Fourteen assays for algal N·P demand were conducted in nutrient enrichment bioassay(NEB) using freshwater algae of the Naktong River. P and N+P enrichment stimulated algal biomass(chlorophyll-a) by 1.9 fold and growth rate (㎛) by 1.7 fold, however, NH_4, and NO_3 enrichment had an insignificant effect in comparison with control samples. NH_4 and PO_4 concentration in the media exponentially decreased with the algal growth, on the other side, NO_3 concentration didn`t show any significant decrease on the contrary of the algal exponential growth. Phosphorus was found to be the predominant limiting nutrient to phytoplankton growth. NEB clearly showed primary phosphorus limitation except HWA station (near the mouth of Kumho River Tributary). The Naktong river has been fertilized and highly eutrophicated with nutrient discharge from the Kumho River Tributary. Such nutrient fertility sharply decreased toward the lower part of the river due to the biological uptake. NH_4 was predominantly utilized as the nitrogen source for phytoplankton growth. We concluded that P is the primary limiting nutrient because the Naktong River has the excessive loading and large supplies of ammonia which can be utilized for phytoplankton growth when the water is enriched with P. Considered the N ·P decrease in the culture media as the biological uptake, algal uptakes (㎍ N or P/㎍ chl-a/day) represented 1.1∼8.0㎍ NH_4-N and 0.10∼0.34㎍ PO_4-P respectively. Furthermore, NH_4 ·PO_4 uptake by heterotrophic bacteria were estimated to be 13∼42% and 7∼13% of total uptakes. Bacteria would make profound effects on NH_4 variation in the Naktong River.