Toxicity of atrazine to the juvenile hard clam, Mercenaria mercenaria

The herbicide atrazine is one of the most heavily used pesticides in the United States. The effects of atrazine on the clam Mercenaria mercenaria were evaluated in aqueous and sediment laboratory assays. Juvenile clams of approximately 1 mm in size were used for all experiments. An acute aqueous bio-assay was used to determine the 96-h LC50 for the juvenile clams. A chronic aqueous bioassay was conducted at lower atrazine concentrations over a 10-day exposure period to examine both lethal and sublethal (dry mass, shell size, and condition index) endpoints. A chronic sediment bioassay examined mortality and sublethal endpoints in a 10-day exposure. The acute 96-h LC50 was 5608 μg/L with 95% confidence intervals ranging from 5003 to 6287 μg/L. Results of the chronic aqueous assay indicated both lethal and sublethal (reduced shell size) effects at high atrazine concentrations. In the 10-day chronic aqueous assay, the no observable effect concentration was 500 μg/L, the lowest observable effect concentration was 1000 μg/L, and the maximum allowable toxicant concentration (MATC) was 707 μg/L. There were no significant effects of atrazine in the chronic sediment exposure. Safe concentrations for the aqueous experiments were estimated by applying an uncertainty factor of 10 to the calculated MATC values. While there were adverse effects of atrazine at high concentrations, these results suggest that atrazine is not directly toxic to M. mercenaria at environmentally relevant concentrations.