Limnology of the Gordon River basin, Tasmania, and its meromictic lakes

In recent years the Gordon River Basin, south west Tasmania, has been the subject of considerable debate between conservationists and the Hydro-Electric Commission. A dam was built on the middle Gordon River and discharge from the power station commenced in October 1977. This development has had a severe impact on the wilderness area of the lower Gordon River. This thesis discusses the water chemistry of the Gordon River system, the limnology of a small holomictic lake elevated above the river and three riverine meromictic lakes, and the effects of the dam on this lower Gordon River area. Those unregulated rivers of the Gordon River Basin whose catchments contain calcareous rocks display a fluctuation in major ion chemistry from alkaline earth bicarbonate dominance at low summer flow to sodium chloride waters at high winter flows. Because Gordon limestone also contains variable amounts of sea salts, the alkaline earth bicarbonate waters sometimes contain appreciable amounts of both sodium and chloride at low flows. Composition of rivers draining catchments not containing calcareous rocks are always similar to sea water. Due to power station release, water chemistry and flow of the Gordon River have been significantly altered. Waters are now more dilute and ionic composition, principally that from Lake Gordon, varies little from that of sea water. High winter flows have been reduced, and most significantly, summer flows elevated, consequently river height during summer is much greater, reducing light penetration to the river bed. Power station discharge has further reduced temperature variability (lowered summer temperatures and raised winter temperatures). Perched Lake is a small lake above the Gordon River near Butler Island, moderately dystrophic and acidic with water chemistry akin to sea water. It is warm monomictic, stratifying in summer and circulating freely in winter. Dissolved oxygen in the surface waters is mostly undersaturated. Late in the period of stratification oxygen in the bottom waters is reduced to about 20% of saturation. Phytoplankton biomass is sparse, dominated by one chrysophyte and two desmid species. The three meromictic lakes occur behind river-deposited levee banks along the tidal section of the lower Gordon River. They are well protected from the prevailing westerly weather by surrounding rainforest. Meromixis results from salt input from the river to the lakes, producing a saline gradient within each of these lakes. The lakes are thermally stratified in summer and inversely stratified in winter. They have two isothermal periods, but do not circulate completely due to the saline gradient. Dissolved oxygen in the mixolimnia is mostly undersaturated, decreasing rapidly to zero in the chemocline. Large amounts of total dissolved sulphides occur in the monimolinnia. Surface waters are acidic to neutral, becoming basic in the monimolimnia. The waters are very dark and chemically akin to sea water. Large amounts of phosphorus are trapped in the monimolimnia. A well developed bacterial plate is present in each lake, composed principally of green sulphur bacteria below an algal layer, including Cryptomonas. Light penetration is severely limited by dissolved organic material, and to a lesser extent by photosynthetic pigments, with blue light being deficient. These lakes are amongst the shallowest meromictic lakes in the world, with the chemocline situated at depths of only 0.1 in in some cases. Discharge from the Gordon Dam has flushed the salt wedge from the river, eliminating salt supply to these lakes. As a result, the halocline of one lake has been seriously eroded and the lake shows all signs of becoming holomictic. This is likely to have severe repercussions for the maintenance of meromixis in the other two lakes.