Temporary rivers are characterized by shifting habitats between flowing, isolated pools and dry phases. Despite the fact that they are currently receiving increasing attention by researchers and managers, the isolated pools phase has been largely disregarded. However, isolated pools in temporary rivers are transitional habitats of major ecological relevance as they support aquatic ecosystems during no-flow periods, and can act as refuges for maintaining local and regional freshwater biodiversity. Pool characteristics such as surface water permanence and size, presence of predators, local physicochemical conditions, time since disconnection from the river flow or distance to other freshwater habitats challenge a comprehensive understanding of the ecology of these habitats, and compromise ecological quality assessments and conservation practices in temporary rivers. In this presentation we will provide a characterization of isolated pools from a hydrological, geomorphological, physicochemical, biogeochemical and biological point of view as a framework to better conceptualize, conserve and manage these habitats.

Long periods of flow recession, streambed drying, and flow resumption are key components of hydrological droughts, but can impact ecological functioning and the stability of lotic communities. In this study, we examined benthic macroinvertebrate diversity and community composition during a flow recession and streambed drying in four distinct habitats within two temperate, bedrock dominated headwaters. The habitats included bare bedrock, bedrock covered with moss (e.g. Fontinalis squamosa), and pebble clusters in a mixed bedrock-alluvial reach, and gravel patches in a nearby alluvial reach. Taxonomic approaches indicated significant differences in macroinvertebrate alpha and beta diversity during lotic conditions in spring and through out the flow recession and upon flow resumption. Post flow resumption, alpha and beta diversity were significantly lower in all habitats, especially in bedrock habitats dominated by Fontinalis squamosa. Faunal traits (including respiration, locomotion, and substrate preferences) also identified the effects of the flow recession on macroinvertebrate communities, but the findings were not as clear. Our results highlight that flow recession and drying initiate decreases in lotic diversity, but recovery of macroinvertebrate communities following a flow recession and drying depends on the substrate and vegetation characteristics of habitat patches. Identifying the ecological recovery of invertebrate communities following streambed drying from different habitat patches can provide useful information for effective biodiversity conservation and management programmes.

Diatoms are photosynthetic organisms of both marine and freshwater habitats that are able to survive and resist desiccation following connectivity loss in temporary streams. The intensive studies in recent years have shown, that ecological and population disruption can have an effect on the ecological quality of the water. Most of the studies on temporary streams have focussed on tropical and subtropical climates, due to their immediate danger regarding climate change driven droughts. However, also continental (oceanic) climates have shown an increasing tendency for summer droughts, and studies are also starting to focus on the rivers of these climates. In this study we characterize a karstic and naturally occurring temporary stream in the Paderborn plateau as a proxy for climate dependent variation through two main seasons, spring and autumn. We found that diatom resistance to drought and survival was higher at the first part of the year. Diatom species and functional composition varied according to aquatic regime more than due to time. In comparison with neighbouring permanent rivers, diatom species diversity did not vary, as long as dampness sufficed. Our results point out that continental karstic rivers could be used as proxies to tropical/subtropical temporary rivers and could also be a base for the creation of a hydrology-independent water quality index.

Due to global change, an increasing proportion of river networks are drying and shifts from perennial to non-perennial flow regimes represent ecological thresholds with dramatic and irreversible changes to community and ecosystem dynamics. However, there is minimal understanding of how biological communities respond functionally to drying, which can weaken the functional integrity of river networks and disrupt the services provided to society. Here, we shed light on the taxonomical and functional responses of aquatic macroinvertebrate communities to flow intermittence across river networks from 3 continents, to test predictions emerging from underlying trait-based conceptual theory. We predicted an high functional redundancy resulting from niche selection filtering taxa from adapted to drying, increasing flow intermittence to be associated with a reduction of taxonomic alpha diversity but not functional diversity, and a selection of specific trait modalities related to resistance or resilience to flow intermittence. Surprisingly, functional redundancy declined sharply with increasing flow intermittence: even weakly intermittent sites were functionally altered relative to perennial ones. Both taxonomic and functional alpha diversity decreased with flow intermittence. Last, a set of functional trait modalities including small body size, short life span and high fecundity were selected with increased flow intermittence. These results demonstrate the functional responses of river communities to drying and suggest that on-going biodiversity reduction due to global change in drying river networks is threatening their functional integrity.

In case of dryings, the hyporheic zone is one of the most important refugia for stream macroinvertebrate communities, including the few Odonata species living in these habitats, such as Cordulegaster species. There is no information on the desiccation resistance strategies and methods for any members of the genus, including Cordulegaster heros. We hypothesized that the larvae use burrowing behaviour to survive droughts. In this study, beyond recording the survival rates of the larvae, we tested the effects of the sediment particle size and the body size of the larvae on burrowing behaviour in a three-week-long simulated drought experiment in an indoor artificial stream system. Eighty larvae were involved in the experiment from which sixty were treated with drought and twenty served as controls. Larvae were put into flowing water, into separate special compartments; one day later the flow was ceased and then the water level was gradually decreased for three weeks. Approximately 15% of larvae could survive the three weeks of drying. The survival probability of drought treated larvae was significantly increased if animals burrowed into the sediment. Also, the survival probability was higher in case of fine substrate material. Size of the larvae only affected the depth of the burrowing, but not the survival rate. However, two thirds of the larvae did not dig into the sediment, which implies that surviving via burrowing is not the only mechanism of the species for withstanding dry periods.

Changing weather patterns and receding glaciers are increasing flow intermittency in alpine streams globally, with strong effects on biodiversity and functional processes. We conducted a before-after-control-impact (BACI) flow experiment to examine the effects of increased summer intermittency on an alpine headwater stream. We monitored macroinvertebrates, periphyton and benthic organic matter over three years. Flow in one channel was manipulated to simulate the onset of increased summer intermittency (June-September) over two consecutive years, while an adjacent channel served as a control. Before manipulation, both channels had similar ecological properties. Multi-year flow manipulation caused a reduction in rheophilic macroinvertebrate density, macroinvertebrate density and taxa number. Periphyton and benthic organic matter did not differ between channels. Recovery of the macroinvertebrate community following the experimental flow intermittency took over a year, which is discussed in the context of macroinvertebrate life histories, dispersal limitation and biotic interactions. Climate change induced shifts in stream flow regimes may lead to a fundamental shift in macroinvertebrate assemblages with local extinctions of specialized rheophilic species.