Assessment of the Potential Impacts of Large-scale Eradication of Spartina anglica from the Tamar Estuary
Creators: name: Sheehan, MR
Publisher: name: University Of Tasmania
Year: 2023
Subjects: Environmental rehabilitation and restoration

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 1947. 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 ìm). 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