Environmental Gradients Shaping the Freshwater Bryophyte Communities of Croatia (Western Balkans)

A comprehensive field survey of 527 sites on 293 watercourses across Croatia revealed 76 sites (14.42%) in which bryophytes were the dominant part of the macrophyte vegetation. Using classification and ordination analyses, we obtained five community types segregated across the gradients of several climatic, physiographic and water chemistry parameters. The Didymodon tophaceus–Apopellia endiviifolia and the Berula erecta-Cratoneuron filicinum communities were mostly confined to the clean and basic karstic rivers of the Dinaric Ecoregion under the influence of the Mediterranean climate, with the Didymodon tophaceus–Apopellia endiviifolia community being a tufa-forming community associated with the seasonally dry watercourses of small catchment areas and cascades along the larger karstic rivers, while the Berula erecta–Cratoneuton filicinum community was mostly associated with rivers with larger catchment areas and permanent flow. On the other hand, the Oxyrrhynchium hians–Chiloscyphus pallescens community and the Fissidens pusillus–Veronica beccabunga community were associated with eutrophic water restricted to small rivers of the Pannonian Ecoregion under the influence of the temperate climate and flowing over silicate bedrock. The most represented and widespread in Croatia was the Cinclidotus community, displaying the widest ecological range in the study. It was mostly associated with the relatively clean karstic rivers of large catchment areas belonging to the Dinaric Ecoregion, with the majority of the sites under the influence of a temperate climate with higher precipitation during the warm period of the year. The geographical patterns of the freshwater bryophyte communities showed that the relatively clean, fast and cold karstic rivers belonging to the Dinaric Ecoregion provide habitats that harbour a greater diversity of bryophyte communities than the watercourses of the Pannonian Ecoregion, where bryophyte-dominated communities are restricted to a small number of small lowland and semi-montane rivers and predominantly occupy periodically flooded microhabitats such as river margins.