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Algae control by using allelochemicals secreted from plants is a safe and novel biological algal inhibiting technology. In this work, Hydrilla verticillata was cultivated with algae in coexisting systems, and its possible inhibitory effect and mechanism on planktonic algae were studied in eutrophic water. The results showed that 12 g L−1 H. verticillata under nutrient sufficient conditions had an allelopathic inhibitory effect on planktonic algae at initial concentrations of 103, 105 and 107 cells mL−1, with an inhibitory ratio of 25.75, 38.49, and 58.17, respectively. H. verticillata played a role of “promoting with low biomass while inhibiting with high biomass” on planktonic algae. The optimal biomass of H. verticillata for the growth inhibition of planktonic algae at initial concentration of 103 cells mL−1 was 12 g L−1. H. verticillata produced possible inhibitory effects on Ankistrodesmus spirali, Closterium sp., Ankistrodesmus angustus, Dactylococcopsis sp. and Microcystis aeruginosa of planktonic algae. The possible inhibitory effects shown in descending order were as A. spirali > Closterium sp. > A. angustus > Dactylococcopsis sp. > M. aeruginosa. However, little effect could be observed on Ankistrodesmus falcatus and Tetrasporales sp. It is predicted that allelochemicals of H. verticillata with algal inhibiting effect were as follows: compounds of lipid, acid, hexanol, amide and ketone.

The diagnostic characters alleged byEttl (1981) andEttl &Komarek (1982) to distinguish the ClassesChlamydophyceae andChlorophyceae are discussed. The recognition of these classes ignores ultrastructural evidence of the close phylogenetic relationships of their members and produces an artificial redistribution of orders, families, and genera that can only lead to confusion in the taxonomy of the green algae. Suggestions for a more natural classification are made.