Effects of planting system design on the toxicological sensitivity of Myriophyllum spicatum and Elodea canadensis to atrazine

The triazine herbicide atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-trazine) was selected as a chemical stressor in an investigation of how toxicological responses of individually grown macrophytes reflect those of plants grown in more natural model populations and two-species communities. Phytotoxicity of the compound to Myriophyllumspicatum L. and Elodeacanadensis Michx. was assessed under semi-natural field conditions using 12 000 l outdoor microcosms. Exposure concentrations of 25, 50, 100, 250 μg l−1 plus controls (n = 3) were evaluated, selected to fall within a range of concentrations known to produce a toxic response in the tested macrophytes, and effective concentrations required to cause a decrease in biomass endpoints by 10%, 25%, and 50% were estimated. The sensitivities of aquatic plants to atrazine did not differ substantially between planting systems, and few interactions between the effects of the planting method and atrazine effects on macrophyte biomass were detected using a two-way ANOVA. A lack of significant differences in biomass and relative growth rate measures between plants grown under the various test systems also indicated that interactions between and among species did not influence growth of plants in the model population and communities. Under these test conditions, the use of the “cone-tainer” method provided estimates of toxicity consistent with those from plants grown in assemblages, and potential interactions between plants were not found to modify the response of macrophytes to atrazine.