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Abstract. Phytoplankton as chlorophyll-containing organisms is the first step of production in most marine processes and food chains. Nutrient enhancement in the seawater due to the discharge of agricultural, industrial, and urban wastes threatens the Caspian Sea environment. Increasing concentrations of chlorophyll-a in seawater, in reaction to the elevation of nutrient supply can have severely damaging effects on the marine environment of the Caspian. In this research, seasonal variability of the chlorophyll-a concentrations in the western part of the southern coastal waters of the Caspian Sea near Iranian coast was examined using field observations. The data showed that the most chlorophyll-a was found below the sea surface. The thermal stratification in water column and outflow of the Anzali Lagoon affect the chlorophyll-a concentrations in the region. Concentrations of chlorophyll-a were recorded in midsummer in a range of 0.2–3.4 mg m−3.

Problem statement: Dissolved Oxygen (DO) and Chlorophyll-a (Chl-a) are two vital requirements for marine ecosystems. Nowadays, discharge of large amounts of industrial, agricultural and urban wastes threats the Caspian Sea (CS) environment. With regard to the elevation of nutrient contents, average of DO concentrations in the deeper layers of the CS is reducing. On the other hand, increased amounts of phytoplankton in water bodies in reaction to the enhancement of nutrient supply can have severely damaging effects on the marine environment. According to the importance of DO and Chl-a in marine ecosystems of the Caspian, we attempted to investigate the distributions of DO and Chl-a in the southern coastal waters of the CS in present study. Approach: DO and Chl-a concentrations were analyzed using field measurements data over the southern continental shelf near Iranian coasts. The data were collected at 13 stations along three transects down to 42 m depth in east part of the southern coasts of the CS, off Babolsar in autumn 2008. Results: In the time of measurements, the DO concentrations ranged between 8.4 and 11.2 mg L^-1 in the area. Amounts of the Chl-a were varied between 1.3-4.2 mg m^-3 with the maximal levels at 15 m depth. The formation and destruction of seasonal thermocline affect the DO and Chl-a concentrations in vertical direction. Conclusion: The collected data present useful knowledge about distributions of DO and Chl-a in the coastal waters of Babolsar and near mouth of Babolrood River. Amounts of DO and Chl-a in the region were under effects of Babolrood River discharge, stratification of water column (seasonal thermocline) and atmospheric processes. The results indicate the need of serious efforts for reducing entrance of human activities wastes and pollutants into the Caspian environment