Discussion
Considering expected future developments of the energy demand and the
concurrent progression of the implementation of climate change goals,
the need for energy storage capabilities such as hydropower reservoirs
will become more and more important in the near future (Boes et al.,
2021). As a result, an increase in HP operation thus poses a further
threat to physical habitats and biodiversity within riverine ecosystems,
highlighting the need for a prudent approach. Even though the mitigation
of HP impacts is a legal requirement in many countries (e.g., Swiss
Water Protection Act, European Water Framework Directive), the complex
interplay of HP-related impacts on MIV still remains poorly understood,
which complicates the implementation of efficient mitigation strategies.
Several studies suggest that river reaches affected by HP are
characterized by reduced benthic abundance, biomass and altered
community composition (Céréghino et al., 2002; Céréghino & Lavandier,
1998;; Elgueta et al., 2021; Leitner et al., 2017; Moog, 1993), which in
consequence may have negative effects on metacommunity-wide functional
diversity (Kjærstad et al., 2018; Ruhi et al., 2018). Within this field
experimental study – conducted in three HP-regulated Swiss rivers – we
could prove short-term (drift and stranding) and long-term (benthic
density, established community composition) effects of HP operation
within the investigated river reaches. Further, we could identify some
HP-related parameters most likely responsible for triggering active
and/or passive drift and stranding, as well as show that MIV response to
HP is highly taxa-, trait- and river-context dependent. In conclusion,
our study adds knowledge about effective implementation of mitigation
measures and more sustainable operation of HP power plants (Bruder et
al., 2016; Moreira et al., 2019; Smokorowski, 2021).
General response of macroinvertebrate to hydropeaking
Consistent with our hypotheses and in accordance to other published
literature (Aksamit, Carolli, Vanzo, Weber, & Schmid, 2021; Bruno et
al., 2010, 2016; Imbert & Perry, 2000; Miller & Judson, 2014;
Schülting et al., 2018a, 2021; Timusk et al., 2016), HP generally led to
increased MIV drift intensities compared to base flow and to the
upstream-located RF reaches with highest drift during the up-ramping
phase and the first part of the peak flow. Furthermore, the analyzed
data partially supports the result of Tanno et al. (2021) that an
increased drift of MIV also increases their risk of stranding
(especially considering the UR phase). Comparison could be biased by the
low number of individuals found in our stranding samples as well as the
different equations used for drift intensity (our study) and drift
density (Tanno et al., 2021). The use of drift density as response
variable artificially decreases the effects of HP scenarios with higher
flow magnitude and corresponding flow velocities because MIV drift
mostly does not increase proportionally to the increase in water volume,
leading to a dilution effect (Naman, Rosenfeld, Richardson, & Way,
2017; Pegel, 1980). This effect was already highlighted in the study of
Naman et al. (2017) who showed that the parameter ‘drift density’
negatively correlates with the investigated water volume, whereas ‘drift
flux’ (i.e., total number of drifting invertebrates) showed opposite
patterns. We therefore decided to use the ‘drift intensity’ (because not
affected by dilution effects) as a parameter for analyzing the effect of
field HP experiments with non-comparable flow conditions.
We found taxa- and river-specific long-term effects on the benthic
community under HP (discussed below). However, in contrast to our
hypothesis, considering all taxa and all study reaches together, we did
not detect a significant reduction in benthic density between RF and HP
reaches, even though drift and/or stranding were increased in all HP
reaches. We conclude that drift and stranding can contribute to a
reduction in benthic density of some taxa, as reported in several
studies (e.g., Céréghino & Lavandier, 1998; Leitner et al., 2017; Moog,
1993), but most likely HP further induces organism stress and hampers
important processes such as feeding and reproduction. The combination of
the HP-induced effects on biota may in consequence lead to a thinning of
the benthic community and to a long-term change in species composition.
Effect of HP intensity and associated hydraulic forces on drift and
stranding
Flow velocity near the surface (v100 ) and mean
up-ramping rate (URmean ) were identified as major
determinants for macroinvertebrate drift propensity, indicating that the
flow magnitude and amplitude (leading to different flow velocities) as
well as the up-ramping rate are important HP-related parameters. A
strong relationship between drift and flow-related hydraulic forces such
as flow velocity is widely known (e.g., Gibbins, 2016). Recently,
Schülting et al. (2021) found that discharge-related parameters, such as
mean-column flow velocity at peak flow, primarily affect MIV drift and
the importance of the up-ramping rate increases only once certain
discharge-related thresholds are exceeded. Following, a reduction of the
up-ramping rate may lead to a lower drift since the MIV have more time
to seek shelter in the interstices during flow increase (Imbert &
Perry, 2000; Schülting et al., 2021; Timusk et al., 2016).
We also found a positive link between down-ramping rate
(DRmax ) and stranding propensity, corresponding
to findings by Kroger (1973) and Perry & Perry (1986). In line with
Tanno et al. (2016), a larger flow ratio
(Qpeak/Qbase ) also led to higher
stranding, possibly due to increased drift and a larger wetted area.
Additionally, we showed that turbidity (NTU ) and water
temperature (T ) most likely influence drift and stranding risk.
However, this could also be an indirect effect as variations in water
temperature and turbidity are influenced by HP operation (e.g., flow
ratio). We conclude that the magnitude of drift and stranding propensity
is most likely affected by a combination of hydrological and hydraulic
factors as well as by their interaction (v100 ,URmean,
Qpeak/Qbase, DRmax ) and
possibly by behavioral (NTU , T ) responses. Indeed,
thermopeaking has been shown to induce behavioral MIV drift of many taxa
and often has a synergic magnifying effect with the HP wave (e.g., Bruno
et al., 2013). Similarly, water temperature has been shown to influence
stranding risk of fish, with greatest effect in winter due to low
temperatures (e.g., Halleraker et al., 2003). HP-induced changes of
turbidity and their effects on MIV are poorly understood. Though,
re-mobilized and re-suspended fine sediments during peak flows are
mainly responsible for an increase in the river’s turbidity and can
induce fine sediment infiltration
and potential clogging (Hauer et al., 2019), thereby exposing MIV to
increased abrasion and reduced potential interstitial habitats (Bo et
al., 2007; Crosa et al., 2010; Jones et al., 2012).
Taxa- and traits-specific responses to hydropeaking
Long-term HP effects were found
only for a few taxa such as Limnephilidae, Nemouridae and Leuctridae,
which were abundant in the RF reaches yet strongly reduced in the HP
reaches. Limnephilidae further showed short-term response reflected in
their high propensity to drift and strand. In contrast, Heptageniidae
seemed to be more resistant in respect to short and long-term HP
effects.
Limnephilidae are adapted to slow flowing lentic habitats on the
substrate surface and, due to their size and shape, they experience high
drag (Rader, 1997). Moreover, for case-building Limnephilidae it has
been shown that an increased flow velocity decreases their ability to
return to the stream bottom from the drift (de Brouwer,
Besse-Lototskaya, ter Braak, Kraak, & Verdonschot, 2017). Therefore,
this taxon is easily detached passively from its habitat and has poor
settling efficiency which might explain its particularly high
sensitivity towards short- and long-term HP effects. In line with these
observations, other studies suggest that HP often contributes to a
decrease in the density of limnophilic taxa associated to lentic
habitats and to strong drift (Graf et al., 2013; Leitner et al., 2017;
Schülting et al., 2021). Correspondingly, in our study, lentic taxa were
more abundant in the RF reaches compared to the HP reaches, confirming
the sensitivity of this trait to HP (Schülting et al., 2021).
Heptageniidae showed higher abundances in the benthic samples of the HP
reaches compared to the RF reaches (Appendix F in Data S1) and lowest
propensity to drift and strand. This emphasized the tolerance towards HP
of this family. Similar observations were reported by other authors
(e.g., Bruno et al., 2013; Graf et al., 2013; Moog, 1993, Schülting et
al., 2018a, 2021). Although different taxa within this family may show
varying responses towards HP, they generally prefer lotic habitats on
the substrate surface and are characterized by high agility and active
settling efficiency (Elliott, 1971) as well as by various morphological
adaptations that enhance attachment (streamlined body shape, sharp
tarsal crawls, sucker-like ventral gills) and reduce drag (Rader, 1997).
Nemouridae and Leuctridae did not show distinctively high drift and
stranding following HP, possibly due to relatively thin source
populations. Schülting et al. (2021) reported high drift induced by HP
for Nemouridae (Nemoura/Nemurella sp.), whereas Leuctridae
(Leuctra sp., a taxon associated with the interstices) showed
comparably low response, and this family is often not affected by HP
(e.g., Graf et al., 2013; Moog, 1993).
Simuliidae and Rhyacophilidae also showed high propensity to drift –
Rhyacophilidae also to strand – but both families were more abundant in
the benthic samples of the HP reaches compared to the RF reaches. Both
taxa are known to prefer lotic habitats and they own adaptations (claws
and silk) to prevent passive drift (Rader, 1997). Nevertheless,
Simuliidae have been reported to often drift (Bruno et al., 2013, 2016;
Imbert & Perry, 2000) and strand (Perry & Perry, 1986; Tanno et al.,
2021) following HP. As surface-dwelling taxon, they are naturally
exposed to high flow, which can likely explain their high drift
propensity, even under natural conditions (Elliot, 1967). Rhyacophilidae
are generally associated to the interstices, but as predators, they may
also actively move on the surface seeking prey and are thus exposed to
abruptly increased hydraulic forces. The cold thermopeaking in the
Hasliaare (Figure 2c) might also explain the high (probably behavioral)
drift and stranding of Rhyacophilidae (Bruno et al., 2013) in this
reach. However, our experimental setup does not allow a separation of
active and passive drift. In conclusion, our data suggests that high
drift and/or stranding induced by HP flow fluctuations do not lead to
distinct density reductions of Simuliidae and Rhyacophilidae. Thus,
these taxa are less sensitive towards HP compared to Limnephilidae.
Taxa associated to lentic areas showed reduced benthic density compared
to lotic taxa. However, on the long-term, HP did not considerably reduce
benthic densities of most taxa, especially of Chironomidae and Baetidae.
Even though these taxa showed comparably high drift and stranding
responses (Appendix E and F in Data S1), they were also dominant in the
respective benthic samples. Matching our results, these two families are
often reported as dominant components of the benthic assemblages and
drift following HP (Bruno et al., 2010, 2013, 2016; Gibbins et al.,
2016; Imbert & Perry, 2000; Moog, 1993; Schülting et al., 2018a, 2021;
Tanno et al., 2016, 2021; Timusk et al., 2016) as well to frequently
strand (Perry & Perry, 1986; Tanno et al., 2016, 2021). Chironomidae
and Baetidae are likely to be represented strongly in the drift and
stranding due to the high density and density dependence of drift
(Waters, 1972). Accordingly, they showed comparably low propensity to
drift and strand and did not show reduced benthic densities under HP,
probably due to their morphological and behavioral adaptations to high
currents (Chironomidae: rapid colonizers with rapid growth rates and
short life cycles; Baetidae: good swimmers with a streamlined body,
rapid settling capabilities, frequent intentional drift; Elliott, 1971;
Milner, 1994; Naman et al., 2016; Rader, 1997; Wilzbach, Cummins, &
Knapp, 1988). Even though, results
regarding Chironimidae cannot be generalized since this family consists
of many different taxa with different adaptations. Our study suggests
that some Chironomid species and Baetidae are resistant and resilient to
long-term HP effects and most likely have flexible habitat needs. We
therefore conclude that high passive drift and/or stranding, especially
of individual-rich taxa, does not necessarily indicate strong HP
sensitivity. Yet it must be stated that the benthic community
composition in all investigated river reaches has already been
significantly affected by the long history and huge variety of
anthropogenic impacts on river ecosystems. The presented results should
therefore only be interpreted in consideration of the limitations
related to the altered source populations.
Some
studies suggest that functional descriptions are maintained, even with
coarser taxonomic resolution, and it barely influences the
discrimination of impact levels (e.g., Dolédec, Olivier, & Statzner,
2000; Gayraud et al., 2003). However, since organisms in our study were
identified only to family-level, taxa-specific results should not be
over-interpreted. This level of taxonomic resolution inevitably leads to
a loss of information, especially for families that have heterogeneous
adaptations among the constituent genera, such as Chironomidae. Thus,
diverse species/genera within a family can react quite differently to HP
or other flow modifications (e.g., RF). Lastly, some taxa were found
only in low numbers (e.g., Limnephilidae), limiting the statistical
power of some of our results. Moreover, the comparably low number of
drifting and stranding individuals can probably be explained by the peak
magnitude of our experimental setup, which were lower as normally found
in Swiss HP rivers (e.g., Tanno et al. 2016, 2021).
River- and reach-specific responses
MIV responses to HP were not only
taxa- and trait-specific but varied widely among the investigated rivers
and reaches. Disparities in the direction and magnitude of responses
among rivers likely resulted from differences in community composition
but also from variability in the HP intensity and river morphological
characteristics. Indeed, it is likely that the lower peak flow in the
Sitter in combination with its higher morphological heterogeneity
compared to the Hasliaare and the Linth lead to lower hydraulic forces
acting on the MIVs favoring benthic abundance in concert with a reduced
drift and stranding risk. The high drift in the RF reach of the Linth
can be explained by the hydraulic forces measured, such as flow
velocities and Froude number, which were similar to the ones measured in
the HP reach during peak flow. In contrast, in the RF reaches of the
Sitter and Hasliaare, they were comparable to the ones measured during
base flow (Appendix B in Data S1). This finding underlines that
discharge-related hydraulic forces (e.g., flow velocity) acting on the
riverbed are most probably the major determinant for MIV drift. Further,
the HP-induced removal and mobilization of organic matter from the
substrate, which is often associated to passive drift of MIV (Aksamit et
al, 2021; Bruno et al., 2016; Miller & Judson, 2014; Timusk et al.,
2016), is a another determining factor. The RF reach of the Linth was
characterized by high algal cover on the substrate (>50%;
Appendix A in Data S1) and showed the highest drifting biomass (ash free
dry mass) of FPOM and CPOM compared to the other two rivers (Tonolla,
Kastenhofer, Vögeli Kummert, & Gufler, 2020).
In general, these river- and reach-specific responses confirm the
importance of structural complexity (such as in the Sitter) for
providing lentic habitats and hydraulic refugia at the locale scale as
well as the retention of organic matter during high flow disturbance
(e.g., Bruno et al., 2016; Hauer et al., 2017; Lancaster, 2000). In
structurally complex rivers with moderately sloping bank, velocity and
bottom shear stress remain relatively constant (Naman et al., 2017),
thereby reducing passive drift. In contrast, fluctuation of the
dewatering area is enhanced if the river-bank slope is low, forcing
organisms to shift habitats which subsequently increase their risk of
stranding.
In addition to river widening and
a general increase in structural complexity, morphological measures
should aim to reduce substrate-deficit and clogging due to fine
substrate infiltration in the dewatering area (often occurring in
combination; Hauer et al., 2019), thereby increasing interstitial space
as potential hyporheic refugial habitat during HP (Bruno et al., 2010).
Further, the reconnection of tributaries with a natural flow and
sediment regime, as well as improving the morphology upstream of HP
impacted reaches (e.g., in the RF reach), may provide refugial habitats
and a connection with benthic source populations for faster
recolonization downstream of HP power plants (Aksamit et al., 2021;
Bruno et al., 2016; Hauer et al., 2017; Kennedy et al., 2016; Milner,
Yarnell, & Peek, 2019).
Overall, our results highlight
that different MIV taxa (and traits) vary in their vulnerability and
response to flow alteration (HP and RF) and that the river-specific
physical context (HP intensity, morphology, distance to potential,
source populations) mediate the magnitude and direction of this
response. Therefore, next to structural and/or operational measures to
reduce HP discharge (and related hydraulic forces) and the
ramping-rates, morphological measures should be considered as an
essential component of HP mitigation.