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

Oldenborg, Kimberly A., Steinman, Alan D.
The Science of the total environment 2019 v.650 pp. 1969-1979
agricultural land, biodiversity, freshwater, habitat conservation, habitats, oxygen, pollution load, reactive phosphorus, risk, sediments, sorption, sorption isotherms, surface water, water quality, water temperature, wetlands
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 μg L−1. Additionally, the dredging depth (~1 m on average) exposed sediment with significantly reduced P sorption capacities. Second, P release rates were measured in sediment cores that were incubated under two water temperatures (ambient; +2 °C) and two oxygen levels (oxic; hypoxic). Average maximum total phosphorus (TP) release rates ranged from 40 to 85 mg m−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.