The cyanobacterial species Raphidiopsis raciborskii (Woloszy ' nska) Aguilera et al. has a high invasiveness potential, which in less than a century leads to its cosmopolitan spread. In the temperate climate of Europe, R. raciborskii has been reported in many countries, but there is still a lack of detailed information about the current status of its distribution in lakes of Bulgaria, as a part of the southeastern range of its spread in Europe. We investigated the distribution of the species using data on the phytoplankton of 122 lakes surveyed during 13years period (2009-2022). The species was found in 33 lakes (up to 324 m asl), and 14 new localities were registered during the studied period. The results reveal that the number of lakes with the presence of R. raciborskii (27 % of all research lakes) and its contribution to the total phytoplankton biomass, has increased significantly over the last decade. The species has successfully adapted and dominated the phytoplankton in 9 lakes, forming a bloom in 8 of them. The dominant position of R. raciborskii causes loss of species and functional diversity of phytoplankton and displaces the native bloom-forming cyanobacteria. Lakes with and without the species were compared based on the available data on bioclimatic and local environmental variables. Statistically significant differences were established with respect to water transparency, conductivity, maximum depth and maximum air temperature in the warmest month. Species distribution models (SDMs) were used to identify lakes in high risk of future invasion by R. raciborskii. The results of the SDMs implementation confirmed the high maximum air temperature and low water transparency to be important predictors of the occurrence of R. raciborskii in freshwater lakes in Bulgaria. In the areas with high summer temperatures the most suitable for R. raciborskii development were found to be shallow polymictic or medium deep lakes with small surface area and low water transparency. In areas with a suitable climate, the large, deep reservoirs with high transparency as well as macrophyte dominated lakes have a low probability of occurrence of R. raciborskii. Future colonization of lakes above 500 m asl (but most likely below 700 m asl) is also possible, especially in the conditions of global warming. SDMs account for climatic and biogeographic differences of lakes and could help in elucidating the underlying factors that control the occurrence and adaptation of R. raciborskii in a given area.

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Spatial, seasonal, and annual bacterioplankton dynamics in recently infested by the speciesDreissena polymorphaOgosta Reservoir were studied for the first time during three year period. Bacterioplankton total number was higher in spring in ecotone zones, than in summer at thermocline. NH₄-N, PO₄-P, turbidity, dissolved oxygen, COD and chlorophyll-a correlate positively, while transparency and Ca²⁺negatively with bacteria. Nematode species composition, included 22 species studied (13 rarely found andRhabditis brevispinanew for Bulgaria) belonging to nine families. TheD. polymorphaimpact is positive on nematodes and phytoplankton, negative on zooplankton and bacterioplankton, but weak positive on larger bacteria, rods and attached bacteria.

Functional Group V (FG V), which comprises anoxygenic phototrophic purple and green sulfur bacteria, has traditionally been associated with stratified lakes, where pronounced vertical redox gradients are typically described. Here, we report the repeated occurrence of anoxygenic phototrophic sulfur bacteria in two shallow riparian wetlands, suggesting that the ecological context of this functional group may extend beyond classical stratified systems. Environmental variables and phytoplankton functional structure were analysed in a lowland and a semi-mountain wetland characterized by dense macrophyte cover, high summer temperatures, elevated organic matter content, and low dissolved oxygen conditions. FG V, represented by Chromatium okenii and Thiospirillum jenense, was detected in all samples and episodically reached high relative abundance. It co-occurred within a functionally coherent assemblage of motile flagellates (W1, X2, Y), filamentous cyanobacteria (TC), and metaphytic diatoms (MP), typical of shallow, weakly mixed systems. Although direct redox measurements were not performed, the combination of low dissolved oxygen, high organic loading, and restricted mixing indicates the potential for reduced conditions and localized redox discontinuities. Our results are consistent with the interpretation that macrophyte-dominated shallow wetlands may locally provide environmental conditions functionally analogous to those of deeper stratified systems, allowing FG V to persist within phytoplankton communities. These findings support a process-based view of FG V ecology, emphasizing the role of redox-related conditions over water depth, and highlight shallow wetlands as potentially important but underexplored habitats for redox-dependent phototrophic communities.

Lake Srebarna is a Danube floodplain lake of exceptional conservation value protected under multiple international conservation frameworks, including the UNESCO World Heritage Convention, the Ramsar Convention, the UNESCO Biosphere Reserve Programme and the Natura 2000 network. Owing to its exceptional ecological importance as a model ecosystem for long-term ecological research, Lake Srebarna is also included in both the Bulgarian Long-Term Ecosystem Research (LTER-Bulgaria) network and the European Long-Term Ecosystem, Critical Zone and Socio-Ecological Research (eLTER) network, providing a valuable platform for long-term monitoring of ecological changes across multiple ecosystem components. Within this long-term monitoring framework, understanding spatial variability in phytoplankton communities is essential for interpreting ecosystem processes, detecting ecological change and supporting reliable ecological status assessment. The present study investigated the spatial organization of phytoplankton functional communities across the main sectors of Lake Srebarna during two consecutive years representing contrasting hydrological conditions to examine how extreme drought influenced spatial patterns in functional composition and the ecological mechanisms underlying relationships among functional composition, biomass and diversity. Phytoplankton was sampled during the vegetation seasons of 2024 and 2025 at four monitoring sites including two open-water sites: Central Lake area (CLA) and Dragayka pool (DRP) and two sites influenced by channels: South Sluice (SS) and Western Channel (WCH). Spatial comparisons were based on sampling months common to all stations within each year. Samples were processed using standard phytoplankton methods. Community composition was analyzed using the Reynolds–Padisák functional group approach. For each sample, total phytoplankton biomass and the Shannon diversity index (H′) were calculated. Spatial patterns in phytoplankton functional composition were visualized using non-metric multidimensional scaling (NMDS) based on Bray–Curtis dissimilarity and tested by one-way PERMANOVA with pairwise comparisons. Functional groups contributing most to spatial differentiation were identified using SIMPER analysis. Differences in total phytoplankton biomass and Shannon diversity among sampling sites were evaluated using the non-parametric Friedman test based on sampling months common to all stations, followed by pairwise comparisons with Bonferroni correction. All statistical analyses were performed using PAST version 5. NMDS ordination (stress = 0.14) revealed a more pronounced separation of samples among stations in 2025 than in 2024 Fig. 1. This pattern was confirmed by PERMANOVA, which indicated significant spatial differences in phytoplankton functional composition in 2025 (F = 2.47, p = 0.003), but not in 2024 (F = 1.15, p = 0.31). Following Bonferroni correction, only the CLA–SS pairwise comparison remained significant. SIMPER analysis indicated that the CLA–SS differentiation was driven mainly by higher contributions of functional groups X1 (small Chlorococcales) and X2 (small flagellates) at the CLA, whereas the SS station was characterized by greater contributions of the cyanobacterial group Sn ( Raphidiopsis raciborskii ) together with C ( mainly Cyclotella spp .) , W1 (Euglenophyceae) and P ( mainly Aulacoseira granulata , Fragilaria sp. ) , jointly explaining more than two-thirds of the average Bray–Curtis dissimilarity. Although spatial differences in phytoplankton biomass Fig. 2 were not statistically significant (Friedman test, p > 0.05), the SS station showed an opposite temporal pattern to the other stations. In contrast, Shannon diversity exhibited a significant overall spatial effect only in 2025 (Friedman test, p = 0.04). At the DRP and SS stations, high phytoplankton biomass was associated either with taxonomically and functionally diverse assemblages (July) or with extremely low diversity during near-monodominance of Raphidiopsis raciborskii (September). In contrast, the CLA station was characterized by extreme shallowing, increased conductivity and high-water transparency, where pronounced environmental stress restricted the community to a small number of tolerant taxa. Under these conditions, both phytoplankton biomass and diversity remained very low, consistent with strong environmental filtering rather than competitive dominance. Extreme drought conditions in 2025 generated contrasting phytoplankton responses within Lake Srebarna. Despite the absence of significant spatial differences in total phytoplankton biomass, functional composition and species diversity varied among sampling stations. The observed patterns demonstrate that similar reductions in diversity may arise through fundamentally different ecological mechanisms, including competitive dominance by Raphidiopsis raciborskii and environmental filtering under severe drought-induced environmental stress. These findings highlight the context-dependent relationship between phytoplankton biomass and diversity in shallow floodplain lakes exposed to extreme hydrological events.