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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.