FATE OF THE TOXIC CYCLIC HEPTAPEPTIDES, THE MICROCYSTINS, FROM BLOOMS OF MICROCYSTIS (CYANOBACTERIA) IN A HYPERTROPHIC LAKE

The in situ fate of the toxic cyclic heptapeptides, the microcystins, produced by blooms of Microcystis was examined at two stations in a hypertrophic Japanese lake. Microcystins were detected in all samples of Microcystis with quantities varying seasonally and spatially (230-950 mug.g dry wt-1 at St. 1 and 160-746 mug.g dry wt-1 at St. 2) and composed of microcystin-LR, -RR, and -YR. Microcystin-RR was the dominant toxin in most samples. A large amount of microcystin (1.1 mug.L-1) was detected in only one sample of filtered lake water. Accumulation of microcystin in zooplankton was indirectly estimated from a newly developed equation model. Large amounts of microcystin (75-1387 mug.g dry wt-1) were accumulated in the zooplankton community, which consisted of two cladocerans, Bosmina fatalis Burckhardt and Diaphanosoma brachyurum Lieve, and a copepod, Cyclops vicinus Uljanin, that co-occurred with the toxic Microcystis blooms. The maximum percent of microcystin content in zooplankton to that in Microcystis was 202%. Among the three species of zooplankton, only B. fatalis seemed to be responsible for accumulation of the microcystins because C. vicinus appeared to avoid contact with Microcystis cells and D. brachyurum did not consume colonies of Microcystis. Microcystins may be transferred to higher trophic levels through B. fatalis.