Investigation of the succession of tufa-forming communities at Skradinski buk waterfall

Investigation of the succession of tufa-forming communities at Skradinski buk waterfall
Investigation of the succession of tufa-forming communities in the area of the Skradinski buk was carried out during 2024, and encompassed five different zones where the intensity of tufa formation, the composition of tufa-forming organisms, and the ecological conditions that influence the dynamics of calcite (tufa crystals) deposition were analysed

The results showed that the tufa-forming processes are most intense in sunny areas with strong water flow, while they are significantly weaker in shaded and hydrodynamically more peaceful areas.

The greatest intensity of tufa formation was recorded in Zone 2, which includes sunlit barriers and waterfalls with strong water flows. The area is dominated by moss communities, primarily the species Palustriella commutata and Didymodon tophaceus, which provide optimal surfaces for calcite encrustation. In this zone, the growth rate of the tufa was the highest, and microscopic analysis showed the presence of extremely porous and layered carbonate deposits. A large role in tufa formation is also played by communities of sessile animals that inhabit moss communities – Bryozoa (bryophytes).

Zone 3, located in the downstream lake below the main waterfall (former swimming area), showed significant successional changes, with mosses and periphyton algae gradually taking over the role of dominant tufa-forming organisms. Microcrystalline calcite layers were identified on underwater substrates, and the expansion of periphyton communities on previously unprotected substrates was confirmed. These data indicate a stable, but somewhat slower process of tufa formation in comparison to Zone 2.

Zone 1, which encompasses shaded forest cascades in the most upstream part of the falls, showed a lower intensity of tufa formation due to reduced light availability and weaker water flow. The tufa-forming communities in the area were predominantly cyanobacterial, with a lower proportion of mosses, which resulted in more compact and dense tufa deposits, but with a lower deposition rate in comparison to sun-exposed barriers.

Zone 4, which includes lateral forest tributaries, had a lower intensity of tufa formation. This area is dominated by cyanobacterial deposits, with the occasional presence of periphyton algae, while mosses were rarely present. The layer of tufa deposits is thin and shows no signs of active encrustation.

The least active process of tufa formstion was recorded in Zone 5, the riverbed downstream of the waterfalls, where hydrological conditions do not support the stable development of tufa-forming communities. Green algae are mainly present in this area, while mosses and cyanobacteria are rare or completely absent. Tufa is present here mainly in the form of old, eroded blocks along the river banks, with the presence of tufa terraces at the border of water and air.

Analysis of tufa-forming organisms has shown that mosses play a key role in the creation of porous carbonate structures in conditions of strong water flow and high insolation, while cyanobacteria dominate in more hydrodynamically stable and shaded areas, where they form more compact tufa deposits.

These results highlight the importance of hydrological and light factors in regulating the succession of tufa-forming communities, and confirm that the stability of the tufa-forming ecosystem at Skradinski buk is directly related to the preservation of natural water flow conditions.