Nature versus mixture: Evaluating changes in benthic assemblages due to insecticides and nutrients

Run-off from agricultural drainages contains complex mixtures of sediments, nutrients, and contaminants.  Although we know that pesticide inputs can reshape the structure and function of aquatic food webs, the role of nutrients as a modifier of pesticide effects is poorly understood.  In a partial answer to this question we are examining the interactions of multiple insecticides in the presence and absence of nutrient addition.  In particular, we are interested in the potential of nutrients to mask the effects of pesticides on benthic invertebrate assemblages.  Using classic approaches, we examined the responses of benthic macroinvertebrate communities and algal responses with respect to nutrients and an insecticide-fungicide mixture.  However, metrics of benthic community structure were insensitive to the sublethal effects of pesticides.   In contrast, the body size of emergent insects collected over the course of the mesocosm experiments were excellent predictors of pesticide-nutrient treatment.  Furthermore, moderate total nitrogen addition was found to be an important driver in the improved success of sensitive species under sublethal exposure regimes.  These findings indicate that existing benthic invertebrate metrics (e.g. EPT, multivariate analysis) are insufficient to detect nutrient-insecticide interactions, particularly in the sublethal effect range, where, presumably most effects occur.  Furthermore, these studies highlight that nitrate addition is necessary and sufficient to mask changes in aquatic larval insects, preventing the detection of changes in benthic invertebrate abundance and diversity.