Reduction of pollution impacts through the control of toxicant release rate must be site- and season-specific

Abstract

The marine environment is commonly used for the deliberate disposal of industrial, mining and metabolic wastes. Managers will benefit from experimental work that identifies ways of reducing environmental impacts by varying the frequency and intensity of toxicant release. Using a field dosing technique we investigated the effects of three frequencies of pulse copper pollution event, and two intensities of copper dose, on developing assemblages of sessile marine invertebrates. The resulting impacts could then be compared in assemblages exposed to the same amount of toxicant via different disturbance regimes. The experiment was replicated simultaneously at three sites within Port Philip Bay, Victoria, Australia. Pulse pollution events altered assemblage composition through a direct negative effect on densities of large solitary ascidians. In response to the removal of the spatially dominant solitary ascidians, there were increases in recruitment of many different phyla, and in the abundance of older individuals of some serpulid and bryozoan taxa. Biodiversity, as measured by total species number and Shannon's H′, did not reflect the dramatic structural changes apparent within assemblages. If pulse pollution events had either a negative or positive effect on a species' density, then that effect was accentuated by increasing the intensity (strength) or the frequency of the pollution disturbance. Within populations, however, adult mortality might benefit new recruits of the same species through the freeing up of settlement space. In this case, the effects of copper pulses were evident as changes in the population size structure rather than the overall density of an organism. There were variable responses to manipulating the output rate of the toxicant between sites that appear to be driven by the rate of recovery of the dominant space occupiers. At one site there was a negligible effect of disturbance intensity and impacts could be minimized through reducing only the frequency of toxicant release. At two other sites both the intensity and frequency of disturbance determined the pollution effect and minimizing impacts was feasible only through an overall reduction in the amount of toxicant released. The management and reduction of pollution impacts through the control of toxicant release will require site- and season-specific modifications.