River networks contribute disproportionately to the global carbon cycle. However, global estimates of carbon emissions from inland waters are based on perennial rivers, even though more than half of the world's river length is prone to drying. We quantified CO ₂ and CH ₄ emissions from flowing water and dry riverbeds across six European drying river networks (DRNs, 120 reaches) and three seasons and identified drivers of emissions using local and regional variables. Drivers of emissions from flowing water differed between perennial and non‐perennial reaches, both CO ₂ and CH ₄ emissions were controlled partly by the annual drying severity, reflecting a drying legacy effect. Upscaled CO ₂ emissions for the six DRNs at the annual scale revealed that dry riverbeds contributed up to 77% of the annual emissions, calling for an urgent need to include non‐perennial rivers in global estimates of greenhouse gas emissions.

River fragmentation is one of the main threats to diadromous fish species. We aimed to create a new and simple connectivity index to calculate habitat accessibility that considers habitat suitability for fish species, using the Bidasoa River basin in the north of Spain and Atlantic salmon (Salmo salar) as an example. A habitat connectivity index (HCI) was calculated for the river basin using upstream passability and segment length as variables. We then calculated a new habitat quality index for each river segment and multiplied it by river segment length to create the Breeding Habitat Connectivity Index (HCIb). These 2 indices were first calculated using only upstream barrier passability and then by adding downstream passability. In each case, the indices show different outcomes but a similar pattern: in all cases, main-stem obstacles closest to the river mouth most affected the connectivity index, even when habitat quality was considered. Although we cannot compare the indices to the real area used by salmon because spatial tracking was not performed during the study years, we consider that including habitat quality in a river connectivity index adds useful information for scientists and managers.

The Mekong River, a global freshwater biodiversity hotspot, has suffered from intensive barrier construction, resulting in major challenges in safeguarding its fauna. Here, we provide a comprehensive evaluation of the impacts of river barriers on the distribution of 952 fish species in the Mekong River Basin. Our analysis revealed that 93% of Mekong fish species analysed suffer from habitat fragmentation, and species with larger habitat range requirements experienced higher river fragmentation impacts. Sub-basins along the main channel in the Lower Mekong had high values of species richness but relatively high barrier impacts. Across all migration types, freshwater-resident migratory (potamodromous) fishes were affected by the greatest levels of habitat fragmentation (Fragmentation Index, 42.45 [95% confidence interval, 38.61–46.62]). Among all International Union for Conservation of Nature conservation status categories, Critically Endangered species experienced the highest habitat fragmentation index (34.48 [19.46–53.52]). Among all barrier types in the Mekong, small dams and sluice gates contributed more to habitat fragmentation than large dams. While the effects of existing individual large dams on habitat fragmentation and fish distribution in the Mekong Basin are greater than for small barriers, the cumulative impacts of small barriers are greater; hence, basin-wide connectivity planning is needed for more effective conservation.

The signal crayfish Pacifastacus leniusculus Dana, 1852 is a successful invasive species in the Iberian Peninsula. This is of particular conservation concern, as fish fauna is highly endemic and also threatened within this ecoregion. The aim of this study was to analyze behavioral interactions and trophic overlap between signal crayfish and native fishes in Iberian rivers (northern Spain). Video cameras were used to record fish “dominance/evasion” after spatial encounters with crayfish. Diet composition and isotopic signatures (δ¹³C and δ¹⁵N) were compared to evaluate the niche overlap. Fish were dominant in 25% of the encounters with juvenile crayfish, whereas this percentage was only 4% with adult crayfish. Observations showed a high fish “evasion” response for Pyrenean stone loach Barbatula quignardi (Bǎcescu-Meşter, 1967) (>30%). Dietary results showed a high trophic overlap between signal crayfish with the pelagic Pyrenean minnow Phoxinus bigerri Kottelat, 2007 and the benthic loach. However, the isotopic niche overlap was low, with brown trout Salmo trutta L., 1758 showing the highest area (only 0.1 ‰²). Overall, our findings suggest that interferences may occur with native species for food (i.e., benthic invertebrates). Consequently, measures should be applied to control invasive crayfish in Iberian rivers.