Tracking phosphorus dynamics: Historical and future trends in eight Lake Erie tributaries

Abstract. Phosphorus (P) pollution in Lake Erie has been a growing concern, yet a comprehensive understanding of long-term P loss patterns is still lacking. We analyzed annual, monthly, and extreme daily P loss trends from 1974 to 2021 across eight major P-contributing tributaries using Seasonal Trend Decomposition with Loess (STL) and Generalized Extreme Value (GEV) models, and projected P loads through 2040 using machine learning (Bagging). Our results indicate no clear reduction in P loading from these tributaries over the past 47 years. Since the late 1980s, soluble reactive P (SRP) loads in the Sandusky and Maumee Rivers have increased from 0.12 and 0.67 tons day^-1 to 0.41 and 1.55 tons day^-1, respectively, with an increasing trend observed between January and June. We found that molar total nitrogen (N) to total P (TP) ratios in most tributaries, except for the Portage River, were generally 2–3 times higher than in the 1970s. Despite increased annual P loads, our analysis indicates a decline trend in daily P loads in the Maumee River during extreme flow events, except at the 2-year flow level, where daily TP tended to increase from 1.4 to 1.7 tons day^-1 and SRP tended to increase from 0.2 to 0.3 tons day^-1. Our future projections suggest that tributary P loads will continue to exceed target thresholds. In conclusion, this study addresses knowledge gaps in understanding long-term P dynamics in Lake Erie and highlights the need for more site-specific research to safeguard its water quality.