Mangrove forests are important coastal ecosystems as they stabilise mudflats, and the edges of waterways. Additionally, they offer ecological niches for important species such as crabs, fishes, molluscs, for which mangroves provide refuge and sources of food. Globally, mangrove forests are being lost at a rapid rate, due to human activities and inappropriate management practices (land development, expansion of industrial activities, exploitation of natural resources, agricultural activities and land clearance), which are the main cause of mangrove loss around the world. The drastic expansion of human activities over the last century has contributed to an increase in the fluxes of heavy metals to aquatic ecosystems. Mangroves are, however, particularly susceptible to heavy metal build up because of their particular habitat, for instance, sediments tend to be fine grained and rich in organic matter. For that reason mangrove sediments are known to be a sink for heavy metals. Moreton Bay in South East Queensland is a highly populated and a much urbanised area. Urbanisation and recent changes in land use may have contributed to high heavy metal contents in the environment with negative consequences for mangrove health. The toxicity of metals is highly influenced by the geochemical factors that influence metal bioavailability. Biogeochemical and physical factors governing the distribution of heavy metals are also crucial for understanding their accumulation in sediment and water sources.
Moreton Bay has been chosen for this study because it is a fast growing region surrounded by a fragile aquatic ecosystem. This makes it an ideal place to investigate how the ecosystem, in particular the mangrove forest, is dealing with human impact. This thesis investigates the distribution of heavy metals in mangrove sediments and their pore-waters, their abundance relative to natural backgrounds, and their potential mobility and adverse effects on the health of mangrove forests. The survey generated in this thesis will help identifying potential heavy metal ‘hot spots’, and assessing their bioavailability in the estuarine ecosystem. Surficial sediments (top 30 cm) and pore-waters were sampled from forty-six locations across Moreton Bay and analysed for ten heavy metals (As, Cd, Co, Cr, Cu, Mo, Ni, Pb, V and Zn). Sedimentological analyses were carried out to establish the distribution of particle sizes (clay, silt and sand) and to determine the amount of organic matter present in the sediment (based on loss-on-ignition analysis). Geochemical analyses including total digestion and sequential extraction (following the first two steps of the BCR 701 procedure) were used to determine the proportion of metals bound to different phases of the sediment matrix. Sedimentological and geochemical data were combined to generate information on the spatial distribution of metals in the surficial sediment. Enrichment factors (EF) and established guideline (ANZECC/ARMCANZ and Queensland Estuarine Sediment) values were used to establish metal enrichment relative to local pre-European backgrounds, and the extent of heavy metal pollution across Moreton Bay mangrove sediments. Heavy metals show a significant positive correlation with organic matter and a strong correlation with Fe, Mn and Ti. The highest enrichment factors (EF) were observed in North and South Moreton Bay for Co (EF 3.74 and 5.10, respectively), Cu (2.33 and 7.74), Mo (4.51 and 2.55) and Zn (2.10 and 2.65). Cobalt and Cd were most abundant in the exchangeable fraction. The highest concentration of exchangeable Co (61.36 %) was found in North Moreton Bay, while 70.98% of Cd was exchangeable in South Moreton Bay. Chromium, Pb and V were more abundant in the reducible fraction (100%, 45.36% and 41.77%, respectively). North Moreton Bay yielded the highest proportions of exchangeable heavy metals, while Deception Bay yielded the lowest. All average values are within the acceptable guidelines, although these were exceeded at some sites for specific heavy metals. Nevertheless, the data suggest that existing metal loads in surficial sediments do not seem to have any negative effect on the health of Avicenna marina, the most abundant mangrove of Moreton Bay.
The highest heavy metal (Cd, Co, Cr, Mo and Ni) concentrations in pore-waters were measured in Waterloo Bay (0.009 μg L-1 Cd, 1.21 μg L-1 Co, 1.16 μg L-1 Cr, 9.45 μg L-1 Mo and 5.10 μg L-1 Ni), while Deception Bay and South Moreton Bay showed the lowest metal content in pore-water. Interestingly, there is no consistent correlation between the concentration of heavy metals in pore-waters and the exchangeable heavy metal fraction extracted from the co-existing sediments. Similarly, there is no correlation between heavy metal loads in pore-waters and water quality measured using the set of ecological indicators adopted by the Healthy Waterways program to assign a certain score to the different Moreton Bay catchments and coastal sites. All pore-water heavy metal concentrations are within the acceptable guideline values.