Background Excess loads of nutrients finding their way into waterbodies can cause rapid and excessive growth of phytoplankton species and lead to the formation of cyanobacterial harmful algal blooms (cyano-HABs). Toxic cyanobacteria produce a broad range of bioactive metabolites, some of which are known as cyanotoxins. These metabolites can negatively impact the ecosystem, and human and animal health, thus their presence needs to be closely monitored and mitigated. This study aimed to monitor St. George Lake (Athalassa National Forest Park, Cyprus) for its water quality characteristics, and initiate a new methodology to control the bloom that occurred in the lake during summer 2019, by comparing hydrogen peroxide treatment with novel metallic peroxide granules as source of hydrogen peroxide. Results Lake monitoring showed that pH, salinity, total dissolved solids and conductivity varied throughout the year, and nutrients concentration was high, indicating a eutrophic lake. The cyanobacterium Merismopedia sp. bloomed in the lake between June and September 2019, comprising up to 99% of the phytoplankton biovolume. The presence of microcystin synthase encoding gene (mcyB, mcyE) was documented, however microcystins were not detected by tandem mass spectroscopy. Treatment with liquid hydrogen peroxide in concentrations 1 to 5 mg L−1 had no effect on the phycocyanin fluorescence (Ft) and quantum yield of PSII (Fv/Fm) indicating an ineffective treatment for the dense Merismopedia bloom (1 million cells mL−1 ± 20%). Metallic peroxide granules tested for their H2O2 releasing capacity in St. George Lake water, showing that CaO2 released higher H2O2 concentration and therefore have better mitigation efficiency than MgO2 granules. Conclusion The present study highlights the importance of monitoring several water parameters to conclude on the different actions to be taken to limit eutrophication in the catchment area. The findings demonstrated that testing for the presence of genes involved in cyanotoxin production may not be sufficient to follow cyanotoxins in the water, therefore it should be accompanied with analytical confirmation. Treatment experiments indicated that slow release of H2O2 from peroxide granules may be an alternative to liquid hydrogen peroxide when applied in appropriate doses, but further investigation is needed before it is applied at the field. Graphic Abstract

Background: Cyanobacteria are phytoplankton microorganisms, also known as blue-green algae, and an essential component of the food web in all aquatic ecosystems. Excess loads of nutrients into waterbodies can cause their rapid and excessive growth which leads to the formation of cyanobacterial harmful algal blooms (cyano-HABs). Toxic species of cyanobacteria genera excrete into the water a broad range of bioactive metabolites, some of which are known as cyanotoxins. These metabolites can negatively affect the ecosystem, and human and animal health in various ways, thus their presence needs to be closely monitored. This study aimed to monitor a lake at the Athalassa National Forest Park in Cyprus, in order to correlate its trophic condition with its water quality characteristics and identify the key environmental variables driving cyanobacteria blooming and their toxicity. In addition, surface water during the blooming period was collected and used in bench-scale experiments in order to test novel hydrogen peroxide releasing granules as mitigation processes for cyano-HABs.Results: The monitoring lasted throughout 2019 with ten sampling events taking place during this period. Samples were mainly analyzed for phytoplankton community, and various physicochemical parameters: pH, conductivity, salinity, total and dissolved nutrients. Obtained data indicated that cyanobacteria blooming lasted for four months (June – September), while microscopic observation of preserved samples showed that 99% of the phytoplankton biovolume was attributed to a single picocyanobacterial species, the Merismopedia sp. Select samples were analysed for the presence of toxins genes with positive results mainly for mcyB and mcyE genes. Further analysis with HPLC MS/MS, revealed that cyanotoxins’ concentration was lower than the method detection limit - MDL (<2-6 ng/L). Conclusion: The present study highlights the importance of monitoring several water characteristics to conclude on the main drivers of a bloom and its toxicity. The findings demonstrated that it is not enough to test cyanotoxin genes as indicator of their presence since, in case of mono-domination, cyanobacteria may not be active on producing the toxins. Treatment experiments of contaminated water indicated that slow realizing peroxide granules may be an alternative to hydrogen peroxide. Treatment with CaO2 granules outperformed MgO2 granules due to higher H2O2 releasing capacity.