Changes in microbial loop components: effects of a harmful algal bloom formation and its decay

Temporal changes in microbial loop components during the formation and deca y of a bloom caused by the harmful microalga Heterosigma akashiwo (Raphidop hyceae) 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 wh ich exceeded 10(4) cells ml(-1), was recorded in early summer at surface te mperatures ranging from 21.6 to 23.2 degrees C. Although the abundance and species diversity of tintinnid ciliates decreased in the surface layer when the density of H. akashiwo exceeded 104 cells ml(-1), aloricate ciliates i ncreased evidently at the end of the bloom. In addition, the mean ciliate b iomass in the surface and near bottom layers reached the same level as the bacterial biomass. Two peaks in bacterial abundance and biomass were record ed during the formation and at the end of the bloom, and fluctuations in th e abundance of heterotrophic nanoflagellates (HNF) clearly corresponded wit h fluctuations in bacterial abundance, with a lag period of 1 to 3 d. The i ncrease of each microbial loop component during the course of the H. akashi wo bloom suggests that the dissolved organic matter produced from H, akashi wo cells temporarily enhanced the energy flow from bacteria through HNF to bacterivorous aloricate ciliates.