Study of physico-chemical Characteristics of water in the southern of Caspian Sea

This study was conducted to physico-chemical characteristics of water in the Iranian coastal of southern Caspian Sea during four seasons at 8 transects (Astra, Anzali, Sefidrud, Tonekabon, Nowshahr, Babolsar, Amirabad and Bandar Turkman) during 2009-2010. 480 samples were collected at different layers of water and then analyzed based on standard methods. Results showed that the mean of water temperature were 18.67±0.32 and 17.82±0.43 ◦C at the surface and euphotic layer, respectively. Minimum and maximum of water temperature were recorded in winter (6.40) and summertime (27.60). In addition, temperature gradient was ranged between 12 to 15 ◦C at 20 to 50 layers in 50 and 100m depths. Mean of salinity was 11.04±0.17 g/l at euphotic layer. Pearson correlation between temperature and salinity was positively significant. Therefore, maximum salinity was recorded in summer and minimum in winter. Mean of transparency (SD) in the present study was 4.35±0.21 m which compared to the pervious sampling period (2008) showed small decrease, but the mean of pH was observed 8.43±0.01 which was higher than the previous sampling period (2008). The mean of DO and DO% were observed 5.72±0.06 ml/l and 130±1 percent at euphotic layer during sampling period. The mean of DO% was recorded 104±5 at euphotic layer in 1996 (before introduction of Mnemeiopsis leidyi), but the study in the years 2004, 2008 and present (one decade after introduction of Mnemeiopsis leidyi) this value was registered higher than 120% at euphotic layer. This shows that trophic status of Caspian ecosystem shifted from oligotrophy (before introduction of Mnemeiopsis leidyi) to meso-eutrophy (after introduction of Mnemiopsis leidyi). Annual concentration of inorganic nitrogen (DIN= NH^4 +, NO^2-, NO^3-) has a fairly wide variation. Percentage of nitrogen components out of DIN were varied 9-98, 0.2-28.2 and 0.0-90.0 respectively. In this study, percentage of DIN was lower than 15% and organic nitrogen (DON) was higher than 80%. Overall, results showed that water temperature, salinity, transparency and DON were recorded lower than the previous study (2008), but DO, DO%, pH, NH^4+, NO^3- and DSi were higher than the previous sampling period. Inorganic phosphorous (DIP) and NO^2- have not changed substantially. N/P ratio of Caspian Sea has a narrow range which order of magnificent lower than other seas. The results of the present study showed that Caspian ecosystem was nitrogen limitation before introduction of Mnemiopsis leidyi, while it seems that after introduction of Mnemiopsis leidyi the system has been shifted to the phosphorous limitation.