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How it all began: travertine builders

Physicochemical processes alone would not be able to create travertine deposits or barriers in the river.

This also requires the living organisms that are able to retain the deposited calcium carbonate crystals, in order to create the travertine deposits. In the splash zones, where water is aerated, these organisms find suitable places to colonise the substrata, as they require wet calcific habitats to survive. Most often, these organisms are calcific mosses (bryophytes have a significant role in trapping the materials for travertine, so they are called travertine-building mosses).

All biological and ecological discussions about the living world of karst waters refer to the travertine-building communities and travertine-builders. The living world at the bottom of the river is called benthos. In the ecological sense, travertine-building communities are based on three fundamental categories: primary producers (plants), consumers (animals), and the decomposers (bacteria, fungi and animals that feed on dead organic matter). The travertine-building mosses inhabit virtually all parts of the waterfall in large quantities, but primarily on surfaces covered by water, at the water’s edge or in wet habitats, and are not found in places of calm water. The Krka waterfalls are inhabited by 41 moss species, of which 33 are porophyte species and the rest are aporophyte species, i.e. those that do not participate in travertine-building.

Based on the plant or animal organisms that participate in the building of limestone deposits, we can differentiate the different forms of travertine: cratoneurumic, briumic, didimodonic, all named after the travertine-building mosses (Cratoneurum commutatum, Brium vebtricosum and Didimodon tophaceus), and chironomidic, gastropodic and trichopteric, named after the travertine-building animal groups, the lake flies (Chironomida), snails (Gastropoda) and caddisflies (Trichoptera).

The aquatic mosses are most abundant in the splash zones, and this is where the most calcium carbonate is deposited. The sedimented calcium carbonate crystals catch onto the moss tufts, with the help of algae, and are built into the moss. This is how travertine forms in so many different shapes. As the moss grows, the deposition surface increases, and so the travertine grows along with the moss. In addition to the travertine-building mosses, different species of algae and cyanobacteria also play important roles in trapping the calcite crystals. These organisms inhabit the moss shoots and secrete a sticky substance that traps the crystals. New crystals accumulate around these trapped crystals, thus beginning to create a travertine deposit. Over time, these deposits grow to form barriers consisting of limestone full of the remains of microscopic algae and petrified aquatic mosses.

In the travertine-building communities, the primary producers are the mosses, filamentous algae and periphyton (algae growing on plants and solid surfaces). The filamentous algae and mosses often form a mosaic structure of vegetation cover of the travertine-building biotope. The periphyton growing on moss clumps and filamentous algae on the rocky substrates is made up of diatomaceous algae.