Long-term sustainable phosphorus (P) retention in a low-P stormwater wetland for Everglades restoration

Highlights: • Whether soil P enrichment could impair treatment wetland performance is an overriding concern. • This SAV-dominated wetland did not become P-saturated under stable operational conditions of low to moderate loadings and continuous hydration. • SAV soils accrued P at a relatively steady rate over the entire 17-yr period. • Close agreement in the soil-water P mass balance confirms the methods used were sufficient to characterize the soils. • Limitations to indefinite P removal are extreme perturbations and accumulation of particles. The Stormwater Treatment Areas (STAs) are large wetlands constructed for phosphorus (P) retention for Everglades restoration in south Florida (USA), and include areas of submerged aquatic vegetation (SAV) at a globally unprecedented scale (~12,000 ha). The goal of this study was to elucidate the fate of P retained in large-scale SAV wetlands, and the associated temporal trends in P removal and retention. In a well-performing, 929-ha SAV-dominated STA surface water flow-through treatment wetland, measurements of accrued soil depth and soil P storage performed every ~4–6 years revealed a steady-state longitudinal soil P enrichment profile established within the first ~4 years of flow-through operation. Subsequently, the SAV soils accrued P at a relatively steady rate (1.13 g P m⁻² yr⁻¹ for the entire 17-year period) without indication of temporal P enrichment, spatial expansion of soil P enrichment in the inflow region, or impairment of water column P removal efficiency. Phosphorus sequestration occurred via accumulation of new sedimentary material (0.9–1.5 cm yr⁻¹), rather than enrichment of existing soil. These soil surveys were accompanied by measurements of porewater SRP concentrations, soil P release under anoxia, and soil P fractions, which demonstrated that soil P release potential and concentrations of highly labile soil P generally decreased over time. These findings demonstrate that the P retention mechanisms operating within this large SAV wetland can be sustainable under managed steady-state conditions. Susceptibility of SAV to extreme environmental perturbations in this and other wetlands, however, remains a research priority.