Acceleration of eutrophication has been observed in the lower Nakdong River since the early 1980s. A longer hydrologic retention time due to the estuary dam construction and high nutrient input were the cause of an increase in phytoplankton biomass. Trophic status (chl. a) changed from meso-eutrophic (28±23㎍/l, 1982-87, n=26) to hypertrophic (58 ±148㎍/l, 1993-98, n=236) with frequent bloom developments. Elevated BOD, COD, and nutrient levels were observed (before 1987: 3.6±0.8mg/l, after 1987: 4.6 ±2.4 BOD, 4.4±1.2 to 8.1 ±2.6 COD, 0.9 ±0.8 to 2.3 ±1.0 NO_3-N, 2.4±0.9 tO 3.7±1.7TN, respectively). Along With the eutrophication Process in the river, phytoplankton community structure has been shifted (Microcystis bloom in summer and Stephanodiscus bloom in winter). Microzooplankton (rotifers) were the dominant group and they appeared to play an important role in regulating phytoplankton biomass. Hydrologic influence on the water quality at the lower part of the river has to be evaluated in spring and fall when discharge is generally low. Evaluation of point and non-point pollutant leadings of entire river basin is urgently needed for the systematic reduction of nutrient inputs.