Why Are Plitvice Lakes Really So Turquoise?

The colour of Plitvice Lakes varies: from deep blue, over emerald green to a very particular kind of turquoise or teal. As a tour guide who has visited the Plitvice Lakes National Park for more than 200 times, I have always wondered: What REALLY lies behind this magical colour?

  1. Fine floating particles absorb and scatter sunlight in ways that can give water a striking blue-green, or turquoise, colour.
  2. These particles, consisting of minerals and algae, come into existence by means of a fascinating natural process.
  3. In a well coordinated play of nature, the deposits of these particles eventually form barriers, or dams, creating lakes and waterfalls
  4. The lakes and waterfalls actually perpetuate the creation of the fine particles, amplifying the entire process.
The amazing turquoise colour of the Plitvice Lakes

Since I heard of several (conflicting) theories on the amazing turquoise colour of some of the Plitvice lakes, I decided to get scientifically based answers. Here is what I found:

1. The Secret Of The Liquid Turquoise

If you visit Plitvice Lakes at different times of the year or at different weather conditions, you will be able to perceive the change in colour of the water. Also, the colour varies from lake to lake. One single lake can even show several distinct colours.

The most amazing and awesome colour appears in Summer at the Lower Lakes. I like to refer to it as fluorescent turquoise, because items immersed into that water seem to release blue light!

What makes blue “bluer” than blue and green “greener” than green?

There are actually two crucial elements that create such an effect:

  • the INTENSITY of light
  • the WAVELENGTH of light
  • tiny particles suspended in water DISPERSE light

The Intensity Of Light

The shiny turquoise colour of the lakes hit its peak in Summer, when the Sun is intense and high in the sky. The sunnier it is, the shinier the colour.

Interestingly, in early Spring or in Autumn, the same water is nearly not as intense in colour. One of the reasons is the lower intensity of sunshine, and the fact that the Sun is lower in the sky.

There are other reasons, though. Read on and find out.

The Wavelength Of Light

Clear seawater, for example absorbs longer wavelengths of visible light (yellow, orange and red), while powerfully reflecting shorter blue and blue-green wavelengths. Therefore, unless there is another pigment present near the surface of the water, like green algae or plankton, it will always appear blue or slightly blue-green.

The blue-green water of the Adriatic Sea in Summer

Many of the Upper Lakes at Plitvice are not turquoise, not even in high Summer. The dramatic difference in colour begins only towards the final part of Lake Kozjak, and continues down through the canyon of the Lower Lakes.

This is where the water becomes turquoise. Not just blue-green. It is thick, dense, shiny kind of blue-green. As if someone spilled a bucket of teal paint.

So there must be something special there. What is going on?

Tiny Particles Suspended In Water Disperse Light

This is probably one of the most essential reasons why the water gets so turquoise.

The light that hits the water is being partially absorbed by tiny particles suspended in water. These particles absorb the shortest wavelengths, dark blues and purples. The water absorbs the longer wavelength reds, oranges, and yellows. What remains are the blues and greens. These colours not only get reflected back to our eyes by these miniature mirrors, but they actually get AMPLIFIED. Why?

Because suspended particles act like tiny prisms, producing an effect similar to the microscopic scales on the wings of the Morpho Butterflies, that cause the light to diffract and interfere.

But what are these suspended particles? Where do they come from?

2. A Fascinating Natural Process

Similar turquoise colour is commonly found at many glacial lakes. The reason why water there gets this colour is the so-called glacial flour, the dust created as the glaciers grind the rocks beneath them and released as the ice melts. Being super light-weight, this flour tends to float in the water of glacial lakes, capturing and dispersing the sunlight.

However, at Plitvice Lakes region there are neither glaciers nor glacial flour. According to the official website of the Plitvice Lakes National Park, water of the Plitvice Lakes is supersaturated in CALCITE.

Calcite – The Star Of Plitvice Lakes

Calcite is a carbonate mineral that forms beautiful crystals.

These microscopic crystals appear as miniature snowflakes in the water (And you know how unique and beautiful each of these snowflakes can be!).

Photo credit: By Rob Lavinsky, iRocks.com – CC-BY-SA-3.0, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10030057

Calcite has some amazing features that may explain my observations about the water in the Lower Lakes, namely, that the water appears to be bluer than blue and greener than green.

Calcite is transparent or opaque and may occasionally show phosphorescence or fluorescence.
Single calcite crystals display an optical property called birefringence (double refraction). This strong birefringence causes objects viewed through a clear piece of calcite to appear doubled. (Wikipedia)

Demonstration of birefringence in calcite, using 445 nm laser

By Jan Pavelka – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=45341180

Surreal beauty of the Plitvice Lakes owes its magic to amazing physical, chemical and biological phenomena.

But Why Is Water Supersaturated With Calcite?

Calcite, like most carbonates, will dissolve with most forms of acid. Calcite can be either dissolved by groundwater or precipitated by groundwater, depending on several factors including the water temperature, pH, and dissolved ion concentrations. Although calcite is fairly insoluble in cold water, acidity can cause dissolution of calcite and release of carbon dioxide gas. Ambient carbon dioxide, due to its acidity, has a slight solubilizing effect on calcite.

However, the MAIN source of ACID dissolving calcite from the limestone and dolomite rocks surrounding the Plitvice Lakes is the organic matter – rotting trees and leaves.

As they feed on fallen leaves, BACTERIA create humic and fulvic acids.

Calcite acts as a buffer, neutralising the acids. More acids=more dissolved calcite. As sinkholes and caves develop, calcite ends up in water.

Why Aren’t All The Plitvice Lakes Turquoise?

But why does the opaque turquoise effect happen predominately at the Lower Lakes, not so much the Upper Lakes?

Because of the higher content of organic matter, the water of the Upper Lakes appears to be darker.

Chemically, this means that the pH level of the Upper Lakes is lower than the pH level of the Lower Lakes.

However, along the way, something will happen to the organic matter! The algae and the bacteria will FEED ON IT! By the time water reaches the Lower Lakes, most of the organic matter will be gone.

More On Calcite

Another answer may be found in the features of calcite. According to Wikipedia,

Calcite exhibits an unusual characteristic called retrograde solubility in which it becomes less soluble in water as the temperature increases.

What does that mean?

Well, it means that the dissolved calcite becomes “heavy”, so to speak, as the water temperature goes up.

Now, when you observe the layout of the cascading Plitvice Lakes, as the water flows through each of the lakes, starting from the top-most, Lake Prošćansko, over Lake Galovac and Burgeti to Lake Kozjak, it gets WARMER.

The large surfaces of the lakes accumulate heat. By the time the water reaches the Lower Lakes, it becomes significantly warmer than at the top.

High water temperature accelerates the precipitation (the process of a liquid becoming solid). As the crystals of calcite precipitate, they become larger and finally fall to the bottom, coating everything with…let’s call it “calcite flour”. This is one of the reasons why the deposits of calcite grow more quickly in Summer.

However, that’s not all! To make things more awesome, biological factors step in! And these biological factors just LOVE warm baths!

Microbiologically Induced Calcite Precipitation

As I was doing research for this article, I found out about this study:

Recently it has been demonstrated that microbially induced precipitation may be more important than physico-chemical precipitation. Pedley et al. (2009) showed with flume experiments that precipitation does not occur unless a biofilm is present, despite supersaturation.

Pedley, M.; Rogerson, M.; Middleton, R. (2009). “Freshwater calcite precipitates from in vitro mesocosm flume experiments: a case for biomediation of tufas”. Sedimentology. 56 (2): 511–527. Bibcode:2009Sedim..56..511P. doi:10.1111/j.1365-3091.2008.00983.x.

So what does this mean? It seem that the colour of the water is not only result of CHEMICAL processes. It appears that, without biological agents, the entire process would NOT occur! No matter how saturated or supersaturated the water is, without LIVING ORGANISMS, the magic wouldn’t happen!

But, which biological agents that would be?

The main groups of microorganisms that can induce the carbonate precipitation are photosynthetic microorganisms such as cyanobacteria and microalgae; sulfate-reducing bacteria; and some species of microorganisms involved in nitrogen cycle.

 Ariyanti, D.; Handayani, N.A.; Hadiyanto (2011). “An overview of biocement production from microalgae”. International Journal of Science and Engineering2 (2): 30–33.

Apparently, the warm lakes of the National Park are ideal habitats for these microorganisms. Similar pools used to commercially grow microalgae, like Spirulina. In the Wikipedia article on Algaculture, I found this piece of information that will finally make the picture complete:

Water, carbon dioxide, minerals and light are all important factors in cultivation, and different algae have different requirements. The basic reaction for algae growth in water is carbon dioxide + light energy + water = glucose + oxygen + water.

“Biology Resources”. Archived from the original on 2008-12-08. Retrieved 2008-06-17.

In order for precipitation of calcite to take place, carbon dioxide has to be low (because it acidifies the water, and acid dissolves calcite)!

Some of the carbon dioxide is removed from the water as it passes down the waterfalls and gets exposed to atmospheric oxygen.

However, the action of the ALGAE is crucial for this process to take place! The algae need carbon dioxide to grow!

And it is their microscopic bodies, together with calcite crystals, that make up the light-projecting particles, causing the water to appear turquoise.

Interestingly, despite the fact that the water that leaves the Plitvice Lakes and flows to the Korana river is supersaturated in calcite, the natural phenomena seem to rapidly come to a halt. Apparently, as the “biofilm” disappears, and the algae get washed away, the organic matter increases (the stuff bacteria and algae eat), pH level goes down and the miracle stops.

So, in order for the algae and bacteria to thrive, water needs to be slowed down and pooled. In other words, there need to be lakes. How did these lakes appear in the first place?

3. The Miracle Of Tufa

Tufa is the stuff waterfall are made of. BTW, it’s not travertine, which is harder and formed in thermal waters. And it’s not TUFF, which is a volcanic rock.

Tufa is a lightweight fragile rock that forms in freshwater at ambient temperature.

There is a great article on TUFA at the official website of the Plitvice Lakes National Park.

I like to think of tufa as the “freshwater coral”, because it consists of organic and mineral matter, living and non-living organisms. In tufa, more “players” join the game. Apart from bacteria, algae and the calcite crystals, there are mosses, grasses, larvae, tree branches, leaves…

These “players” thrive on abundant OXYGEN provided by the waterfall, organic matter provided by the water and the protection of each other. Their feeding frenzy results in massive barriers of petrified moss, fossilised trees and branches and leaves turned to stone. This makes the barriers bigger and higher, lakes behind them deeper and waterfall taller.

Initially, it may have be just a fallen tree blocking a stream and disturbing the water. Or the growth of reeds that slows the flow.

Other organisms colonise this area, as other debris gets trapped in it.

The Mali Prštavac waterfall consists of fallen trees turned into pipes, with pressurised water flowing through the petrified trees.

4. Fantastic Self-perpetuating Choreography Of The Plitvice Lakes

The Plitvice Lakes are apparently an integral part of our amazing planet and our awe-inspiring universe.

Like a well-coordinated ballet performance, the rocks, the trees, the water, bacteria, algae, carbon dioxide, oxygen, light… and probably many more other participants… create this wonder together.

But what makes this miracle even more fascinating is that it is self-perpetuating and inter-dependent.

There was a lady from Argentina who once joined my tour. This is what she said at the end of the visit:

At Plitvice Lakes God is not whispering.

At Plitvice Lakes, God is SHOUTING

Plitvice Lakes in Autumn

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