Nitrogen dynamics of a large-scale constructed wetland used to remove excess nitrogen from eutrophic lake water

Abstract Nitrogen loss from landscapes affects downstream freshwater and marine systems. Constructed wetlands, which are ecologically-engineered systems, can mitigate nitrogen loss from various landscape types. We examined the efficacy of a large-scale constructed wetland (the marsh flow-way at Lake Apopka) to remove particulate nitrogen and other nitrogen species from eutrophic lake water. During the first seven years of operation, the hydraulic loading rate to the flow-way was high (mean ± SD; 34 ± 10 m yr −1 ). Inflow concentrations of total nitrogen (TN) were also high and variable (mean ± SD; 4 ± 1 mg L −1 ). Both hydraulic loading rate and concentration of N species were affected by variations in lake level. Inflow nitrogen was mostly in particulate forms (particulate organic nitrogen [PON] was 58% of TN). Subsequently, the dominant biogeochemical mechanism for nitrogen removal was sedimentation of PON. Average annual removal rates were 56 g PON m −2  yr −1 , while TN and total Kjeldahl nitrogen removal rates were similar, about 30 g N m −2  yr −1 . Nitrogen removal was seasonally dependent, with greatest removal rates occurring during cool periods (October through May). Although the marsh flow-way showed substantial removal of particulate N, the wetland released dissolved inorganic N fractions. Release of ammonium (NH 4 ) averaged 21 g N m −2  yr −1 . Release of NH 4 was greatest during warm periods (June through September), which was probably related to increased water temperature, associated increased decomposition, and decreased dissolved oxygen content of wetland waters. Long-term first order rate constants ( k ) for removal were also high. Annual k values for PON were 85 m yr −1 . Managing the marsh flow-way to maximize particulate N removal in the long-term, while mitigating the release of inorganic N during warm periods, will require dynamic wetland management.