The elongate, endorheic Caspian Lake has a north-south orientation and its main freshwater inflow, the Volga River, enters at the shallow north end. Two deep basins occupy its central and southern regions. These facts lead to horizontal differences in temperature, salinity, and ecology. Nutrient levels and primary production are low. Historically, lake level has fluctuated by -6 m, but on the geological timescale, fluctuations of >200 m have occurred. The lake formed in the late Miocene, first went through a long shrinking phase, expanded to three times its present size in the late Pliocene, and repeatedly rose and fell throughout the Pleistocene, with corresponding freshenings and salinizations. The lake surface has remained below sea level since the last pleniglacial. This dynamic history led to the assemblage of a set of euryhaline biota of Tethyan and freshwater origin, besides Baltic elements, that invaded with glacial meltwater. Endemism is as high as in Lake Baikal, but typical marine groups are absent. Because the lake is oxygenated to the bottom, its biota show a vertical zonation, but without a true abyssal community, suggesting catastrophic episodes of deep anoxia.
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