Stormwater ponds: Unaccounted environmental challenges of a widely-adopted best management practice in urban landscapes.

Stormwater ponds (SWPs) are an increasingly common management tool for flood control and water quality protection in urban areas. They are designed to buffer the impacts to downstream environments caused by altered hydrologic, chemical, biological, and ecological processes in developed watersheds. While small in size, they can have disproportionately large impacts on watersheds because they store, transform, and release inputs of carbon (C) and nutrients, mainly nitrogen (N) and phosphorus (P). In this review, we discuss how SWPs are not passive conveyances of nutrients and C, where minimal internal processing occurs. Rather, they are active hotspots of biogeochemical processing, with implications for downstream water quality protection. We highlight how processes of assimilation, sedimentation, erosion, filtration, remineralization and remobilization, gaseous transformations, and the activities of living organisms all transform nutrients and C in SWPs, sometimes making ponds net exporters of nutrients, rather than net sinks or removers, as is often believed. There are numerous unaccounted challenges in SWP management, such as in-pond processes that decouple pond effluent and influent quality; that sedimentation often fails as a proxy indicator for nutrient removal; how optimizing for removal of one nutrient (nitrogen or phosphorus) may reduce removal efficiencies of the other; or how nutrient removal strategies may be at odds with strategies to mitigate greenhouse gas emissions from SWPs. Our goal is to show that SWPs play large roles in constraining and mediating the fluxes of materials and energy in urban ecosystems and that their effluent water quality is driven not only by inflowing water quality but largely also by in-pond processes that warrant increased future research.