Shifting regional development scenarios amplify legacy phosphorus threats to water quality.

Legacy phosphorus, accumulated from past anthropogenic activities, poses persistent and complex threats to global water quality. Despite extensive efforts to control phosphorus inputs, legacy phosphorus can persist for decades and undermine restoration goals. Emerging evidence suggests that shifts in regional development patterns profoundly reshape the dynamics and environmental risks of legacy phosphorus accumulation and mobilization. However, the mechanisms by which development pattern shifts reshape legacy phosphorus trajectories remain poorly understood. Here we show the complex pathways linking development-driven land-use changes, biogeochemical buffering capacities, and legacy phosphorus mobilization through an integrative modeling framework that couples developmental shift coefficients, anthropogenic phosphorus inventories, and riverine time-lag modeling to diagnose and predict long-term legacy phosphorus risks. Using the Songhua River as a case study, our results reveal that shifts from industrial to agricultural dominance significantly amplify legacy phosphorus accumulation by 86 times. Consequently, legacy phosphorus accounts for 65.4 %-69.9 %, surpassing current-year inputs and becoming the primary driver of riverine pollution. Furthermore, we demonstrate that development shifts systematically alter the dominant controlling factors, from fossil fuel emissions and drainage infrastructure to soil retention characteristics and agricultural practices, reshaping mitigation priorities. Our framework provides a generalizable methodology for quantifying legacy phosphorus risks under dynamic development patterns, offering immediate applications for water quality management. More broadly, this framework offers critical insights that can guide sustainable management strategies for linking evolving regional development patterns with long-term ecological restoration.