Beyond the Mass Balance: Watershed Phosphorus Legacies and the Evolution of the Current Water Quality Policy Challenge.

Increased use of phosphorus (P) fertilizers and detergents and intensified livestock production have more than doubled P inputs to human‐impacted watersheds over pre‐industrial levels. While P fertilizer use and manure application help to maximize crop yields, excess P is lost to runoff, leading to eutrophication of downstream waters. Excess P also accumulates across the landscape, leading to legacies that serve as long‐term sources of P to surface waters, even after inputs to the watershed are reduced. Here, we have developed, for the first time, a process‐based model, Exploration of Long‐tErM Nutrient Trajectories‐Phosphorus, designed to capture legacy P accumulation and depletion trajectories along the land‐aquatic continuum. To drive the model, we have developed a more than 100‐year trajectory of watershed P inputs to the Grand River Watershed (GRW), Canada's largest watershed draining directly to Lake Erie. Our results first show that net P inputs to the watershed approximately tripled between 1900 and the late‐1970s, when P surplus magnitudes peaked at approximately 15 kg ha−1 y−1. During this same period, stream P loads have increased more than fourfold, from 0.11 kg ha−1 y−1 in 1900 to 0.80 kg ha−1 y−1 in the 1970s. Since 1900, the GRW has served as a net P sink, with approximately 96% of net P inputs having been retained within the basin. Future simulations suggest that while 40% reductions in P loading in Lake Erie watersheds are possible under aggressive management scenarios, legacy P will continue to elevate P loads to Lake Erie for many decades to come. Plain Language Summary: Phosphorus is a major driver of algal blooms and water quality challenges throughout the Great Lakes region. Excess phosphorus from crop and livestock production is a particular problem in agricultural areas, and much of the phosphorus applied to the landscape is accumulating as legacy within our watersheds. We have developed a model demonstrating that watershed phosphorus legacies continue to degrade water quality over many decades and can make it difficult to meet water quality goals. Our modeling results show that we must take legacy phosphorus into account when setting policy goals for water quality improvement. Our results also show that better manure management can provide relatively fast improvements and should therefore be emphasized when implementing conservation measures. Key Points: More than 95% of surplus phosphorus has been retained within the watershed since 1900Legacy phosphorus is a major contributor to current watershed phosphorus loadsReducing manure handling losses can provide substantial reductions in watershed P loads [ABSTRACT FROM AUTHOR]