Effects of five rice herbicides on the growth of two threatened aquatic ferns

The effects of five rice herbicides bensulfuron methyl, mefenacet, quinoclamine, simetryn, and thiobencarb on the growth of two threatened aquatic ferns Azolla japonica and Salvinia natans were tested using 12-day exposure experiments at 0.1–100 nM which are expected to be present in drainages and rivers in Japan. As a reference species, Lenma minor was also used to examine the toxicity of bensulfuron methyl. Bensulfuron methyl had the most pronounced effect on the relative growth rate (RGR) of A. japonica, S. natans, and L. minor with an EC50 of 5.0, 0.54, and 10 nM, respectively. The other herbicides reduced the RGR of the aquatic ferns only at the highest concentration (100 nM) or not at all. S. natans showed the highest susceptibility to bensulfuron methyl among the three species, and the EC50 for this species was comparable to or below the maximum concentration (0.49–5.6 nM) that had been previously detected in 7 of 11 rivers in Japan. These results suggest that bensulfuron methyl runoff in drainages and rivers in Japan is expected to have adverse effects on the growth of threatened aquatic ferns in some cases, and that no or small effects occur for the other four herbicides tested.