Sediment storage of phosphorus in a northern prairie wetland receiving municipal and agro-industrial wastewater

Abstract

The ability of wetlands to function as sinks for phosphorus (P) has been the subject of much debate. We measured the ability of a 1246 ha restored northern prairie wetland to store P from beef slaughter and municipal sewage wastewater. Sediment cores were collected from the Frank Lake marsh to quantify P accumulation, sedimentation rates and sediment adsorption ability. Approximately 60% of P inputs into the marsh since restoration in 1990 have been stored in the sediments (79 662 kg out of 141 760 kg applied). Sites near the wastewater inflow had greater sedimentation rates (3.0 cm year −1) and P burial rates (38.5 g P m −2 year −1) than other sites across the marsh (24 g P m −2 year −1). Surface sediments from the marsh and reference wetlands were collected and spiked experimentally with 25–500 μg l −1 P (as NaH 2PO 4 3−) to determine the ability of the sediments to take up additional P. Sorption isotherms showed that the sediments near the inflow had a limited ability for additional P-sorption. When exposed to 500 μg l −1 P, inflow sites sorbed a maximum of 1000 μg P g sediment −1. In contrast, the rest of the sites in the marsh sorbed up to 1700 μg P g sediment −1, while nearby reference wetland sites sorbed more than 2500 μg P g sediment −1. Approximately 66% of the marsh sediments still had high sorption ability. Inflow sites had a reduced ability for additional P uptake due to the high P loadings applied to that area. Frank Lake has provided effective P retention, however, future treatment efficacy may decrease if the remaining sediments become saturated. Continued high P loading to the marsh may lead to eutrophication problems and downstream P export from the wetland.