Results
Short-term response of macroinvertebrate (drift, stranding) to
hydropeaking
Across all reaches, drift
intensity was 2.5 times higher during HP (12.5 ± 0.9
ind./m2min) compared to base flow (4.9 ± 0.8
ind./m2min, p< 0.001) and approximately 1.5
times higher than in the RF reaches (7.4 ± 0.8
ind./m2min, p < 0.001, ‘ALL’ in Figure 3).
Drift intensity in the RF reaches was also significantly higher than at
base flow in the HP reaches (p < 0.05). At the river level,
significantly higher drift intensities were found during HP compared to
base flow of the Sitter (p < 0.01), Hasliaare (p <
0.001) and Linth (p < 0.01) as well as compared to the RF
reaches of the Sitter (p < 0.001) and Hasliaare (p <
0.01). The Linth showed a different pattern with higher drift intensity
in the RF reach compared to HP (p = 0.06) and to base flow (p
< 0.001).
Across
all HP reaches, drift intensities during all HP phases (UR, P1, P2, DR)
were significantly higher than during base flow (p ≤ 0.001, Appendix D
in Data S1). Moreover, drift intensity in the UR phase was significantly
higher compared to the DR phase (p < 0.01). The highest
average drift intensity was found for the UR phase (17.1 ± 1.7
ind./m2min) and the first part of the peak phase (P1;
13.7 ± 1.3 ind./m2min) which showed drift intensities
3.5 times and approximately three times higher, respectively, than
during base flow (4.9 ± 0.8
ind./m2min).
Across
all HP reaches, and considering the entire HP scenario (i.e., UR, P1, P2
and DR phase), stranding density was not significantly related to drift
intensity (‘ALL’ in Figure 4a). However, if considering only the UR
phase, a positive significant relationship was found (R = 0.368, p
< 0.01, ‘ALL’ in Figure 4b). At the reach level, a positive
significant relationship was found in the Hasliaare and in the Sitter
(but only for the UR phase) but not in Linth. In general, stranding was
much less pronounced than drift and many stranding samples contained
only few individuals (Appendix C in Data S1).
The CCA analyses revealed that taxa drift and stranding composition
significantly differed between the three rivers (drift:
r2 = 0.23, p < 0.01; stranding:
r2 = 0.32, p < 0.01; Figure 5).
Additionally, but to a lesser extent, the lateral sampling location (day
1 vs day 2: higher peak flow velocities at day 2, see Table 1;
drift: r2 = 0.16, p < 0.01; stranding:
r2 = 0.27, p < 0.01) significantly
contributed to the differences. Drift composition in the Sitter and
Linth were more similar than in the Hasliaare (Figure 5a, confidence
ellipses). The HP scenario also significantly contributed to the drift
(r2 = 0.16, p = 0.05) but not to the stranding
differences. 43.5% and 40.8% of the total variation of drift and
stranding taxa distributions, respectively, can be explained by the
selected environmental variables.
The
main environmental variables that could affect the taxa drift propensity
to the greatest extent were the flow velocity near the surface
(v100 ; F = 12.5, p < 0.001), mean
up-ramping rate (URmean; F = 10.4, p <
0.001), turbidity (NTU ; F = 6.4, p < 0.001), and water
temperature (T ; F = 6.0, p < 0.001) which explained
8.0, 9.8, 9.0, and 8.5% of the variation, respectively (Figure 5a).
Froude number (Fr ) also contributed to the explained variation
(8.2%) but not significantly (F = 1.7, p = 0.119). The main
environmental variables that could affect the taxa stranding propensity
to the greatest extent were the flow ratio
(Qpeak/Qbase ; F = 9.7, p
< 0.001), max. down-ramping rate
(DRmax ; F = 8.8, p < 0.001), water
temperature (T ; F = 5.5, p < 0.001), turbidity
(NTU ; F = 4.1, p < 0.001) and flow velocity near the
surface (v100 ; F = 3.7, p < 0.01),
which explained 7.3, 8.8, 8.2, 8.1, and 8.3 % of the variation,
respectively (Figure 5b).
Across all HP reaches, Rhyacophilidae and Limnephilidae showed high
propensity to drift and strand, whereas Oligochaeta and Heptageniidae
showed the lowest propensity (‘ALL’ in Table 2). Simulidae also showed
high propensity to drift and Perlodidae to strand. However, drift and
stranding propensity varied considerably between reaches. Limnephilidae
and Simuliidae showed considerable propensity to drift in all three HP
reaches, whereas Rhyacophilidae only in the Hasliaare. Limnephilidae,
Rhyacophilidae and Nemouridae showed considerable propensity to strand
only in the Hasliaare, whereas Empididae and Perlodidae in the Linth.
Almost all taxa showed highest propensity to drift and strand in the
Hasliaare and lowest propensity to drift and strand in the Sitter.
Long-term response of macroinvertebrate (density and community
composition) to hydropeaking
Average benthic density was five
times higher in the Hasliaare RF reach (1146 ± 229
ind./m2) compared to the HP reach (227 ± 23
ind./m2, p < 0.001), whereas in the Sitter a
contrasting trend was observed with average density approximately 3.5
times higher in the HP reach (1838 ± 357 ind./m2) than
in the RF reach (493 ± 97 ind./m2, p <
0.001, Figure 6). The density variability of the Sitter HP samples was
the largest of all reaches. No significant differences were found for
the Linth and considering all reaches together (‘ALL’ in Figure 6).
The NMDS analysis showed that benthic community composition was
significantly different between the six reaches (p = 0.001), whereby it
was more similar in the three RF reaches than in the three HP reaches
(Figure 7, overlap vs no overlap of the confidence ellipses;
lower R2 and F values in Appendix E in Data S1).
The Hasliaare showed the largest
dissimilarity between RF and HP reaches, whereas the other two rivers
grouped more together, indicating more considerable consistency in
benthic community composition.
Four taxa (Heptageniidae,
Chironomidae, Baetidae and Leuctridae) remarkably contributed to
differences in community composition among RF and HP reaches of all
three rivers (Table 3). Among these taxa, Heptageniidae, Chironomidae
and Baetidae in the Sitter, and Chironomidae, Baetidae and Leuctridae in
the Linth cumulatively contributed for approximately 70% of the
dissimilarities. In the Hasliaare two other taxa, Limnephilidae and
Nemouridae, accounted for 41% of the differences among RF and HP
reaches, and Chironomidae additionally contributed for approximately
15%.
Across all reaches, Leuctridae,
Nemouridae and Limnephilidae showed significantly higher benthic
densities in the RF than in the HP reaches (p < 0.001,
Appendix E in Data S1). In contrast, Simuliidae showed significantly
higher benthic densities in the HP reaches (p < 0.05).
However, differences between HP and RF reaches varied widely among taxa
and river (Appendix E in Data S1).
On
average, lentic taxa were 5.5 times more abundant in the benthic samples
of the RF reaches (191.2 ± 47.7 ind./m2) compared to
the HP reaches (35.0 ± 6.1 ind./m2; p <
0.001), whereas lotic taxa were two times more abundant in the HP
reaches (437.8 ± 105.3 ind./m2) than in the RF reaches
(222.2 ± 35.1 ind./m2), but the difference was not
significant (‘ALL’ in Figure 8). Differences between RF and HP reaches
for taxa classified as surface dwelling or interstitial were also not
statistically different.