Summary

Acid sulfate soils (ASS) are coastal soils that contain the mineral iron pyrite. When ASS are drained or excavated in a way that exposes the iron pyrite to air, a reaction occurs that produces sulfuric acid.  During wet periods the sulfuric acid is flushed out of the soil and into nearby waterways causing estuarine acidification. Affected waters cause a range of environmental impacts due to their low pH and high concentrations of iron, aluminium and manganese.  Sydney rock oyster production occurs in areas of estuaries that are acidified following rainfall. This paper investigated the effects of acidification on survival and growth of oysters on the Manning River, NSW, Australia. Water quality and oyster survival and growth were measured for seven months at seven sites in this estuary. Sites located close to ASS outflow sources had low pHs (< 4.5), reduced salinities and high levels of iron and aluminium in the water.  These sites had significantly higher oyster mortality compared to sites that were some distance from ASS outflow sources and not acidified.  Small oyster mortality was significantly higher than large oyster mortality during the experiment.  Also, small and large oysters at sites that were acidified had significantly reduced growth compared to the oysters at sites which were not acidified. This work highlights the importance of proper management of ASS to maintain estuarine water quality and protect aquatic ecosystems.