Water nitrogen and phosphorus may exhibit pronounced diel changes as affected by diel temperature and DO fluctuations due to algal productivity (photosynthesis and respiration) in eutrophic lakes. Here, we used a laboratory experiment to evaluate the diel dynamics of water ammonium (NH₄⁺), nitrate (NO₃^-), and soluble reactive phosphorus (SRP) in different periods of algal bloom in the highly polluted region of Taihu Lake, China. Results showed that diel changes of water NH₄⁺, NO₃^-, and SRP depended on the different periods of algal bloom. Diel variations of NH₄⁺, NO₃^-, and SRP showed increased trends during algal decomposition, and decreased trends at the beginning of algal growth, suggesting that their diel variations were mainly controlled by algae self through nutrients released by decomposing algae or algal assimilation. However, water NH₄⁺, NO₃^-, and SRP decreased from predawn maxima to afternoon minima and subsequently increased when Chl-a was high, which might be governed by the combined effects of algal assimilation and geochemical processes (e.g., nitrification, denitrification, and Fe-bound P cycles). Overall, the diel biogeochemical cycles of inorganic nitrogen and SRP have important implications for the accuracy of pollution assessment and provide important support for the control and management of eutrophic lakes.