Legacy phosphorus dynamics in subtropical river sediments: Impacts of dredging and water column aeration status

Excessive loading of phosphorus (P) in coastal systems has been a growing concern for watershed managers due to the link with harmful algal blooms. In particular, legacy P creates a persistent challenge for eutrophication management in a range of aquatic systems, including wetlands, lakes, and estuaries, as a source of indirect, internal P loading. Following reductions in external P loads, internal sediment sources have been known to release bioavailable P back into the water column, undermining nutrient restoration goals. This study investigates the flux dynamics and longevity of sediment legacy P in three subtropical rivers—Amite, Tangipahoa, and Tickfaw—draining into the Lake Pontchartrain estuary. We employed a controlled laboratory incubation using intact and dredged sediment cores subjected to both aerobic and anaerobic treatments to quantify the flux of soluble reactive P over an 8‐week period with regular surface water replacements. The soluble reactive phosphorus (SRP) flux under anaerobic water column conditions was three to five times higher than under aerobic conditions, with the most significant release occurring during the first 4 weeks. Following the fourth week, cores across all treatments observed a significant decrease in the rate of SRP released. Dredged cores showed consistently lower SRP flux across both aerobic and anaerobic treatments. These findings underscore the salient role of redox conditions and recent sediments in the mobilization of legacy P in river networks. Our work provides new evidence of the temporal limitation of internal P loading and the potential for strategic sediment management to complement external nutrient load reduction efforts to improve surface water quality.