The impact of a hydro-electric dam on the stability of meromictic lakes in south-west Tasmania, Australia

Three small meromictic lakes adjacent to the lower reaches of the Gordon River in the World Heritage Area of south west Tasmania have been of interest to the scientific community for almost twenty years on account 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 extent that it inhibited the natural process maintaining meromixis in the lakes and their meromictic stabilities declined. Two of the lakes, Lake Morrison and Sulphide Pool, now vacillate between holomixis and meromixis and the third, Lake Fidler, maintains meromixis but its chemocline is located at greater depth. The decline in meromictic stability resulted in profound biological changes in all three lakes and the elimination of many of the micro-organisms associated with meromixis. In order to assess the impact of the Gordon Power Development remote data-loggers were installed in Lake Fidler and in the Gordon River catchment in 1992 to monitor the key hydrological variables. The results of this study identify the complex series of events necessary for maintenance of meromixis. They also show how minor changes in the operating regime of the Gordon Power Development could assist in the conservation of these natural processes and help protect the unusual ecosystems of these meromictic lakes which are a biologically-significant component of the World Heritage Area of south west Tasmania.