The scientific and public awareness of hazardous photosynthetic prokaryotes (cyanobacteria/cyanoprokaryotes) and especially the contamination of drinking-water reservoirs with cyanotoxins is world-wide increasing. Recently much more attention has been paid to the events and results of mass proliferation of these toxic organisms even in South-East European countries in spite of the fact that, as a rule, they are not controlled by national legislation.The present paper presents a summary of results of such studies carried out in summer-autumn periods of the last 15 years (2000-2015) in Bulgarian water bodies differing by location, morphometry and trophic status, incl. drinking water reservoirs, recreational lakes and sites of nature conservation importance. A multivariate analysis allowed to outline the distribution patterns and environmental drivers of the planktonic cyanoprokaryote assemblages in relation with the available data on the water bodies, highlighting species composition and abundance of the main taxa, including potentially toxic species. Samples analysis by HPLC-DAD and/or LC/MS, ELISA and in vitro cytotoxicity tests allowed detection of microcystins, nodularins and saxitoxins. Toxin concentration ranged between 0.1 and 26.5 µg L-1 in water samples and between 10.9 and 1070 µg g-1 (d.w.) in concentrated (net) samples. Despite the fact that microcystins were not found in all studied water bodies and that the recorded levels were still lower in comparison with some other European countries, the fact that cyanotoxins were detected in 16 water bodies (incl. 3 drinking water reservoirs) could serve as an alert for the need of recognition of cyanotoxins as a new health risk factor in the country. Therefore, permanent monitoring with identification of toxins in water bodies at risk and activities for limitation and control of toxic blooms are urgently needed, in combination with increase of the attention to the effects of cyanotoxins on both human health and health of aquatic ecosystems in Bulgaria.

This paper describes the first use of aerial observations by a drone as an additional means for choosing sampling points during field studies of cyanobacterial harmful algal blooms (CyanoHABs) in selected Bulgarian waterbodies and the use of HPLC analysis of marker pigments for the fast determination of phytoplankton composition and biomass. The selection of waterbodies was based on the authors’ personal expertise and data collected over a 25-year period. In all sites chosen by drone, there were high levels of cyanobacteria and cyanotoxins were present: microcystins (MC-LR, MC-RR, MC-YR in Durankulak Lake and MC-LR and MC-RR in the Sinyata Reka Reservoir), cylindrospermopsin (in the Vaya Lake and in the Mandra Reservoir) and saxitoxins (in Durankulak Lake). The finding of cylindrospermopsin is the first in Bulgaria, the detection of saxitoxins is the first for Durankulak Lake and the microcystins records are the first for Sinyata Reka Reservoir. Considering the high total number of wetlands in Bulgaria, many of which are lowland, small and shallow and therefore vulnerable to CyanoHABs, we recommend further use of drones and HPLC in monitoring, which should speed up detection and reduce sampling efforts while enabling valuable information to be gathered.

This paper presents the results from analyses of water samples from 15 Bulgarian reservoirs and lakes for detection of microcystins presence by high-performance liquid chromatography (HPLC) coupled to a ultraviolet (UV) and photo diode array (PDA) detector. In 3 of the studied drinking water reservoirs cyanobacterial blooms were not detected. Cyanobacterial blooms were observed in 6 of 12 investigated water bodies used for fishing and water sports. Microcystis was the dominant cyanobacterial genus. The concentration of total microcystins in the biomasses ranged from 8 to 1070 lg g -1 (d.w.). Microcystin-LR and microcystin-RR were the predominant microcystins, followed by microcystin-YR. Presence of dissolved microcystins was obtained in one water sample with concentration of total microcystins 1.64 lg L -1. Presence of microcystin-LF and -LW or unknown peaks possessing characteristic microcystin-like UV-spectra were not detected. Those first results for occurrence of microcystins in some Bulgarian reservoirs and lakes are a contribution to the knowledge on their distribution.

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The rising interest in harmful cyanoprokaryote blooms promotes an increase of phycological and ecological research on potentially toxic species and their hazardous substances. The present study aimed to identify the main microcystin (MC) producers and their contribution to the phytoplankton of shallow waterbodies in Bulgaria, applying different methods. The sampling was performed in August 2019 in nine lakes and reservoirs, two of which (reservoirs Kriva Reka and Izvornik 2) were studied for the first time. The high contribution of cyanoprokaryotes to the total species composition and phytoplankton abundance was proved by light microscopic (LM) observations and HPLC analysis of marker pigments. The LM identification of potential MC-producers was supported by PCR amplification of mcyE and mcyB genes. The MCs amounts, detected by HPLC-DAD, varied by sites with a range from undetectable concentrations to 0.46 µg L−1 with only one recorded variant, namely MC-LR. It was found only in the reservoirs Mandra and Durankulak, while toxigenic MC-strains were obtained by PCR from five more waterbodies. Both LM and PCR demonstrated that the MC-producers were Microcystis aeruginosa and M. wesenbergii, despite their occurrence in low amounts (<0.5–5% of the total biomass) when filamentous cyanoprokaryotes dominated.