Response of benthic macroinvertebrates to different hydropower off‐stream diversion schemes

Abstract

Despite significant advancement in environmental flow science, the release of minimum flows (MFs) still represents one of the main measures globally adopted to mitigate off‐stream diversion in regulated rivers. In the Lake Como basin (Italy), we monitored water‐depleted reaches below hydropower reservoirs and intakes and reaches unimpacted by water withdrawal. Our aim was to verify if the gradient in streamflow alterations resulted in detectable differences in the structure and functions of benthic assemblages, possibly identifying streamflow metrics explaining the observed differences. The macroinvertebrate assemblages sampled below reservoirs showed significant differences from those from unimpacted reaches, including the reduced relative richness of Ephemeroptera, Plecoptera and Trichoptera (mainly stoneflies) and the higher abundance and richness of dipterans. These differences were well explained by the corresponding streamflow pattern, characterized by a rather constant flow throughout the year, occasionally interrupted by sharp peaks related to spilling episodes. In contrast, the shift from unimpacted conditions was minor for benthic assemblages collected at reaches below intakes, where the seasonal streamflow variation was partly preserved. Our results suggest that an increase of the e‐flow released by hydropower dams during the seasons characterized by larger run‐offs could support improved benthic assemblages. Dewatered river sections below reservoirs could thus be prioritized for updating the current MF management, in the study area as well as in comparable contexts. At least to our knowledge, specific investigations are so far unavailable regarding the exploration of the different responses of benthic macroinvertebrates to intakes and reservoirs operations.

Climate Change and Water Exploitation as Co-Impact Sources on River Benthic Macroinvertebrates

“…The highlighted differences in taxonomical composition between sampling periods include the poorer variability among samples in the I-warm, compared to the NI-cold period. Differences in taxonomical composition, as well as in variability among samples, were already described by Quadroni et al [36], both at unperturbed sites and at sites characterized by mild hydrological impairment partly preserving the seasonal flow variability, and they were ascribed to natural seasonal dynamics in assemblages.…”

“…Similarly, in another Italian regulated lowland river, benthos sampled after periods of increasing duration of stable flows (equal to MFs) showed a strong increase in overall density [18]. Low flows can also sustain the development of richer assemblages, while flows of high magnitude can negatively affect family richness, partially in contrast to expectations [36,55]. Difference of flow magnitude controlled the relative abundance of EPT and multivoltine taxa during the NI-cold period, comparably to what was previously reported for increases of nitrate nitrogen in the I-warm period but in the opposite way.…”

“…Overall, diversity (i.e., H) and richness (i.e., S and EPT) metrics were higher on average during the I-warm compared to the NI-cold period and followed an increasing trend from the site with lower discharge minima to that with flow increased by the residual basin contribution. The increase in the abundance of multivoltine taxa and the decrease in Shannon-Wiener index, taxa richness, and EPT richness were already indicated among the effects of reduced discharge on benthic macroinvertebrates [36,[55][56][57][58], although the response of taxa richness to hydrologic impairment reported in the literature is not univocal as an effect of other environmental factors affecting this community metric [59,60].…”

“…The latter comprised the above-mentioned spot measurements, carried out concurrently to benthos sampling. In the case of hydrological variables, twenty metrics (selected and modified from Schneider et al [35]) were computed over the 90-days time-span before macroinvertebrate sampling, an adequate time interval for the detection of the effects of water diversion on macroinvertebrate assemblages [36]. Sorted by the five major categories of the hydrological regime (magnitude, rate of change, frequency, duration, and timing, [11]), the adopted flow metrics were:…”

Monitoring and Management of Inland Waters: Insights from the Most Inhabited Italian Region

“…Current state-of the-art approaches, e.g., [83][84][85][86], specifically advocate that e-flow recommendations should be based on the mechanistic relationships between flows and ecological outcomes. For instance, a recent study accounting for the status of the benthic macroinvertebrate communities of both unregulated and regulated Alpine streams in Lombardy highlighted that the shift from unimpacted conditions was more significant at reaches below reservoirs than at reaches below intakes, since seasonal streamflow variation was partly preserved in the latter [87].…”