Floating wetlands and their potential for nitrogen removal in estuarine waters

Abstract

Nutrient enrichment of estuarine waters remains a problem globally. New efforts have sought to apply principles of ecological engineering to promote nutrient removal within degraded aquatic systems by constructing habitats that may support enhanced nitrogen removal. Several technologies and approaches are certified as best management practices (BMPs) by the Environmental Protection Agency (EPA). These include the restoration of oyster reefs, stream and estuarine wetlands, and riparian buffers, as well as the implementation of wet retention ponds. Each of these habitats has been suggested to be a “hotspot” for nitrogen removal, but they are also biologically and chemically complex environments. Thus, the quantification of their impact on nutrient removal is difficult due to interactions with other processes that influence nitrogen transformation and loss within tidal estuarine waters. Floating wetlands have traditionally been implemented to increase the nutrient removal efficiency in freshwater retention ponds, where nutrient removal is presumed to be accomplished by plant assimilation and sediment deposition. Similar installations of floating wetlands in tidal waters have recently been implemented to achieve the same ecosystem services as in retention ponds, but little is known about their efficacy. In this dissertation, I quantified floating wetland nitrogen removal and cycling through a series of mesocosm experiments and numerical modeling. The mesocosm experiments were carried out for 90 days at the Chesapeake Biological Laboratory in both spring and summer periods during the years 2019, 2021, and 2022. My results indicate that denitrification was the major nitrogen removal process, removing ~4 times as much nitrogen as plant uptake. Denitrification rates were comparable between mesocosms with wetland plants and control mesocosm with only the media, suggesting that denitrification was occurring in microzones within the high-surface area media. Thus, a new formulation to incorporate this high surface area media was necessary to reproduce these high denitrification rates in model representations of the experiments. This dissertation research thus clearly demonstrates that floating treatment wetlands can remove nitrogen at rates that are often higher than subtidal sediments, but future work on their durability and efficacy under a wider range of conditions is needed to better assess their potential to become a certified best management practice tool for nutrient management.