Water-quality outcomes of wetland restoration depend on hydroperiod rather than restoration strategy

Land managers increasingly use wetland restoration to improve water quality, particularly in cultivated landscapes. In agricultural wetland restoration, managers regularly excavate accumulated sediments eroded from the surrounding landscape to increase water storage capacity, decrease invasive species cover, or improve water quality. However, it is unclear whether the effects of sediment excavation are influenced by wetland hydroperiod. Additionally, we lack data on how long excavation effects persist in restored wetlands. We examined dissolved nutrient concentrations (i.e., NH4+, NO3–, total dissolved N, soluble reactive P, total dissolved P, and dissolved organic C) as proxies for water quality in 54 restored agricultural wetlands ranging from 1 to 10 y post-restoration in the Prairie Pothole Region of west central Minnesota, USA. In 26 of these wetlands, restoration practitioners restored natural (i.e., either seasonal or semipermanent inundation) hydrological regimes by removing subsurface tile drainage and plugging surface drainage ditches (business-as-usual treatment). In 28 wetlands, practitioners removed accumulated sediment and redeposited it on the surrounding landscape (excavated treatment) prior to restoring hydrology. We found that wetlands in the excavated treatment group initially experienced reduced dissolved P concentrations, but over time P levels increased, particularly in wetlands with shorter hydroperiods. Excavated wetlands had lower NH4+ and dissolved organic C concentrations compared with business-as-usual wetlands, but the trend was driven by differences between restoration treatments in semipermanent wetlands. N and P dynamics were almost universally related to hydroperiod, both immediately following restoration and over the ensuing years. We postulate that the effects of hydroperiod are likely related to differences in redox conditions via direct mechanisms (water level fluctuations related to hydroperiod) and indirect mechanisms (development of dense emergent macrophyte communities in seasonal wetlands). In basins with seasonal hydroperiod, inorganic N concentrations decreased over time and inorganic P concentrations increased, suggesting net P mobilization concurrent with growing N limitation. Our results illustrate that hydroperiod regulates the expression of legacy P following wetland restoration, with little long-term effect of sediment removal on water quality outcomes.