Is it blue dye? or a Bunyip?
A range of explanations have been proposed over the last century. It was also proposed that the blue colour was caused by fluorescence of dissolved organic matter which builds up seasonally in the upper layers of the lake. Another theory was that the blue colour was caused by absorption of all incident visible radiation except blue by finely crystalline calcite in the surface of the lake.
However, the natural colour of the water is blue, and for the same reason the sky is blue. Therefore, the Blue Lake (and all lakes) should be blue. The question to answer therefore:
Why does the Blue Lake become less blue in winter?
Winter – August
In winter the lake is green-grey. It is thoroughly stirred up with dead algae which are mixed into the top of the lake at its lowest. The lake appears less blue due to absorption of blue light by humic substances in the near- surface water. Low rates of calcite precipitation are insufficient to remove the humic substances from the water column hence the water clarity is poor.
Spring – November
The lake commences stratification in spring forming a thin layer of warm water which cuts off deep circulation in the lake. Degassing of CO2 modifies the pH of the surface water promoting precipitation of calcite. An abundance of white calcite forms near the surface of the lake, making the water appear brighter blue. The calcite comes from the groundwater inflow from the adjacent limestone and dolomite aquifer. The high rate of calcite production removes the humic acids from the water column. The spring colour change results principally from the introduction of significant quantities of fine crystalline calcite.
Summer – February
In summer, the calcite continues to form. The calcite continues to strip the humic substances (which come mostly from dead algae in the lake) from the water (through a chemical reaction) in the top of the lake, and the calcite and humic substances settle to the bottom of the lake. As more and more humic substances are removed through the summer, the lake becomes deeper and deeper blue. (The humic substances being produced by algae deeper in the lake don’t get into the water near the top of the lake because the lake stratification in summer prevents mixing of the two water layers.)
Autumn – May
As the top layer of water cools in autumn the calcite production slows dramatically but the mass flux of organic material continues. The concentrations of algae in the near surface water increases and the colour returns to aqua. New and old humic substances are mixed into the surface water from below, which is the probable cause of the colour change. The addition of humic substances to the water increases the absorbency of light in the blue end of the spectrum.