Trend of Macronutrients Fluctuation of Waterin the Iranian Coasts of Southern Caspian Sea

This study was conducted to nutrient concentrations of water in the southern Caspian Sea waters 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. Annual concentration of inorganic nitrogen (DIN= NH4+, NO2-, NO3-) 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 DON was recorded lower than the previous study (2008), but NH4+, NO3- and DSi were higher than the previous sampling period. Inorganic phosphorous (DIP) and NO2- 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 (N/P<10) before introduction of Mnemeiopsis leidyi, while it seems that after introduction of Mnemeiopsis leidyi the system has been shifted to the phosphorous limitation (N/P>20).