THE PALEOLIMNOLOGY OF LAKE FIDLER, A MEROMICTIC LAKE IN SOUTH-WEST TASMANIA AND THE SIGNIFICANCE OF RECENT HUMAN IMPACT

Three meromictic lakes in the World Heritage Area of south-west Tasman ia possess unusual microbiological communities. Their meromixis is mai ntained by periodic incursions of brackish water from the nearby Gordo n River which, in its lower reaches, is a salt-wedge estuary. In 1977 the construction of a dam in the middle reaches of the river restricte d penetration of the salt-wedge and meromixis rapidly declined in all three lakes. A palaeolimnological study was carried out on one of the lakes, Lake I;idler, firstly to determine the history of meromixis and its associated microbiological communities, and secondly to assess wh ether the recent and rapid decline of meromixis is inconsistent with n atural rates of development of the Gordon River meromictic lakes. One part of this study included the analysis of the stratigraphy of fossil diatoms from a 17-metre sediment core dating back 8000 yrs. Detrended Correspondence Analysis and Analog Matching were used to compare diat om species assemblages in core samples with diatom samples from a refe rence dataset consisting of a selection of lake and river sites in the lower Gordon River valley. Five distinct stratigraphic zones were ide ntified in the core. These zones indicated specific stages in the deve lopment of the Gordon River lakes from river backwaters to ectogenical ly-maintained meromictic lakes which will, finally, become terrestrial ised by encroaching rainforest. The onset of a stratified water column was identified by the emergence of a dominant freshwater algal flora which suggested that the lake had developed a mixolimnion and become m eromictic ca. 2070 +/- 50 C-14 yrs ago. In the context of this long hi story of meromixis, the rapid demise in meromictic stability following construction of the dam is judged to be inconsistent with natural rat es of development. The palaeolimnological studies, of which this paper is one part, prompt recommendations for a management strategy to prev ent the further decay of these meromictic lakes in the World Heritage Area.