Hiking
How does a lake get its color?
Why is Lake Achensee blue? Or isn't it rather turquoise? And didn't the lake look a little different in spring? A lake scientist explains how lakes get their colors and what they mean.
Even if it's hard to imagine, there are more than 600 lakes, ponds and pools in mountainous Tyrol. 35 of these are officially classified as bathing waters by the province of Tyrol. Anyone who has ever lain on a lake beach and tried to assign a color to the surface of the water will have noticed that this is often not clear. And anyone who has ever wondered why a high mountain lake is turquoise and a moorland lake is black is unlikely to have come up with a green branch, or at best swum past one.
"If books say that the original lake color is blue, then that's not true," says Ruben Sommaruga, Professor of Limnology and Director of the Institute of Ecology at the University of Innsbruck. His job is to research lakes. For a limnologist, the color of a lake is important for identifying fundamental processes in the ecosystem of a body of water. "The color," says Sommaruga, "says a lot about the lake." To determine the surface coloration, it is essential to distinguish between external influences such as the reflection of the sky color and the shoreline vegetation and the actual color of the water. If the sky is cloudy or there are dark trees at the edge of the lake, the water automatically appears grayer and darker. In addition to these factors, the depth of a lake also plays a role.
"In general, our perception of a lake's color depends on the turbidity, the algae content and the humic substances, also known as 'yellow substances'," explains Sommaruga. Bog lakes such as the Lanser Moor are typical examples of waters with a high yellow substance content. These substances are a kind of remnant of poorly degradable dissolved organic substances, a harmless waste product that absorbs light. For the lake, a higher yellow substance content means a brown coloration. If you measure the color of the water with a so-called Secchi disk, which forms a white contrasting background in the lake, it looks yellow.
There are physical reasons why the lake as a whole appears to be brown: As many people remember from school, different colors of light have different wavelengths. When light hits the surface of a lake, some of it is reflected. The light of the wavelength that penetrates the deepest is reflected most strongly and is absorbed and scattered by the water and its dissolved substances and particles. "This is also known as selective scattering - selective because shorter wavelengths are scattered or absorbed more than longer ones," explains the limnologist. In practice, you can imagine it like this: The sun is shining and light hits the Lanser Moor. Yellow substances and other particles ensure that the color blue (low wavelength) is swallowed or attenuated, while green and red light (higher wavelength) is reflected back. Green and red mixed together make brown - the mystery of the color of the Lanser Moor is solved.
Waters such as Lake Piburger See or Lake Berglsteiner See stand out due to their green to dark green color in places. This is not only due to the lush vegetation on the banks of the two lakes. In addition to the yellow color, the algae content plays a major role in these lakes. If there are relatively high concentrations of nutrients in a lake (e.g. phosphorus and nitrogen), algae can grow well. They multiply and give the lake a green color due to the plant pigment chlorophyll.
A prime example of this process is Lake Piburger See in Ötztal. "Lake Piburger See was a lake that became very nutrient-rich in the 1970s to 1980s," says Sommaruga. Back then, when environmental pollution was not yet on the socio-political agenda, people and farms gave free rein to their "nutrient input" and caused algae blooms. In technical jargon: an intensive process of "eutrophication" took place. Today, the lake has partially recovered. Ruben Sommaruga gives the all-clear: "All lakes in Tyrol have bathing water quality."
Nevertheless, the formation of algae and cyanobacteria is usually problematic for the large organisms that live in a lake. When lake plants and algae die, they sink to the bottom, where their decomposition consumes oxygen. For fish and other animals that also need oxygen in the water, oxygen-free areas are dead zones. In addition, global warming is causing the water surface in lakes to heat up more and more, so that there is less oxygen diffusion in the water, especially during heat waves. A water rearrangement or circulation in lakes can only take place in the cooler fall - when the water gets colder and the natural wind energy is strong enough for this process. According to Sommaruga, it has already been discovered that global warming is reducing the oxygen concentration in lakes around the world. Algae and bacteria sometimes cause bizarre discolorations on the surface of the water. A small lake on the Issboden above the Sistranser Alm, for example, is probably turning bright red due to the blooming of an algae.
If, like Ruben Sommaruga, you take a closer look at Lake Achensee , you realize: "Some lakes change their colors. The color of a lake is not constant." Lake Achensee was also affected by eutrophication in the past. Due to the blooming of algae, its color was in the blue-green range. Thanks to the restoration of lakes in the 1990s and the canalization of catchment areas, Lake Achensee recovered and is now even considered "oligotrophic", i.e. low in nutrients. A blue-turquoise surface color characterizes the lake, which is sometimes due to the lack of calcium carbonate (very fine lime crystals, which increase the scattering). Nutrient-poor lakes in calcareous areas, such as the Karwendel, are white-blue due to "biogenic decalcification through photosynthesis" and resemble Caribbean beaches, especially near the shore. One example of this is the Bavarian Walchensee.
Deep blue lakes are often found in the high mountains. There, the lakes are sometimes so deep that the nutrient-poor water appears black. "Around 10000-12000 years ago, many lakes were formed due to the retreat of the glaciers. Today we can observe this live," says Sommaruga, explaining the formation of high mountain lakes. One factor in particular plays a major role here that has not yet been addressed: turbidity. If a lake formed by melting has contact with the glacier, its water is coloured grey by rock flour and thus has the original color of a lake. Limnologists call this cloudy glacier runoff water "glacier milk". Penetrating light scatters on mineral particles and makes the water appear gray. If the particles of glacial milk partially settle, the lake appears turquoise. If the lake finally loses contact with the glacier, it is blue.
Lakes are full of viruses, bacteria and living organisms. However, this is completely normal and does not usually harm people. It is safe to swim in the 35 Tyrolean lakes listed on the website of the province of Tyrol. In practice, however, humans can damage the sensitive ecosystems of lakes. As a limnologist, Ruben Sommaruga wishes "that people would not jump into a high mountain lake". Large groups of people in small lakes are, like the dumping of organic waste in lakes, an understandable outrage. Responsibility in dealing with nature is important so that both admirers and residents of the Tyrolean lakes can enjoy them for a long time to come.
