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.