The Speciation of Zooplankton and Its Effects on the Distribution of Colony Size of Microcystis in Hsin-Shan Reservoir

Microcystis, one of the most notorious bloom-forming cyanobacteria occurring in freshwaters, is able to form colonial aggregates. Large sizes of colonies are helpful to their vertical migration, nutrient storage, and to defend themselves against predation pressure. Therefore, formation of colonies is believed to be a special ability of Microcysits to dominate over other species of phytoplanktons in the thermally stratified lakes or reservoirs. The previous study in this laboratory also found that the particle size of Microcystis in Hsin-Shan Reservoir is not uniform, but a specific distribution. The mechanism with which Microcystis maintain a specific particle size distribution is not clear. In this study we hypothesize that predation by herbivorous zooplankton can consume a large amount of Microcystis colonies, therefore, control the size distribution of Microcystis. Experiments were conducted with zooplanktons and Microcystis colonies collected from Hsin-Shan Reservoir. The results or the investigation of the variation of size distribution and cell numbers of Microcystis indicated that Microcystis colonies are not consumed as food by zooplankton. Predation is not the main reason for Microcystis to maintain its size distribution. We developed a model to simulate the variation of colonies size distribution by using the real observed patterns of colonial division. The results of the simulation indicate that Microcystis can maintain a specific size distribution. That is to say, Microcystis may preserve a dynamic equilibrium of colonial distribution by following a division pattern we proposed. Investigation of the vertical distributions of the composition and abundance of zooplankton had been performed monthly from July 2015 to June 2016. It was found that zooplankton abundance had positive correlation with the concentration of chlorophyll a. Since zooplanktons did not consume algae, the reason of this phenomenon could be that planktonic small animals closely follow their food sources, which may be dead bodies of algae, organic matters excreted by phytoplanktons or some heterotrophic bacteria in vicinity of the colonies and their concentration, which are all related to the concentration of algae.