Epipelic and Planktonic Diatom Communities in the Limnocrene Spring Zelenci Reveal an Increase in Trophic Values

Zelenci is a limnocrene spring in the South-Eastern Alps, attractive for its unique structure and known among researchers for its high diatom diversity. Our aim was to assess how abiotic factors influence the structure of diatom communities in different habitats and to compare the trophic status over 10 years of investigation. Four sampling sites were chosen: two for tychoplankton and two for epipelon. Achnanthidium minutissimum was the most abundant species in both habitats, while Navicula was the most diverse genus (17 species). Planktonic diatoms dominated plankton samples, while the motile ecological type dominated the epipelon. Of all diatom taxa, 23.5% had some endangerment status. Key factors influencing the Shannon–Wiener diversity index were water level, temperature, and concentration of NH4+. The most important abiotic factors for the tychoplankton community were temperature and NH4+, while the most important abiotic factors influencing the structure of epipelon communities were water level, NH4+, pH, and dissolved oxygen concentration. Trophic index revealed increasing inflow of nutrients to the spring; 10 years ago, they were oligotrophic to oligo-mesotrophic, whereas in the present, they are predominantly eu-mesotrophic to eutrophic, indicating human pressure from the catchment area.