Survivorship Patterns of Larval Amphibians Exposed to Low Concentrations of Atrazine
Storrs, Sara I., Kiesecker, Joseph M., Environmental Health Perspectives
Amphibians can be exposed to contaminants in nature by many routes, but perhaps the most likely route is agricultural runoff in amphibian breeding sites. This runoff results in high-level pulses of pesticides. For example, atrazine, the most widely used pesticide in the United States, can be present at several parts per million in agricultural runoff. However, pesticide levels are likely to remain in the environment at low levels for longer periods. Nevertheless, most studies designed to examine the impacts of contaminants are limited to short-term (~ 4 days) tests conducted at relatively high concentrations. To investigate longer-term (~ 30 days) exposure of amphibians to low pesticide levels, we exposed tadpoles of four species of frogs--spring peepers (Pseudacris cructfer), American toads (Bulb americanus), green frogs (Rana elamitans), and wood frogs (Rana sylvatica)--at early and late developmental stages to low concentrations of a commercial preparation of atrazine (3, 30, or 100 ppb; the U.S. Environmental Protection Agency drinking water standard is 3 ppb). We found counterintuitive patterns in rate of survivorship. Survival was significantly lower for all animals exposed to 3 ppb compared with either 30 or 100 ppb, except the late stages of B. americanus and R. sylvatica. These survival patterns highlight the importance of investigating the impacts of contaminants with realistic exposures and at various developmental stages. This may be particularly important for compounds that produce greater mortality at lower doses than higher doses, a pattern characteristic of many endocrine disruptors. Key words: amphibian, atrazine, endocrine disruption, NMDRC, nonmonotonic dose-response curve. doi:10.1289/ehp.6821 available via http://dx.doi.org/[Online 25 March 2004]
Although pesticides are used on a local scale, they are ubiquitous and spread regionally and globally. In some areas, the transport and deposition of pesticides from agriculturally intensive areas to adjacent nonagricultural areas are well documented (Davidson et al. 2002; LeNoir et al. 1999; Zabik and Seiber 1993). Moreover, pesticides have been found in the bodies of frogs from areas where pesticide use has not occurred historically or in the past 25 years (Cory et al. 1970; Datta et al. 1998; Russell et al. 1995, 1997). Agricultural runoff of pesticides can also have an effect on amphibians because pesticides have been detected at amphibian sites almost a year after being applied (Hayes et al. 2003). This measure of persistence combined with their high susceptibility to exposure, because they have a complex life cycle and permeable skin (Cooke 1981; Hall and Henry 1992), demonstrates that amphibians are excellent model organisms for testing pesticide exposure.
Runoff from agricultural lands can expose pond-breeding amphibians to various levels of pesticides. For example, atrazine, the most widely used pesticide in the United States [U.S. Department of Agriculture (USDA) 2002], has been reported at levels from 0.1 to 6.7 ppb in amphibian breeding ponds in mid- to late July (Hayes et al. 2003), but during storm events agricultural runoff has been reported to be as high as 480 ppb (Huber 1993).
Moreover, it has been measured in rainfall at levels up to 40 ppb in agricultural areas (Nations and Hallberg 1992). Atrazine can be present at several parts per million in agricultural runoff for short periods of time (days). However, atrazine levels are likely to remain relatively low (parts per billion) for longer periods of time (Huber 1993). Nevertheless, most studies designed to examine the impacts of contaminants are limited to short-term (~ 4 days) tests conducted with relatively high concentrations (parts per million). Studies that have examined larval amphibian exposure to atrazine have found effects on plasma thyroxine, plasma corticosterone, larval size (Larson et al. 1998), developmental stage (Howe et al. 1998; Larson et al. …