Dam-driven phosphorus sinks reversed the anticipated increase in phosphorus export from the Yangtze River Basin

Excessive anthropogenic phosphorus inputs create nutrient surpluses along the terrestrial-aquatic continuum, triggering worldwide water quality challenges. River damming enhances phosphorus retention within river networks and redistributes it among reservoirs, rivers, and coastal oceans. However, the role of reservoirs as phosphorus sinks and their effect on phosphorus export to estuaries remain poorly quantified. We developed a coupled terrestrial-aquatic model to trace phosphorus fate in the highly dammed and fertilized Yangtze River Basin from 1980 to 2018. Our results show that cumulative dam-driven phosphorus retention reached $${4.7}_{3.7}^{5.5}$$ teragrams of phosphorus (median and range), exceeding the cumulative rise in phosphorus loading ($${2.1}_{1.3}^{2.9}$$ teragrams of phosphorus), thereby reversing the anticipated rise in phosphorus export to the estuary. Notably, 40% of dam-induced retention occurred during 2011–2018, despite fertilizer use plateauing around 2010. This strengthened inland water phosphorus sink arises not only from increased damming but also from the continuous release of previously stored phosphorus in terrestrial ecosystems, highlighting the key role of legacy effects in controlling the phosphorus temporal dynamics. Dam-assisted phosphorus retention occurs in a considerable amount along the terrestrial aquatic continuum within the Yangtze River Basin, suggesting it be included in terrestrial biogeochemical and watershed models