Cyanobacterial blooms in freshwater bodies in England are currently monitored reactively, with samples containing more than 20,000 cells/mL of potentially toxin-producing species by light microscopy resulting in action by the water body owner. Whilst significantly reducing the risk of microcystin exposure, there is little data describing the levels of these toxins present in cyanobacterial blooms. This study focused on the quantitative LC-MS/MS analysis of microcystins in freshwater samples, collected across England during 2016 and found to contain potentially toxin-producing cyanobacteria. More than 50% of samples contained quantifiable concentrations of microcystins, with approximately 13% exceeding the WHO medium health threshold of 20 μg/L. Toxic samples were confirmed over a nine-month period, with a clear increase in toxins during late summer, but with no apparent geographical patterns. No statistical relationships were found between total toxin concentrations and environmental parameters. Complex toxin profiles were determined and profile clusters were unrelated to cyanobacterial species, although a dominance of MC-RR was determined in water samples from sites associated with lower rainfall. 100% of samples with toxins above the 20 μg/L limit contained cell densities above 20,000 cells/mL or cyanobacterial scum, showing the current regime is suitable for public health. Conversely, with only 18% of cell density threshold samples having total microcystins above 20 μg/L, there is the potential for reactive water closures to unnecessarily impact upon the socio-economics of the local population. In the future, routine analysis of bloom samples by LC-MS/MS would provide a beneficial confirmatory approach to the current microscopic assessment, aiding both public health and the needs of water users and industry.

Cyanobacteria are microorganisms that can produce natural toxins with significant impacts on human, animal and ecosystem health. Portishead Marine Lake is a small freshwater lake in southwest England frequented by thousands of people each year for recreational activities but was associated periodically with prolific cyanobacterial blooms with potential risk to visitor health. Water samples were tested weekly between January and December 2016 for cyanobacteria and cyanotoxins. Citizen scientists undertook testing for nutrients, dissolved oxygen, pH, temperature and meteorological parameters. Four major blooms of Microcystis spp. were observed in the warmer months between May and September and were associated with high concentrations of microcystins. Complex toxin congener profiles were dominated by MC-LR and MC-RR, with lower contributions from MC-WR and MC-YR. Cyanobacterial cell densities and summed microcystin concentrations ranged from 1600 to 5.6 × 10⁶ cells/mL and 0.1 to 18,069 µg/L, respectively, and correlated significantly. Toxin concentrations were positively correlated with nitrate levels and inversely correlated with pH, although mean pH levels did not fall below 7.0. There was little statistical evidence for influence of weather conditions overall, although significant intra-lake differences in cell densities and toxin concentrations were noted. Intervention measures employed by local authorities included the use of barley straw, hydrological manipulation, debris removal and ultrasound treatment. Whilst no statistical assessment of intervention effectiveness was possible without effective control samples, there was some limited evidence for the temporary reduction in toxin levels after hydrological intervention, but none following installation of ultrasound. Overall, the study utilized LC-MS/MS and microscopy to quantify microcystins and Microcystis spp., respectively, and evidenced risks to lake visitors and the importance of toxin testing in recreational waters and the use of appropriate risk mitigation approaches. Further work incorporating appropriate experimental controls would be needed to determine the effectiveness of hazard mitigation measures for reducing or eliminating human health threats.