Broad screening of toxic and bioactive metabolites in cyanobacterial and harmful algal blooms in Lake of the Woods (Canada and USA), 2016–2019

Cyanobacteria produce toxic/bioactive metabolites that affect human and ecosystem health via inhibition of proteases, carboxypeptidases, or phosphatases. From surface water samples collected between 2016 and 2019 inclusive (n = 78), we report intracellular concentrations of more than 30 such compounds in Lake of the Woods (LOW), a large lake system of multinational importance. Although most surface biomass blooms were dominated by the cyanobacteria Dolichospermum (max. ∼50 mg/L) and Aphanizomenon (max. ∼60 mg/L), there was also significant biomass of other cyanobacteria (Gloeotrichia echinulate along shoreline, max. ∼40,000 mg/L; Woronichinia naegeliana max. ∼100 mg/L; Lyngbya max. ∼10 mg/L; Planktothrix agardhii max. ∼6 mg/L) and phytoplankton (Stephanodiscus niagarae max. ∼6 mg/L). Microcystins (MCs) were present in ∼80 % of samples, peaking at ∼33 µg/L (MC variant-LR). Approximately 50 % of samples contained MC concentrations below 0.1 µg/L. Alkaloid metabolites detected included neurotoxin anatoxin-a (∼60 % < 0.1 µg/L) and cytotoxic cylindrospermopsins (∼30 % < 0.1 µg/L). Anabaenopeptins A/B/F were prevalent (>96 %) and exceeded MCs by >100 fold (max. >4,000 µg/L). Aeruginosamide B was present in ∼75 % of samples (max. ∼10 µg/L). Cyanopeptolin 1040 MB, microginin 690 methyl ester, and oscillaginin A were not detected however, traces of micropeptin 1106 were detected. Microcystin and anabaenopeptin genes were also detected in the majority of samples (78 % and 79 %, respectively), while saxitoxin, cylindrospermopsin, and anatoxin genes occurred at lower frequencies with 59 %, 36 %, and 38 % detection, respectively. These results will aid assessment of risk from cyanobacterial blooms in LOW and inform ongoing binational lake management and policy development.