Assessment of the potential impacts of the large-scale eradication of Spartina anglica from the Tamar Estuary, Tasmania
Creators: name: Sheehan, MR
Publisher: name: University Of Tasmania
Year: 2023

description: Estuarine environments worldwide have long been utilised as centres for urban and industrial development. The Tamar Estuary supports Tasmania's second most densely populated region, and over the past 200 years has been affected by a number of anthropogenic activities, including mining, agriculture, urban and industrial development, dredging and the introduction of exotic species. The introduction of Spartina anglica to the Tamar Estuary in 1947 brought about significant changes to both the ecology and geomorphology of the intertidal zone, and has resulted in the retention of sediments from previous industrial eras. The development of an appropriate management strategy to address the continued spread of S. anglica requires a thorough understanding of the physical and biological nature of the intertidal zone, the hydrodynamics of the estuary, and the influence that S. anglica has on each of these. This study applies a multidisciplinary approach to examine the geomorphic change that has occurred within the intertidal zone following the introduction of S. anglica, and to assess the potential consequences of its large-scale eradication with respect to sediment erosion and the possible release of contaminated sediment. Topographic surveys of S. anglica marshes were conducted along transects perpendicular to the shore to determine marsh morphology and stability. Two marsh morphologies were identified within the Tamar Estuary. Type-1 marshes occur in the upper estuary and are characterised by having accreted between 0.5 m and 2.0 m of sediment above the pre-Spartina surface. Surface topography of Type-1 marshes were found to be independent of the pre-Spartina surface morphology, exhibiting a flat to slightly concave-up upper marsh, a convex-up ridge in the outer mid marsh, and a relatively steeply graded convex-up lower marsh. Conversely, Type-2 marshes of the lower estuary are considerably thinner, with surface topography generally dictated by the underlying pre-Spartina surface, often with the basement material outcropping along the transect. It is predicted that the presence of extensive shoals seaward of the marsh and a lack of fine grain sediment in the lower estuary have resulted in the development of Type-2 marshes .The differences in morphology between these marsh types is attributed to the variations in environmental conditions between the upper and lower Estuary, and is not an expression of maturity as previously thought. Topographic profiles, stratigraphic and geospacial data were used to estimate total infestations size and sediment volumes trapped by S. anglica in the Tamar Estuary. The current S. anglica infestation of the Tamar was estimated at 374 hectares, with significant expansion into the lower reaches of the Estuary. Approximately 1,193,441 m3 of material has been trapped within marshes since the introduction of S. anglica in 194 7. It was estimated that between 14 and 28 percent of this material was Spartina-derived organic matter, while the remainder was predominantly silts and clays (&lt;63 ˜í¬¿m). While S. anglica marshes are accretionary, surveys demonstrated retreat of the seaward margins throughout the estuary since 1989, and the development of erosional scarps in Type-1 marshes. The degree of anthropogenic enrichment of major trace metals (Al, As, Cd, Co. Cr, Cu, Fe, Mn Ni, Pb, Zn) was determined through aqua regia digestion of Spartinatrapped sediments obtained from sediment cores at four sites within the Tamar Estuary. Cd, Cr, Cu, Pb, Ni and Zn occur at concentrations within the trigger range of the ANZECC/ ARMCANZ (2000) interim sediment quality guidelines for several sites within the estuary. Spatial distribution of metals is highly variable, with a general increase in contaminant concentration with increasing distance from the mouth of the Estuary. This trend is likely to reflect distance from contaminant sources, while anomalies are explained by point sources along the estuary and the redistribution of dredge material. Cd, Cu, Pb and Zn concentrations were shown to increase with depth from the surface at all four sites, supporting previous findings that show a reduction in these contaminants over the last 10-25 years. The concentration of selected organic pollutants (PCB, TPH, BTEX, PAHs, and DDTs) in Spartina marsh sediments at the four study sites were also investigated and found to be well below ANZECC/ ARMCANZ (2000) interim sediment quality guideline trigger values and often below detection levels. This suggests that Spartina marsh sediments have not acted as a sink for these organic compounds, or that organic pollution detected in previous studies of the lower estuary, is highly localised. Finally, erosion rates were monitored within a 0.5 ha experimental plot where S. . anglica cover was removed, and compared with erosion rates at a control plot. Erosion rates were estimated at 13 .2 mm/yr for the experimental plot, compared to 2.0 mm/yr for the control. The difference in elevation loss betweem the marsh surface of the two treatments was found to be statistically significant (P = 0.001). Elevation loss was found to increase by a factor of 1.06 with every 10 m seaward of MHW at both sites. Net erosion occurred in the outer 40 m of the control plot, suggesting that while defoliation has increased the rate and magnitude of intertidal erosion, other environmental factors such as increased storm intensity or frequency are contributing to retreat of intertidal marshes. The findings of this study have allowed for a review of current S. anglica management in the Tamar Estuary, which sets out to contain the infestation and prevent its further spread. This research concludes that while this is an appropriate management strategy which should be continued, efforts to reduce the current infestation by removing S. anglica from selected developing Type-2 marshes should also be implemented. A greater understanding of estuarine hydrodynamics and the ecological role of Spartina anglica is required and should be the focus of future research efforts.