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

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 w h ~ c h exceeded 104 cells ml-', 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 decreased in the surface layer when the density of H. akashiwo exceeded 104 cells rnl-', 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. aka-shiwo cells temporarily enhanced the energy flow from bacteria through HNF to bacterivorous alori-cate ciliates.