Rivers are highly dynamic ecosystems, in which both human impacts and climate-driven drying events are increasingly common. Disentangling ecological responses to these interacting stressors is crucial to guide effective monitoring and management. We analysed the independent and interactive effects of human impacts and river drying on taxonomic richness, functional diversity and biomonitoring indices representing aquatic invertebrate assemblages in nine European countries. We calculated response metrics for the whole community and for an assemblage of drying-resistant and resilient (‘high RR’) taxa. Most community metrics decreased independently in response to impacts and drying. Biomonitoring index totals and ‘ASPTs’ showed the strongest responses, indicating their potential for adaption and use in biomonitoring. High RR taxonomic richness responded only to impacts, but little variance in high RR metrics was explained. The responsiveness of high RR metrics could be enhanced by recognizing region-specific environmental conditions and high RR taxa, to inform the development of richness metrics that respond to ecological status. To enable effective ecological status assessments in dynamic river ecosystems, including those that dry, we thus recommend the consideration of richness-independent biomonitoring indices alongside new metrics that characterize the richness of region-specific groups of resistant and resilient taxa. We also observed interactions between aridity, human impacts and drying, evidencing the need for new metrics to be flexible, to enable their ongoing adaptation to protect river ecosystems as they respond to climate change.

As saline streams are geographically widespread in arid and semiarid regions, flow intermittence frequently occurs as a stressor factor in addition to salinity. Both salinity and flow intermittence are natural stressors that shape biological communities. However, we know little about the effect of drying on aquatic invertebrates and even less on terrestrial invertebrates, especially in saline streams. In this study, we analyzed structural and compositional responses of aquatic and terrestrial invertebrate communities along drying in two intermittent saline streams (SE Spain), distinguishing among the wet, contraction and dry phases. Aquatic invertebrates were sampled during the aquatic phases (wet and contraction) with hand nets and terrestrial invertebrates were sampled with pitfall traps in the channels and their shoreline during the three phases. The highest abundance and richness values were significantly higher in contraction than in wet in both streams for both communities in the two study streams. Aquatic and terrestrial composition differed significantly among phases in both streams. For the aquatic community, Coleoptera, Ephemeroptera, Heteroptera and Odonata were the most important taxa that explained the differences, with their highest abundances in the contraction phase. For the terrestrial community, Formicidae, Araneae and Hemiptera were responsible for the detected differences, with greater abundance in the contraction and dry phases than in the wet phase. These findings evidence the significant effect of drying in both aquatic and terrestrial invertebrate communities in saline streams, with the contraction phase particularly contributing to the diversity of the communities.

The disruption of natural flow regimes by the combined effects of extensive anthropogenic pressures and climate change is transforming biodiversity patterns worldwide, due to the increasing spatial and temporal extent of drying events. Thus, characterizing changes in lotic communities as a consequence of flow intermittence is vital for their conservation. In this framework, we studied the distribution of aquatic macroinvertebrates in 24 perennial (P) and 21 intermittent (I) stream sites from northern Apennines (Po River basin, Northern Italy), aiming to evaluate changes in macroinvertebrate metrics and community structure as a consequence of flow intermittence. Macroinvertebrates were sampled before the summer dry phase in the P and I-sites for two following years (2017 and 2018), and several environmental variables (at patch, reach and basin scales) were recorded for each site. A flow permanence score (FPS) was computed using all environmental variables. Taxa richness and abundance were significantly lower in I-sites and also decreased during the second sampling year, as a consequence of an extremely dry summer. Beta-diversity was higher in I-sites but generally decreased after 2017 summer drought, with many taxa showing a decline with decreasing FPS. Conversely, few taxa increased with decreasing FPS, highlighting weak adaptation of macroinvertebrate communities to flow intermittence. Furthermore, we found that communities of I-sites were mainly affected by reach-scale environmental variables, whereas communities of P-site were driven by patch-scale variables. These findings represent valuable insight in the effects of flow alterations in the perspective of best-strategy planning to face the flow intermittence phenomenon.

Intermittent rivers and ephemeral streams, characterized by the presence of a dry phase, are widespread and abundant ecosystems worldwide. When rivers are dry, traditional indicators of river health cannot be measured, however indicators to determine their ecological quality during the dry phase are barely starting to be developed. Here, we analyzed both physico-chemical parameters of sediment and terrestrial invertebrates present in dry channels to evaluate their potential as indicators of river health. To this end, we monitored 47 intermittent and ephemeral streams located at Segura River Basin (SE Spain) during the dry phase. We analyzed the sensitivity of different physico-chemical parameters (extractable nitrate, ammonium, pH, conductivity and dissolved organic carbon) and invertebrate metrics (Total richnes and abundance, Araneae richness and abundance, Coleoptera richness and abundance, and Formicidae richness and abundance) to a multiple stressor gradient reflecting main pressures in the study area. Nitrate and ammonium were the parameters which significantly discriminated among different levels of quality classes, showing higher levels in most disturbed sites. Similarly, several invertebrate metrics
discriminated among levels of degradation. Our findings support the incorporation of dry riverbeds into ecosystem monitoring and assessment works through government policy and legislation.

Climate change represents a major challenge for the management of native fish communities in Mediterranean rivers, as reductions in discharge may lead to a change in habitat availability, both in temporary and large rivers. Moreover, the efficacy of small barriers such as weirs to fish passage may also be affected, as it will depend on the incoming flows upstream. Through physical habitat modelling, we investigated how the flows released from a large hydropower plant in the Tagus River are expected to affect the passability of native freshwater fish species through a rock weir equipped with a fish ramp. We considered not only mean daily flow data retrieved from nearby gauging stations (1991–2005), but also predicted flows supported on climatic projections (RCP 4.5 and RCP 8.5) for the Tagus River Basin. Results showed that a minimum flow of 3 m³/s may be required to ensure the passability of all species through the fish ramp of the weir and that, overall, passability is likely to occur exclusively through the fish ramp for flows lower than 30 m³/s. The habitat suitability index for each species was significantly different between the reference and climate change scenarios, with lower values in the latter ones. This study suggests that climate change may reduce the passability of native freshwater fish species in weirs, meaning that the construction of small barriers in rivers should consider the decreases in discharge that are likely to occur in global change scenarios for a suitable management of fish populations.

Animals are important in nutrient cycling. Fish represents steady pools of nutrients in aquatic ecosystem recognized as nutrient sinks, accumulating over time more nutrients than is recycled by releasing. Nevertheless, considering fish as nutrient sink or source depends on emigration and immigration, biomass changes, and fate of post mortem nutrients. Annual cease of flow in Intermittent rivers can promotes fish kills that potentially causes sources of nutrients. The goal of this study is to investigate if fish carcasses represents a Phosphorus source in intermittent rivers. We quantified phosphorus composition of the most abundant fish species in a intermittent river and temporal biomass change of these species to estimate P potentially released by fish mortality. Furthermore, we test whether fish carcasses were predated or not and identified fish scavengers in artificial river pools. The results shows a fish biomass temporal decrease in the middle reach of the river only in the dry season very end. On the other hand, in the low river reachs the overall fish biomass increase over time. Moreover, fish carcasses were highly predated in artificial river pools. River intermittence can enhance the fish role as sink of nutrients by different mecanisms that depends on the presence or absence of remanent pools. On the basis of these results, it can be concluded that fish carcasses in intermittent rivers are nutrient subsidies to terrestrial ecosystem in sites where total flow cease, whereas fish are steady pool of Phosphorus capable to accumulate nutrients during dry season remanent river pools sites.