Risks to Recreational Water Users from Microcystis spp. and Microcystins in Portishead Lake (SW England) as Determined by LC-MS/MS and Citizen Science Monitoring

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.