AbstractIn the south western corner of the Gulf of Carpentaria at Bing Bong is the Pb/Zn ore concentrate loadout facility for McArthur River Mine. Ore concentrate is barged approximately 35 km offshore, via a chaimel dredged through the seagrass beds, for transhipment to bulk carriers. Spatial, seasonal and temporal effects on tissue heavy metal concentrations in three local seagrass species Syringodium isoetfolium, Thalassia hemprichii and Enhalus acoroides were determined over the period 1996-1999. Concentrations of Fe, Mn, Co, Ni, Cu, Zn, Cd and Pb as well as Pb isotope ratios 207 Pb/ 206Pb and 208Pb/206Pb in seagrass leaf, root and rhizome tissues were determined by ICP-MS and compared and correlated with similar analysis for sediments and seawater.
Differences in heavy metal distribution within and between the three seagrass species were attributed to inherent physiological and morphological characteristics as collection was simultaneous at each site. Spatial variability of total acid extractable metal concentrations in sediments was high with differences in concentrations east or west of the dredged channel significant as a function of one or two naturally high concentrations. Similar patterns of heterogeneity were observed for seagrass tissues and differences were attributed to natural background variation and not due to anthropogenic inputs. There were seasonal differences between Wet (November - April) and Dry (May - October) seasons with no indication of metal dilution due to growth spurts, observed differences were ascribed to inherent physiological and morphological characteristics. Metal concentrations in S. isoetifolium increased from 1996 to 1997 remaining stable over the following two years, indicating no significant impacts from anthropogenic heavy metal inputs. Sediments had low concentrations of metals held within bioavailable chemical fractions resulting in low concentrations of metals in seagrasses. Correlations between tissue fractions and sediments for Cd and Mn indicated sediments as the major uptake source for these metals. No significant correlations between tissue sections and seawater for Cd, Cu, Pb or Zn were found although this may be due to the time integrated nature of seagrass metal uptake as opposed to single point in time measurement of seawater metal concentration.
Lead isotope ratios 207 Pb/ 206Pb and 208Pb/206Pb were determined by ICP-MS and used to track the fate of anthropogenic Pb inputs from MRM Pb/Zn ore concentrate in the Bing Bong seagrass ecosystem. The radiogenic signature of MRM ore concentrate was significantly different from modern crustal Pb allowing small changes in background Pb isotope ratios as a result of MRM ore concentrate to be readily distinguished. An increasing trend of Pb isotope ratios from background to MRM ore concentrate was evident in tissues of the three seagrass species analysed, most pronounced in leaves and a temporal Pb concentration and Pb isotope ratio increase in S. isoetifolium tissues from 1996-1997. These Pb isotope ratio results indicated there had been low level Pb impact by contaminated seawater exchanged from the loadout swing basin even though there was not clear evidence from concentration data alone. The sensitivity of Pb isotope ratio analysis coupled with concentration analysis provides a valuable tool for monitoring low level impacts of Pb in the marine environment.
Experiments were conducted using S. isoetifolium to identify uptake and translocation mechanisms for Cd, Cu, Pb and Zn. Metal uptake was determined from both seawater and sediment mediums and the chemical composition of these substrates influenced the extent of tissue accumulation. Sediment metal bioavailablity was determined by sequential chemical extraction, acid volatile sulfide/simultaneously extracted metals ratio (AVS/SEM) and by diffusive gradients in thin films (DGT). It was found that although sediments had high total metal concentrations labile metal concentrations were very low with the greatest concentrations held in the less accessible sulfidic/organic fractions or lithogenic crystalline structures and therefore concentrations in seagrass tissues were relatively low.
Metal uptake experiments showed rapid initial uptake followed by a plateau for Cd and Zn in leaf, root and rhizome tissues and Pb in all but leaf tissues. This trend was also observed with DGT labile metal concentrations in sediments indicating an active uptake mechanism with metal cations rapidly adsorbed to surficial binding sites of the seagrass until an equilibrium was reached. Pb uptake by S. isoetifolium leaf showed a positive linear trend with time and a passive uptake mechanism was proposed.
|Date of Award||2002|
|Supervisor||David Parry (Supervisor)|