Tufa (also called travertine) is the term for calcium carbonate that is formed in rivers when minerals from the water settle onto different types of surfaces.
The deposition of tufa is a dynamic process of the interactions of physical and chemical factors and living organisms in water. Tufa formations are created in water with a high concentration of dissolved calcium bicarbonate (Ca(HCO3)2). The deposition of tufa begins on rapids, irregular surfaces, flooded branches, etc., because of the splashing of water and the release of CO2, creating a disturbance of the chemical balance that leads to the extraction of calcium carbonate or calcite (CaCO3), and its deposition in the form of tiny crystals on the submerged surface.
The deposition of tufa and the growth of tufa barriers are possible in water supersaturated with calcium carbonates (carbonates with a hardness over 3 HD), with a concentration of organic matter less than 10 mg/L of carbon, and a pH greater than 8. In addition to these three basic conditions, the formation of tufa is accelerated by higher temperatures and water flowing at a speed of 0.5 to 3.5 m/s. An important role in the process of the deposition of the calcite crystals on the surfaces is played by the microorganisms caught on aquatic plants. The immersed surfaces are inhabited by pioneering microflora or rather bacteria that create the conditions for the settlement of algae and the formation of growth communities. Deposition begins on the surface of the mosses with the fixation of calcite microcrystals on the sticky secretions of cyanobacteria and diatom algae. Organic and inorganic particles (diatom shells, fragments of animal shells, grains of limestone, etc.) stick to them, creating a nucleus on which the calcite crystals will grow.
According to the participation of certain organisms in the construction of limestone deposits, several forms of tufa are distinguished: cratoneurum, bryum, and didymodon. The names come from individual tufa forming mosses (Cratoneurum commutatum, Bryum vebtricosum, and Didymodon tophaceus). Chironomid, Gastropod, and Trichopter tufa are differentiated according to the tufa forming animal groups of midges (Chironomidae), snails (Gastropoda), and caddis flies (Trichoptera). Species that have the ability to deposit cyrstals of calcite are called “tufa formers”. The most common forms of tufa in karst water flows are underwater covers, sills, and barriers, which are found at the base of karst rapids, curtains, and consoles, which are formed in places where water pours down a perpendicular rock, and ridges, grooves, and pipes, which are found in places where a large amount of water cascades, forming waterfalls.
The tufa barriers of the karst rivers of the Dinarides (Natura 32A0) are the target habitat type of the ecological network in the area of the Krka National Park, and as such represent a great asset and a widely recognised phenomenon of this protected area.