THE IMPACT OF A HYDROELECTRIC DAM ON THE STABILITY OF MEROMICTIC LAKES IN SOUTH-WEST TASMANIA, AUSTRALIA

Three small meromictic lakes adjacent to the lower reaches of the Gord on River in the World Heritage Area of south west Tasmania have been o f interest to the scientific community for almost twenty years on acco unt of their unusual microbiological communities. In 1977, a dam, the Gordon Power Development, was constructed in the middle reaches of the river. This modified the flow regime in the lower reaches to the exte nt that it inhibited the natural process maintaining meromixis in the lakes and their meromictic stabilities declined. Two of the lakes, Lak e Morrison and Sulphide Pool, now vacillate between holomixis and mero mixis and the third, Lake Fidler, maintains meromixis but its chemocli ne is located at greater depth. The decline in meromictic stability re sulted in profound biological changes in all three lakes and the elimi nation of many of the micro-organisms associated with meromixis. In or der to assess the impact of the Gordon Power Development remote data-l oggers were installed in Lake Fidler and in the Gordon River catchment in 1992 to monitor the key hydrological variables. The results of thi s study identify the complex series of events necessary for maintenanc e of meromixis. They also show how minor changes in the operating regi me of the Gordon Power Development could assist in the conservation of these natural processes and help protect the unusual ecosystems of th ese meromictic lakes which are a biologically-significant component of the World Heritage Area of south west Tasmania.