Rogoznica Lake is small, closed and partly isolated saline lake placed at Gradina peninsula near Rogoznica. Dynamics within the lake is characterized by the extreme and highly variable environmental conditions (seasonal variations in salinity and temperature, water stratification and mixing, redox and euxinic conditions, concentrations of nutrients) which significantly influence the biology in the lake. Seasonal vertical mixing usually occurs during the autumn/winter upon the breakdown of the stratification, injecting oxygen-rich water from the surface into the deeper layers. Depending on the intensity and duration of the vertical dynamics (slower diffusion and/or faster turnover of the water layers) anoxic conditions could developed within the whole water column. Extreme ecological conditions, which prevail in the lake, control phytoplankton activity represented by low species diversity and high single species abundance. Phytoplankton was reaching high abundances up to 107 cells L-1 and was dominated by diatoms Chaetoceros curvisetus, Dactyliosolen fragilissimus, Thalassionema nitzschioides and Cyclotella choctawhatcheeana. The species alternated with domination in the phytoplankton assemblages, reflecting the environmental conditions in the lake. Rogoznica lake, due its size and isolation, extreme environmental conditions, low species diversity and high single species abundance can serve as an natural laboratory for testing the possible outcomes of ongoing climate changes.