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.

A survey of the macrozoobenthos communities in the Maritsa River (South Bulgaria) was carried out in the summer of 2021. Benthic samples were collected and physicochemical parameters (water temperature, pH, conductivity, dissolved oxygen and nutrients) were measured at 15 sites located on the main river and its tributary system. The studied sites belonged to different river types and characterised the diversity of the ecological conditions—from unaffected to anthropogenically influenced river stretches. In addition, data from a study conducted in the summer of 2020 were used to analyse species–factor interactions in the river ecosystems and to assess the bio-indicative potential of the aquatic invertebrates. The dynamics of the taxonomic composition and abundance of the macrozoobenthos were analysed in relation to environmental factors. The physicochemical conditions of the water environment changed during the period of high water, which led to a reduction in the composition of the macrozoobenthos. Plecoptera and Trichoptera decreased in richness and abundance downstream and under human impacts. Ephemeroptera and Chironomidae were permanently present along the whole river. Oligochaeta increased in the lower river reaches and at sites with a greater amount of organic matter. The ecological status determined by the macrozoobenthos varied from high (site 1) to good, moderate and bad (site 13) at the studied sites.

The importance of adequate biological assessments of rivers based on aquatic assemblages is essential to establish recovery measures. Macrophyte and macroinvertebrate communities react differently in time and in response strength to diverse stressors. Our hypothesis was that each group response is a result of specific and combined abiotic factors and each stressor’s impact. To address the above, both biological quality elements (BQEs) and values of the ecological quality ratio (EQR) were studied in relation to four abiotic parameters and five physico-chemical stressors. Discrepancies of more than one degree between the ecological status assessments of Bulgarian river sites determined using macrophytes and macrozoobenthos were discussed. The RDA analysis showed that altitude had a determining role in shaping the abundance of macrophyte and macrozoobenthos communities. Aquatic flora richness positively correlated with nitrogen enrichment and macroinvertebrate fauna—with altitude and biochemical oxygen demand (BOD). Nutrients and shading were most significant for the ecological status evaluation defined with both macrophytes and macrozoobenthos. Macrophyte-based EQR was related to oxygen concentration and shading, while macroinvertebrate-based EQR was better at sites with coarser substrates. Among tested stressors, mainly total nitrogen and BOD explained the lower macrophyte-based assessment at half of the studied sites. In conditions of increased nitrogen and BOD, but remaining in the range of good status, macrophytes as primary producers gave a faster and stronger response. Despite the differences in the assessment, both BQEs have higher values in conditions of lower BOD and total phosphorus.

The assessment of the current state of lakes and reservoirs is a prerequisite for the conservation and management of these ecosystems as well as for their restoration and mitigation of the damages triggered by various pressures. The aim of the present study was to explore associations of the zooplankton communities with the trophic state and (or) ecological potential (EP) of reservoirs, incorporating also standard physical and chemical factors and primary production. We recorded 48 zooplankton taxa: 32 rotifer, nine cladoceran and seven copepod taxa. Our results suggested a significant relationship of both zooplankton community and univariate metrics with trophic state index (TSI) and conductivity. Reservoirs with the highest correlation with TSI and chlorophyll a were with the highest trophic state. Most of the rotifers identified as indicator taxa evidenced eu-mesotrophic conditions or moderate-poor EP. The indicator cladocerans were significantly associated with moderate EP. The indicator copepods were associated with higher EP and oligo-mesotrophic conditions. Policy-driven approaches identified littoral macrozoobenthos as the key invertebrate group for the assessment of lentic water bodies, while science-driven studies focus more on the pelagial zooplankton communities in assessments of ecosystem health. The pelagial and the littoral of a lake (or a reservoir) are very different in the conditions, which they provide; they could be affected by distinct stressors, thus triggering a specific response of their communities. Studying lentic ecosystems in a more holistic way, across various water body types and for longer periods, would result in rigorous assessment schemes and improved management of the ecosystem health.

Reliable assessment of the ecological status of standing waters is key for the effective implementation of the Water Framework Directive and for supporting water management and conservation. Biological quality elements, including benthic macroinvertebrates, provide an integrated measure of ecological condition by reflecting the cumulative impacts of environmental pressures over time. However, assessment methods developed for running waters are not readily applicable to standing waters, highlighting the need for category-specific approaches. We conducted a large-scale survey of representative stagnant water bodies (lakes and reservoirs) on the territory of Bulgaria. The goal was to develop a relevant type-specific methodology for the ecological status assessment of standing waters based on macrozoobenthos. We sampled 79 monitoring lake sites between May and November 2020 through an adapted version of the multi-habitat sampling methodology developed within the European AQEM/STAR projects in accordance with applicable standards. A composite sample was taken from the littoral zone of lakes and reservoirs, representative of the substrates available at each location. The sites represent 19 lake types. Eleven biological indices for lakes and seven for rivers were calculated in order to identify the most suitable metric. For each of the metrics, the pressure–response relationship was estimated. Being the first operational national index based on macroinvertebrates, the proposed Bulgarian multi-metric index for lakes is an adapted version of the Hungarian multi-metric macroinvertebrate index. It consists of three metrics, which have been separately tested in Bulgaria for monitoring purposes as early as 2011–2012 as well as in subsequent scientific research. Ecological quality class boundaries for each type of water body were determined. The compatibility of the BMMI with the existing national monitoring programme would facilitate its future integration into the biological water assessment system.