Toxicity of chlorpyrifos, diazinon, malathion, carbaryl and acrolein alone and in mixtures on the fairy shrimp <i>Thamnocephalus platyurus</i>

Assessments of potential risk to non-target organisms from pesticide exposure are required for federal actions, including registering pesticides and setting water quality criteria. Where threatened or endangered (listed) species are potentially impacted, pesticide-related assessments should include environmentally relevant exposure scenarios, yet experimental toxicity data for surrogate species often focuses on single-chemical laboratory exposures. Vernal pools in California, USA are home to several ecologically important listed species, including the vernal pool fairy shrimp (Branchinecta lynchi), and coincide with areas of intense agricultural pesticide use. We tested individual and combined toxicity of pesticides chlorpyrifos, malathion, diazinon, carbaryl and acrolein in 24-hr acute bioassays to Thamnocephalus platyurus, a surrogate fairy shrimp species to endangered B. lynchi. Acrolein, an herbicide, was the most toxic and was associated with the highest % lethality of total effect which included death and morbidity. Chlorpyrifos was the most toxic of the remaining acetylcholinesterase-inhibiting pesticides. For pesticide mixture experiments in which proportions of acetylcholinesterase inhibitors varied, the expected additive effective concentrations affecting half of the population (EC50s) overestimated the observed effect. Malathion and diazinon mixtures with relatively high percent diazinon were closer to expected effects than high malathion mixtures, whereas in mixtures with carbaryl, treatments with higher diazinon percentage had less than additive effects. Variation within treatments was high, and overall deviations from expected values were moderate, supporting a general additive approach for evaluating risks from these pesticides where they may co-occur. Pesticide EC50s to T. platyurus are further compared with other aquatic invertebrates, to modelled concentrations in vernal pools, and with environmental concentrations from a paucity of in situ field-collected data from vernal pool habitats in the Western US. Our results can be applied to ecological risk assessments of pesticide impact to B. lynchi or its critical habitat, and to those involving the wider vernal pool habitat range for T. platyurus.