PERSISTENCE OF CYANOBACTERIAL HEPATOTOXIN, MICROCYSTIN-LR IN PARTICULATE MATERIAL AND DISSOLVED IN LAKE WATER

The persistence of cyanobacterial hepatotoxin, microcystin-LR, was inv estigated in Lake Tuusulanjarvi in southern Finland from August to Oct ober, 1993 and 1994. The amount of toxin in particulate material and d issolved in water were determined by HPLC from samples collected from mesocosm enclosures and from the surrounding lake water. In the beginn ing of the experiments over 80% of the phytoplankton biomass consisted of cyanobacteria. The main species were Microcystis wesenbergii (Chro ococcales, Cyanobacteria), M. viridis and M. aeruginosa. The microcyst in-LR concentration in particulate material varied from 2.7 to 3.2 mu g l(-1) and the corresponding concentration of microcystin LR dissolve d in water from 0.06 to 0.21 mu gl(-1). The cyanobacterial biomass dec reased towards the middle of September and simultaneously the microcys tin concentration in freeze dried particulate material decreased below the detection limit of 10 mu g g(-1), corresponding 0.01 mu gl(-1). D issolved 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 cya nobacterial biomass was less than 1 mg l(-1). Thus, dissolved microcys tin was more persistent compared to microcystin in particulate materia l: the decimal reduction time for dissolved toxin was 30 d and for tox in in particulate material about 15 d. (C) 1997 Elsevier Science Ltd.