Effectiveness of the lake Okeechobee Basin Management Action Plan in reducing phosphorus loading: A 25‐year assessment of watershed sources, nutrient retention, and management outcomes

Abstract

Lake Okeechobee, the largest freshwater lake in the southeastern United States, has experienced chronic eutrophication driven by excessive phosphorus (P) loading from its 14,000‐km ² watershed. A total maximum daily load (TMDL) of 140 metric tons per year (t year ⁻¹ ) for total P (TP) was established, and the Basin Management Action Plan (BMAP) was adopted in 2014 to coordinate nutrient‐reduction strategies. This study analyzed 25 years (WY2000–WY2024, where WY is water year) of hydrologic and water‐quality data from the South Florida Water Management District (SFWMD) DBHYDRO database to evaluate long‐term trends in TP loading, inflow–outflow mass balances, sub‐watershed contributions, downstream P routing, and management effectiveness. The 5‐year average TP load for WY2020–WY2024 was 390 t, reduced from the peak of 742 t (WY2002–WY2006) but still approximately 250 t above the TMDL. Annual TP loads ranged from 131 t (WY2001) to 1081 t (WY2018), reflecting strong hydrologic control by rainfall variability and storm events. The Lower Kissimmee sub‐watershed contributed 29% of TP loads from only 13% of the drainage area, while Taylor Creek/Nubbin Slough exhibited the highest flow‐weighted mean concentration (340 µg L ⁻¹ ) and contributed 15% of loads from 6% of the inflow. Coordinating‐agency projects removed approximately 37.5 t of TP in WY2024, but approximately 206 t of additional reduction remains necessary. For the first time, the 5‐year average outflow TP concentration exceeded inflow concentrations. Therefore, achieving the TMDL will require spatially targeted interventions in high‐loading sub‐watersheds, expanded best management practices enrollment, scaled‐up treatment infrastructure, and adaptive management that accounts for hydrologic variability of the watershed.