The Bear Lake in Sovata

Fascinating haloclines, patterned biofilms

Abstract

Heliothermic lakes, like the Bear Lake in Transylvania, have a stable stratification with a concentrated saline layer at the bottom and a less saline (thus lower density) surface layer. Due to solar radiation, the transition layer (the halocline) is heated up to 40–45 °C. The pronounced physico-chemical gradients result in a series of biological niches. Scientific diving was used to investigate this unique ecosystem. Biofilms of cyanobacteria are especially interesting. They follow a complex annual cycle by forming reticulated patterns in late autumn presenting characteristic red Fe₂O₃ spots on the mud, probably formed from the black FeS-containing mud by iron-oxidizing bacteria. Diatom taxa, whose relatives are known for inhabiting saline habitats, have also been found in the biofilms.