A literature review of pesticide toxicity to aquatic plants was conducted in order to arrive at a set of species for use in preregistration testing. Criteria used in the selection process were: (1) existing database; (2) interspecies variability in response to pesticides; (3) availability of well-characterized plant cultures; and (4) ease of culture. Freshwater and marine algae and freshwater and estuarine macrophytes were included in the study. Results showed that the three practical criteria [1,3,4] all resulted in the selection of unicellular green algae (Chlorophyceae) for the freshwater algae tests. However, variability among freshwater species and classes was so great and so unpredictable that it became obvious that a species battery approach was required. The battery should consist of representatives of each of the major algal classes. Research into culture methods and alternative endpoints is required before species other than unicellular green algae can be routinely incorporated into species battery tests. The studies in the marine algal database had much more consistent methodology; therefore, it was possible to include relative sensitivity and variability as well as practical considerations in the selection of marine tests species. The golden-brown algae (Chrysophyceae) were generally the most sensitive. However, because variability in response was still high, we recommend a species battery for marine algae as well. Four species of golden brown alga, two diatoms (Bacillariophyceae) and two green algae, were chosen based on the four criteria. Macrophytes should be part of preregistration testing because they can be more sensitive than algae and because of their ecological importance. The database showed a great variety of test methods and very few laboratory methods. No species emerged as the most consistently sensitive among either freshwater or estuarine macrophytes. Lemna gibba and Lemna minor are fast growing, easy to culture, and available commercially; they are logical test species for effects of pesticide drift and surface films. However, they are not always very sensitive. Rooted macrophytes belong in a comprehensive test program; however, test methods have to be developed. One criterion not included in this study was ecological importance. We recommend that ecological importance become part of any selection process in order to facilitate extrapolation to the field during hazard assessments.

Four reference toxicants (Cr6+, Cu2+, Zn2+, 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 EC50s 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 (EC50 = 4.9 μg · L−1) was nearly 7 times that of the flask assay (EC50 = 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 EC50s of 17.6 and 16.9 μg · L−1, respectively) and the least toxic (imazamethabenz, flask, and microplate EC50s 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.

Guidelines are currently being developed to outline requirements for nontarget plant testing for pesticide registration in Canada. The Saskatchewan Research Council (SRC), under contract to Environment Canada, evaluated international regulatory requirements and test protocols, identified research needs, and recommended guidelines for Canada, by reviewing the scientific literature and databases, and consulting with regulators and scientists involved in phytotoxicity testing. Highlights of the SRC reports and a review by a federal working committee are presented to stimulate additional research, and to solicit critical review from scientists and regulators prior to formal drafting of Canadian regulatory guidelines.