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Assessment of the potential impacts of the large-scale eradication of Spartina anglica from the Tamar Estuary, Tasmania

Sheehan, MR 2008 , 'Assessment of the potential impacts of the large-scale eradication of Spartina anglica from the Tamar Estuary, Tasmania', PhD thesis, University of Tasmania.

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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 (<63 μm). 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.

Item Type: Thesis - PhD
Authors/Creators:Sheehan, MR
Keywords: Spartina anglica, Estuarina plants, Weeds
Copyright Holders: The Author
Copyright Information:

Copyright 2008 the author

Additional Information:

Available for library use only and copying in accordance with the Copyright Act 1968, as amended. Thesis (PhD)--University of Tasmania, 2008. Includes bibliographical references

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