Stressor dominance and sensitivity-dependent antagonism: disentangling the freshwater effects of an insecticide among co-occuring agricultural stressors

1. Pesticide concentrations are correlated with regional declines in stream invertebrate diversity. Experimental studies have identified that pesticides can have strong and persistent negative effects on aquatic ecosystems. These effects may occur at concentrations orders of magnitude lower than laboratory toxicity studies predict. Synergism among stressors is one explanation for the observed laboratory-field difference. However, the true effect of pesticides on stream invertebrates remains uncertain, given interactions between stressors and natural environmental conditions. 2. We experimentally examined multiple-stressor effects on stream invertebrate assemblages and leaf-litter breakdown using 24 independent ~900 L re-circulating outdoor mesocosms in a semi-orthogonal design. Two pulses of the pesticide malathion (C10H19O6PS2) were delivered at low and high concentrations (Pulse 1: low at 0.1 and high at 1 µg L-1¬; Pulse 2: at 2.5 and 25 µg L-1). These were crossed with a treatment mi...