Individual and Mixture Toxicity of Three Pesticides; Atrazine, Chlorpyrifos, and Chlorothalonil to the Marine Phytoplankton Species Dunaliella tertiolecta

Abstract

This study analyzed the toxicity of three pesticides (the herbicide atrazine, the insecticide chlorpyrifos and the fungicide chlorothalonil) individually, and in two mixtures (atrazine and chlorpyrifos; atrazine and chlorothalonil) to the marine phytoplankton species Dunaliella tertiolecta (Chlorophyta). A standard 96 h static algal bioassay was used to determine pesticide effects on the population growth rate of D. tertiolecta. Mixture toxicity was assessed using the additive index approach. Atrazine and chlorothalonil concentrations ≥ 25 µg/L and 33.3 µg/L, respectively, caused significant decreases in D. tertiolecta population growth rate. At much higher concentrations (≥ 400 µg/L) chlorpyrifos also elicited a significant effect on D. tertiolecta population growth rate, but toxicity would not be expected at typical environmental concentrations. The population growth rate EC₅₀ values determined for D. tertiolecta were 64 µg/L for chlorothalonil, 69 µg/L for atrazine, and 769 µg/L for chlorpyrifos. Atrazine and chlorpyrifos in mixture displayed additive toxicity, whereas atrazine and chlorothalonil in mixture had a synergistic effect. The toxicity of atrazine and chlorothalonil combined was approximately 2 times greater than that of the individual chemicals. Therefore, decreases in phytoplankton populations resulting from pesticide exposure could occur at lower than expected concentrations in aquatic systems where atrazine and chlorothalonil are present in mixture. Detrimental effects on phytoplankton population growth rate could impact nutrient cycling rates and food availability to higher trophic levels. Characterizing the toxicity of chemical mixtures likely to be encountered in the environment may benefit the pesticide registration and regulation process.

Keywords:

• Pesticides
• Mixtures
• Phytoplankton
• Toxicity

Acknowledgments

Funding for this research was provided by NSF Research Experiences for Undergraduates Program Grant # DBI‐9876926 and NOAA. The authors are very grateful to Jeannette Weiner, Meagan Leatherbury, and Jennifer Emblidge for laboratory assistance.

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