Floating treatment wetland nutrient removal through vegetation harvest and observations from a field study

• A six-month in-situ FTW experiment was conducted in an urban retention pond.
• We evaluate temporal changes of plant biomass and phosphorus distribution in FTWs.
• We provide suggestions to optimize phosphorus removal by plant harvest.
• We evaluate sustainability of plants with low DO and ice encasement stresses.
• We document observed wildlife species and activities associated with the FTW.

Nonpoint source pollution from urban areas has been identified as a leading contributor to impaired water quality. Floating treatment wetlands (FTWs) are cultivated plants growing on floating mats in open water. FTWs can be used to remove pollutants from runoff, but data on their effectiveness is limited. We conducted a field study of FTWs in a nutrient enriched urban wet pond to investigate vegetation biomass and phosphorus (P) accumulation/distribution, sustainability under ice encasement stress (which is a concern in temperate regions), and to assess the use of the FTW by species. Planted perennial macrophytes successfully adapted to stresses of the low dissolved oxygen (DO) concentrations (minimum: 1.2 mg/L) in summer, ice encasement in winter, and relatively low nutrient concentrations in the water (median: 0.15 mg/L TP and 1.15 mg/L TN). Pickerelweed produced more biomass and demonstrated higher P removal performance (10.44 g dry weight/plant and 7.58 mg P/plant) than softstem bulrush (2.20 g dry weight/plant and 1.62 mg P/plant). Based on the observed seasonal changes in biomass and P, we recommend harvest of above-ground vegetation is conducted in June for maximum P removal or in September to prevent P release due to senescence. Submerged tissues of pickerelweed, softstem bulrush, and yellow iris (Iris pseudacorus) survived ice encasement and regrew in the second year. Additionally, plant diversity increased during the study period through recruitment of both native and exotic wetland plants. Systematic observation of wildlife activities indicated eight classes of organisms inhabiting, foraging, breeding, nursing, or resting in the FTWs. This study suggests above-ground plant harvest can enhance P removal, and that softstem bulrush, yellow iris, and pickerelweed can be sustained over winter on the FTW. Future study is recommended to investigate the feasibility of multiple vegetation harvest and document the possible habitat creation by the use of FTWs.