FATE OF THE TOXIC CYCLIC HEPTAPEPTIDES, THE MICROCYSTINS, FROM BLOOMS OF <i>MICROCYSTIS</i> (CYANOBACTERIA) IN A HYPERTROPHIC LAKE<sup>1</sup>

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<jats:title>ABSTRACT</jats:title><jats:p>The <jats:italic>in situ</jats:italic> fate of the toxic cyclic heptapeptides, the microcystins, produced by blooms of <jats:italic>Microcystis</jats:italic> was examined at two stations in a hypertrophic Japanese lake. Microcystins were detected in all samples of <jats:italic>Microcystis</jats:italic> with quantities varying seasonally and spatially (230–950 μg · g dry wt<jats:sup>−1</jats:sup> at St. 1 and 160–746 μg · g dry wt<jats:sup>−1</jats:sup> 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 μg · L<jats:sup>−1</jats:sup>) 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 μg · g dry wt<jats:sup>−1</jats:sup>) were accumulated in the zooplankton community, which consisted of two cladocerans, <jats:italic>Bosmina fatalis</jats:italic> Burckhardt and <jats:italic>Diaphanosoma brachyurum</jats:italic> Lieve, and a copepod, <jats:italic>Cyclops vicinus</jats:italic> Uljanin, that co‐occurred with the toxic <jats:italic>Microcystis</jats:italic> blooms. The maximum percent of microcystin content in zooplankton to that in <jats:italic>Microcystis</jats:italic> was 202%. Among the three species of zooplankton, only <jats:italic>B. fatalis</jats:italic> seemed to be responsible for accumulation of the microcystins because <jats:italic>C. vicinus</jats:italic> appeared to avoid contact with <jats:italic>Microcystis</jats:italic> cells and <jats:italic>D. brachyurum</jats:italic> did not consume colonies of <jats:italic>Microcystis.</jats:italic> Microcystins may be transferred to higher trophic levels through <jats:italic>B. fatalis</jats:italic>.</jats:p>

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