Nitrogen and phosphorus compounds in water column oj a waterbody are important nutrient Jor the growth ofphytoplankton. Based on literature, it was noted that the NIP ratio in a waterbody can be useful as a diagnostic tool Jor assessing the types of algae likely to exist under different condition. it was also said that a low NIP ratio between 2 - 10, community phytoplankton would be dominated by Cyanophyta (blue green algae) group. in applying in cascade reservoirs at Citarum River Basin indicated there is a particular link between NIP ratio and the tendency oj phytoplankton population dominating the reservoirs. in Jatiluhur Reservoir, the total N to total P ratio (TNITP) is low around 4 and the phytoplankton communities are dominated by Pyrrophyta group with species Ceratium hirudinella. On the contrary, the low TNITP ratio in Saguling and Cirata Reservoirs show that the reservoirs are dominated by Cyanophyta group.