The Caspian Sea, the largest enclosed body of water on Earth, is neither truly freshwater nor truly saltwater, but is categorized as the world’s largest brackish lake. It is situated in a basin that borders five countries—Kazakhstan, Russia, Azerbaijan, Iran, and Turkmenistan—and spans approximately 371,000 square kilometers. The designation “sea” is a historical and cultural title rather than a scientific classification based on its salinity.
Defining Brackish Water
Brackish water represents a salinity level between that of freshwater and typical oceanic saltwater. Salinity is measured in parts per thousand (PPT). Freshwater is defined as having a salinity of less than 0.5 PPT, while the world’s oceans maintain a salinity of around 35 PPT. Brackish water generally falls within the range of 0.5 to 30 PPT, a condition typical of estuaries where rivers meet the sea.
The Caspian Sea’s average salinity is about 12 to 13 PPT, making it roughly one-third as salty as the open ocean. This concentration places it firmly in the brackish category. The chemical composition of the Caspian’s dissolved salts differs from that of ocean water, which is dominated by sodium chloride. The Caspian Sea’s water is relatively poor in sodium and chlorine ions. Instead, its salt content contains a higher proportion of calcium and sulfates, reflecting its geological history and isolation from the global ocean system.
How the Caspian Sea Became Landlocked
The Caspian Sea is a remnant of the ancient Paratethys Sea, an inland sea that covered much of Europe and parts of western Asia millions of years ago. The isolation process began in the Late Miocene epoch, approximately 5.5 million years ago. Tectonic forces, specifically the collision of the Arabian peninsula with West Asia, caused uplift that raised the surrounding mountains, including the Caucasus and Kopet Dag ranges.
This uplift gradually severed the Paratethys from the global ocean system, turning it into a closed basin. Without a continuous connection to the ocean, the water balance became dependent solely on river inflow and evaporation. Over time, the continuous influx of river water carrying dissolved minerals, combined with subsequent evaporation, led to the accumulation of salts in the remaining basin. While isolation allowed salts to concentrate, the lack of oceanic replenishment prevented the salinity from reaching the high levels found in modern oceans.
Salinity Variation Across the Basin
The classification of the Caspian as brackish is an overall average that masks significant internal variations in salinity. The sea is divided into three distinct basins—Northern, Middle, and Southern—each exhibiting different salt concentrations. The shallow Northern basin receives approximately 80 to 85% of the total river inflow, primarily from the Volga River. This freshwater input creates near-freshwater conditions in the north, with salinity levels near the river delta often dropping below 0.5 PPT.
Moving south, the water becomes progressively more saline as the influx of freshwater diminishes and the effects of evaporation increase. The deeper Middle and Southern basins, which hold the vast majority of the water volume, have a consistent salinity of around 12.8 to 13.5 PPT. The Garabogazköl lagoon on the extreme eastern edge represents the upper limit of this variation. This highly restricted, shallow lagoon acts as a natural evaporator, where salinities can exceed 200 PPT, far surpassing that of the open ocean.
The Unique Caspian Ecosystem
The brackish environment has led to the development of a unique, isolated ecosystem. Organisms have adapted over millions of years to tolerate this low salinity, which is too high for most freshwater species and too low for typical marine life. The fauna is characterized by a high degree of endemism, meaning many species are found nowhere else in the world.
The Caspian seal, the only mammal in the sea, is an endemic species adapted to the brackish water and often ventures into the deltas of inflowing rivers. The sea is also home to the Ponto-Caspian faunal complex, including several species of sturgeon. These fish are an ancient lineage that evolved in the Paratethys and adapted to the low-salt conditions. The specialized nature of this isolated ecosystem makes it sensitive to changes in salinity, temperature, and introduced non-native species.