changing community composition with increasing intermittency is a reduction in rheophilic taxa such as many EPT species (Ephemeroptera, Plecoptera, Trichoptera) in most systems (Boulton, 2003; Datry et al, 2014; Stubbington et al, 2009), and in alpine streams in particular (Siebers et al, 2020). Alpine macroinvertebrate communities may take several months to recover from drying events (Drost et al, 2022; Siebers et al, 2020), probably because alpine macroinvertebrates have low resistance to drying.…”