Integrating effects of stressors across levels of biological organization: examples using organophosphorus insecticide mixtures in field-level exposures

An important uncertainty often identified inecological risk assessment is the lack of ecologicalconnectivity between endpoints measured across themany levels of biological organization within anecosystem. In the present study, we address thisissue by quantitatively linking acetylcholinesterase(AChE) activity, a common biomarker of exposure toorganophosphorus (OP) insecticides, with endpoints athigher levels of biological organization in fish andinvertebrates, and to assess the utility of thisendpoint as a predictive biomarker of effects underfield conditions. In 1997, we conducted three fieldstudies in outdoor microcosms to assess binary andternary mixtures of chlorpyrifos (CLP),azinphos-methyl (AZM), and diazinon (DIA). The firststudy (14 days) used a regression design(concentration-response) to assess direct and indirectpopulation-level responses of zooplankton andphytoplankton to a binary mixture of DIA and CLP atnominal concentrations of 0.44–44.0 μg/L. A secondregression study (7 days) was conducted to assesslethality (organismal response) in fathead minnows(Pimephales promelas) to a ternary mixture ofthe three OP's at nominal concentrations of 50–1750 μg/L. An ANOVA study (7 days) was also conducted toassess lethality in fathead minnows. In this study,the concentrations of the components of the mixturewere determined from exposure and toxicitydistributions using a toxic equivalent (TE) approachand apportioned to achieve equipotent mixtures at80:10:10 ratios. The abundance of Cladocera declinedby close to 100% within 24 hours of application atthe four highest concentrations; consequently, AChEcould not be measured in these treatments. At the twolowest treatments, AChE activity exhibited aconcentration-dependent decline over the study period;however, AChE activity increased over the first 24hours of exposure, while abundance decreased. Infathead minnows, mortality was significantlycorrelated with brain AChE activity in both studies. The form of this relationship was remarkablyconsistent between studies, with a 50% reduction inAChE activity corresponding to 10–15% mortality anda 90% inhibition of AChE corresponding to 50%mortality. The results of these studies show thatAChE activity, a biochemical endpoint, can be used asa reliable biomarker of effect at the organismal leveland may be useful as a predictor of population-levelresponses in invertebrates.