A significant fraction of Earth's ecosystems undergoes periodic wet‐dry alternating transitional states. These globally distributed water‐driven transitional ecosystems, such as intermittent rivers and coastal shorelines, have traditionally been studied as two distinct entities, whereas they constitute a single, interconnected meta‐ecosystem. This has resulted in a poor conceptual and empirical understanding of water‐driven transitional ecosystems. Here, we develop a conceptual framework that places the temporal availability of water as the core driver of biodiversity and functional patterns of transitional ecosystems at the global scale. Biological covers (e.g., aquatic biofilms and biocrusts) serve as an excellent model system thriving in both aquatic and terrestrial states, where their succession underscores the intricate interplay between these two states. The duration, frequency, and rate of change of wet‐dry cycles impose distinct plausible scenarios where different types of biological covers can occur depending on their desiccation/hydration resistance traits. This implies that the distinct eco‐evolutionary potential of biological covers, represented by their trait profiles, would support different functions while maintaining similar multifunctionality levels. By embracing multiple alternating transitional states as interconnected entities, our approach can help to better understand and manage global change impacts on biodiversity and multifunctionality in water‐driven transitional ecosystems, while providing new avenues for interdisciplinary studies.

The study of leaf litter as a resource for shredders has emerged as a key topic in trophic links in ecology. However, thus far, most studies have emphasized the leaf quality as one of the main determinants of shredder behaviour and growth without simultaneously considering the leaf quantity availability. Nevertheless, the combined effects of leaf quantity and quality on shredder behaviour and growth is particularly crucial to further understand how ecosystem functioning may respond to the increasing flow intermittency due to climate change. In this study, we explore how changes in the leaf litter quality and quantity influence the feeding preferences and growth of an invertebrate shredder (Potamophylax latipennis). To do so, we used black poplar leaves conditioned in two streams with different flow regimens as a food resource. Afterwards, using a microcosm approach, we offered leaf discs that varied in terms of leaf quantity and quality to P. latipennis. Our results showed that flow intermittency had a negative effect on the quality of the food resource, and a lower quality had a negative effect on the consumption and growth rates of P. latipennis. Furthermore, we found that P. latipennis fed selectively on higher quality leaves even though the availability (quantity) of this resource was lower. In the context of climate change, with higher aridity/drier conditions/scenarios, our findings suggest that a decrease in the availability (quantity) of high-quality resources could potentially threaten links in global fluvial food webs and thus threaten ecosystem functioning.

Predicting the impacts of global change on highly dynamic ecosystems requires a better understanding of how communities respond to disturbance duration, frequency and timing. Intermittent rivers and ephemeral streams are dynamic ecosystems that are recognized as the most common fluvial ecosystem globally. The complexity of the drying process can give rise to different annual and antecedent hydrological conditions, but their effect on aquatic communities remains unclear. Here, using aquatic invertebrates from 33 streams across a flow‐intermittence gradient, we assessed how annual (drying duration and frequency) and recent drying characteristics (duration of the last dry period and flowing duration since the last rewetting) affect the density and diversity metrics of communities and trophic groups while controlling for other key abiotic factors (dissolved oxygen and altitude). We characterized invertebrate communities using taxonomy and functional traits to capture biological features that increase vulnerability to drying. In addition, using structural equation modelling (SEM), we evaluated pathways by which drying characteristics directly impact invertebrate density and whether diversity indirectly mediates such relationships. We show that drying frequency drove reductions in diversity at the community level and within trophic groups, whereas both the drying duration and frequency had a negative influence on density metrics. Reductions in taxonomic richness were linked to increased annual drying duration, whereas functional diversity declined in response to annual drying frequency. Filterer, predator and shredder trophic groups exhibited the strongest negative responses to drying. Recent drying characteristics had a minor effect on density and diversity metrics. Our SEM results demonstrated that diversity mediates the negative impacts of annual drying duration and frequency on invertebrate density through reductions in their taxonomic richness and functional diversity. Our results underscore the importance of considering multiple drying characteristics together with the interdependence of density and diversity to better anticipate drying responses in freshwater ecosystems.

Summary: Microbes inhabiting intermittent streambeds are responsible for controlling and developing many biogeochemical processes essential for the ecosystem functions. Although streambed microbiota is adapted to intermittency the intensification of water scarcity and prolonged dry periods may jeopardise their capacity to cope with hydrological changes. This study aims to evaluate whether, and to what extent, the duration of dry periods affects streambed microbial density, diversity, composition (16S rRNA gene diversity) and functions (extracellular enzyme activities and respiration). Our results highlight the fact that hydrology modulates the community composition and, to some extent, the functions carried out under different environmental conditions. The relative abundance of certain taxa inhabiting the driest intermittent communities differs significantly from those found at sites with continuous flow. Microbial functional metrics revealed a progressive increase in recalcitrant carbon degradation activity at sites with an extended dry phase. In contrast, bacterial density and diversity were mainly influenced by the catchment land use, agriculture enhanced density but reduced diversity, and the presence of riparian vegetation supported greater streambed bacterial diversity. From this perspective, a combination of prolonged dryness with reduced riparian vegetation and increased agricultural land cover could compromise the ecosystem functioning by threaten microbially mediated processes linked to the carbon cycle.