First report on the occurrence of a single species cyanobacterial bloom in a lake in Cyprus: Monitoring and treatment with hydrogen peroxide releasing granules

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.