Nowadays, great values of urban sewage and industrial pollutants discharge into the Caspian Sea and threaten its marine environment. This study evaluates characteristics and variability of dissolved oxygen and active reaction (pH, is a parameter for measuring of the acidity or basicity of a solution or seawater) in deep waters of the southern Caspian Sea near the Iranian coast during March to November 2008. The vertical distribution of active reaction showed a general reducing trend with depths that reached 8.2 at depth of 350 m in midsummer. Moreover, observations in November showed active reaction value of 8.3 at 450 m depth. In comparison, amounts of active reaction (pH) in mid-summer (August) were less than that in other seasons. Dissolved oxygen concentrations ranged between 2.0-7.5 mg·L⁻¹ at the surface layers. Below 100 m, a rapid drop in values of dissolved oxygen was observed and measured about 1.2 mg·L⁻¹ at 450 m in mid-summer and mid-autumn, while 2.0 mg·L⁻¹ was recorded below 300 m in late winter. The seasonal variations of dissolved oxygen and active reaction (pH) were considerable, especially in intermediate and deepwater layers. According to levels of dissolved oxygen concentrations and active reaction (pH) values in deepwater of the Caspian Sea, increased observations and serious efforts for decreasing entry of pollutants into the marine environment are necessary.
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