Dominance of Microcystis with Special Reference to Carbon Availability in Lake Water

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The mechanism of dominance by Microcystis spp. (Cyanophyceae) was examined with special reference to the concentration of dissolved inorganic carbon (DIC), in in situ experiments using lake water. Lake water collected from mesotrophic Lake Biwa was enriched with nitrogen and phosphorus nutrients prior to the experiments, and use of a NaHCO_3-Na_2CO_3 buffer was successful in allowing a change of DIC concentrations for phytoplankton without lowering the pH. The experiments consisted of a control (no DIC addition) and a treatment. There were no significant differences in levels or patterns of change in pH values, nutrient (nitrogen and phosphorus) or chlorophyll concentrations between the two systems. Microcystis became dominant in the control system, while Scenedesmus sp. dominated in the DIC system. This suggested that the low DIC concentration in surrounding water favored dominance by Microcystis although this cyanobacterium needed some other carbon sources in order to proliferate. The buoyancy regulation of Microcystis may be the strategy by which it floats up to the surface layer, where there is abundant CO2, and dominates when the DIC concentration in the water column is low.