To understand the effect of cyanobacterial blooms on nitrogen transformations in eutrophic Lake Taihu, the variation in nitrate concentration during a bloom was observed in a field simulation experiment. This result showed that the cyanobacterial bloom might cause nitrate depletion in an aquatic ecosystem. To further investigate this field result, nitrate transformations after the addition of cyanobacteria collected from Lake Taihu were traced in laboratory microcosms with the ¹⁵N isotope addition method. About 81.2% and 98.4% of nitrate was lost when 2 × 10⁹ and 4 × 10⁹ cells L⁻¹ of cyanobacteria were added, respectively. The nitrate concentration decrease followed first-order kinetics with the rate constant 0.23 and 1.41 d⁻¹ in the two treatments, respectively. Conventional denitrification was found to play a major role in the nitrate removal process while other pathways (e.g., dissimilatory nitrate reduction to ammonium [DNRA] and assimilation of nitrate into microbial biomass) were negligible. It was likely that the cyanobacterial respiration as well as their decomposition resulted in anoxic conditions and that cyanobacteria also served as a carbon source for denitrification. Based on the above results, it was estimated that 10⁹ cells of cyanobacteria were enough for denitrifiers to remove 0.53 mmol nitrate in the eutrophic lake ecosystem. Therefore, cyanobacterial blooms have the potential for nitrogen removal by denitrification and this process could cause nitrogen limitation of primary production in summer in Lake Taihu. This interactive relationship of nitrogen and cyanobacteria potentially constitutes a negative feedback for mitigating cyanobacterial blooms in this eutrophic lake environment.