The Rogoznica Lake is a small, marine, karstic lake 15 m deep and the size of 5300 square meters. There is no fresh water inflow or drifts because it is situated in a valley, protected from strong winds. Large seasonal amplitudes of salinity, temperature and dissolved organic matter make it an extreme habitat. It is a meromictic lake with anoxic deeper layers. The biological community of the lake consists of a small number of phytoplankton species and invertebrates. In late September of 1997 there was a sudden mixing of the water column that caused anoxic conditions in the whole lake. After this event, almost the whole population of the lakes phytoplankton had died. With numerous sampling the recovery of phytoplankton was monitored. The first examples of microflagellates appear already two weeks after the mass mortality in areas of increased concentration of ammonia. Subsequent oxidation of the water column and larger nitrate concentrations favored the creation of larger phytoplankton, such as diatoms and dinoflagellates. Sampling a year after shows that the lake´s phytoplankton populations fully recovered and is the same as before the anoxia. In the Rogoznica Lake there are two rare species: Prorocentrum arcuatum Issel and Hermesinum adriaticum Zaharis. These marine species of phytoplankton are rare throughout the Mediterranean. We can conclude that specific microclimate in the lake favoures the growth of those rare species. The complete regeneration of 14 phytoplankton communities indicates that the Rogoznica Lake, is a selfsustainable environment.