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Owing to a long history of anthropogenic pressures, freshwater ecosystems are among the most vulnerable to biodiversity loss1. Mitigation measures, including wastewater treatment and hydromorphological restoration, have aimed to improve environmental quality and foster the recovery of freshwater biodiversity2. 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.

Natural ecosystems typically consist of many small and few large organisms. The scaling of this negative relationship between body mass and abundance has important implications for resource partitioning and energy usage. Global warming over the next century is predicted to favour smaller organisms, producing steeper mass–abundance scaling and a less efficient transfer of biomass through the food web. Here, we show that the opposite effect occurs in a natural warming experiment involving 13 whole-stream ecosystems within the same catchment, which span a temperature gradient of 5–25 °C. We introduce a mechanistic model that shows how the temperature dependence of basal resource carrying capacity can account for these previously unexpected results. If nutrient supply increases with temperature to offset the rising metabolic demand of primary producers, there will be sufficient resources to sustain larger consumers at higher trophic levels. These new data and the model that explains them highlight important exceptions to some commonly assumed ‘rules’ about responses to warming in natural ecosystems.

This is a policy discussion paper aimed at addressing possible alternative approaches for environmental flows (e- Flows) assessment and identification within the context of best strategies for fluvial restoration. We focus on dammed rivers in Mediterranean regions. Fluvial species and their ecological integrity are the result of their evolutionary adaptation to river habitats. Flowingwater is themain driver for development and maintenance of these habitats, which is why e-Flows are needed where societal demands are depleting water resources. Fluvial habitats are also shaped by the combined interaction ofwater, sediments, woody/organicmaterial, and riparian vegetation. Water abstraction, flowregulation by dams, gravel pits or siltation by fine sediments eroded fromhillslopes are pressures that can disturb interactions among water, sediments, and other constituents that create the habitats needed by fluvial communities. Present e-Flow design criteria are based only on water flow requirements. Here we argue that sediment dynamics need to be considered when specifying instream flows, thereby expanding the environmental objectives and definition of e-Flows to include sediments (extended e-Flows). To this aim, a hydromorphological framework for e-Flows assessment and identification of best strategies for fluvial restoration, including the context of rivers regulated by large dams, is presented. [ABSTRACT FROM AUTHOR]

Streams and rivers are an important source of CO2emissions. One important control of these emissions is the metabolic balance between photosynthesis, which converts CO2to organic carbon, and respiration, which converts organic carbon into CO2(refs ,). Carbon emissions from rivers could increase with warming, independently of organic carbon inputs, because the apparent activation energy is predicted to be higher for respiration than photosynthesis. However, physiological CO2-concentrating mechanisms may prevent the increase in photorespiration, limiting photosynthesis with warming. Here we report the thermal response of aquatic photosynthesis from streams located in geothermal areas of North America, Iceland and Kamchatka with water temperatures ranging between 4 and 70 °C. Based on a thermodynamic theory of enzyme kinetics, we show that the apparent activation energy of aquatic ecosystem photosynthesis is approximately 0.57 electron volts (eV) for temperatures ranging from 4 to 45 °C, which is similar to that of respiration. This result and a global synthesis of 222 streams suggest that warming will not create increased stream and river CO2emissions from a warming-induced imbalance between photosynthesis and respiration. However, temperature could affect annual CO2emissions from streams if ecosystem respiration is independent of gross primary production, and may be amplified by increasing organic carbon supply.

Dredging or channelization has physically modified the majority (90%) of the 64 000 km of Danish stream network with substantial habitat degradation as a result. Analyses of physical habitat structure in streams, biota, catchment features and regional differences in hydrology, topography and geology have never been carried out in Denmark. Therefore, there is little knowledge of processes, interactions and patterns across the different scales. Physical habitats, catchment parameters and macroinvertebrates were sampled at 39 sites in three major river systems during summer and winter 1993.

Algal biomass, standing stocks of particulate organic matter (POM) and benthic invertebrate communities of 20 streams draining native forest, exotic conifer forest and tussock grassland catchments in the South Island, New Zealand, were examined in spring/summer 1994-95. Sixteen were forest streams, 8 in native and 8 in exotic forest, with 4 of each kind located on each side of the Main Alpine Divide and therefore in different climatic (including rainfall) zones. Chemistry of stream water reflected geographic location more strongly than forest type with western streams being slightly more acid and having lower ionic concentrations than those in the east. Algal biomass on stones in forest streams ranged from 7-51 mg chl · m-2, and regardless of forest type, appeared to be limited primarily by light and substrate stability. POM standing stocks were low (22-254 g AFDW m-2) and dominated by coarse materials (> 13 mm) in both native and exotic forest streams. Benthic faunas of all stream groups were dominated numerically by Chironomidae and Ephemeroptera and contained similar numbers of species (35-40). However, differences in community composition were found between eastern and western streams, and these were greater than between forest types. Total invertebrate abundances were moderately low (2008-6083 m-2) and were significantly correlated with epilithic algal biomass. In contrast, correlations between invertebrate numbers and detrital biomass were weak, perhaps reflecting poor retention of riparian inputs. Our results support the view that forest type has little effect on stream benthic communities in the South Island, New Zealand, and that geographical factors (climate, location) are of greater importance.

Freshwater habitats and organisms are among the most threatened on Earth, and freshwater ecosystems have been subject to large biodiversity losses. We developed a Climate Change Sensitivity (CCS) indicator based on trait information for a selection of stream- and lake-dwelling Ephemeroptera, Plecoptera and Trichoptera taxa. We calculated the CCS scores based on ten species traits identified as sensitive to global climate change. We then assessed climate change sensitivity between the six main ecoregions of Sweden as well as the three Swedish regions based on Illies. This was done using biological data from 1, 382 stream and lake sites where we compared large-scale (ecoregional) patterns in climate change sensitivity with potential future exposure of these ecosystems to increased temperatures using ensemble-modelled future changes in air temperature. Current (1961~1990) measured temperature and ensemble-modelled future (2100) temperature showed an increase from the northernmost towards the southern ecoregions, whereas the predicted temperature change increased from south to north. The CCS indicator scores were highest in the two northernmost boreal ecoregions where we also can expect the largest global climate change-induced increase in temperature, indicating an unfortunate congruence of exposure and sensitivity to climate change. These results are of vital importance when planning and implementing management and conservation strategies in freshwater ecosystems, e.g., to mitigate increased temperatures using riparian buffer strips. We conclude that traits information on taxa specialization, e.g., in terms of feeding specialism or taxa having a preference for high altitudes as well as sensitivity to changes in temperature are important when assessing the risk from future global climate change to freshwater ecosystems.

Algal biomass, microbial activity and invertebrate colonization were investigated in 20 streams in the South Island, New Zealand. Sixteen streams drained catchments with native or exotic forest and four were unshaded, non-forested sites. Algal biomass on stones was highest at the unshaded sites and was greater at forested sites east of the Alpine Divide than at forested sites on the western side. Algal biomass on nutrient-diffusion substrata also showed significant location (east > west) and nutrient effects. However, responses to nutrient additions were variable among stream groups, with unshaded and eastern native-forest streams showing the strongest response. Abundances of invertebrates (mainly Chironomidae) colonizing diffusion substrata were positively correlated with algal biomass in eastern native forest streams and unshaded streams but not the other treatments. Microbial activity, expressed as loss in weight of cellulose cloth over a three-week period, was unaffected by location/vegetation type but increased significantly in response to nutrient additions. Extra keywords: algal biomass, cellulose breakdown, forest type, invertebrates, nutrient additions.