The origin of the colour of the Wörthersee

"The beautiful turquoise blue colouring of the Wörthersee but also of many other lakes depends on the content of turbidity and algae and on the scattering and absorption of light in the water. The colours of the surroundings only apparently influence the water colour by reflection.

Especially in the summer months, Lake Wörthersee has a whitish to turquoise turbidity. The colour is caused by finest lime particles (calcium carbonate). Lime is introduced 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 there is increased algae growth, carbon dioxide becomes scarce. Then the hydrogen or bicarbonate serves as a carbon source for the build-up of organic matter. The formation 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 hydrogen carbonate and carbonic acid shifts and lime is formed. This conglomerates into differently shaped crystals and aggregates, the calcites, and precipitates. This process is called biogenic decalcification. In simple terms: because floating algae extract carbon dioxide from seawater during their growth, lime precipitates in the form of very fine particles of lime.