An Evaluation of Appropriate Expressions of Toxicity in Aquatic Plant Bioassays as Demonstrated by the Effects of Atrazine on Algae and Duckweed

Abstract

This investigation was undertaken to determine the toxicity of the herbicide atrazine to a variety of aquatic plant species as expressed by different endpoints, including the five-day EC50, the five-day no-observed effect concentration (NOEC), and the phytostatic and phytocidal concentrations. The validity of each of these endpoints and the related issues of test duration and biological response are discussed. Tests were conducted with two freshwater algae (Anabaena flos-aquae and Navicula pelliculosa), a marine alga (Dunaliella tertiolecta), and a freshwater vascular plant, Lemna gibba (duckweed). The test procedure consists of a five-day exposure phase followed by a nine-day recovery phase, with standing crop as the measure of effect on growth. Of the four endpoints, the NOEC is the most conservative and is also the most sensitive to the experimental design and statistical procedures used. A 50% reduction in population growth (EC50) is also conservative and is an acceptable parameter for a screening test. However, determination of the EC50 alone does not indicate the lethality of the test material or the recovery potential of the test species. Due to environmental significance and statistical considerations, the phytostatic and phytocidal concentrations are recommended as the primary responses.