Meromictic lakes of south-west Tasmania

Lake Fidler, Lake Morrison and Sulphide Pool are three small, shallow lakes adjacent to the lower reaches of the Gordon River. They are levee lakes, and may be remnants of once more extensive water bodies. Eah of them shows the usual features of meromictic lakes, with a persistent chemocline acioss which the concentration of electrolytes and hydrogen sulfide increases sharply, dissolved oxygen concentration falls to zero, and redox potential changes from positive to negative. In all three lakes the change in electrolyte concentrations takes place over a comparatively thick water stratum, but the redoxcline and oxycline are pronounced, and in their vicinity is a 'plate' of microflagellates and photosynthetic sulfur bacteria, marked by sharp peaks in the profiles of chlorophyll concentration and of turbidity. Their waters are dystrophic, severely attenuating light and limiting plankton populations in the mixolimnia. Photosynthetically active radiation at the depth of the plates of photosynthetic bacteria is considerably less than 1% of surface values. With the chemocline in each case so close to the surface (1.0-2.5 m), and with their shallowness, these are among the most dramatic cases of meromixis yet recorded. Their survival is in doubt because operation of a power station now prevents the entry of estuarine salt on which the state of meromixis appears to depend.