Comparative assessment of herbicide phytotoxicity to Selenastrum capricornutum using microplate and flask bioassay procedures

Four reference toxicants (Cr 6+ , Cu 2+ , Zn 2+ , phenol) and 9 herbicides (imazamethabenz, 2,4‐dichlorophenoxyacetic acid, picloram, glyphosate, bromoxynil, metolachlor, diquat dibromide, hexazinone, cyanazine) were appraised using both the microplate and flask assay Ninety‐six hour EC 50 s determined with Selenastrum capricornutum as the indicator species essentially demonstrated good intermethodological data concordance for all chemicals, with the exception of diquat dibromide, whose phytotoxicity in the microplate assay (EC 50 = 4.9 μg · L −1 ) was nearly 7 times that of the flask assay (EC 50 = 34.2 μg · L −1 ). Comparisons with other data in the scientific literature relating to similar herbicides with the same or different green algal indicator species appeared to corroborate the overall data obtained in our study. More than 4 orders of magnitude separated the most toxic (cyanazine, flask, and microplate EC 50 s of 17.6 and 16.9 μg · L −1 , respectively) and the least toxic (imazamethabenz, flask, and microplate EC 50 s of 89.1 and 91.1 mg · L −1 , respectively) herbicides. The biprocedural phytotoxicity comparison described in this work suggests that the simpler algal microplate assay can be an appropriate alternative to the flask technique to evaluate the algal growth inhibition effects of herbicides.