Metal tolerance and predator-prey interactions in benthic macroinvertebrate stream communities

Predicting responses of benthic communities to contaminants requires an understanding of both direct and indirect effects. I examined the influence of previous exposure to heavy metals on metal tolerance and susceptibility of benthic communities to stonefly predation in experimental streams and field cages. Benthic communities obtained from a reference stream, the Cache la Poudre River (PR), and a metal-polluted stream, the Arkansas River (AR), in Colorado were exposed to a mixture of Cd, Cu, and Zn at similar to 2x the U.S. EPA chronic criterion level (2.2 mu g Cd/L, 24 mu g Cu/L, and 220 mu g Zn/L) for 10 d in stream microcosms. Effects of metals on survival were significantly greater on mayflies (Ephemeroptera) from the unpolluted PR than from the metal-polluted AR. Metal exposure also significantly increased drift of Rhithrogena hageni, the dominant mayfly in the unpolluted stream but had no effect on invertebrates from the AR. In a second experiment, communities from the two locations were exposed to predatory stoneflies (Classenia sabulosa) in stream microcosms. Stonefly predation significantly reduced total macroinvertebrate abundance, number of taxa, and abundance of Baetis tricaudatus (Ephemeroptera) and R. hageni. Mayflies from the metal-polluted AR were also more susceptible to stonefly predation, but these results were probably influenced by differences in initial prey density. Results of caging experiments conducted at PR and AR field sites were consistent with results of laboratory experiments and showed that stonefly predation significantly reduced abundance of B. tricaudatus and R. hageni; however, effects of predation in the AR were relatively weak because of high prey immigration. Results of these experiments suggest that abiotic factors influence the outcome of species interactions in metal-polluted streams. Ecosystems disturbed by contaminants, particularly those polluted for relatively long periods of time, provide excellent opportunities to study these interactions.