Introduction
High-head storage hydropower plants often have detrimental ecological
impacts on downstream river ecosystems because of their high-volatile
electricity production, which cause severe daily and sub-daily
downstream fluctuations in discharge and water levels due to so-called
‘hydropeaking’ (hereafter, HP; Bruder et al., 2016; Young, Cech, &
Thompson, 2011). Those hydropower operations influence the flow regime
not only during peak flow. Due to the temporal water storage in the
reservoir, the river segment located between storage and hydropower
plant experiences relatively constant base-flow conditions, so-called
residual flow (hereafter, RF). An increase of 1.2 TWh is forecasted by
2050 in the energy storage capability of the Swiss hydropower
reservoirs, corresponding to 20% of today’s energy storage capacity
(Boes et al., 2021). Consequently, further ecological impacts within
riverine ecosystems are expected with an increase in HP operation.
In alpine rivers, HP often occurs in high frequency and its intensity
can be characterized by several hydrological parameters (Greimel et al.,
2016; Li & Pasternack, 2021). HP operation in the up-ramping phase
often leads to abrupt increase in discharge and related hydraulic forces
(e.g., flow velocity, bed shear stress). This can promote unintentional
and mechanical dislodgement of aquatic macroinvertebrates (hereafter,
MIV) from the substrate and their downstream transport in the water
column until passive (e.g., in hydraulic dead zones) or active
(behavioral or through morphological adaptations) exit of the drift
(Ciborowski, 1987; Naman, Rosenfeld, & Richardson, 2016). The causes
for MIV drift by HP operation are manifold and complex and often
strongly taxon- and trait-specific (e.g., Kjærstad, Arnekleiv, Speed, &
Herland, 2018; Leitner, Hauer, & Graf, 2017). The following parameters
are of major importance regarding HP-induced passive MIV drift:
up-ramping rate, flow magnitude and amplitude, flow velocity, and to a
lesser extent also Froude number and bed shear stress (Bruno, Cashman,
Maiolini, Biffi, & Zolezzi, 2016; Bruno, Maiolini, Carolli, & Silveri,
2010; Gibbins, Vericat, Batalla, & Buendia, 2016; Imbert & Perry,
2000; Miller & Judson, 2014; Schülting, Feld, Zeiringer, Huđek, &
Graf, 2018a; Timusk, Smokorowski, & Jones, 2016). Moreover, sudden
variations in water temperature (i.e., thermopeaking) may induce
behavioral MIV drift (Bruno, Siviglia, Carolli, & Maiolini, 2013;
Schülting, Feld, & Graf, 2016), and re-suspension of fine sediments
increases turbidity and clogging risk (Hauer, Holzapfel, Tonolla,
Habersack, & Zolezzi, 2019) further affecting MIV (Bo, Fenoglio,
Malacarne, Pessino, & Sgariboldi, 2007; Crosa, Castelli, Gentili, &
Espa, 2010; Jones et al., 2012). In addition
to drift, stranding is a possible
consequence of HP operations. MIV stranding has been documented, also if
not unequivocally, to be related to down-ramping rate, flow magnitude,
amplitude and rate, flow velocity and the extension of the dewatering
area (Kroger, 1973; Perry & Perry, 1986; Tanno, Wächter, & Gerber,
2021; Tanno, Wächter, & Schmidlin, 2016). Tanno et al. (2021) further
found a positive correlation between MIV drift and stranding. In a
similar vein as for drift, water temperature and turbidity are probably
likely to influence stranding. For example, water temperature has been
shown to influence stranding risk of fish (e.g., Halleraker et al.,
2003). Besides the short-term effects of MIV drift and stranding,
long-term effects of altered
hydromorphological habitat conditions in HP rivers are reported to
affect colonization patterns of benthic populations (Bretschko & Moog,
1990; Cushman, 1985; Kjærstad et al., 2018). Morphological
heterogeneity, for instance, is known to be crucial in providing diverse
habitats and refuges for MIV communities under HP conditions (Hauer,
Holzapfel, Leitner, & Graf, 2017). The
complex interplay of drift,
stranding and altered hydromorphological habitat conditions caused by HP
operation, most likely contribute to reductions in MIV abundance and
biomasses (Céréghino, Cugny, & Lavandier, 2002; Céréghino & Lavandier,
1998; Elgueta et al., 2021; Leitner et al., 2017; Moog, 1993). Another
possible consequence is the alteration of the MIV community composition
due to the evolution of specific behavioral (e.g., mobility, ability to
regain a foothold, sinking postures) and morphological (e.g., body
shape, hooks) traits as well as life history strategies. HP may, for
example, contributes to a selection of rheobiont and rheophilic taxa
(Bretschko & Moog, 1990; Cushman, 1985; Ruhi, Dong, McDaniel, Batzer,
& Sabo, 2018) against limnophilic taxa or of taxa associated with
lentic and substrate surface areas (Graf et al., 2013; Leitner et al.,
2017; Ruhi et al., 2018; Schülting et al., 2018a; Schülting, Dossi,
Graf, & Tonolla, 2021). Community changes due to HP may consequently
affect the local food web structure and the ecological functioning of
river systems (e.g., Holzapfel, Leitner, Habersack, Graf, & Hauer,
2017; Pearce et al., 2019).
The complex interactions between
MIV drift, stranding and established benthic community remain poorly
understood, and field studies are rare (but see for example Miller &
Judson, 2014; Tanno et al., 2021; Timusk et al., 2016). Such a
comprehensive evaluation is yet of importance for an exhaustive
understanding of HP-related impacts on MIV and finally for
implementation of associated mitigation measures and sustainable
operation of HP power plants (Bruder et al., 2016; Moreira et al., 2019;
Tonolla, Bruder, & Schweizer, 2017).
The main goal of our field
experimental study – conducted in three HP-regulated Swiss rivers –
was therefore to quantify the short-term effects of three HP scenarios
with different intensity (simulated through an increase in flow
amplitude and up-ramping rate) on MIV drift and stranding in addition to
the long-term HP effects on the established community composition. As
comparison, MIV drift and community composition was quantified in
HP-unaffected RF reaches
upstream.
We hypothesize that: (i) drift
increases during the HP scenarios compared to base flow and to drift in
the RF reaches as well as a positive relationship between drift and
stranding; (ii) HP-intensity and associated hydraulic forces, as
summarized by pre-selected environmental variables, can explain any
observed differences in drift and stranding propensity; (iii) compared
to the RF reaches with almost constant hydrological conditions, flow
fluctuations due to HP operation lead to lower benthic densities and a
different MIV community composition, linked to drift and stranding.
Finally, we expected taxa-, trait- and river-specific patterns.