Acute Toxicity of Methyl-Parathion in Wetland Mesocosms: Assessing the Influence of Aquatic Plants Using Laboratory Testing with Hyalella azteca

Methyl-parathion (MeP) was introduced into con- structed wetlands for the purpose of assessing the importance of distance from the source of contamination and the role of emergent vegetation on the acute toxicity to Hyalella azteca (Crustacea: Amphipoda). A vegetated (90% cover: mainly Jun- cus effuses) and a nonvegetated wetland (each with a water body of 50 5.5 0.2 m) were each exposed to a simulated MeP storm runoff event. H. azteca was exposed for 48 h in the laboratory to water samples taken from the wetlands at a distance of 5, 10, 20, and 40 m from the pesticide inlet 3 h, 24 h, 96 h, and 10 days following application. Methyl-para- thion was detected throughout the nonvegetated wetland, whereas the pesticide was only transported halfway through the vegetated wetland. A repeated-measure three-way analysis of variance (ANOVA) using time, location, and vegetation indi- cated significantly lower toxicity in the vegetated wetland. Furthermore, the mortality decreased significantly with both increasing distance from the inlet and time (48-h LC50 95% CI: 9.0 0.3 g/L). A significant three-way interaction of time vegetation location confirmed higher toxicity at the inlet area of the nonvegetated wetland immediately after con- tamination. Significant linear regressions of maximum mortal- ity (independent of time) versus distance from the pesticide inlet indicated that 44 m of vegetated and 111 m of nonveg- etated wetland would reduce H. azteca mortality to 5%. These results suggest that vegetation contributes to reduced MeP effects in constructed wetlands.