Acute Toxicity of Methyl-Parathion in Wetland Mesocosms: Assessing the Inﬂuence of Aquatic Plants Using Laboratory Testing with Hyalella azteca R. Schulz, 1 M. T. Moore, 1 E. R. Bennett, 2 C. D. Milam, 3 J. L. Bouldin, 3 J. L. Farris, 3 S. Smith Jr., 1 C. M. Cooper 1 1 USDA-Agricultural Research Service, National Sedimentation Laboratory, P.O. Box 1157, Oxford, Mississippi 38655, USA 2 GLIER, University of Windsor, Windsor, Ontario, Canada N9B 3P4 3 Environmental Science Program, Arkansas State University, P.O. Box 847, State University, Arkansas 72467, USA Received: 31 July 2002 /Accepted: 27 March 2003 Abstract. 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 LC 50  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. The utility of aquatic plants for removal of insecticides from water has been shown (Hand et al. 2001; Karen et al. 1998; Wolverton and Harrison 1975). Processes important for re- moval of nonpoint-source pesticide runoff in vegetated wet- lands may include adsorption, decomposition, and microbial metabolism (Rodgers et al. 1999). The macrophytes present in the wetland may play an important role in providing an in- creased surface area for sorption as well as for microbial activity (Hand et al. 2001; Karen et al. 1998; Luckeydoo et al. 2002). Furthermore, they may contribute directly to chemical metabolism (Wetzel 1993). It has also been demonstrated that emergent vegetation reduces resuspension of sediments in wet- lands (Dieter 1990). The effects of the organophosphate phorate have been as- sessed using littoral mesocosms in South Dakota wetlands (Dieter et al. 1996). Recent studies have emphasized the ability of constructed wetlands to retain nonpoint-source insecticide pollution, preventing it from entering receiving aquatic habitats (Moore et al. 2002; Schulz and Peall 2001; Schulz et al. 2001). The implementation of retention ponds in agricultural water- sheds was mentioned by Scott et al. (1999) as part of an integrated strategy to reduce the amount and toxicity of runoff- related insecticide pollution discharging into estuaries. How- ever, there are few other studies in the published literature dealing with the fate or effects of agricultural insecticide input in constructed wetlands. Runoff is the major source of insecticide input to aquatic systems in the intensively cultivated Mississippi delta region (Cooper and Lipe 1992). Constructed wetlands may serve as a suitable risk mitigation strategy for agricultural runoff, given that enough information is made available on their effective- ness with speciﬁc reference to the importance of the wetland vegetation. Biological effects of pesticides in wetlands have been stud- ied under experimental conditions using mesocosms (Deten- beck et al. 1996), in littoral enclosures (Dieter et al. 1996), or in the ﬁeld employing organisms in situ (Schulz and Peall 2001; Schulz et al. 2001; Scott et al. 1999). Unfortunately, no information is available on the design of wetlands with respect to the resulting toxicity reduction. This implies a need to provide quantitative data on the effectiveness of wetlands in reducing insecticide toxicity in relation to characteristics such as the presence of emergent macrophyte vegetation. The fol- lowing study was undertaken for this purpose. Methyl-parathion (MeP), an organophosphate insecticide primarily applied to cotton, was chosen as the test substance for Correspondence to: R. Schulz, Syngenta Crop Protection AG, Eco- logical Sciences, Jealott’s Hill International Research Centre, Brack- nell, Berkshire, RG42 6EY, UK; email: Ralf.Schulz@syngenta.com Arch. Environ. Contam. Toxicol. 45, 331–336 (2003) DOI: 10.1007/s00244-003-2170-2 ARCHIVES OF Environmental Contamination and T oxicology © 2003 Springer-Verlag New York Inc.