River Basin Simulations Reveal Wide-Ranging Wetland-Mediated Nitrate Reductions

Theprojected water quality benefits of basin-scale wetlandrestoration varied widely across contrasting assumptions for the extentand characteristics of the yet-restored wetlands, particularly forcharacteristics controlling the amount of nutrients that will enterthe wetlands. River basin-scale wetland restoration and creation isa primarymanagement option for mitigating nitrogen-based water quality challenges.However, the magnitude of nitrogen reduction that will result fromadding wetlands across large river basins is uncertain, partly becausethe areal extent, location, and physical and functional characteristicsof the wetlands are unknown. We simulated over 3600 wetland restorationscenarios across the & SIM;450,000 km(2) Upper MississippiRiver Basin (UMRB) depicting varied assumptions for wetland arealextent, physical and functional characteristics, and placement strategy.These simulations indicated that restoring wetlands will reduce localnitrate yields and nitrate loads at the UMRB outlet. However, theprojected magnitude of nitrate reduction varied widely across disparatescenario assumptions e.g., restoring 4500 km(2) ofwetlands (i.e., 1% of UMRB area) decreased mean annual nitrate loadsat the UMRB outlet between 3 and 42%. Higher magnitude nitrate reductionscorrelated with best-case assumptions, particularly for characteristicscontrolling nitrate loading rates to the wetlands. These results showthat simplified claims about basin-scale wetland-mediated water qualityimprovements discount the breadth of possible wetland impacts acrossdisparate wetland physical and functional conditions and highlighta need for greater clarity regarding the likelihood of these conditionsat river basin scales.