The Isles of Scilly are an archipelago of islands in the far southwest of the UK which contain numerous beds of wild bivalve molluscs which are recreationally harvested for local consumption. However, the islands have never previously been assessed for the presence of harmful algae and their shellfish toxin metabolites which can cause serious human health impacts. This study sought to address these knowledge gaps through the analysis of seawater and shellfish tissues for microalgae and toxins utilizing portable and lab-based microscopy, nanopore sequencing, chemical analysis and immunoassay kits. The study design was affected by the national COVID-19 lockdown which enforced implementation of citizen-led sampling and in-field microscopy. Microscopy and sequencing approaches led to the confirmation of multiple HAB species of concern, including those potentially responsible for production of neurotoxic and diarrhetic shellfish toxins. A portable microscope was successfully utilized in the field for recognition of microalgae and for early warning of potential shellfish toxicity events. Chemical analysis of cockle, clam and mussel samples confirmed the detection of paralytic, diarrhetic and amnesic shellfish toxins, with an unusual okadaic acid group toxin profile reaching a maximum toxicity of approximately half the regulatory limit as defined by EU law. The Sensoreal Alert Lateral Flow Assay was used to screen and highlight samples containing higher concentrations of DSP toxins. Furthermore, Tetrodotoxin was detected for the first time in the UK in cockle and grooved carpet shells. Multiple saxitoxin analogues were also detected in two echinoderm species, with this providing the first ever report of paralytic shellfish toxins in the spiny starfish, Marthasterias glacialis. The toxin profiles in the two species varied significantly with a dominance of GTX4 in Luidia ciliaris as opposed to a dominance of STX in Marthasterias glacialis. Overall, the study showed that a multi-method assessment of a previously unexplored region within the UK territory contained microalgae and toxins of concern to human health, and that a citizen-led programme could be instigated using portable microscopy and rapid toxin testing to assess the early warning for potentially harmful microalgae and toxins in the region, with confirmatory analysis being conducted to establish actual levels of risk for local consumers of seafood.

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.