Sydney Harbour has some of the most contaminated sediments in the world. These sediments have flowed into the harbour from surrounding industrial and urban areas for more than a century. It is well documented that contaminated sediments affect the ecology of organisms living in the sediments. Very little is known however, about the potential for the resuspension of contaminated sediments to pose a threat to the ecology of organisms in the water column or on surrounding rocky reefs. SACF wanted to help investigate...

This is surprising because there are a range of natural (storms, tidal currents) and human (shipping movements, dredging) distrubances that can resuspend (i.e. lift) sediments into the water column, potentially releasing a range of contaminants and dispersing them well away from the harbour floor. Only two studies worldwide to date have attempted to assess the effects of the resuspension of contaminated sediments in the field.  

Drs Nathan Knott and Emma Johnston of UNSW wanted to investigate the potential for these sediments to impact upon resident species. The aim of this study was to assess whether repeated short-term resuspension of contaminated sediments would affect the diverse assemblages of rocky reef sessile invertebrates (e.g. sea-squirts, barnacles and sponges) in Sydney Harbour. Sessile (fixed) invertebrates were specifically used as 'mine canaries' as they are sensitive filter-feeders and cannot move away from water-borne sediment plumes. The experimental method was designed to attempt to mimic the resuspension of contaminated sediment created by short-term distrubances, such as frequrent shipping movements.

Despite being exposed to resuspensions of some of the most contaminated sediments in the world, a diverse range of Sydney Harbour sessile invertebrates clearly showed no short-term ecological effects. The soft bodied invertebrates (e.g. colonial sea-squirts and sponges) were predicted to shrink or decompose rapidly if the resuspensions lethally stressed these organisms. Similarly, the densities of the live invertebrates with hard bodies (e.g. barnacles and polychaete tube worms which can clearly be identified as live or dead) were predicted to decrease, with dead individuals increasing. The abundances and area that the invertebrates covered did not differ among the assemblages exposed to the the resuspengion and control treatments indicating that there were no immediate impacts of the resuspension of contaminated sediments. Also, not even the high levels of relatively clean sediments affected the invertebrates, which is surprising as suspended sediments at the level used in this experiment are generally thought to have substantial negative effects.

Hence, this study provides no support for the model that intense and repeated dosing of relatively clean or contanimated sediments over hourly, daily and weekly timescales have strong short-term effects on sessile invertebrates. This indicates that small infrequent disturbance of contaminated or relatively uncontaminated sediments may not be an ecological problem, possibly as the invertebrates have evolved to deal with ferquent natural changes in the water condition of estuaries (e.g. sainity and turbidity). Nevertheless, as the curretns study focused on the immediate impacts of resuspension over a relatively short time period, further research is required to assess the potential impacts long-term exposure of resuspension of contaminated sediments.

Click here to read the full report of this study.