Change in the microzooplankton community during decay of aHeterosigma akashiwo bloom

The temporal change in the microzooplankton community during the decay of a bloom ofHeterosigma akashiwo was investigated over a 10-day period in June–July 1991 at a site in western Hiroshima Bay, the Seto Inland Sea, Japan. The density ofH. akashiwo was at the order of 10⁴ cells·ml^–1 during the first 5 days, and decreased drastically when vertical mixing occurred.Heterosigma akashiwo cells disappeared after the eighth day. When the density ofH. akashiwo decreased suddenly, the density of microzooplankton increased rapidly. The increase in microzooplankton density was due to non-tintinnid ciliates. The species diversity of tintinnids was significantly higher in 1 and 2-m layers during the post-bloom period than during the bloom period. One species of tintinnid,Codonellopsis nipponica, was the most abundant in the bottom layer during the bloom period but decreased in abundance during the decay of the bloom. Populations of two species of hyaline tintinnids,Eutintinnus tubulosus andHelicostomella longa, were at low levels during the bloom period, and then developed suddenly after the bloom disappeared. It was demonstrated that the abundance and species composition of the ciliate community change dramatically during the decay of theHeterosigma bloom.