Effects of hydropeaking on drift, stranding and community composition of
macroinvertebrates: a field experimental approach in three regulated
Swiss rivers
Abstract
Hydropeaking operation leads to fluctuations in wetted area between base
and peak flow and increases discharge-related hydraulic forces (e.g.,
flow velocity). These processes promote macroinvertebrate drift and
stranding, often affecting benthic abundance and biomass. Our field
experimental study – conducted in three hydropeaking-regulated Swiss
rivers – aimed to quantify (i) the short-term effects of the combined
increase in flow amplitude and up-ramping rate based on
macroinvertebrate drift and stranding, as well as (ii) long-term effects
based on the established community composition. Hydropeaking led to
increased macroinvertebrate drift compared to base flow and to
unaffected residual flow reaches. Moreover, stranding of
macroinvertebrates was positively related to drift, especially during
the up-ramping phase. Flow velocity and up-ramping rate were identified
as major determinants for macroinvertebrate drift, while flow ratio and
down-ramping rate for stranding. Particularly high sensitivity towards
HP was found for Limnephilidae, whereas Heptageniidae seemed to be
resistant in respect to short and long-term hydropeaking effects. In the
long-term, hydropeaking did not considerably reduce benthic density of
most taxa, especially of some highly resistant and resilient taxa such
as Chironomidae and Baetidae, which dominated the community composition
even though they showed comparably high drift and stranding responses.
Therefore, we argue that high passive drift and/or stranding, especially
of individual-rich taxa, does not necessarily indicate strong
hydropeaking sensitivity. Finally, our results demonstrate the necessity
to consider the differences in river-specific morphological complexity
and hydropeaking intensity, since these factors strongly influence the
community composition and short-term drift and stranding response of
macroinvertebrates to hydropower pressure.