Growth inhibition as indicator of stress because of atrazine following multiple toxicant exposure of the freshwater macrophyte, Juncus effusus L.

Atrazine is a herbicide used most frequently in North America, but it usually is encountered in mixtures of agrochemicals. Few atrazine exposure studies have been conducted using mixed pesticides; therefore, little data are available to suggest reliable means of discerning effects attributable to atrazine. The common freshwater macrophyte, Juncus effusus L., was exposed in 66 mesocosms to atrazine at two nominal concentrations (96 and 192 microg/L) with varying concentrations of chlorpyrifos, monosodium methanearsonate, and monomethylmercury. Exposure levels represented typical levels that might follow runoff or direct-spray application in enclosed waterbodies. Using shoot density and number of shoots shorter than 25 cm per unit area as response measurements, the growth effects of atrazine, even in varying pesticide mixes, could be detected as early as 16 d after initial exposure. Further growth effects specifically caused by atrazine also could be detected following a second exposure to the same toxicant mixture. Mesocosm tests offer greater control of natural variability than would be found in the natural environment and offer more realistic conditions than traditional laboratory/greenhouse studies. Therefore, in field testing, growth measurements should be accompanied by other confirmatory tests, such as pesticide concentrations in tissue and, possibly, chlorophyll concentrations, for measuring specific toxic effects of atrazine and other pesticides.