Nutrient retention via sedimentation in a created urban stormwater treatment wetland

Nutrient removal by a 4.6-ha urban stormwater treatment wetland system in a 20-ha water/nature park in southwest Florida has been investigated for several years, suggesting that the wetlands are significant sinks of both phosphorus and nitrogen although with a slightly decreased total phosphorus retention in recent years. This study investigates the role of sedimentation on changes in nutrient concentrations and fluxes through these wetlands. Sedimentation bottles along with sediment nutrient analyses every six months allowed us to estimate gross sedimentation rates of 9.9 ± 0.1 cm yr^-1 and nutrient sedimentation rates of approximately 7.8 g-P m^-2 yr^-1 and 81.7 g-N m^-2 yr^-1. Using a horizon marker method to account for lack of resuspension in the sedimentation bottles suggested that net nutrient retention by sedimentation may be closer to 1.5 g-P m^-2 yr^-1 and 33.2 g-N m^-2 yr^-1. Annual nutrient retention of the wetland system determined from water quality measurements at the inflow and outflow averaged 4.23 g-P m^-2 yr^-1 and 11.91 g-N m^-2 yr^-1, suggesting that sedimentation is a significant pathway for nutrient retention in these urban wetlands and that resuspension is playing a significant role in reintroducing nutrients, especially phosphorus, to the water column. These results also suggest that additional sources of nitrogen not in our current nutrient budgets may be affecting overall nutrient retention.