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

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 (NH4) averaged 21 g N m−2 yr−1. Release of NH4 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.