Stressor dominance and sensitivity‐dependent antagonism: Disentangling the freshwater effects of an insecticide among co‐occurring 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 observed laboratory‐field differences. 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 ~900L 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 combining stressors commonly associated with agricultural development; nitrogen, phosphorus and sediment (kaolinite). 3.Malathion degradation was rapid (