Direct and Indirect Effects of Repeated Pollution Events on Marine Hard-Substrate Assemblages

Testing the effects on organisms of a constant exposure to toxicants in uniform environments is unlikely to reflect accurately the majority of toxicant exposures in the field, and environmental managers need tools to help predict impacts especially of pulse toxicant inputs into highly variable natural environments. We investigated the effect of pulse copper pollution events on the development of sessile marine invertebrate assemblages at two sites in Port Phillip Bay, Victoria, Australia. Using a field dosing technique we created localized 2-d copper pollution events close to settlement plates. Pulses were delivered every 2, 4, 8, or 16 wk for the duration of the 16-wk experiments, and the assemblages allowed to develop under these dosing regimes. Pulse pollution events dramatically altered the assemblages at both sites predominantly through a direct negative effect on the densities of large space-occupying tunicates. Most other taxonomic groups responded positively to multiple copper pulses, which was considered a density-mediated indirect effect of the toxicant. In particular, organisms known to be good colonizers but poor competitors for space such as serpulid polychaetes occurred in densities an order of magnitude higher on plates exposed to copper pulses. The responses of the assemblages were predominantly independent of the frequency of pulse pollution events, although a single pulse exposure at the beginning of the experiment had no observable effect when censused at week 16. The impact of these transient pollution events were manifest as changes in the structure of invertebrate assemblages and could persist for some time after the agent of disturbance was removed. The effects of a single pulse copper pollution event on an established (8-wk-old) assemblage were evident for at least a further 8 wk after it occurred.