Freshwater macroinvertebrates are a diverse group and play key ecological roles, including accelerating nutrient cycling, filtering water, controlling primary producers, and providing food for predators. Their differences in tolerances and short generation times manifest in rapid community responses to change. Macroinvertebrate community composition is an indicator of water quality. In Europe, efforts to improve water quality following environmental legislation, primarily starting in the 1980s, may have driven a recovery of macroinvertebrate communities. Towards understanding temporal and spatial variation of these organisms, we compiled the TREAM dataset (Time seRies of European freshwAter Macroinvertebrates), consisting of macroinvertebrate community time series from 1,816 river and stream sites (mean length of 19.2 years and 14.9 sampling years) of 22 European countries sampled between 1968 and 2020. In total, the data include >93 million sampled individuals of 2,648 taxa from 959 genera and 212 families. These data can be used to test questions ranging from identifying drivers of the population dynamics of specific taxa to assessing the success of legislative and management restoration efforts.

Roads constitute a worldwide network of ecological barriers traversing countless streams and rivers. A large fraction of the land area lies in close proximity to roads. Ecological effects of roads likely extend well beyond the road network, suggesting wide ranging impacts on lotic ecosystems. Road impacts are multifaceted including fragmentation, changing hydrology, sedimentation and pollution. Yet, the ecological impacts are incompletely documented. We examined the effects of roads traversing lotic ecosystems on the structure and function of benthic macroinvertebrates in small temperate headwater streams. Ecological effects differed between reaches upstream and downstream of roads compared to upstream and downstream reaches in control streams. Total macroinvertebrate density and mayfly density were lower downstream of roads than upstream of roads, but they were similar at downstream and upstream reaches of control streams. Species density, but not species richness, tended to be lower downstream than upstream of roads, likely due to the lower macroinvertebrate densities at downstream sites. There were no comparable effects in control streams. Species composition and species abundances differed between road impacted streams and control streams likely because streams that were traversed by roads selected for a different set of species compared to control streams as indicated by checkerboard distribution of species in streams that were traversed by roads. Functional impacts included a greater prevalence of predators in control streams, and a higher proportion of grazers and shredders in streams that were traversed by roads. The study is inconclusive regarding the mechanisms mediating the ecological impact of roads. The ecological effects are likely caused by a combination of factors including fragmentation, pollution and hydrological change among others. Given the vast global road network, the quantitative significance of road effects driving freshwater biodiversity loss may be highly underestimated especially in smaller headwater streams comprising the major part of fluvial ecosystems.

Owing to a long history of anthropogenic pressures, freshwater ecosystems are among the most vulnerable to biodiversity loss 1 . Mitigation measures, including wastewater treatment and hydromorphological restoration, have aimed to improve environmental quality and foster the recovery of freshwater biodiversity 2 . Here, using 1,816 time series of freshwater invertebrate communities collected across 22 European countries between 1968 and 2020, we quantified temporal trends in taxonomic and functional diversity and their responses to environmental pressures and gradients. We observed overall increases in taxon richness (0.73% per year), functional richness (2.4% per year) and abundance (1.17% per year). However, these increases primarily occurred before the 2010s, and have since plateaued. Freshwater communities downstream of dams, urban areas and cropland were less likely to experience recovery. Communities at sites with faster rates of warming had fewer gains in taxon richness, functional richness and abundance. Although biodiversity gains in the 1990s and 2000s probably reflect the effectiveness of water-quality improvements and restoration projects, the decelerating trajectory in the 2010s suggests that the current measures offer diminishing returns. Given new and persistent pressures on freshwater ecosystems, including emerging pollutants, climate change and the spread of invasive species, we call for additional mitigation to revive the recovery of freshwater biodiversity.

Use of fast-growing domesticated and/or genetically modified strains of fish is becoming increasingly common in aquaculture, increasing the likelihood of deliberate or accidental introductions into the wild. To date, their ecological impacts on ecosystems remain to be quantified. Here, using a controlled phenotype manipulation by implanting growth hormone in juvenile Atlantic salmon (Salmo salar), we found that growth-enhanced fish display changes in several phenotypic traits known to be important for ecosystem functioning, such as habitat use, morphology and excretion rate. Furthermore, these phenotypic changes were associated with significant impacts on the invertebrate community and key stream ecosystem functions such as primary production and leaf-litter decomposition. These findings provide novel evidence that introductions of growth-enhanced fish into the wild can affect the functioning of natural ecosystems and represent a form of intraspecific invasion. Consequently, environmental impact assessments of growth-enhanced organisms need to explicitly consider ecosystem-level effects.