If Alpine streams run dry: the drought memory of benthic communities

Abstract

Several mountain streams are currently changing from perennial to temporary due to increasing water abstraction and global climate change with expected detrimental effects on stream biodiversity and functionality. We here examined whether macroinvertebrates and diatoms, experiencing recurring non-flow periods, show alterations even after complete flow resumption in 13 mountain streams in SW Italian Alps. Benthic communities were sampled after complete flow resumption in April 2017 in a control section, with permanent flow, and an intermittent section, which experiences a non-flow period during summer, in each stream. We tested for differences in terms of taxonomic and functional composition, diversity and functional group composition between permanent and temporary sections. Our results showed a significant alteration of benthic invertebrate, but not diatom communities in temporary sections. Different species composition and low diversity values in temporary sections were due to the replacement of monovoltine taxa, with aquatic respiration, preferring medium to fast flowing, oligotrophic waters by plurivoltine taxa, with aerial respiration preferring lentic habitats. Such results provide some insights into the mechanisms by which non-flow periods impact Alpine streams, and further investigations in mountain areas are required in the future to better unravel the repercussions on stream ecosystem processes.

Flow intermittency negatively affects three phylogenetically related shredder stoneflies by reducing CPOM availability in recently intermittent Alpine streams in SW-Italian Alps

“…When considering the effect of flow intermittency on the three examined genera, our results showed that abundances of the three genera were lower in intermittent than in permanent reaches, in agreement with our second hypothesis. This is in accordance with the results observed in a related study performed in the same study area, where we observed a significant negative effect of flow intermittency on the relative abundance of scrapers taxa (Piano et al, 2019a). This reduction is due to the lower availability of organic matter in intermittent than in permanent sites, which is in turn potentially determined by the lower retention capacity of the riverbed substrate in intermittent sites.…”

“…These newly temporary streams are characterized with recurrent non-flow events, occurring in summer, followed by rewetting phases in late autumn (Fenoglio et al, 2010) due to the interactive effects of both climate change and anthropogenic disturbance (Belmar et al, 2019;Bruno et al, 2019). These recurrent drying events are expected to alter the distribution of lotic biota by influencing physical conditions and distribution of trophic resources (e.g., Calapez et al, 2014;Elias et al, 2015;Milner et al, 2017;Doretto et al, 2018;Falasco et al, 2018;Piano et al, 2019a;Doretto et al, 2020a).…”

“…In addition, lower flow velocity reduces the removal of fine sediments, with consequent high fine sediment deposition, which can alter the quality and quantity of resources, in terms of both reduced instream production, due to the abrasion of autotrophic biofilms (Henley et al, 2000;Bona et al, 2016), and allochthonous coarse organic matter availability through burial (Doretto et al, 2016). On top of that, such dramatic consequences on organic matter availability and decomposition may persist even after several months following flow resuming, because of the so-called ''drying memory'' (Datry et al, 2011;Pinna et al, 2016), thus affecting also the late instars of shredders (Piano et al, 2019a). Physical alterations and consequent changes in food resources induced by flow intermittency thus represent strong environmental filters that may influence the ecological niche of shredders.…”

“…Physical alterations and consequent changes in food resources induced by flow intermittency thus represent strong environmental filters that may influence the ecological niche of shredders. Although some studies investigated the effect of flow intermittency on the diversity of this trophic group in Alpine streams (e.g., Fenoglio et al, 2007;Doretto et al, 2018;Piano et al, 2019a), little is known about how these events may alter the ecological parameters that determine the niches of shredder taxa.…”

The River Continuum Concept: lessons from the past and perspectives for the future

“…Flow cessation results in the loss of the longitudinal connectivity and isolation of stream reaches with consequences on population survival, the diversity of aquatic communities, and transport and storage of organic matter subsidies (Datry et al 2016b;Doretto et al 2020). Such adverse conditions may be exacerbated by the effect of anthropogenic-induced hydromorphological alterations at the local scale, such as damming and water abstraction (Grubbs and Taylor 2004;Holt et al 2015;Doretto et al 2019;Piano et al 2019). Regardless of the nature of these alterations (i.e., natural or human-induced), RCC poorly adapts to lotic ecosystems experiencing changes in flow regime, with deviations from its predictions probably depending on the components of the flow regime that are mostly modified (i.e., magnitude, frequency, duration, timing, and flashiness).…”