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


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King, RD 1980 , 'Limnology of the Gordon River basin, Tasmania, and its meromictic lakes', PhD thesis, University of Tasmania.

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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.

Item Type: Thesis - PhD
Authors/Creators:King, RD
Keywords: Limnology, Water chemistry, Hydrology, Dams
Copyright Holders: The Author
Copyright Information:

Copyright 1980 the author

Additional Information:

Thesis (PhD)--University of Tasmania, 1981. Chapters 2-4 previously pub. by Hydro-Electric Commission, Tasmania, 1978. Bibliography: l. 306-316

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