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The origin of the color of Lake Wörthersee
"The beautiful turquoise-blue color of Lake Wörthersee, but also of many other lakes, depends on the turbidity and algae content and on the scattering and absorption of light in the water. The colors of the surroundings only seem to influence the color of the water through reflection.
In the summer months in particular, Lake Wörthersee has a whitish to turquoise turbidity. The color is caused by very fine lime particles (calcium carbonate). Lime is carried into the lake by the lime-rich Alpine tributaries, among other things. Lime is formed directly in the lake with the help of algae. The algae need carbon dioxide for photosynthesis. If algae growth increases, carbon dioxide becomes scarce. The hydrogen or bicarbonate then serves as a carbon source for the formation of organic matter. The build-up of organic matter from carbon dioxide and water using solar energy is called assimilation. With the consumption of carbon dioxide or the use of bicarbonate as a carbon source, the pH value of the lake water rises, with the result that the balance between calcium bicarbonate and carbonic acid shifts and lime is formed. This clusters into differently shaped crystals and aggregates, the calcites, and precipitates. This process is known as biogenic decalcification. Put simply, because suspended algae remove carbon dioxide from the lake water as they grow, lime precipitates in the form of very fine lime particles.
The light of the sun also determines the color of the water. The sun's rays penetrate the water or are reflected by the water's surface. The proportion of light that is reflected determines the color of the water. This process is called selective transmission. Short-wave light (blue) is scattered much more strongly and reflected the most, which is why pure water appears blue from above in a thicker layer. The deep blue water also contains hardly any suspended matter that would change the blue color.
In calcareous waters such as Wörthersee, Faaker See and Weißensee, the scattered light is reflected by the small, scattered lime crystals in the water, the scattering effect is intensified and the water appears turquoise blue. When these lime particles sediment in the shore regions, the white-blue lake chalk banks visible from afar are formed.
Blue water colors only occur in water bodies with low organic production, because plant plankton (algae) makes the water appear in various shades of green to yellow-brown or even blue-green."
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