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Temperature in the two lakes showed significant seasonal fluctuation (Fig 3.3a &
b). Both lakes were ice free throughout the year, therefore surface waters were subject to
considerable seasonal temperature changes. The temperature profiles in both lakes (Fig
3.3a & b) shows a close relationship with the seasonal ambient temperature chart (Fig
3.21). Extreme low temperatures recorded during the winter (Deep = -13.8°C,
July 2 000;
Club = -14.5°C, July, 2000) probably had an inhibitory effect on biological activity.
Previously, the lowest recorded surface temperatures were -19.6°C (9th' August 1973) for
Club Lake and -18.3°C (9th August 1973) for Deep Lake (Kerry et al., 1977). Deep Lake
water has been shown to freeze at about -28°C
(Kerry et al., 1977) and exhibits a
monomictic stratification cycle (Ferris & Burton, 1988). Increase in temperature in the
surface waters during summer produces a pycnocline with the colder denser water and
the lakes becomes thermally stratified (Kerry et al., 1977). Summer temperatures
recorded between the surface and bottom of the lake can differ by up to 26°C (Ferris &
Burton, 1988). Surface water reached temperatures of 3.9°C in Deep Lake and 3.7°C in
Club Lake. However, temperatures have been shown to exceed 11 °C in Deep Lake (Kerry
et al., 1977). As surface waters cool during autumn and winter the pycnocline increase in
depth until the column is isothermal, allowing vertical mixing of the whole water column
possibly due to wind induced mixing (Kerry et al., 1977).
Salinity in Deep Lake and Club Lake (Fig 3.3a & b) was relatively stable
throughout the year (McLeod, 1964; Kerry et al., 1977), but it was generally lower during
summer (Jan-Feb, 2000), probably due to snowdrift and melt water input - these produce
a thin layer of less dense, lower salinity water over the main body of water, which is
quickly assimilated with the surface waters due to wind induced mixing (Kerry et al.,
1977; Hand, 1980; Ferris & Burton, 1988). Burton & Campbell (1980) suggest that Deep
Lake is a snow-drift-trap, which might explain why there is a small decrease in surface
salinity from February to July, a period which comprised the highest snow fall for the
year 2000 (courtesy of the Bureau of Meteorology, Melbourne, Australia). The extremely
high salinity causes interesting density variations within the water column. Compared
with freshwater, density change in water from Deep Lake is 11 times greater over a
temperature increase of 4 to 9°C (Ferris & Burton, 1988). This means that density change
per °C is more rapid in Deep Lake compared with lower salinities. the same is
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