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The stability of shallow coastal sediments with and without seagrasses

Chisholm, Warren John (2009) The stability of shallow coastal sediments with and without seagrasses. PhD thesis, Murdoch University.

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Seagrass meadows in south-western Australia exist along a gradient of hydrodynamic disturbance and may be subjected to varying degrees of sediment movement leading to burial and erosion. Such disturbance may play a role in structuring seagrass assemblages and has contributed to the failure of some attempts at seagrass restoration in the region. The seagrasses in south-western Australia occur in a microtidal environment dominated by swell waves, locally generated wind waves and oscillatory currents. This poses challenges not experienced or studied in environments of the northern hemisphere where most other seagrass rehabilitation has taken place, which are dominated by unidirectional tidal currents. This thesis examined the relationship between seagrass assemblages and hydrodynamic disturbance, along with seagrass growth responses to changes in sediment height and the role of shoot density in determining grain size characteristics in natural and transplanted seagrass meadows.

The distribution of seagrass assemblages in Owen Anchorage was examined in relation to hydrodynamic disturbance, approximated by the calculation of a Relative Exposure Index (REI), variants of which considered effective fetch, depth and the long term wind record. Of the variants examined, REI including the full wind record best differentiated between seagrass assemblages and the significant differences between assemblages support the notion that hydrodynamic processes play a role in structuring seagrass distribution in the study area. Short term changes in sediment height were monitored at six sites using electronic sediment height loggers during summer and winter. Highest levels of sediment movement occurred in winter and were associated with westerly winds. Summer observations showed lower levels of sediment movement, with no particular wind characteristics associated with changes in height. Wind direction and uninterrupted fetch were more important than wind speed when determining the level of sediment movement in the study area.

In order to assess the extent to which transplanted Posidonia australis tolerates burial and erosion, transplanted sprigs were subjected to experimental changes in sediment height (-2, 0, 4, 8 & 16 cm) between November 2007 and April 2008 in the relatively sheltered Southern Flats area of Cockburn Sound. Transplants exhibited high survival rates (> 90%) in all treatments except the 16 cm burial treatment, which experienced total mortality. Active rhizome extension was observed in all treatments, along with the production of new shoots, except in the highest burial treatment. There was little evidence of an increase in vertical growth or the production of vertical shoots. This study indicated that P. australis transplants are tolerant to changes in sediment height between -2 and 8 cm, however a critical threshold exists between 8 and 16 cm, above which transplant mortality is high. As such, consideration of the scale of sediment movement will play an important role in site selection for seagrass restoration projects using this species.

The role of seagrass shoot density in influencing changes in sediment height and grain size characteristics was examined in both transplanted and natural Posidonia australis meadows over the course of one year. Transplanted seagrass patches at spacings of 1.0, 0.5, 0.25 and 0.125 m were established, along with natural meadows adjusted to 100, 75, 50, 25 and 0% of their initial shoot density, with all sites situated in the relatively sheltered Southern Flats region of Cockburn Sound. Short term changes in sediment height were similar between all transplanted and natural seagrass patches, with features such as sand ripples associated with the south-westerly sea breeze observed. Long term monitoring indicated that higher shoot densities experienced greater accretion than lower ones. Within the density manipulated natural meadows, higher shoot densities coincided with a relatively higher proportion of fine sediments (250 µm, 125 µm and <63 µm). Sediments within the transplanted patches became increasingly fine over the course of the experiment, with higher densities showing the greatest proportion of fine sediment, although sediments within all transplant patches were significantly coarser than natural meadows. As such, grain size characteristics of the transplants represented an intermediate condition between natural meadows and unvegetated sediments. These results show that seagrass shoot density plays a role in structuring the sedimentary environment within seagrass meadows.

These findings can be applied to aid in the future planning and execution of seagrass restoration techniques using Posidonia australis in the Perth metropolitan region. In particular, sites can be assessed for suitability in terms of exposure and sediment movement prior to undertaking restoration. This may enhance the success of restoration activities by ensuring that optimal sites are selected.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Environmental Science
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): UNSPECIFIED
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