The impact of climate change in Serbia, as found throughout the world, is reflected in the rapid changes in drought and heavy rain periods. We show the influence of high rainfall levels on the water quality, composition and seasonal succession of Cyanobacteria in the Šumarice reservoir, Serbia (44° 01'49'' N, 20° 52'28'' E). The changes in physical and chemical parameters of water, Chl-a, Cyanobacteria biomass, abundance of phytoplankton and Cyanobacteria along a temporal gradient are visualized using a self-organizing map (SOM). Data was collected monthly during 2013 and 2014. During 2013, the reservoir was characterized as mesotrophic and eutrophic, with a low level of nutrients (the maximum value of total phosphorus was 0.068 mg L ^–1 and total nitrogen was 0.764 mg L ^–1 in August), while the maximum values of Cyanobacteria biomass (12.7 mg L ^–1) and abundance (1.36 × 10 6 ind L^–1) were in September, when Cyanobacteria represented 21% of the whole phytoplankton. In the following year (2014), after the extreme precipitation (227 L m ^–2 in May and 138 L m ^–2 in July), the concentration of nutrients in the reservoir increased. In September, the maximum value of total phosphorus was 0.310 mg L ^–1, and the maximum value of total nitrogen was 6.828 mg L ^–1. In October, the maximum values of Cyanobacteria biomass (1567.28 mg L ^–1) and abundance (2.36 × 10⁸ ind L ^–1) were recorded when the bloom of Aphanizomenon flosaquae was observed, representing 98 % of the whole phytoplankton. This blooming is strongly positively correlated with the increase of total phosphorus and nitrogen. The extreme and long-term precipitation was significant enough to change the state of the reservoir from eutrophic to hypereutrophic.

For 50 years persistent cyanobacterial blooms have been observed in Lake Ludoš (Serbia), a wetland area of international significance listed as a Ramsar site. Cyanobacteria and cyanotoxins can affect many organisms, including valuable flora and fauna, such as rare and endangered bird species living or visiting the lake. The aim was to carry out monitoring, estimate the current status of the lake, and discuss potential resolutions. Results obtained showed: (a) the poor chemical state of the lake; (b) the presence of potentially toxic (genera Dolichospermum, Microcystis, Planktothrix, Chroococcus, Oscillatoria, Woronichinia and dominant species Limnothrix redekei and Pseudanabaena limnetica) and invasive cyanobacterial species Raphidiopsis raciborskii; (c) the detection of microcystin (MC) and saxitoxin (STX) coding genes in biomass samples; (d) the detection of several microcystin variants (MC-LR, MC-dmLR, MC-RR, MC-dmRR, MC-LF) in water samples; (e) histopathological alterations in fish liver, kidney and gills. The potential health risk to all organisms in the ecosystem and the ecosystem itself is thus still real and present. Although there is still no resolution in sight, urgent remediation measures are needed to alleviate the incessant cyanobacterial problem in Lake Ludoš to break this ecosystem out of the perpetual state of limbo in which it has been trapped for quite some time.

Lake Balaton is the largest shallow lake in Central Europe. Its water quality is affected by its biggest inflow, the Zala River. During late 20th century, a wetland area named the Kis-Balaton Water Protection System (KBWPS) was constructed in the hopes that it would act as a filter zone and thus ameliorate the water quality of Lake Balaton. The aim of the present study was to test whether the KBWPS effectively safeguards Lake Balaton against toxic cyanobacterial blooms. During April, May, July and September 2018, severe cyanobacterial blooming was observed in the KBWPS with numbers reaching up to 13 million cells/mL at the peak of the bloom (July 2018). MC- and STX-coding genes were detected in the cyanobacterial biomass. Five out of nine tested microcystin congeners were detected at the peak of the bloom with the concentrations of MC-LR reaching 1.29 µg/L; however, accumulation of MCs was not detected in fish tissues. Histopathological analyses displayed severe hepatopancreas, kidney and gill alterations in fish obtained throughout the investigated period. In Lake Balaton, on the other hand, cyanobacterial numbers were much lower; more than 400-fold fewer cells/mL were detected during June 2018 and cyanotoxins were not detected in the water. Hepatic, kidney and gill tissue displayed few alterations and resembled the structure of control fish. We can conclude that the KBWPS acts as a significant buffering zone, thus protecting the water quality of Lake Balaton. However, as MC- and STX-coding genes in the cyanobacterial biomass were detected at both sites, regular monitoring of this valuable ecosystem for the presence of cyanobacteria and cyanotoxins is of paramount importance.