Persistence of cyanobacterial hepatotoxin, microcystin-LR in particulate material and dissolved in lake water

The persistence of cyanobacterial hepatotoxin, microcystin-LR, was investigated in Lake Tuusulanjärvi in southern Finland from August to October, 1993 and 1994. The amount of toxin in particulate material and dissolved in water were determined by HPLC from samples collected from mesocosm enclosures and from the surrounding lake water. In the beginning of the experiments over 80% of the phytoplankton biomass consisted of cyanobacteria. The main species were Microcystis wesenbergii (Chroococcales, Cyanobacteria), M. viridis and M. aeruginosa. The microcystin-LR concentration in particulate material varied from 2.7 to 3.2 μg l−1 and the corresponding concentration of microcystin LR dissolved in water from 0.06 to 0.21 μg l−1. The cyanobacterial biomass decreased towards the middle of September and simultaneously the microcystin concentration in freeze dried particulate material decreased below the detection limit of 10 μg g−1, corresponding 0.01 μg l−1. Dissolved microcystin-LR was detected in a concentration range of 1 to 5 ng l−1 even at the end of the experiments in October, when the cyanobacterial biomass was less than 1 mg l−1. Thus, dissolved microcystin was more persistent compared to microcystin in particulate material: the decimal reduction time for dissolved toxin was 30 d and for toxin in particulate material about 15 d.