Towards ecological flows: status of the benthic macroinvertebrate community during summer low-flow periods in a regulated lowland river

Climate change along with the increasing exploitation of water resources exacerbates low-flow periods, causing detrimental effects on riverine communities. The main mitigation measure currently adopted to counteract hydrological alterations induced by off-stream diversion is the release of minimum flows (MFs), even if within the European Union Water Framework Directive an upgrade towards ecological flows is urgently required to achieve good ecological status (GES). In this study, we investigated the temporal evolution of the benthic macroinvertebrate community in an Italian-regulated lowland river (Ticino River) to clarify the ecological effects of summer low flows, and we evaluated the current MFs in the perspective of meeting GES standard. Biomonitoring was carried out for four consecutive years (2019-2022), in a river site immediately below a large off-stream diversion. The four study years were characterized by different streamflow patterns, thus allowing us to compare the temporal trajectories of the community under different flow conditions. Moreover, the interruption of the low-flow periods due to overflow spilled by the upstream dam gave us the opportunity to assess the effects of experimental flow peaks. Contrary to the expectation, the macroinvertebrate assemblage kept almost unvaried across the years, showing great resistance and resilience to hydrological changes. Even in extraordinarily dry 2022, the community composition varied only slightly, with a reduction of mayflies and an increase of mollusks. However, a deterioration of the ecological status below GES standard was recorded that summer, indicating the need for an upgrading of the current MFs. This upgrade would include experimental flow peaks in critical periods, which act as intermediate disturbances, enhancing community richness, diversity, and overall quality, as well as compliance with a threshold of an index specifically developed for the hydrological pressure

Assessing the Impacts of Hydropeaking on River Benthic Macroinvertebrates: A State-of-the-Art Methodological Overview

As the global demand for renewable electricity grows, hydropower development of river basins increases across the world. Hydropeaking, i.e., streamflow alteration consisting of daily or subdaily rapid and marked discharge fluctuations, can affect river reaches below hydropower units. Environmental effects of hydropeaking include geomorphological alterations and possible modifications of the freshwater biota. Among affected instream communities, benthic macroinvertebrates are receiving increasing attention and the related scientific research has experienced significant progress in the last decade. In this context, this paper aims to summarize state-of-the-art methods for the assessment of hydropeaking impacts on benthic macroinvertebrate communities. The present review could support the proper design of monitoring plans aimed at assessing the ecological impacts of hydropeaking and the effects of possible mitigation strategies

What does it Happen to Mesohabitats of an Upland Stream after a Controlled Sediment Flushing Operation?

In the Alps, several controlled sediment flushing operations (CSFOs) have been carried out in the last decade to counteract the loss of storage of reservoirs due to siltation. These operations are characterized by the control of the suspended sediment concentration in the downstream watercourse, aimed to reducing the ecological impact. In this study, we assessed the effects of a CSFO from a small Alpine hydropower reservoir at the mesohabitat scale. Specifically, we repeatedly sampled three mesohabitats (one pool, one riffle, and one step-pool) in the final stretch of the stream subjected to the CSFO. In each mesohabitat, we measured both the amount of deposited fine sediment and the related effects on the lower levels of the food web (periphyton and benthic macroinvertebrates), before and up to two years after the CSFO. As expected, after the CSFO, sediment deposition was larger in the pool and a decreasing trend was observed in all mesohabitats. Both periphyton and benthic macroinvertebrates were negatively affected by the CSFO in the short-term. The lowest contraction of benthic macroinvertebrate communities, in terms of density and richness, was observed in the riffle. However, the invertebrate richness of this mesohabitat recovered slowly (more than one year after the CSFO). Differences in the impact and recovery patterns of the zoobenthic assemblages were clearly detected by the Siltation Index for LoTic EcoSystems (SILTES). An improved knowledge about the ecological impact of CSFOs and the related recovery patterns is evidently required to support sediment management practices in regulated river systems

Towards an upgrading of minimun flows: the ecological effects of summer low flows in a aregulated lowland river

In this study, we investigated the temporal evolution of the benthic macroinvertebrate community in an Italian regulated lowland river to clarify the ecological effects of summer low flows. Biomonitoring was carried out for three consecutive years, in a river site immediately below a large off-stream diversion, where minimum flows are released as an environmental protection measure. The three study years were characterized by different streamflow patterns, thus allowing us to compare the temporal trajectories of the community under different flow conditions. Moreover, the interruption of the low-flow periods due to overflow spilled by the upstream dam gave us the opportunity to assess the effects of experimental flow peaks. The analysis of standard macroinvertebrate community metrics showed that larger values of minimum flows favored the taxa belonging to the most sensitive insect orders (Ephemeroptera, Plecoptera and Trichoptera). Moreover, flow peaks interrupting prolonged low-flow periods acted as intermediate disturbance, enhancing community richness and diversity while decreasing density. These results can support the upgrading of current environmental-flow schemes