Impact of sediment dredging on sediment phosphorus flux in a restored riparian wetland

Many riverine wetlands have been drained for the creation of agricultural land; however, global declines in freshwater biodiversity have begun to motivate wetland restoration projects around the world. Legacy phosphorus (P) increases the risk that wetland restoration may liberate excess P to the water column and connecting waterbodies, resulting in a trade-off of restored habitat for degraded water quality. To avoid this trade-off, we dredged a former agricultural parcel prior to hydrologic reconnection, and evaluated restoration success by comparing sediment P dynamics before and after dredging. First, results from P adsorption isotherm experiments suggested that after dredging, the sediment would act as a sink for dissolved P only when water column soluble reactive phosphorus (SRP) concentrations exceeded 40 mu g L-1. Additionally, the dredging depth (similar to 1 m on average) exposed sedimentwith significantly reduced P sorption capacities. Second, P release ratesweremeasured in sediment cores that were incubated under two water temperatures (ambient; + 2 degrees C) and two oxygen levels (oxic; hypoxic). Average maximumtotal phosphorus (TP) release rates ranged from 40 to 85 mgm(-2) d(-1) before dredging and from 0 to 7 mg m(-2) d(-1) after dredging, resulting in a 95-99% reduction in TP release rates after dredging. Similar reductions were measured also for SRP release rates. The significant reduction in sediment P release after dredging now creates a high potential for this restored wetland to reduce net P loads into downstream waters by facilitating the deposition and burial of particulate P. We conclude that sediment dredging can be a useful technique for balancing the goals of habitat restoration and water quality improvements in wetlands restored on former agricultural lands. (c) 2018 Elsevier B.V. All rights reserved.