AME 21:21-30 (2000)  -  doi:10.3354/ame021021

ABSTRACT: Temporal changes in microbial loop components during the formation and decay of a bloom caused by the harmful microalga Heterosigma akashiwo (Raphidophyceae) were investigated in 1995 at a site in northern Hiroshima Bay, the Seto Inland Sea of Japan. A surface bloom of H. akashiwo, the density of which exceeded 10⁴ cells ml^-1, was recorded in early summer at surface temperatures ranging from 21.6 to 23.2°C. Although the abundance and species diversity of tintinnid ciliates de- creased in the surface layer when the density of H. akashiwo exceeded 10⁴ cells ml^-1, aloricate ciliates increased evidently at the end of the bloom. In addition, the mean ciliate biomass in the surface and near bottom layers reached the same level as the bacterial biomass. Two peaks in bacterial abundance and biomass were recorded during the formation and at the end of the bloom, and fluctuations in the abundance of heterotrophic nanoflagellates (HNF) clearly corresponded with fluctuations in bacterial abundance, with a lag period of 1 to 3 d. The increase of each microbial loop component during the course of the H. akashiwo bloom suggests that the dissolved organic matter produced from H. akashiwo cells temporarily enhanced the energy flow from bacteria through HNF to bacterivorous aloricate ciliates.

KEY WORDS: Heterosigma akashiwo · Bloom · Red tide · Microbial loop · Bacteria · Flagellate · Ciliate · Abundance