The effects of single and joint toxicity of atrazine and alachlor on three non-target aquatic organisms

Abstract

The acute and chronic toxic effects of two commonly used herbicides were determined singularly and combined on three non-target aquatic organisms. Test organisms, Lemna minor (plant), Ceriodaphnia reticulata (invertebrate) and Pimephales promelas (fish) representing the different ecological trophic levels, were exposed to various concentrations of atrazine and alachlor to determine the EC50 or LC50 for each species and when possible determine the NOEC and LOEC values associated with each test. Short-term chronic and acute testing procedures utilizing static and static renewal test methods were employed throughout the study. Atrazine test results suggest that the EC5O for Lemna was under 200 ug/L. Acute LC50 for Ceriodaphnia was 7.8 mg/L; and approximately 15.0 mg/L for Pimephales larvae. The acute effects of alachlor were found to occur at 35.4 ug/L (EC50) for Lemna, 3.6 mg/L for Ceriodaphnia (LC50) and 4.9 mg/L for Pimephales (LC50). The combined effect of these herbicides was determined by joint tests using a 1:1 ratio of Toxic Unit concentrations demonstrated that a greater than additive effect occurred on Lemna and an additive or greater than additive effect was observed on the Pimephales larvae. Testing with Ceriodaphnia was discontinued after single testing due to problems in the stock culture maintenance.

Chronic effects were observed in all test species at concentrations significantly lower than the calculated LC50 or EC50 values. LOEC and NOEC values for Lemna indicate a chronic effect (frond development) occurs between 25 and 100 ug/L for atrazine and at or below 7.5 ug/L for alachlor. The lowest observable effect for Pimephales occurred at 3.0 mg/L for atrazine and 2.0 mg/L for alachlor.

Alachlor appears to be more toxic than atrazine to all organisms tested and is known to occur in the environment at levels found to be directly toxic in this study. The occurrence of a greater than additive effect is of concern since these herbicides commonly co-occur in aquatic environments.