Alpine river ecosystem response to glacial and anthropogenic flow pulses

Alpine glacier-fed river hydrology, chemistry, and biology can vary significantly in space and over diurnal to interannual time scales as a function of dynamic inputs of water from snow, ice, and ground water. The sensitivity of biota to these water-source dynamics potentially makes them susceptible to hydrological changes induced by anthropogenic activities, such as flow regulation, but most alpine studies have been focused on intact rivers during summer only. We examined the spatiotemporal dynamics of physicochemical habitat and macroinvertebrate assemblages in a high-altitude (>2000 m) floodplain in the European Alps over an 18-mo period. We present a novel insight into the river system and macroinvertebrate assemblage responses to natural glacier-melt-driven expansion–contraction of unregulated river sites and to intermittent flow pulses caused by hydropower regulation. Mainstem glacier-fed sites had cyclical seasonal dynamics in macroinvertebrate assemblage composition that shifted to be partly reminiscent of groundwater tributaries in winter then back to melt water in the following spring. Significant unimodal relationships were found between glacial influence and macroinvertebrate assemblage density, richness, Simpson’s diversity, evenness, and β diversity. These relationships suggest that glacial influence can positively affect biodiversity where glacier melt water mixes with nonglacial water and habitat diversity is maximized. Regulation-induced flow pulses led to inconsistent responses among macroinvertebrates, with no significant effects in summer 2008 but increased density and decreased taxonomic richness in 2009. Assemblage composition was unaffected by reservoir releases despite significant increases in water temperature and discharge at these times. The effects of alpine river management for hydropower production on macroinvertebrate assemblages in this river system appear to be relatively minor, but further studies should be undertaken in other alpine locations to assess the generality of this finding.