Floating Wetlands beyond Retention Ponds: Estimating Nitrogen Cycling and Removal in Tidal Waters

Efforts to meet nutrient reduction goals in the Chesapeake Bay watershed involve a variety of practices that remove nutrients before they are delivered to estuarine systems. Floating treatment wetlands (FTWs) have been a certified best management practice (BMP) implemented for decades in retention ponds and wastewater treatment plants but have only recently been placed in tidal waters to remove nutrients directly. Given the paucity of data on nitrogen (N) cycling within tidal FTWs, we measured N transformation and removal within FTWs deployed in estuarine-like mesocosms during a 12-week experiment in summer 2019 and a 10-week experiment in spring 2021. Results indicate a comparable, net removal of total N (TN) for both summer 2019 and spring 2021, but substantial transformations of nitrogen within the FTW. Nitrate + nitrite (NO2+3-) was generated while ammonium (NH4+) and particulate N (PN) were removed from the mesocosm. Nitrogen concentrations measured in different parts of the mesocosms and wetland media also indicate signs of transformation, where NO2+3- concentrations were 0.2 mg L-1 higher in the media porewater than the inflowing water for both control and experimental mesocosms. This suggests relatively high rates of nitrification within the media. This nitrification could support measured denitrification rates (2.4 mg N2-N m2h-1 – 10.9 mg N2-N m2h-1), which were at least 4 times higher than in oligohaline marshes and almost half the rates reported in restored oyster reefs that are also considered a BMP. This research has shown that floating wetlands have the capability to transform and remove N in estuarine-like environments, potentially expanding the areas in which floating wetlands can be deployed. Furthermore, this study provides measurements of these transformation and removal rates to inform future estimates of impact and remediation following FTW use in estuarine environments.