Data for: Internal Loading in Stormwater Ponds as a Phosphorus Source to Downstream Waters

Abstract

Stormwater ponds remove phosphorus through sedimentation before releasing captured water downstream. Internal loading can impair net phosphorus removal but is understudied in these highly modified systems. Using a combination of methods, we assessed the prevalence and potential causes of sediment phosphorus release in urban ponds. In a three-year, 98-pond dataset, nearly 40% of ponds had median water column total phosphorus concentrations exceeding the 95% confidence interval for runoff values (0.38 mg/L), suggesting widespread internal loading. In a subsequent intensive monitoring study of four ponds, strong stratification prevented spring and summer diurnal mixing, resulting in persistent hypolimnion anoxia (<1 mg/L dissolved oxygen). Incubated sediment cores from seven ponds demonstrated high anoxic phosphorus release. Sediment analysis revealed high labile organic and redox-sensitive phosphorus fractions with release potential at anoxia onset. Our analyses suggest phosphorus accumulated in stormwater ponds is highly sensitive to internal loading, reducing net removal and contributing to downstream eutrophication.