Habitat conditions and spring-associated limestones (SALs) formed by ambient precipitation with the biotic contribution of the desmid Oocardium stratum Nägeli 1843 (Zygnematophyceae) were studied in 5 springs along a north–south transect across the central Eastern European Alps. Spring waters were characterized by permanent flow and temporally almost stable physicochemical conditions, but variable relationships among major ions (especially HCO₃⁻:SO₄²⁻ and Ca² :Mg² ) among sites. In most cases, CO₂ supersaturation (0.2–1.8 µM/L) caused in situ depression of pH from near equilibrium (8.3) to <8, with a minimum pH of 7.1. SAL areas dominated by Oocardium growth were greatest some distance downstream of the spring origin where degassing of excess CO₂ had occurred and pH had risen to >8.1. The calcified segments of spring streams were limited to areas <300 m from the spring mouth. Within the uppermost segment of larger rheocrenes (CO₂ supersaturated areas), extensive weakly calcified bryophyte crops (Eucladium verticillatum, Palustriella commutata) were replaced further downstream by Oocardium-spiked calcified coatings. The various morphologies of Oocardium cells (assessed with light microscopy [LM], scanning electron microscopy [SEM], and petrographic thin sections) revealed different types of calcification, from micritic (<1–4 µm) to sparitic calcites (>100–1000 µm) whose ultrastructural features were best seen with SEM. The distribution of SAL types within and among sites was evaluated in relation to water-chemistry and additional environmental variables. The findings were compared with earlier records from the Alps and other mountain areas of Europe (in particular, Croatia and UK) to facilitate recognition and delimitation of Oocardium niches in the environment and to foster protection of these rare habitats.