ABSTRACT: Transient pollution events have the potential to cause both direct and indirect effects on exposed assemblages, but the test designs used in the majority of toxicity studies have precluded an examination of potentially important indirecteffects. Field experiments conducted in marine epifaunal assemblages have revealed that pulse chemical disturbances from copper decrease the densities of large solitary ascidians while increasing the densities of serpulid polychaetes. Field manipulativeexperiments were carried out to test whether the positive effects of transient pollution events represent the response of serpulids to the release from competition for space (an indirect effect of the toxicant) or a direct positive response to theputative toxicant. Three species of serpulid polychaetes (Ficopomatus enigmaticus, Hydroides sp., and Pileolaria pseudomilitaris) responded positively to pulse copper exposures in the presence of competition, but did not display anydensity response to pulse copper exposures in the absence of competition for space from other recruits. Pomatoceros taeniata displayed no response to copper exposure in the presence of competition, but was dramatically reduced in density in theabsence of competition for space. This was the only species deemed sensitive to the frequency and intensity of pollution disturbance employed. There was no indication of a direct positive effect of the toxicant on any serpulid, confirming the hypothesisthat positive responses of serpulids to copper exposure in the whole assemblage are an indirect effect of the toxicant mediated through competition for space. Our results show that the interaction between a pollution disturbance and competition for alimiting resource can modify the response of organisms to a toxicant. Such results could not be predicted from conventional laboratory-based toxicity tests and they suggest that small-scale ecotoxicological field tests are a useful tool in predicting theimpacts of pollution events in complex systems.