Purpose <p>This research has three objectives: (1) to elucidate the phosphorus (P) sink–source processes of sediments during cyanobacterial decomposition and growth; (2) to uncover the mechanisms governing the transition of sediments from P sinks to P sources; and (3) to identify the implications of the sink–source processes and controlling mechanisms for P management in shallow lakes.</p> Materials and methods <p>Two sets of experiments were carried out. (1) Sediments and ferrihydrite were added to the culture during cyanobacterial decomposition to characterize their roles as P sinks. (2) Sediments and ferrihydrite with adsorbed P were added to the culture as the sole P sources during cyanobacterial growth to assess their roles as P sources. The culture without the addition of sediment or ferrihydrite served as the control.</p> Results and discussion <p>In the control group, dissolved inorganic phosphorus (DIP) was the dominant form of P, accounting for over 90% of the total dissolved phosphorus. In the sediment or ferrihydrite treatments, most of the DIP was adsorbed onto the sediments or ferrihydrite. Here, Fe/Al oxides functioned as a sink for the released P. When sediments or ferrihydrite-adsorbed P was used as the sole P source, the maximum cyanobacterial cell yield was linearly correlated with the amount of P adsorbed by the sediments or ferrihydrite.</p> Conclusion <p>The desorption and adsorption of P on Fe/Al oxides at surface sediments plays a crucial role in regulating P “sink–source” processes of sediments in shallow lakes, accelerating P cycling even under aerobic condition.</p>