THE CASPIAN LAKE - HISTORY, BIOTA, STRUCTURE, AND FUNCTION

The elongate, endorheic Caspian Lake has a north-south orientation and its main freshwater inflow, the Volga River, enters at the shallow no rth end. Two deep basins occupy its central and southern regions. Thes e facts lead to horizontal differences in temperature, salinity, and e cology. Nutrient levels and primary production are low. Historically, lake level has fluctuated by similar to 6 m, but on the geological tim escale, fluctuations of >200 m have occurred. The lake formed in the l ate Miocene, first went through a long shrinking phase, expanded to th ree times its present size in the late Pliocene, and repeatedly rose a nd fell throughout the Pleistocene, with corresponding freshenings and salinizations. The lake surface has remained below sea level since th e last pleniglacial. This dynamic history led to the assemblage of a s et of euryhaline biota of Tethyan and freshwater origin, besides Balti c elements, that invaded with glacial meltwater. Endemism is as high a s in Lake Baikal, but typical marine groups are absent. Because the la ke is oxygenated to the bottom, its biota show a vertical zonation, bu t without a true abyssal community, suggesting catastrophic episodes o f deep anoxia.