Do Pulse Events Build a Legacy Phosphorus Reservoir? A Case Study of the Long-Term Fate and Transport of Sediment Phosphorus in a Subtropical Estuary Subject to Repeated River Diversions

Phosphorus (P) delivered to estuaries by pulse events such as river diversions is widely assumed to accumulate as legacy P in receiving sediments, yet its long-term fate remains poorly resolved. Repeated openings of the Bonnet Carré Spillway deliver large sediment, P, and freshwater loads to the Lake Pontchartrain estuary (southern Louisiana), driving harmful algal blooms (HABs) and short-term water-quality decline. To test this, we mapped surface sediment (160 sites), compared it with an earlier survey, and weighed spillway loading (pulse) against tributary (press) loading and internal fluxes over 25 years. Cumulative spillway loading across seven openings (268 days) contributed more total P (TP; 14,100 MT) than 25 years of tributary inflow (13,000 MT), yet change mapping revealed spatially structured redistribution of TP away from the spillway toward the southern and western boundaries rather than basin-scale enrichment. Sediment P remained predominantly inorganic (~ 90%), while organic matter doubled from 2003 (3.1%) to 2023 (5.95%), consistent with burial of phytoplankton-derived biomass following spillway-driven blooms. Advective flux dominated internal release, averaging 30.8 mg P m − 2 d − 1 after wind events exceeding 6 m s − 1 versus 0.13 mg P m − 2 d − 1 for diffusion. Together these results support a mass balance in which spillway-delivered P is transiently retained, then redistributed, partly sequestered as organic P, and partly exported to the coastal ocean by wind-driven flushing. Sediment mapping alone is therefore insufficient to track legacy P under pulse loading; watershed-scale loading budgets coupled with internal release-rate measurements are needed to resolve the fate of diverted nutrients.