Transition of phosphorus balance steady-state redirects the relationship of phosphorus mediums in Poyang Lake, China

• Phosphorus balance and retention affected phosphorus variation in aquatic and sediment. • The intervals between quasi-balance states became shorter after transition. • Sediment redirects as a source from net sink of phosphorus for water body in quasi-balance state (2014). • Phosphorus retention and sediment burial redirect to synergism from antagonism on increasing of aquatic phosphorus. The phosphorus fate is critical to aquatic environment and ecosystem in lake. The relationship of phosphorus distribution in different mediums remains unknown under the fluctuation of phosphorus balance steady-state in lake. This study investigates the phosphorus balance of import and export in the Poyang Lake from 1990 to 2019, uncovering the transitional process of phosphorus balance steady-state and its influence on the phosphorus distribution relationship in different mediums. Our finding demonstrates that the phosphorus balance steady-state underwent four transitions during the study period of 1990–2019, resulting in a more fragile lake environment and ecosystem, with shorter intervals between quasi-balance states in Poyang Lake. The rate of sediment phosphorus burial and net phosphorus deposition increased significantly before the third phosphorus balance steady-state (2011–2013). However, the net deposition rate decreased by 5.3% with an increase in aquatic phosphorus after the third transition of phosphorus balance steady-state (2014) in Poyang Lake. Sediment played a significant role in redirecting from a net phosphorus sink to a net phosphorus source for water environment. The influence of phosphorus retention and sediment burial shifted from antagonism to synergism in increasing phosphorus content of waterbody, leading to a 5.4% rise in potential sources of aquatic phosphorus loading. These findings strengthen our understanding of phosphorus cycling and offer guidance for managing water environments in lakes. This work plays a vital role in regional water quality maintenance and water supply safety.