In aquatic environments stressors typically occur simultaneously and have the potential to interact. Recently, the effect of nonconsumptive predator stress as a modifying factor of contaminant toxicity has received increased attention, and a number of studies have suggested that predator stress plays an important role in how contaminants exert their effects in aquatic ecosystems. For this presentation we have two objectives: 1) present results of a literature review on tests of interaction between nonconsumptive predator stress and contaminants in freshwater systems in order to identify trends and patterns, and 2) report data on experiments testing for interactions between predator cues and pesticides.

Results/Conclusions For objective 1, 29 papers published between 1992 and 2008 were identified that examined predator-contaminant interactions in freshwater environments. Within these, 51 cases of 182 (28%) reported an interaction between nonconsumptive predator stress and contaminants. It was found that different combinations of nonconsumptive predator stress and contaminants had a variety of effects on prey responses. Although the mechanisms of the predator-contaminant interaction remain largely unknown, several hypotheses have been proposed. For objective 2, the influence of predatory fish (Lepomis machrochirus) cue on the toxicity of bifenthrin (pyrethroid insecticide) and malathion (organophosphorus pesticide) was examined. The prey species was the cladoceran Ceriodaphnia dubia. Dose-response experiments were conducted for pesticides alone and in the presence of predator conditioned water. Predator cues increased the toxicity of malathion: the 48-h malathion LC50 with and without the predator cue was 1.8 ug/L (95% CI:1.6-1.9) and 2.1 ug/L (95% CI:1.9-2.3), respectively. Predator cue also affected malathion toxicity on a temporal scale with C. dubia LT50s shifting from 8.7 h (3.6 ug/L malathion) to 6.4 h (predator cue and 3.6 ug/L malathion). Alternatively, predator cue reduced the toxicity of bifenthrin. The 48-h LC50 values of bifenthrin with and without the presence of predatory cue were 0.36 ug/L and 0.22 ug/L, respectively. These results indicate a possible synergistic interaction between predatory cue and malathion and an antagonistic interaction with bifenthrin. The latter may be a result of increased organic matter influencing bioavailability of bifenthrin, while the former supports previous work that the toxicity of some OPs may be increased by predation stress. Overall, these results should provide a better understanding of the patterns of predator-contaminant interactions in freshwater ecosystems and provides further evidence that ecological factors, such as species interactions, should be considered when assessing the risk of pesticides within aquatic ecosystems.