Violeta Tyufekchieva

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.

Nature-Based Solutions (NbS) are increasingly recognised as an effective approach for restoring freshwater ecosystems and enhancing river basin resilience under intensified anthropogenic and climatic pressures. However, evaluating their effectiveness requires harmonised long-term ecological monitoring and sustainable approach integrating environmental and socio-economic data. The transboundary Mesta–Nestos River Basin is challenged by agricultural intensification, hydro-morphological alterations, nutrient losses, erosion processes and increasing climate variability. These pressures threaten water quality, groundwater resources and the ecological integrity of riparian and wetland ecosystems, making the balance between agricultural productivity, water resource protection and ecosystem services a key Water–Energy–Food–Ecosystem (WEFE) Nexus challenge. This contribution presents the conceptual development of an integrated long-term environmental monitoring framework for the Bulgarian-Greek demonstration site of the PRIMA-funded NBS360 project. The framework aims to establish harmonised multidisciplinary monitoring to support the implementation, future assessment and management of Nature-Based Solutions within the transboundary Mesta–Nestos River Basin under the WEFE Nexus approach. The monitoring framework integrates observations of hydrological parameters, surface and groundwater quality, soil characteristics, biodiversity, climate and socio-economic indicators through standardised field surveys, laboratory analyses, automated monitoring systems, remote sensing and stakeholder engagement. It is designed to provide baseline environmental information prior to NbS implementation and to initiate (when lacking) the generation of long-term datasets for future ecosystem assessment. The planned Nature-Based Solutions include: restoration of alluvial forests (Habitat 91E0 and 92A0) through invasive alien species control to improve habitat quality, biodiversity conservation, riverbank stabilisation, erosion control and water quality; and assessment of existing riparian wetlands (former gravel pits) as ecosystem service hotspots supporting water and nutrient retention, sediment trapping, biodiversity conservation and groundwater recharge. restoration of alluvial forests (Habitat 91E0 and 92A0) through invasive alien species control to improve habitat quality, biodiversity conservation, riverbank stabilisation, erosion control and water quality; and assessment of existing riparian wetlands (former gravel pits) as ecosystem service hotspots supporting water and nutrient retention, sediment trapping, biodiversity conservation and groundwater recharge. The monitoring system will also provide environmental data for the development of a Digital Twin supporting integrated river basin management and decision-making. Collaboration among research institutions, environmental authorities, NGOs and local stakeholders will facilitate long-term data sharing and participatory implementation of Nature-Based Solutions in accordance with LTER (Long-Term Ecosystem Research) principles. As the NBS360 project is currently in its initial implementation phase, this contribution focuses on the conceptual framework and baseline monitoring strategy that will underpin future assessment of ecological responses to Nature-Based Solutions. By establishing harmonised long-term observations across environmental compartments, the proposed approach provides the foundation for evaluating restoration effectiveness, understanding ecosystem dynamics and supporting adaptive management under changing environmental conditions. More broadly, it contributes to strengthening the Bulgarian Long-Term Ecosystem Research (LTER-Bulgaria) network, promoting methodological harmonisation across freshwater ecosystems, and advancing evidence-based ecosystem restoration and sustainable transboundary river basin management through the integration of field observations, digital technologies, stakeholder engagement and decision-support tools.

Understanding the combined effects of longitudinal environmental gradients and seasonal variability on river communities remains a major challenge in freshwater ecology. Long-term ecological research (LTER) sites provide a unique opportunity to track long-term ecological change through integrated biological and physicochemical observations collected over extended periods. Among the biological quality elements used in river assessment, benthic algae and macroinvertebrates are particularly sensitive and suitable for long-term ecological investigations because they respond predictably to changes in water quality, habitat heterogeneity, substrate composition and flow regime. The transboundary Mesta River (known in Greece as the Nestos) represents one of the most ecologically significant mountain river systems in the Balkan Peninsula and the only riverine site within the Bulgarian Long-Term Ecosystem Research Network (LTER-BG). Originating at the confluence of the Cherna Mesta and Biala Mesta rivers in the Rila Mountains, the Bulgarian section drains approximately 2,770 km² and encompasses pronounced altitudinal, hydrological and geomorphological gradients, extending from alpine headwaters through the Razlog and Gotse Delchev valleys to the lower reaches near the Greek border. Large parts of the catchment preserve relatively natural hydromorphological conditions and belong to the Natura 2000 network, providing favorable conditions for investigating longitudinal patterns of ecosystem organization while reflecting local anthropogenic influences. Since 1978, the Mesta River has served as model lotic ecosystem for hydrobiological monitoring in Bulgaria. Macroinvertebrates are indicative of ecosystem transformstion and recovery and key communities for saprobiological characterisation and ecological status evaluation. During the period of intensive industrial pollution (1978–early 1990s), untreated wastewater from the pulp and paper mill, yeast factory and textile industry in the Razlog region transformed the river reach downstream of the Iztok River into polysaprobic zone. Following the closure of major industrial facilities and the construction of municipal wastewater treatment plants, organic loading declined, dissolved oxygen concentrations recovered and the self-purification zone became substantially shorter and the β-mesosaprobic conditions of the aquatic ecosystem began to recover. These results are also confirmed by long-term studies on the phytobenthic communities. Long-term observations have also documented significant hydroclimatic changes. Water temperature has increased by approximately 1.2–1.8°C during 1979–2026, shifting cold-water species towards upstream tributaries. Oxygen conditions improved from severe summer hypoxia/anoxia (<2–4 mg L⁻¹) during the 1980s to present-day normoxic saturation (85–110%). Mean annual discharge near Hadzhidimovo decreased from approximately 32.3 to 30.7 m³ s⁻¹, accompanied by earlier spring runoff, prolonged summer low-flow periods and increased sediment accumulation, factors that increasingly affect rheophilic macroinvertebrates. Despite nearly five decades of investigations documenting pollution, ecological situation impovement and long-term trends, contemporary analyses integrating longitudinal gradients and seasonal variability in diatom and macroinvertebrate communities’ organization remain limited. Therefore, this study extends the long-term monitoring program by investigating diatom and macrozoobenthic communities and physicochemical characteristics over two consecutive years at three representative stations located in the upper, middle and lower stretches of the river continuum. Seasonal sampling during spring, summer, autumn and winter enabled evaluation of spatial, seasonal and interannual variation in communities’ structure and its relationships with physicochemical variables using multivariate statistical analyses. The study contributes updated information on the distriburion of diatoms and benthic macroinvertebrates of Mesta River and provides new evidence for understanding the functioning and resilience of long-term monitored mountain river ecosystems.

The evaluation of nitrogen pathways in river systems and the identification of suitable aquatic ecosystem response indicators are essential for maintaining river health. Ni-trogen, in its various forms, is a key physico-chemical quality element that underpins the biological communities determining the ecological status of surface waters. Ele-vated nitrogen loads in freshwater ecosystems can trigger adverse processes such as eutrophication and acidification, both of which are linked to biodiversity loss. We hy-pothesize that specific biotic indicators can reflect nitrogen-related stress in aquatic ecosystems, acting simultaneously as biodiversity components and as bioaccumulating indicators. To test this hypothesis, we examined four study sites within the Koiliaris River watershed (Crete, Greece). Our approach aims to enhance understanding of ni-trogen dynamics at the river-basin scale by providing complementary insights into how nitrogen is processed, stored, and transferred within the aquatic ecosystem. To this end, total nitrogen content was quantified in four matrices: water, sediments, aquatic mosses, and macroinvertebrates. This study represents a pilot, exploratory assessment indicating that water may act as the primary pathway of nitrogen availability for aquatic biota in the studied streams.

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.

Humans impact terrestrial, marine and freshwater ecosystems, yet many broad-scale studies have found no systematic, negative biodiversity changes (for example, decreasing abundance or taxon richness). Here we show that mixed biodiversity responses may arise because community metrics show variable responses to anthropogenic impacts across broad spatial scales. We first quantified temporal trends in anthropogenic impacts for 1,365 riverine invertebrate communities from 23 European countries, based on similarity to least-impacted reference communities. Reference comparisons provide necessary, but often missing, baselines for evaluating whether communities are negatively impacted or have improved (less or more similar, respectively). We then determined whether changing impacts were consistently reflected in metrics of community abundance, taxon richness, evenness and composition. Invertebrate communities improved, that is, became more similar to reference conditions, from 1992 until the 2010s, after which improvements plateaued. Improvements were generally reflected by higher taxon richness, providing evidence that certain community metrics can broadly indicate anthropogenic impacts. However, richness responses were highly variable among sites, and we found no consistent responses in community abundance, evenness or composition. These findings suggest that, without sufficient data and careful metric selection, many common community metrics cannot reliably reflect anthropogenic impacts, helping explain the prevalence of mixed biodiversity trends.

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.

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.

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.