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

Abstract

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.