Changing phosphorus dynamics across subtropical river networks in an urban watershed: insights from a 25-year record under a changing climate

Phosphorus (P) in aquatic systems is a key nutrient for primary productivity and has become increasingly important in global studies of water quality and eutrophication in coastal systems. This study examines P dynamics and watershed changes in four major tributaries of the Lake Pontchartrain estuary: the Amite, Tangipahoa, Tickfaw, and Tchefuncte rivers. Long-term monitoring data (2000–2024) and a targeted 2024 field campaign were utilized to evaluate nutrient concentrations, discharge, land-use changes, and climate patterns across the watersheds. Mean discharge ranged from 81.6 ± 201 m 3 s⁻ 1 in the Amite to 8.5 ± 29.1 m 3 s⁻ 1 in the Tchefuncte, with winter–spring flows accounting for 58–66% of annual totals. Total phosphorus (TP) concentrations averaged 0.15 ± 0.06 mg L⁻ 1 in the Amite and 0.11 ± 0.04 mg L⁻ 1 in the three remaining rivers. Nitrate (NOx; NO 2 + NO 3 ) was greatest in the Tangipahoa (0.25 ± 0.07mg L⁻ 1 ) compared to 0.16 ± 0.07 mg L⁻ 1 average across the three other rivers. Across all samples from the 2024 field sampling year, soluble reactive phosphorus (SRP) comprised an average of 57% of TP during the 2024 observational study, remaining constant at 58% during dry months relative to 52% in wet months, and accounted for 90% of total dissolved phosphorus (TDP) during the observational year. Land-use analysis (2000–2024) revealed a 12% increase of development land area, alongside declines in shrubland (–22%), pasture (−18%), and wetlands (−2%). El Niño-Southern Oscillation (ENSO) variability influenced discharge and precipitation patterns, with El Niño years characterized by 27% greater precipitation and elevated P and NOx loading compared to neutral years, and La Niña years associated with relatively lower flow dry years and a reversal of the typical relative wet and dry seasons. These results are consistent with the notion that P transport is temporally correlated with both hydrologic variability and watershed development. Additionally, the prevalence of SRP as the primary dissolved P fraction indicates a potential for rapid mobilization of bioavailable P under changing watershed and climate conditions.