Abstract: Nitrogen and phosphorus are limiting and essential macronutrients to maintain the growth of phytoplankton, and monitoring their changes are fundamental to lake management. The relationship of TN and phytoplankton chlorophyll a (Chl. a) was reported in some research; however, TN may be underestimated in some research due to measurement errors in bloom-occurring water bodies. The alkaline potassium persulfate digestion-UV spectrophotometry method is widely used for water quality determination for TN. The organic nitrogen in cyanobacterial cells may be difficult to be completely degraded, which should result in the underestimation of TN. However, there is no detailed study on TN determination of bloom water containing high-density algal cells.The TN was underestimated by directly measuring the samples from Taihu Lake, although the TN was within the allowable concentration range of the standard method (<7.0 mg/L, HJ636-2012). In order to explore the effect of cell densities on TN determination, the lab-cultured cyanobacterium and green algae was measured. The results showed that TN of high-density cells was significantly underestimated. The density of cells must be less than 10⁸/L to meet the requirements of accurate TN determination. We measured several samples from natural blooms to estimate the TN in the Microcystis blooms. The TN value obtained from the original water sample was underestimated by 26.59%—40.42% compared to the measured value of the water sample diluted 10—30 times. Our results demonstrated that sufficient dilution is necessary for the TN measuring of cyanobacterial blooms.In order to estimate the contribution of Microcystis biomass to TN, the TN and Chl. a in four bloom samples collected from Taihu Lake were measured. The mean value was 0.15 mg N/μg Chl. a (spiked recovery rate was 98.84%—109.39%). The average nitrogen content of dry Microcystis blooms from Taihu Lake was about 16%. These data should be useful for the ecological research of and the control of Microcystis blooms.