Impact of thin deposits of terrigenous clay on benthic communities

This study assessed the impact of the deposition of thin layers of terrigenous clay on estuarine communities and ecosystem processes. 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.

Potential ecological threats include loss of sensitive species, changes in biodiversity, reduced oxygenation of surficial sediment, shifting microbial activity, diminished light levels and restricted photosynthesis within the sediments and interference with animal feeding processes across the sediment surface.

These issues are focused into three major questions that were addressed by field experiments: 1. What is the critical depth of thin (<1 cm) clay deposits that cause chronic effects? 2. How do benthic communities in different habitats respond to thin clay deposits? 3. How does frequent deposition of thin layers of terrigenous clay affect benthic communities?

The experimental sites encompassed a variety of common habitats that cover a range of hydrodynamic conditions, sediment properties and benthic community composition.

Results showed that thin clay deposits ranging from 3–7 mm depth have an impact on benthic communities, with negative effects increasing with the depth of deposited clay. However, these responses were not of the magnitude recorded from earlier studies in Okura estuary which described the impact of > 2 cm deposits of clay.

The response of the macrobenthic community varied with habitat, with the subtidal habitat, in particular, showing little response to the experimental additions of clay. Nevertheless, community analysis indicated that repeated additions of 3 mm layers of clay over a 6-month period had a cumulative effect on the macrobenthic community.

Of the biogeochemical sediment properties measured variations in response dependent on habitat and timing of clay deposition were observed. At some sites and some times we recorded ecologically important changes in nutrient fluxes and in the abundance of microphytes in the sediment.

It is clear from the experiments that the frequency of disturbance and thus the time available for recovery between disturbance events is critical in assessing whether the deposition of thin layers of terrigenous clay pose a threat of broad-scale degradation to the Whitford embayment.

Low load depositional events that produce thin layers of clay are likely to be more frequent and to cover a larger area than rare catastrophic events. It will be important to use the catchment and hydrodynamic modelling to assess the risk of low-load events depositing clay in different parts of the embayment. The experiments presented in this report have provided us with information that will enable us to refine the rules for assessing the risk of ecological change as a result of sediment inputs.