Hydrochemistry and nutrient distribution in southern deep-water basin of the Caspian Sea

In this work, the results of hydrochemical studies aboard the R/V Iran Behshahr in southern Caspian Sea in late-winter 2014 were presented. Salinity, temperature, dissolved oxygen, pH, total dissolved inorganic carbon, total alkalinity, nitrate, phosphate and silicate concentrations in water column of Neka-Amir Kabir oil platform section in the southern Caspian Sea were measured to study the status of hydrochemistry of this area. Results showed that the hypoxia continues to intensify in the deep-water basin of the South Caspian Sea. Near-zero concentration of dissolved oxygen and accumulation of phosphate, silicate and total dissolved inorganic carbon in near-bottom layers in the study area showed that vertical winter mixing of water column did not reach the near-bottom layers at the time of this survey. Nitrate showed its maximum concentration at the intermediate maximum depth of 300m. •Hydrochemical studies in southern Caspian Sea in late-winter 2014 were performed.•Expansion of the hypoxic condition layer up to horizon level of 300–400m was observed.•The accumulation of phosphate, silicate and dissolved inorganic carbon in the bottom layers was observed.•Nitrate intermediate maximum depth of 300m was recorded.