Hydrological alteration is among the principal drivers of ecological change in floodplain wetlands, affecting ecosystem functioning, biodiversity, nutrient cycling and the provision of ecosystem services. Distinguishing natural variability from long-term environmental change requires sustained observations based on harmonised monitoring protocols. As part of the Bulgarian Long-Term Ecosystem Research Network (LTER-BG) and the Integrated European Long-Term Ecosystem, Critical Zone and Socio-Ecological Research Infrastructure (eLTER), Lake Srebarna Biosphere Reserve provides a unique natural laboratory for studying ecosystem responses to hydrological modification, ecological restoration and climate variability in one of Europe’s most valuable floodplain wetlands. The hydrology and ecological functioning of Lake Srebarna have been closely linked to its connection with the Danube River. The interruption of this connection following the construction of the protective embankment in 1949 fundamentally changed the lake’s hydrological regime. In 1978, restoration activities were limited to partial dyke removal, while the construction of the Eastern (1994) and Western (2023) Channel—have aimed to improve the water exchange and recover natural ecosystem processes. These successive alterations make Srebarna an exceptional model system for evaluating ecological responses to both hydrological degradation and restoration efforts under increasing climatic variability. The study presents recent observations (2023-2026) generated as part of the long-term environmental monitoring at Lake Srebarna eLTER Site. Approximately 250 monthly samples collected from seven representative monitoring stations were analysed, encompassing open-water areas, channels, groundwater springs, and the Danube River. The monitoring network was designed to capture the principal hydrological gradients and environmental heterogeneity of the ecosystem. Standardised field observations and laboratory analyses were used to characterise the hydrological, physicochemical and hydrochemical conditions, the nutrient dynamics and the indicators of ecosystem functioning, providing one of the most comprehensive up-to-date datasets describing the overall current status of the wetland. The obtained data revealed pronounced seasonal variability together with considerable spatial heterogeneity in the studied abiotic variables. The period was characterised by lack of sufficient flooding events and persistently low water levels, resulting in the desiccation of vast areas of peripheral pools, reduced or absent lateral connectivity within the lake and increased physicochemical differentiation among the sampling sites. These hydrological conditions influence the nutrient dynamics and physicochemical processes, and ultimately affect the overall ecosystem functioning. The spatial differences among monitoring stations further demonstrate the combined effects of natural environmental gradients and anthropogenic hydromorphological modifications, highlighting the importance of maintaining an integrated long-term monitoring network capable of detecting ecological responses to ongoing environmental changes. The observed shifts in the hydrological connectivity and physicochemical conditions indicate changes in the habitat quality and ecosystem processes, revealing the role of the water regime as the principal driver of ecological functioning in the wetland. The generated dataset establishes a baseline for distinguishing natural seasonal variability from long-term changes associated with hydrological alteration and provides a valuable basis for evaluating the effectiveness of recent restoration measures aimed at improving the hydrological connectivity between the wetland and the Danube. Beyond documenting environmental conditions, the data demonstrates the value of long-term ecological observations for translating scientific evidence into informed adaptive management. By contributing harmonised long-term observations to the eLTER research infrastructures, the study supports ecosystem restoration, biodiversity conservation and climate-change adaptation while reinforcing the role of long-term ecosystem research in the sustainable management of protected floodplain wetlands.

Srebarna is a shallow freshwater lake situated along the right bank of the Lower Danube (391–393 rkm), northeastern Bulgaria. It formed approximately 8000 years ago after the inundation of the riverside terrace, which has shaped its unique hydromorphological and ecological characteristics. In recent times, the natural hydrology of the lake has been modified and managed through a series of embankments and artificial canals which regulate and control the water exchange with the Danube, since their natural connection was interrupted with a dyke built in 1948. Nevertheless, the fundamental importance of the lake’s ecosystem has even increased, considering the degradation and desiccation of the majority of the wetlands on the Lower Danube. Srebarna is a Nature Reserve, designated as a Wetland of International Importance (Ramsar Site) due to its role in biodiversity conservation. The lake is also recognized as a UNESCO World Heritage Site for its ecological significance and function as a key habitat for a wide range of species. Despite its protected status, Srebarna faces numerous environmental challenges, e.g. eutrophication caused by sediment accumulation, nutrient enrichment and pollution often leading to algal blooms, reduced oxygen levels, and overall poor water quality, especially during the vegetation season. Rapid spread of invasive species also contributes to the degradation of the wetland ecosystem and negatively impact native species diversity. Climate change is a growing threat, as shifts in temperature and precipitation patterns alter the lake's hydrology. Desiccation of periphery pools and overgrowth with common reed and grey willow has been observed in the last years, due to the ever-decreasing water levels of the Danube. In an effort to negate these effects, during 2022 and 2023 three main activities were planned and executed: an excavation of new canal in the western part of the lake, with the main purpose to enhance the flush of accumulated sediments by creating a natural flow between the west and east canals and the main river. In addition, floating mats with reeds, along with willow overgrowth, clogging the canals were removed in order to unblock the canal flow. Significant areas of reed and reedmace were harvested in the peripheral pools in order to increase habitat heterogeneity and to open more water surfaces. About 350000 m³ sapropel sediments were planned for excavation in order to remove nutrients and increase the lake’s water volume. As part of ongoing conservation efforts for monitoring and assessment of the effectiveness of these restoration activities, advanced techniques to collect up-to-date spatial data were utilised. One key aspect is the use of UAS, which employ airborne LiDAR scanning and photogrammetry for high-resolution data collection. The primary objective is to map the variations in the altitude of the lakebed and to assess the dimensions of the artificial channels used to regulate the water flow between the lake and the main river. Scanning is performed during low-water season when environmental conditions and features can be accurately captured. In the areas where LiDAR scanning is ineffective, e.g within the waterbody, sonar technology, using a boat is employed to gather bathymetric data, providing detailed information about the underwater topography. These tools allow assessment of changes in the lakebed, particularly in the sections that are permanently under water. The lake and its floodplains are divided into 31 non-equal but similar by area sections, covering overall 981.5 ha. To capture comprehensive data for conservation and monitoring purposes, each section is scanned and photographed individually in a single autonomous flight mission using a battery-powered UAV equipped with both LiDAR and RGB camera sensors. The UAV follows flight paths that consist of two to four parallel trajectories, spanning from shore-to-shore across the lake and its adjacent floodplains. This approach is designed to optimize time efficiency and reduce risks, with each flight starting and ending on the same shore, ensuring the UAV’s safe operation. The flight trajectories maintain an average gap of approximately 70 m, and the UAV operates at an altitude of 130 m asl (approx. 120 m above the target surface), with a forward speed of 6.5 m/s and 70% side overlap, allowing dual-mode scanning. The LiDAR data collected is processed to generate an initial LAS point cloud, which is then converted into a digital elevation model (DEM). Sonar data is integrated into a GIS layer, aligning with the DEM grid cell centres to produce a seamless, improved DEM. This advanced processing technique ensures that the transition between the lakebed and the surrounding dryland surface is smooth and accurate in the final DEM, aiding the evaluation of hydrological changes over time. Photogrammetric data is also processed to generate a complete RGB orthomosaic of the entire lake and its surrounding floodplains. The orthomosaic serves as a visual representation for further analysis, enabling the monitoring of water body changes and the assessment of the wetland vegetation dynamics.

Coal-fired power plants can adversely affect aquatic ecosystems through wastewater discharge, waste landfills, and the atmospheric deposition of toxic substances released during coal combustion. These processes degrade the water quality of nearby surface and underground water bodies. The study presents the impact of the coal-fired power plant Contour Global Maritza East 3 on the ecological status of the Sokolitsa River, reflected by changes in the composition and structure of the sensitive phytobenthos and macrozoobenthos communities and supporting environmental variables, including water temperature, pH, dissolved oxygen, conductivity, nutrients, sulfates, calcium, and calcium carbonate hardness. Methods for monitoring and assessing the ecological status of surface water bodies compliant with European and national legislation were applied to the studied biological quality elements and key physicochemical variables. Historical monitoring data from a ten-year period, 2013–2022, together with data collected during the study in 2023 and 2024 were analyzed and evaluated. The results indicated a significant increase in most physicochemical variables downstream of the CFPP compared with the upstream site, including water temperature, conductivity, calcium carbonate hardness, calcium, sulfates and nitrogen (N) nutrients (ammonium N, nitrite N, nitrate N, total N). The ecological status of the river deteriorated, as indicated by the negatively affected aquatic habitats and the changes in the taxonomic richness and abundance of the studied organism groups.

Introduction: Aquatic floodplain ecosystems along the Lower Danube are among most biodiverse freshwater habitats in Europe, yet they are increasingly affected by eutrophication, hydrological alteration and climate change. Long-Term Ecosystem Research (LTER) provides a framework for integrating biodiversity observations with environmental drivers across these ecosystems. Scope: The study aimed to assess the biodiversity and ecological status of aquatic ecosystems on two of the largest Danube floodpains - Brăila islands and Danube Delta, Romania. The objectives were to: characterize the qualitative and quantitative composition of phytoplankton, phytobenthos, macrophytes, zooplankton, and macrozoobenthos, and assess ecological status/potential using phytobenthos and macrophytes as biological quality elements. evaluate sensor-based versus microscopy-based monitoring methods for detection and assessment of the main algal groups. characterize the qualitative and quantitative composition of phytoplankton, phytobenthos, macrophytes, zooplankton, and macrozoobenthos, and assess ecological status/potential using phytobenthos and macrophytes as biological quality elements. evaluate sensor-based versus microscopy-based monitoring methods for detection and assessment of the main algal groups. Materials and Methods: Ecological monitoring was conducted in July 2025 at ten sites along the Lower Danube, including two Brăila Island lakes (Blasova and Zaton), two Danube sidearm stations along Brăila islands, three stations at Carasuhat Canal and three in Razim Lake. In situ measurements of temperature, dissolved oxygen, electrical conductivity, pH, as well as concentration of photosynthetic pigments like chlorophyll a , phycocyanin, and fluorescence-based algal group were measured using YSI EXO2 multiparameter probe and FluoroProbe BBE fluorometer. Five groups of organisms - phytoplankton, phytobenthos, zooplankton, macroinvertebrates, and macrophytes were sampled and taxonomically identified. Ecological status was assessed for phytobenthos (IPS/EQR) and macrophytes (AIM, RI/EQR) using national assessment methods for river type R6 and lake type L5. Phytoplankton abundance and biomass, as well as the richness and abundance of macroinvertebrate, zooplankton and macrophyte communities, were also determined. Results: Considerable differences in biodiversity and ecological status were observed among the investigated ecosystems. Phytoplankton richness reached 148 taxa (100 identified to species level): Cyanoprokaryota (35 taxa), Chlorophyta (72), Bacillariophyta (17), Cryptophyta (6), Euglenophyta (12), Chrysophyta (1), Pyrrhophyta (2), and Xanthophyta (3). Species diversity (Shannon index, H’) ranged from 1.59 at lake Blasova to 4.62 in the Danube Delta. Overall, 236 diatom taxa were identified, with nearly half exceeding 1% relative abundance in at least one of the samples. Most of the sites were dominated by halobious taxa, tolerant of higher salinity, particularly Lake Blasova, where marine-brackish species comprised approximately 90% of the diatom flora. The highest macrophyte diversity was recorded in Razim Lake, including the rare species Nuphar lutea and Trapa natans . The Danube sidearm stations were dominated by Vallisneria spiralis and Butomus umbellatus. Macroinvertebrates were most diverse (with more than 10 systematic groups) in Zaton and Blasova Lakes and the coastal part of Razim Lake, whereas the Danube sidearm upstream of Brăila and Carasuhat Canal supported less diverse communities dominated by molluscs, with bivalves prevailing at the river sites. Overall, 74 zooplankton taxa were identified, including 14 Crustacea (7 Cladocera and 7 Copepoda) and 60 Rotifera. Brachionus angularis was the dominant species in Brăila lakes, Brachionus forficula dominated Razim lake , while nauplii and copepodites co-dominated along with several Brachionus species in the Danube sites. The ecological status/potential based on phytobenthos ranged from bad (Lake Blasova and Carasuhat Canal) to poor (Carasuhat Site 2) and moderate (Danube, Razim Lake and Zaton Lake). The assessment based on macrophytes community classified Carasuhat Canal, Zaton Lake and Blasova Lake in bad ecological status/potential, while Razim Lake in moderate ecological status and Danube sidearm stations in moderate and good status. Fluorescence-based estimates of the major phytoplankton groups were comparable with conventional microscopy, with both methods generally identifying the same dominant groups, despite differences in the calculated contributions of cyanobacteria, green algae, and diatoms (PERMANOVA, p = 0.8616). Conclusion: The biodiversity and ecological condition of the investigated floodplain ecosystems differed, reflecting habitat heterogeneity and hydrological connectivity. Most sites exhibited poor to moderate ecological status despite their high biodiversity. The good comparability between fluorescence-based sensor measurements and conventional microscopic analyses indicates that sensor-based monitoring could provide a robust approach for long-term ecosystem assessment within the LTER framework.

Srebarna is a shallow freshwater lake situated along the right bank of the Lower Danube, northeastern Bulgaria. In recent times, the natural hydrology of the lake has been modified and managed through a series of embankments and artificial canal which regulate and control the water exchange with the Danube, since their natural connection was interrupted with a dyke built in 1948. Nevertheless, the fundamental importance of the lake’s ecosystem has even increased, considering the degradation of the majority of Lower Danube’s wetlands. Despite its protected status, the lake faces numerous environmental challenges, e.g. eutrophication caused by nutrient enrichment and sediment entrapment, leading to algal blooms, oxygen deficiency, and overall poor water quality, especially during the vegetation season. Desiccation of periphery pools and overgrowth with common reed and grey willow has been observed in the last years, due to the decreased flood potential of the Danube River. Succession in biodiversity like rapid spread of invasive and alien species, followed the degradation of the wetland ecosystem and negatively impact native species diversity. Climate change is a growing threat, as shifts in temperature and precipitation patterns alter the lake's hydrology. In an effort to negate these effects, during 2022 and 2023 several main activities were manged as restoration acitivies: an excavation of second canal in the western part of the lake, floating mats cut off, reed harvest and sapropel excavation. As part of ongoing conservation efforts for monitoring and assessment of the morphological changes after these restoration activities, advanced techniques to collect up-to-date spatial data were utilised. One key aspect is the use of UAS, which employ airborne LiDAR scanning and photogrammetry for high-resolution data collection. The primary objective is to map the variations in the altitude of the lakebed and to assess the dimensions of the artificial channels used to regulate the water flow between the lake and the main river. Scanning is performed during low-water season when environmental conditions and features can be accurately captured. In the areas where LiDAR scanning is ineffective, e.g within the waterbody, sonar CHIRP technology on a boat is employed to gather bathymetric data, providing detailed information about the underwater topography. These tools allow assessment of changes in the lakebed, particularly in the sections that are permanently under water. The lake and its surrounding floodplains are divided into 31 non-equal but similar by area sections, covering overall 981.5 ha. To capture comprehensive data for conservation and monitoring purposes, each section is scanned and photographed individually in a single autonomous flight mission using a battery-powered UAV equipped with both LiDAR and RGB camera sensors. The UAV follows flight paths that consist of two to four parallel trajectories, spanning from shore-to-shore across the lake and its adjacent floodplains. This approach is designed to optimize time efficiency and reduce risks, with each flight starting and ending on the same shore, ensuring the UAV’s safe operation. The flight trajectories maintain an average gap of approximately 70 m, and the UAV operates at an altitude of 130 m asl (approx. 120 m above the target surface), with a forward speed of 6.5 m/s and 70% side overlap, allowing dual-mode scanning. The LiDAR data collected is processed to generate an initial LAS point cloud, which is then converted into a digital elevation model (DEM). Sonar data is integrated into a GIS layer, aligning with the DEM grid cell centres to produce a seamless, improved DEM. This advanced processing technique ensures that the transition between the lakebed and the surrounding dryland surface is smooth and accurate in the final DEM, aiding the evaluation of morphological changes over time. Photogrammetric data is also processed to generate a complete RGB orthomosaic of the entire lake and its surrounding floodplains. The orthomosaic serves as a visual representation for further analysis, enabling the monitoring of water body changes and the assessment of the wetland vegetation dynamics. The high-resolution imagery provides a comprehensive overview of the lake’s current state and facilitates informed decision-making for future conservation activities. Long-term ecological and biological monitoring, the use of advanced technologies, continuous collection and analysis of spatial data, and the application of sustainable management practices, are essential for the preservation of the integrity of this unique wetland, it’s biodiversity and the overall health of the Danube’s floodplain ecosystem.

Introduction: Freshwater ecosystems of the Lower Danube and Black Sea coastal region are among the most valuable aquatic habitats in southeastern Europe, supporting exceptional biodiversity and providing essential ecosystem services. They comprise interconnected riverine, floodplain, and coastal freshwater ecosystems that differ in hydrological connectivity, physicochemical conditions, and ecological functioning. Many of these ecosystems belong to the Bulgarian and Romanian LTER/eLTER network, where long-term ecological observations improve our understanding of ecological processes and support biodiversity conservation, restoration, and sustainable management. However, hydrological alteration, floodplain regulation, eutrophication, habitat modification, and climate-related changes have increasingly affected these ecosystems, highlighting the need for ecological assessment. Diatoms are widely recognized as sensitive indicators of environmental conditions and ecological status and are routinely applied in biomonitoring under the European Water Framework Directive (WFD). Nevertheless, regional assessments of benthic diatom diversity and ecological status across these representative freshwater ecosystems of the Lower Danube and Black Sea coastal region remain limited. Scope: The study aimed to assess the ecological status, diatom diversity, and community composition of freshwater ecosystems in the Lower Danube and Black Sea coastal region of Bulgaria and Romania. Particular emphasis was placed on evaluating the relationships between benthic diatom assemblages, ecological status, and key environmental gradients, especially water depth and electrical conductivity. Materials and Methods: The study included representative freshwater ecosystems from the Bulgarian LTER network and the Romanian LTSER Bălțile Brăilei Research Platform, comprising two Lower Danube River sites (Bulgaria and Romania), Srebarna Lake, Brăila lakes (Blasova and Zaton), Carasuhat Canal, Razim Lake, Durankulak Lake, Shabla Lake, and Ezerets Lake. These sites represent complementary riverine, floodplain, and coastal freshwater ecosystems within a common Lower Danube–Black Sea biogeographical region. Srebarna is a UNESCO World Heritage Site, Ramsar Site, and Biosphere Reserve, while Blasova and Zaton are relict lakes of the historical Lower Danube floodplain. Razim Lake represents the freshwater component of the Razim–Sinoie lagoon system, and the Bulgarian coastal lakes are protected wetlands of international conservation importance. At each site, in situ measurements of water temperature, dissolved oxygen, electrical conductivity, pH, water depth, and other key physicochemical variables were performed. Benthic diatom samples were collected, processed, and identified according to the most recent European standards. Species richness, Shannon diversity (H′), community composition, and ecological status based on the Indice de Polluo-sensibilité (IPS), expressed as the Ecological Quality Ratio (EQR), were determined. Relationships between benthic diatom assemblages, ecological status, and environmental variables were evaluated. Results: A total of over 300 diatom taxa were identified across the investigated ecosystems, demonstrating considerable diversity and variation in community composition among riverine, floodplain, and coastal freshwater habitats. The analyses indicate differences in ecological status and benthic diatom assemblages among the investigated ecosystems, corresponding to variation in environmental conditions. Taxa indicative of elevated ionic conditions were particularly well represented in Brăila lakes, where halobious species comprised a high proportion of the recorded flora. Conclusions: The investigated freshwater ecosystems encompass a broad range of ecological conditions representative of the Lower Danube and Black Sea coastal region. Preliminary findings suggest that water depth and electrical conductivity are among the principal environmental gradients influencing benthic diatom assemblages and ecological status. This study provides a regional baseline for assessing freshwater biodiversity and ecological quality across Bulgaria and Romania and contributes to long-term ecological research of freshwater ecosystems.