Subtropical stormwater ponds are more frequently net nitrogen fixing compared to natural ponds

Urban stormwater ponds (SWPs) are engineered ecosystems designed to prevent flooding and protect downstream ecosystems by retaining nutrients associated with stormwater runoff, including nitrogen (N). Despite these expectations, multiple studies have found that SWPs have low N removal efficiencies and can be sources of N to downstream ecosystems. To understand mechanisms controlling the fate of N in SWPs, we quantified dinitrogen (N₂) gas saturation to characterize net N₂ exchange as either net denitrification or net N-fixation. We assessed temporal and spatial patterns of N₂ dynamics in fifteen SWPs and six naturally occurring ponds in undisturbed watersheds (Florida, USA) by sampling in two seasons (dry and wet) and from multiple depths of the water column. Samples from SWPs were equally likely to exhibit N₂ supersaturation (net denitrification; 50%) or undersaturation (net N-fixation; 50%). In contrast, the majority (82%) of samples from natural ponds were supersaturated with N₂, indicating net denitrification. The mean SWP air–water N₂ flux was − 1.7 μg N₂-N m⁻² h⁻¹ (range − 500 to 433 μg N₂-N m⁻² h⁻¹), which was lower than clear (40 μg N₂-N m⁻² h⁻¹; range − 68 to 74 μg N₂-N m⁻² h⁻¹) and humic (202 μg N₂-N m⁻² h⁻¹; range 41 to 407 μg N₂-N m⁻² h⁻¹) natural ponds despite considerably higher variation in SWPs. These results indicate that SWPs may have low N removal efficiencies in part due to N-fixation adding new N to the system. Overall, this study shows that SWPs are less effective than natural ponds at removing reactive N from the environment, potentially impacting downstream water quality.