Quantifying Summer Internal Phosphorus Loading in Large Lakes across the United States

Area of Science:

• Environmental Science
• Limnology
• Water Quality Management

Background:

• Internal phosphorus loading (IPL) is a major phosphorus source in freshwater systems, delaying water quality improvements.
• Estimating IPL is challenging due to measurement and modeling difficulties, leaving many systems unassessed.
• Phosphorus (P) is a key nutrient driving eutrophication in aquatic ecosystems.

Purpose of the Study:

• To develop a modeling framework for estimating summer anoxic sediment release rates (SRRs) of phosphorus.
• To assess IPL across 5899 large lakes and reservoirs in the contiguous US (CONUS).
• To identify regions where IPL is a critical factor for nutrient management.

Main Methods:

• Developed a framework combining random forest models for bottom-water temperature and total phosphorus (TP) with a mixed-effects regression model for SRR.
• Incorporated uncertainty propagation across all model components.
• Analyzed data from 5899 lakes and reservoirs (surface area > 1.0 km², mixing depth < maximum depth) across the CONUS.

Main Results:

• Mean summer SRR ranged from 1 to 37 mg/m²/day across CONUS lakes.
• 31% of waterbodies exhibited SRR > 10 mg/m²/day, often linked to high predicted surface-water TP, especially in agricultural regions.
• In dry summers, IPL exceeded external loading in 26% of watersheds.

Conclusions:

• The developed framework provides crucial estimates for IPL, addressing a significant data gap.
• High SRR is prevalent in agricultural areas, highlighting the need for targeted watershed management.
• IPL can be a dominant phosphorus source, necessitating its consideration in water quality improvement strategies.