13.2 The Wallis Lake Estuary Management Committee - Great Lakes ...

Wallis Lake Estuary Management Plan
The project focuses on identifying nutrient sources in impacted catchments and determining the extent to which
these nutrients are entering the estuarine food chain. The flow of nitrogen has been traced using a technique called
stable isotope analysis. This technique uses the ratio of the heavy to light isotopes of nitrogen ( 15 N: 14 N) to provide a
‘marker’ to trace energy transfers through food chains. Being stable, the isotopes do not degrade, are not
radioactive and occur naturally in the environment.
To determine the contribution of nutrients from the Wallamba River catchment to the estuarine food chain, the
Pygmy mussel is used as a biological indicator. The nitrogen stable isotope ratio for various man-made nutrient
sources is compared with that of the Pygmy mussel. It has been found that dense beds of Pygmy mussels are
acting as a sink for much of the nutrient load entering the river. Both the stable isotope work and the results of a
simple coupled physical-biological model developed for the Wallamba River support this. Therefore the Pygmy
mussel could be playing a substantial role in the ability of the Wallamba River to assimilate much of the nutrient
loads entering the estuary from this sub-catchment.
The development of the Pygmy mussel as a biological indicator will be beneficial for the cost/benefit of the
rehabilitation and preventative management of Wallis and Smiths Lakes by GLC.
This project also used zooplankton size frequency distribution as an indicator for nutrient enrichment in Wallis Lake.
Zooplankton are microscopic animals, mostly crustaceans, that live in the water column and have little control over
their position in the water column. As a result of their poor swimming ability they are largely at the mercy of the tides
and currents.
13.1.17 Seagrass change assessment using satellite data for Wallis Lake – Chief
Investigators: Dekker A. G., Anstee J. M. and Brando V. E. (2003)
Seagrasses are important in maintaining the healthy functioning of estuarine ecosystems. Nutrient cycling, provision
of habitat for larval fish and crustaceans and stabilisation of sediments are just some of the benefits of seagrass
beds. Seagrasses are also very sensitive indicators of pollution and are hence considered to be a barometer of
catchment health.
Before the distribution of various seagrass species can be used in the ongoing assessment of catchment and
estuarine health the natural variability in seagrass coverage must be known. To assess this long term data sets are
required to achieve this as seagrass coverage can change naturally following wet years with higher frequencies of
storms. This increases sediment loads to the seagrass causing shading and scouring.
This study used 14 years of satellite imagery of Wallis Lake to assess long term changes in seagrass communities
due to both natural and anthropogenic causes. Landsat satellite images are ‘calibrated’ with advanced processing
methodologies using in situ field data to achieve optical closure.
Between 1988 and 2002, there seems to be a gross loss Zostera spp, and little change in coverage of other
seagrass species such as Posidonia australis, Ruppia sp and Halophila sp. Therefore future management and
research efforts should be invested in determining the reasons for this change, be it natural or anthropogenic. This
methodology is now a valuable tool in estuarine management and can be used to assess and predict any effects of
current and future management practices.
127