Why is there no oxygen in the north of the Caspian Sea?

The lack of oxygen in the northern Caspian Sea is primarily due to significant water stratification and high levels of salinity, which limit the mixing of water layers. Additionally, the inflow of nutrient-rich freshwater from rivers can lead to algal blooms, which consume oxygen during decomposition. The resulting hypoxic conditions create an environment where oxygen levels are insufficient to support most aquatic life.