Toxicity of hexazinone and diquat to green algae, diatoms, cyanobacteria and duckweed

Hexazinone and diquat are two broad-spectrum contact herbicides used in a variety of crop and non-crop applications. Both pesticides are highly water soluble and persistent in the aquatic system. Hexazinone is mobile in soil and, thus, the potential for leaching into ground water and for overland runoff into surface water is high; diquat, however, is rapidly bound by soil and sediment to a biologically inactive form, at least temporarily. Toxicity to green algae (two species), diatoms (two species) and cyanobacteria (five species) was determined using inhibition of C-14 uptake. Toxicity to the floating vascular plant, duckweed, was tested by measuring seven-day growth inhibition. Test organisms exhibited large differences in sensitivity to both herbicides. The green algae, diatoms, and duckweed (all eukaryotes) were more sensitive to hexazinone than were cyanobacteria (prokaryotes). Mean concentrations at which 50% inhibition occurred in these groups were 0.01 (green algae), 0.05 (diatoms), 0.07 (duckweed), and 0.6 (cyanobacteria) mg hexazinone per litre, respectively. In contrast, green algae were comparatively tolerant of diquat (EC50 values approximately 0.6 mg l(-1)), while the cyanobacteria and diatoms were much more sensitive (mean EC50 values of 0.074 and 0.079 mg diquat per litre). Duckweed showed the greatest sensitivity to diquat, with 50% inhibition of growth occurring at 0.004 mg l(-1). That some non-target aquatic plants are susceptible to diquat toxicity at less than 0.01 mg l(-1) is significant, as most methods of diquat analysis have minimum detection limits of 0.01 to 0.05 mg l(-1). Therefore, algae and vascular aquatic plants may suffer phytotoxic effects in a contaminated water body even when diquat is undetectable chemically. Differential sensitivity to herbicide contaminants among taxonomic groups of plants may have ecological consequences. Cyanobacteria may produce neuro-and hepato-toxins and constitute a poor food resource for higher trophic levels relative to the other organisms tested. Contamination of surface water with hexazinone may degrade water quality for wildlife, both by causing food reduction and habitat loss through inhibition of green algae, diatoms and macrophytes, and by allowing the proliferation of cyanobacteria. Varying responses among the different test algae to the two herbicides under study indicated that a wide taxonomic range of test species is necessary in evaluating the impacts of contaminants in aquatic systems. (C) 1997 Elsevier Science B.V.