Evaluation of terrestrial sediment spatial patterns throughout the Whitford embayment and its benthic food web using stable isotope techniques

This study uses stable isotope techniques to track the movement of terrestrial sediments through an estuary and to examine the impact of thin layers of terrigenous clay on the microscopic plant communities in the sediments. It is one of a series of studies commissioned by ARC to provide information on the threats to estuarine ecology posed by changes in land use (see also Berkenbusch et al. 2001).

Stable isotopic techniques were able to provide an estimate of the % clay distribution through the estuarine arms and across Whitford embayment. These measurements were closely correlated with the % clay distribution measured by particle size analysis. Although further work is required to refine the transformation algorithm, these results suggest that the use of stable isotopes may be a valid technique for estimating the % terrigenous clay in estuarine sediments.

While the stable isotope transformation data provided empirical evidence of terrigenous clay movement through the estuary, the natural abundance isotopic C and N data also provided information on a broader range of terrigenous inputs including possible waste-water contamination of beaches from urban storm-water drains and small stream inflows. There were also indications of correlations between benthic communities and their habitats.

Isotopic enrichment experiments provided information on the effect of thin layers (5 mm) of clay on sediment hydrology and the microscopic plant communities that live in the sediments. The results indicate that there is a very delicate balance between chemical fluxes across the sediment-water interface and primary production in the sediments for benthic communities. This balance can be affected by even small inputs of terrigenous clay. The degree of that effect is dependent upon the habitat impacted and the thickness of the clay layer. Significant ecological changes would depend on the frequency of clay deposition events.

The isotopic enrichment experiments demonstrated small scale (mm range) changes in hydrodynamics, and the natural abundance spatial patterns indicated large scale (km range) circulation and sedimentation patterns across the embayment.

This study has demonstrated that stable isotope techniques are valuable and highly versatile tools for assessment of coastal and estuarine environments, and evaluation of human impacts from catchment development.