Phosphorus removal in created wetland ponds receiving river overflow

Abstract

Water quality changes and biogeochemical development were evaluated over 2 years in two newly created freshwater riparian wetland ponds (1 ha each) in an agricultural and urban watershed. Both wetlands received pumped river water and had similar hydrologic regimes. One wetland was planted with 13 species of vegetation typical of Midwestern US marshes; the other received no planted vegetation. Water quality sampling was conducted weekly and detailed hydrologic budgets were developed from data collected twice daily. Hydrologic budgets were dominated by pumped surface flows (mean inflow=1480 m 3 day −1). Two floods accounted for 32% of inflow in 1 year. Both wetlands significantly decreased turbidity (62 to 27 NTU) and increased dissolved oxygen (9–11 mg l −1). Inflow dissolved reactive phosphorus (DRP) and total phosphorus (TP) concentrations (17±3 and 169±11 μg P l −1) were significantly higher ( P<0.05) than outflow concentrations (DRP: 5±1 and 6±1 μg P l −1; TP: 69±8 and 74±9 μg P l −1) for planted and unplanted wetlands, respectively. Phosphorus removal was related to decreases in turbidity and the level of biological activity. Extensive and highly productive algal coverage in both wetlands and the subsequent deposition and decomposition of the algal mat influenced P retention through biological uptake and chemical sorption and coprecipitation. Mean removal rates were 1.0 g P m −2 year −1 for DRP and 5.4 g P m −2 year −1 for TP and did not differ significantly between wetlands ( P<0.05). Approximately 35% of TP mass removal occurred during two floods. A conservative tracer (Cl) indicated limited and negligible effects of dilution on decreases in P concentration. Water flow rate and P concentration did not affect P removal which was loading-limited and seasonal. Initial development of macrophytic vegetation demonstrated no influence on water quality changes. Both wetlands acted as effective P sinks in the initial 2 years of operation.