Shifting nutrient sink and source functions of stormwater detention areas in sub-tropics

- Stormwater detention areas (SDAs) function as both phosphorus (P) source and sink.
- SDAs are steady sinks of nitrogen (N), denitrification is the main chemical process.
- Soil P saturation from long-term inputs of P results in its release in wet years.
- Sink function is mainly due to ≈50% water retention, not the biogeochemical processes.
- Biomass harvesting can mine legacy P to maintain SDAs as a P sink in the long-term.

Two-year field measured water and nutrient fluxes from an agricultural stormwater detention area (SDA) in Florida's Everglades region showed that it was a source of phosphorus (P) for the first year (Y1 retention efficiency = −12%) and a sink for the second year (Y2, 54%). The SDA remained a consistent sink of nitrogen (N). Source function was a combined effect of dilution of incoming drainage from a tropical storm and legacy-based soil P saturation. Denitrification was the main biochemical process contributing to N retention (Y1 = 23%, Y2 = 56%). Volume reduction was the main reason for nutrient retention, especially for P because of limited to no remaining soil P sorption capacity. Although a net sink of P for Y2, an event-wise analysis showed the source function for 40% of the outflow events indicating intermittent P release from soil. Because surface P treatment efficiency during both years was either less than or close to the water retention efficiency, volume reduction and not sorption or biological assimilation controlled P treatment. Almost a third of the incoming P was lost through subsurface pathways, highlighting the significance of groundwater P losses from SDAs. Harvesting and removal of biomass can mine P and restore SDA's sink function. For an example sub-basin of Florida Everglades, potentially harvestable P from SDAs was more than the basin P loads. A payment for environmental services project to treat additional P through biomass harvesting is a sustainable approach, especially under future climate projections of more frequent high-intensity storms for the Everglades and beyond.