The water quality response to implementation of conservation measures across watersheds has been slower and smaller than expected. This has led many to question the efficacy of these measures and to call for stricter land and nutrient management strategies. In many cases, this limited response has been due to the legacies of past management activities, where sinks and stores of P along the land–freshwater continuum mask the effects of reductions in edge‐of‐field losses of P. Accounting for legacy P along this continuum is important to correctly apportion sources and to develop successful watershed remediation. In this study, we examined the drivers of legacy P at the watershed scale, specifically in relation to the physical cascades and biogeochemical spirals of P along the continuum from soils to rivers and lakes and via surface and subsurface flow pathways. Terrestrial P legacies encompass prior nutrient and land management activities that have built up soil P to levels that exceed crop requirements and modified the connectivity between terrestrial P sources and fluvial transport. River and lake P legacies encompass a range of processes that control retention and remobilization of P, and these are linked to water and sediment residence times. We provide case studies that highlight the major processes and varying timescales across which legacy P continues to contribute P to receiving waters and undermine restoration efforts, and we discuss how these P legacies could be managed in future conservation programs.

Abbreviations

BMP best management practice

Conclusions The legacies of previous management and the remobilization of P stored in land, river, and lake systems are clearly important issues. Because of the lag time between BMP implementation and water quality improvements, remedial strategies should consider the time necessary for re‐equilibration of watersheds and water bodies, where nutrient sinks may become sources of P with only slight changes in watershed management and hydrologic response. Clearly, hotspots of P accumulation and legacy across watersheds are subject to complex interactions of hydraulics, hydrology, geomorphology, and land management. It will take time to address the P legacy at the field or headwater scale to “cascade” and “spiral” down to the larger watershed scale. The lags in water‐quality response to BMP implementation at the downstream watershed scale will be determined by the key rate‐limiting (slowest responding) parts of the system. Furthermore, environmental change (e.g., impacts of climate change on C cycles and hydrology) will have important implications for recovery, because of shifting baselines in hydrologic response, for instance. A better understanding of the spatial and temporal aspects of watershed response to nutrient load reductions in both flowing and standing water bodies is needed, as well as an understanding of the scale at which responses may occur in a more timely fashion. It is likely that local water quality and quantity benefits may become evident more quickly at a smaller subwatershed scale. This is an important outcome in itself to help demonstrate change and foster accountability and ultimately wider adoption of conservation practices. As shown in the Coastal Plain poultry litter experiments, however, even at smaller scales, improvements at the field level may not immediately translate to the subwatershed. It is also important to accept in any watershed‐P loss reduction strategy that it is essential to address the overall physical and social complexity of individual systems and the mitigation of non‐agricultural sources of P. Only this will bring about lasting improvements in water quality as evidenced under all hydrologic (storm and non‐storm) conditions.