Effects of Zooplankton Grazing on the Suppression of Harmful Algal Blooms by the Rotifer Brachionus calyciflorus in Freshwater Ecosystems

To study the influence of the rotifer Brachionus calyciflorus on harmful algal bloom suppression, we focused on assessing the rotifer's abilities using several prey species : Microcystis aeruginosa, Synechocystis sp., Chlorella vulgaris and Coelastrum sp. of the warm-weather species and the cold-weather centric diatom Stephanodiscus hantzchii. Grazing effects and growth rates of rotifers B. calyciflorus were 94.5% and $1.29d^{-1}$, respectively, for Synechocystis sp., 87.4% and $0.60d^{-1}$, respectively, for M. aeruginosa, 95.2% and $0.65d^{-1}$, respectively, for C. $vulgaris^{TM}$, 78.6% and $0.45d^{-1}$, respectively, for C. vulgaris UTEX., 86.5% and $0.99d^{-1}$, respectively, for Coelastrum sp., and 82.6% and $0.40d^{-1}$, respectively, for S. hantzchii. Of these, although the growth of Synechocystis and Coelastrum was effectively suppressed by rotifer grazing, efficient suppression effects on Stephanodiscus blooms were unexpected. The present study revealed that reproduction of B. calyciflorus was greatly influenced by its food types in the initial stages and the efficiencies of bio-agents as sole food sources vary depending on the target algae and the agent.