Assessment of duckweed Lemna aequinoctialis as a toxicological bioassay for tropical environments in developing countries

The tropical duckweed Lemna aequinoctialis Welwitsch (Lemnaceae) was collec ted in Thailand and the Philippines, so that an efficient toxicological bio assay for tropical environments could be developed. Optimal conditions requ ired for conducting ecotoxicological growth tests were investigated and the resulting test manual was used to test the effects of heavy metals (Cd2+, Cr6+, Cu2+, Hg2+, Zn2+), ClO3-; 3,5-dichlorophenol and phenol on the growth of the two strains of duckweed. The effect on growth was measured as 96-hr EC50 for biomass production and the number of fronds (leaf-like process) p roduced. The sensitivity of the two duckweed strains to the tested compound s was quite similar. Cd2+, Cu2+ and Hg-2 demonstrated the highest toxicity, ClO3- and phenol the lowest toxicity, while Cr6+ and Zn2+ demonstrated int ermediate toxicity.