A landscape in motion
The sixteen terraced lakes of Plitvice are not static geological features but a dynamic, living system constantly building upon itself. The secret to this continuous creation is the formation of tufa, a porous carbonate rock that precipitates directly from the water. This process constructs the barriers that separate the lakes, causing the entire system to grow and change.
The water here, flowing through a karst landscape of limestone and dolomite, becomes supersaturated with calcium carbonate. For this dissolved mineral to become solid rock, specific conditions are necessary: the water's pH must be above 8.0, the concentration of dissolved organic carbon must be below 10 mg/L, and the water must be agitated. As water flows over obstacles and turns into waterfalls, carbon dioxide is released, the pH changes, and calcite (CaCO3) precipitates out of the solution. This biodynamic process has been active for millennia, with the current tufa barriers estimated to be between 6,000 and 7,000 years old. Ancient, inactive "palaeobarriers" found at higher elevations date back as far as 250,000 to 300,000 years, showing the process was also active during past interglacial periods.
The microbial architects
The precipitation of calcite is a chemical process; it is heavily mediated by biology. Microscopic life, including cyanobacteria, diatoms, and other bacteria, contributes directly. These organisms form biofilms on submerged surfaces, and their metabolic activities, particularly photosynthesis, alter the local water chemistry, encouraging calcite to crystallize. The sticky extracellular substances secreted by these microbes act as nucleation sites, providing a surface for calcite microcrystals to attach and accumulate.
Larger organisms, especially aquatic mosses, provide the structural framework for the barriers. Species like Palustriella commutata and Ptychostomum pseudotriquetrum thrive in the spray of the falls. As calcite encrusts the lower parts of the moss, the plant continues to grow upwards, leading to a steady accumulation of tufa. This living component makes the barriers a self-constructing habitat. The growth is measurable, with an average rate of around 13.5 mm per year, though this varies between barriers. For instance, the barrier at Prošćansko Lake grows about three times faster than the exit barrier of Kozjak Lake, which grows at about 5.6 mm annually. This differential growth can even lead to dramatic landscape changes; about 400 years ago, the faster-growing downstream barrier of Kozjak Lake raised the water level enough to submerge an upstream barrier, merging two separate lakes into the single large one seen today.