Phosphorus Legacies and Water Quality Trajectories Across Canada

Abstract

Phosphorus (P) pollution in freshwater is a critical environmental issue, primarily driven by agricultural runoff, wastewater discharge, and industrial effluents. Across Canada, lakes such as Lake Erie and Lake Winnipeg experience severe and persistent algal blooms driven mainly due to excess phosphorus loading. Excessive phosphorus loading leads to eutrophication, which causes harmful algal blooms and hypoxia which disrupt aquatic life, reduce biodiversity, and impair water quality, making human consumption and recreational activities unsafe. Despite policies aimed at reducing phosphorus loading, such as improved farming practices and wastewater treatment upgrades, we have not seen a marked decrease in riverine loads. Phosphorus management goals often fall short due to the persistence of legacies – phosphorus that has accumulated in soils and sediments over decades of agricultural applications – which continue to release phosphorus into water bodies for decades after its initial application. Despite recognizing the existence and significant regional and global impact of legacy P on water quality and aquatic ecosystems, our understanding of the magnitude and spatial distribution of these P stores remains limited. Understanding the legacy P stores and their contributors is crucial for efficiently managing water quality, highlighting the importance of studying these factors to develop more effective and sustainable management strategies. The central theme of this thesis is the exploration of the phosphorus legacy across various landscapes. My work has three objectives. First, I explore phosphorus legacies and water quality trajectories across the Lake Erie basin. Second, I quantify various legacy P stores and evaluate their current and future impacts on water quality. Third, I quantified phosphorus accumulation for Pan Canada. In the first objective, I develop a comprehensive phosphorus budget for the Lake Erie Basin, a 60,000 km² transboundary region between the U.S. and Canada, by collecting, harmonizing, and synthesizing agricultural, climate, and population data. The phosphorus inputs included fertilizer, livestock manure, human waste, detergents, and atmospheric deposition, while outputs focused on crop and pasture uptake, covering a historical period from 1930 to 2016. The budget allowed us to calculate excess phosphorus as phosphorus surplus– surplus defined as the difference between P inputs and non-hydrological exports. A random forest model was then employed to describe in-stream phosphorus export as a function of cumulative P surplus and streamflow. The results indicated a significant accumulation of legacy P in the watersheds of the Lake Erie Basin. Notably, higher legacy P accumulation corresponded strongly with greater manure inputs (R²=0.46, p < 0.05), whereas fertilizer inputs showed a weaker relationship. For the second objective, I model the long-term nutrient dynamics of phosphorus across 45 watersheds in the Lake Erie basin using the ELEMeNT-P model. This aimed to quantify legacy phosphorus accumulation and depletion across different landscape compartments, including soils, landfills, reservoirs, and riparian zones, and to assess the potential for phosphorus load reductions under future management scenarios. The model sought to identify key legacy phosphorus pools and explore the feasibility of achieving significant reductions in phosphorus loading, with results indicating that 40% reductions are attainable only through aggressive management efforts. For the last objective, I develop a high-resolution phosphorus budget dataset for Canada, spanning the years 1961 to 2021, at both county and 250-meter spatial scales. This dataset aimed to capture phosphorus inputs from fertilizers, manure, and domestic waste, along with phosphorus uptake by crops and pastureland, across all ten provinces. With this dataset, I aim to better understand the state and progress of phosphorus management across space and time. The results reveal significant variation in P surplus attributable to differences in land use and management practices. The highest surpluses were observed in southern Ontario and Quebec, with approximately 50 kilotons in 2021, contributing to an accumulation of over 2 tera tons of phosphorus over the past 60 years.