The purpose of this study is to examine the allelopathic potential of emergent macrophytes in actual lakes or ponds. Water was collected around the roots of emergent macrophytes Phragmites australis, Carex dispalata and Typha domingensis vegetated on floating beds or areas without macrophytes in a pond every second month for a year, and examined by bioassay using Microcystis aeruginosa. The water at areas without macrophytes did not exhibit growth-inhibiting effect throughout the year, but almost all of the water collected at the floating beds with the emergent macrophytes had the effect when the macrophytes existed. These results showed an obvious tendency that the exhibition of the growth-inhibiting effect correlated with the existence of the emergent macrophytes. To confirm such effect observed in actual ponds, three kinds of macrophytes were cultured hydroponically in which the roots were submerged in water and their culture filtrates were collected. All of the culture filtrates showed allelopathically growth-inhibiting effect on M. aeruginosa. From the comparison between the bioassay results of the water in the pond and the culture filtrates, the growth-inhibiting effect shown by the water at the floating beds with the emergent macrophytes would be responsible for the allelopathic effect of the macrophytes.
