Hepatotoxic cyanobacteria in the shallow lake Muggelsee

Since 1996 we have studied the occurrence of cyanobacterial hepatotoxins, m icrocystins (Mcyst), in the shallow lake Muggelsee. Intracellular (cell-bou nd) Mcyst's were detectable from late summer to autumn in 2 years, and maxi mum concentrations of about 3 mu g/l of total cell-bound Mcyst were measure d. The concentration of Mcyst's was correlated to the biovolume of Microcys tis sp. (R-2=0.560 in 1996 and R-2=0.483 in 1997; P<0.001). Between years, the ratio of Mcyst'ss per biovolume of Microcystis differed significantly ( P<0,001; 0.913 mu g/mm(3) in 1996 and 0.171 mu g/mm(3) in 1997). The ratio of the detected Mcyst variants, Mcyst-LR and Mcyst-RR, was constant in all samples with Mcyst-LR:Mcyst-RR approximate to 1:2.9. One out of 22 strains of Microcystis sp. we isolated in 1997 contained Mcyst-LR and Mcyst-RR at a ratio of Mcyst-LR:Mcyst-RR=1:1.6. The Mcyst'ss found in lake samples were likely to have been produced by a number of a few strains that were present in the lake for extended periods of time in both years.