Inhibitory effects of macrophytes on the growth of blue-green algae (i.e. Microcystis aeruginosa, Anabaena flos-aquae, or Phormidium tenue) were evaluated in a coexistence culture system in which concentrations of different macrophyte species were varied (i.e. Egeria densa, Cabomba caroliniana, Myriophyllum spicatum, Ceratophyllum demersum, Eleocharis acicularis, Potamogeton oxyphyllus, Potamogeton crispus, Limnophila sessiliflora, or Vallisneria denseserrulata). Coexistence assay results showed that only the macrophytes C. caroliniana or M. spicatum inhibited the growth of all blue-green algae, with the inhibitory effects of M. spicatum being stronger than those of C. caroliniana and being produced by the release of allelopathic compounds. In subsequent initial addition assays using M. spicatum with the alga M. aeruginosa, no significant growth inhibition was observed; whereas, in contrast, quasi-continuous addition assays showed strong growth inhibition by M. spicatum. These results provide the first evidence that unstable, growth-inhibiting allelopathic compounds are continuously secreted by M. spicatum.

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