Seasonal Variation in Nitrate Removal Mechanisms in Coastal Stormwater Ponds

Stormwater wet ponds (SWPs) are engineered structures used to collect and retain stormwater runoff from developed areas. SWPs are generally regarded as important nitrogen (N) sinks, but seasonal variation in SWP N cycling that influences pond nitrogen removal has not been characterized. To inform SWP function across seasons, we sampled the sediments and water columns of three stormwater ponds in the southeastern US coastal plain and measured gas and nutrient fluxes from the sediment-water interface during ambient conditions and nitrate (NO3)-enriched “simulated storm” conditions. Dissolved organic nitrogen (DON) was the dominant form of dissolved N in the water column, while nitrate + nitrite (NOx) was typically below detection. SWP sediment organic matter properties varied by study site but had minimal impact on sediment N processes or estimated NO3 fate. SWP sediments generally functioned as TN sinks during NO3-enriched conditions, but the estimated fate of NO3 varied based on water temperature, DON concentrations, and sediment O2 uptake. These results suggest that permanent N removal (denitrification) by SWPs varies seasonally, with retention of NOx becoming more important during hotter conditions when NOx uptake is largest. Low ambient NOx concentrations and rapid NO3 uptake suggest that coastal stormwater ponds can host reduced conditions that may promote NO3 retention over denitrification. Additional research is needed to determine the fate of retained NO3 in SWP sediments and how variation in NO3 fate might impact downstream water quality