Seasonal variations in microcystin contents of Danish cyanobacteria.

One hundred ninety-eight of 296 phytoplankton net samples, collected from Danish fresh waters during 1993-1995, contained microcystins when analyzed by high-performance liquid chromatography (HPLC), and the seasonal variations in microcystin contents were examined. Three genera, Anabaena, Microcystis, and Planktothrix, with microcystin-production potential were found. The highest percentage of the samples dominated by Anabaena and Microcystis was found in June-July and July-November, respectively, and those by Planktothrix in May and October-November. The highest microcystin contents were found in samples collected in July and September. When pooling all samples by month of collection, no major differences in mean toxin contents were seen in samples collected from May through November, with the exception of slightly higher microcystin contents in September and October. The frequency of samples (all samples grouped by month of collection) with detected microcystins ranged from 62% in July (n = 116) to 86% in May (n = 7). Variations in toxin profiles were found between 5 lakes sampled several times during 1993-1995, and in addition, quantitative and qualitative seasonal changes in microcystin content accompanied species composition changes in 3 of these lakes. Consistent blooms dominated by single species were characterized by more or less constant toxin profiles but variable toxin concentrations. Analysis of 6 samples from Lake Bryrup Langsø 1995 by enzyme linked immunoabsorbent assay and HPLC revealed 1-70 and 0-737 micrograms microcystins/g dry wt, respectively. The discrepancy between the two detection methods may be attributed to the types of microcystin variants present, emphasizing the possible influence of seasonal variations in toxin profiles on the quantification of microcystins.