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United States Department of Agriculture

Agricultural Research Service

Title: Dissolved Phosphorus Retention and Release from a Coastal Plain in-Stream Wetland

item Novak, Jeffrey
item Stone, Kenneth
item Szogi, Ariel
item Watts, Donald
item Johnson, Melvin

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 10, 2003
Publication Date: January 20, 2004

Interpretive Summary: Runoff from agricultural areas and water discharges from urban sources can transport phosphorus (P) into surface water systems. It is important that low P concentrations occur in these surface water systems to maintain a healthy aquatic habitat. Otherwise, high P concentrations can increase the growth of aquatic weeds and harmful algae blooms. Because these surface water systems will flow through a network of forested wetlands, P concentrations can be reduced by plant uptake. Although wetlands can reduce P concentrations, they are drained to provide water for irrigation and other needs. Draining wetlands will disturb plant growth conditions, which could result in high surface water P concentrations and a reduction in water quality. Our goal was to determine the effects of draining and flooding a wetland on P concentrations in a stream that flows through the wetland. This stream collects drainage from land that has received P from fertilizer and animal manure sources. We measured stream P concentrations at the inlet and outlet of the in-stream wetland over a 4-year period. In the first year, we determined that the wetland was too small to effectively reduce P concentrations. During the second and third years, the wetland¿s area was enlarged by flooding in anticipation of more P removal. We found that flooding the wetland resulted in lower P concentrations in the stream outflow. In the fourth year, the owner of the wetland required water for crop irrigation, so the wetland¿s area was reduced by draining. Draining the wetland resulted in higher P concentrations in the outflowing stream. Our results suggest that for this wetland to effectively reduce stream P concentrations, it should not be drained.

Technical Abstract: Wetlands systems can retain phosphorus (P) depending on their P assimilation capacities. P-saturated wetlands can release dissolved phosphorus (DP) across the sediment-water interface as a result of flooding and/or fluctuations in DP concentrations. Our objectives were to determine the long-term (1460 days) DP retention and release characteristics of an in-stream wetland, and to evaluate how these characteristics respond to flooding, draining and changes in DP concentrations. The studied in-stream wetland drains an agriculturally intensive subwatershed in the NC Coastal Plain region. The in-stream wetland DP retention and release characteristics were evaluated by measuring inflow and outflow DP concentrations, DP mass balance, and DP movement across the sediment-water column interface. P-sorption isotherms were measured to determine the sediments equilibria P concentration (EPC) and passive samplers were used to measure DP concentrations in sediment pore water. Initially, the in-stream wetland was undersized (0.31 ha) and released 1.5 kg of DP. Increasing the in-stream wetland area to 0.67 ha by flooding resulted in more DP retention (28 kg) and low outflow DP concentrations. Draining the in-stream wetland from 0.67 to 0.33 ha caused the release of stored DP (12.1 kg). Shifts in sediment pore water DP concentrations and sediment EPC values both support the release of stored DP. Re-flooding the in-stream wetland from 0.33 to 0.85 ha caused additional release of stored DP into the outflowing stream (10.9 kg). We conclude that, for a time period, this DP-laden in-stream wetland did provide DP retention. However, management-induced variations in wetland area, rainfall, and changes in DP concentrations during other times resulted in DP releases. This indicates that the in-stream wetland did not provide long-term effective DP retention.

Last Modified: 4/21/2015
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