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

Abstract

River basin-scale wetland restoration and creation is a primary management option for mitigating nitrogen-based water quality challenges. However, the magnitude of nitrogen reduction that will result from adding wetlands across large river basins is uncertain, partly because the areal extent, location, and physical and functional characteristics of the wetlands are unknown. We simulated over 3600 wetland restoration scenarios across the ∼450,000 km 2 Upper Mississippi River Basin (UMRB) depicting varied assumptions for wetland areal extent, physical and functional characteristics, and placement strategy. These simulations indicated that restoring wetlands will reduce local nitrate yields and nitrate loads at the UMRB outlet. However, the projected magnitude of nitrate reduction varied widely across disparate scenario assumptions�e.g., restoring 4500 km 2 of wetlands (i.e., 1% of UMRB area) decreased mean annual nitrate loads at the UMRB outlet between 3 and 42%. Higher magnitude nitrate reductions correlated with best-case assumptions, particularly for characteristics controlling nitrate loading rates to the wetlands. These results show that simplified claims about basin-scale wetlandmediated water quality improvements discount the breadth of possible wetland impacts across disparate wetland physical and functional conditions and highlight a need for greater clarity regarding the likelihood of these conditions at river basin scales.

Wetland Destruction in a Headwater River Leads to Disturbing Decline of In-stream Nitrogen Removal

“…9 The ecological and environmental benefits offered by riverine wetlands have been extensively documented, 6 sparking global interest in constructing more of these wetlands to achieve water quality and ecological objectives at reach or watershed scales. 6,10,11 While many studies have shown the effectiveness of constructed wetlands in purifying water quality during their operational phase, 3,11 there is a scarcity of research on the environmental consequences once they are no longer in operation. Constructed wetlands, like their natural counterparts, are prone to damage from disasters and/or recession in their ecological functions.…”

“…Constructing riverine wetlands often involves modifying riverine geomorphology, especially in flashy headwater rivers where benthic and hydrological conditions are unfavorable for vegetation growth . The ecological and environmental benefits offered by riverine wetlands have been extensively documented, sparking global interest in constructing more of these wetlands to achieve water quality and ecological objectives at reach or watershed scales. ,, …”