The geochemistry and minerology of estuarine sediments : implications for Rice Grass removal in the Rubicon Estuary

Saltmarshes are incredibly valuable ecosystems which support large ecosystems of fish, birds and plants, they provide a buffer between marine, terrestrial and riverine environments (Sheehan and Ellison, 2015). The aim of this study was to assess the risks associated with a large-scale removal of Spartina anglica from the Rubicon estuary in central northern Tasmania. The removal of this invasive species would allow hectares of sediment to remobilise into the surface waters of the estuary and any metals currently held in the sediment would be mobile and could become toxic to the ecosystem. The results of the study showed elevated levels of Cu and Zn in the surface waters, with Cu being above ANZECC guidelines. The sediment also showed elevated metals, particularly Al, Cu, Cr, Fe, Ni, Pb, Ti and Zn, these metals of concern were the focus of the study. It was found that although metals were elevated in the sediment, they were held in immobile mineral phases which are unlikely to enter the ecosystem, such as the high proportion of Ti that was found to be held in ilmenite and rutile. The shaker flask testing resulted in metals being resorbed into the sediment rather than releasing into the surface waters. This study demonstrated that a large-scale eradication of Spartina anglica from the Rubicon estuary is unlikely to cause a mass remobilisation of metals into the estuary. This suggests that removing the invasive grass will have an overall positive benefit to the ecosystem.