Past and Current Changes in the Largest Lake of the World: The Caspian Sea

Abstract

The Caspian Sea (CS), located between Europe and Asia, is the largest lake in the world; however, its physical environment and its floor have oceanic characteristics. The CS is composed of a very shallow north sub-basin with a very low salinity mostly below 5 psu. The middle and southern sub-basins are deep and have a salinity of c. 13 psu. To the east, the Kara-Bogaz-Gol, a hypersaline lagoon, is connected to the middle sub-basin. The CS is endorheic and therefore very sensitive to changes in hydrography and climate. Because of its long history of isolation following the disconnection of the Caspian Sea from the Paratethys c. 6 million years ago, this ancient lake has many endemic species. The harsh environment of its brackish waters and the repeated salinity changes over the millennia, however, do not allow for a high biodiversity. The benthos is more varied than the plankton. The history of water-level changes remains poorly known even for the last centuries. Nevertheless, the amplitude was of >150 m in the Quaternary, several tens of meters in the Holocene and several meters in the last century. Many factors affect its natural state, such as petroleum pollution (an industry dating back to Antiquity), nutrient increase (alongside >14 million inhabitants along the coast), invasive species (e.g. the comb jelly Mnemiopsis leidyi), overfishing (including sturgeon) and modifications of its coastline (e.g. sand extraction). In comparison to other ancient lakes, the CS surface temperature has suffered from the fastest increase on record. Owing to the complex natural state of the CS, it is not easy to identify the Holocene-Anthropocene transition, although it may be suggested that is was approximately AD1950 when intense human activity started to modify the lake.