Lake Vostok, isolated from direct exchange with the atmosphere for millions of years, provides a unique, so far inaccessible habitat. By using a numerical model, and recent geometry information, the lake circulation was investigated for different salinites. For freshwater, thermally driven circulation occurs, resulting from pressure-dependent melting point differences at the inclined ice ceiling. North to south ice pumping provides a steady supply of glacial water. The weak circulation is driven by very small density contrasts, but requires no unusual geothermal input. For low salinity conditions, however, circulation intensifies, occupying the entire lake. The maximum amplitudes of melting/freezing increase by about 50% and melting extends further south. For both conditions approximately 200 m of refrozen ice accumulates beneath Vostok Station. The lake habitat will be affected clearly by salinity. It is essential to establish the specific chemistry for comprehending this unique environment and planning in situ experiments.