Ecotoxicology of metals in estuarine plants

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The increasing population pressures in the coastal zones of the world create increasing pollution pressures on the biota of the coastal areas, of which estuaries are the more productive components. The focus of this project was the response of the estuarine plants, mangroves and saltmarsh species, to the increasing metal load in the estuaries. An ecotoxicological approach was used, based on the ‘triad’ approach recommended by the USEPA and Environment Canada, for use in ecological risk assessment. This methodology integrates the results of ecological surveys, chemical analyses and toxicity testing to evaluate the effects of pollutants on the environment. Three estuaries were selected in the Sydney region of NSW to examine sediment metal levels and plant uptake of copper, zinc and lead, as the major components of metal pollution in the estuaries. Lake Macquarie sites were found to be highly contaminated with zinc (≥ 800 ppm) and lead (≥ 1100 ppm), while the Sydney Estuary was of medium level of contamination, and Port Hacking was a ‘clean’ site. All the species selected for study, the two mangroves of the Sydney region (Avicennia marina and Aegiceras corniculatum) and two saltmarsh eo-dominants, Sarcocornia quinqueflora and Sporobolus virginicus, accumulated higher levels of metals in their roots than in the shoots when growing on metal contaminated sediments. In both mangroves, uptake of metals into the shoots was restricted, while in S. quinqueflora, the shoots had the highest levels of metals of all four plants. The results of field studies indicated that metal toxicity was probably ameliorated by sediment factors, and no correlation was found between above-ground biomass or productivity and metals in the sediments. The influence of the sediment type on toxicity was also demonstrated in the incremental bioassay using A. marina, in which toxicity of zinc was examined in the presence of variable organic content in the soils. The results of the bioassays revealed the high capacity of mangroves to take up zinc without gross toxicity effects, and indicated the advantages of using physiological and biochemical biomarkers, such as chlorophyll fluorescence, to detect early signs of metal stress in plants. Additional experiments examining the influence of site-specific factors on metal tolerance of plants grown from propagules of various provenances of metal contamination in estuaries, indicated that genetic factors should also be investigated. The outcomes of this research to environmental management and restoration ecology of estuarine plants is discussed, and directions for further work are presented.
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