The Bear Lake in Sowata (Transylvania) is one of the biggest heliothermic lakes in the world with a unique underwater ecosystem. This solar heated lake has a very stable density stratification with a concentrated saline layer at the bottom and a much less concentrated water in its 2.5 meter thick surface layer. In the transition layer (halocline), where the salt concentration changes very rapidly with depth, temperature and light intensity also change very fast, resulting in multiple ecological habitats over a relative short distance. Cyanobacteria, living in the form of biofilms on the yellow sludge that drifts above the halocline, have a unique complex annual cycle. They form geometrical reticulated patterns, presenting characteristic red Fe2O3 spots, during late autumn. Inside the warm halocline, huge amounts of little brine shrimps (Artemia salina) swim through the sludge debris together with abundant larvae of midges and salt flies. Diatom taxa, whose relatives are known for inhabiting saline habitats, have also been found in the biofilms.