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

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 km2 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 km2 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 wetland-mediated 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.

Considerations on the use of carbon and nitrogen isotopic ratios for sediment fingerprinting.

Carbon and nitrogen stable isotopic ratios are increasingly used in sediment fingerprinting studies. However, questions remain regarding tracer conservativeness during sediment transport and other error considerations. We investigate conservativeness processes, including carbon oxidation and nitrogen mineralization, using experiments. We also test how other considerations impact the isotopic ratios including algae accrual into temporary sediment deposits in the river, the physical loss of organic matter via disaggregation, concentration dependent mixing, and time-varying isotopic ratios of sediment sources. Results show all processes and considerations can change isotope abundance, however, significance varied. Carbon oxidation, nitrogen mineralization and upland seasonality of sediment sources did not significantly change isotopic ratios. Algae accrual, concentration dependency mixing, physical loss of organic matter during transport, and seasonality of the in-stream sediment source significantly changed the isotopic ratios for the conditions tested. Fertilization significantly impacted the stable carbon isotopic ratio in one case considered. Results from sediment fingerprinting simulations and testing how well the virtual mixture fits the mass balance equation agreed with significance results for tracer changes, and some uncertainty considerations changed fractional contribution of sources by as much as 50%. A noteworthy recommendation is the mean isotopic ratios of sediment sources should be separated by at least 1‰ to lessen tracer conservativeness concerns in fingerprinting simulation. We recommend concentration dependent mixing becomes the accepted practice when using isotopic ratios, however, we warn against using particle size corrections. We recommend the loss of organic matter during disaggregation be accounted for in fingerprinting estimates. We recommend algae accrual in in-stream sediment deposits should either be accounted for or in-stream sediment should be treated as a time-varying source in sediment fingerprinting simulations. Finally, we recommend both the carbon and nitrogen isotopic ratio should be tested as potential tracers because the two tracers performed similarly when testing how well the virtual mixture fits the mass balance equations.

Wetland restoration yields dynamic nitrate responses across the Upper Mississippi river basin.

Wetland restoration is a primary management option for removing surplus nitrogen draining from agricultural landscapes. However, wetland capacity to mitigate nitrogen losses at large river-basin scales remains uncertain. This is largely due to a limited number of studies that address the cumulative and dynamic effects of restored wetlands across the landscape on downstream nutrient conditions. We analyzed wetland restoration impacts on modeled nitrate dynamics across 279 subbasins comprising the ∼0.5 million km2 Upper Mississippi River Basin (UMRB), USA, which covers eight states and houses ∼30 million people. Restoring ∼8,000 km2 of wetlands will reduce mean annual nitrate loads to the UMRB outlet by 12%, a substantial improvement over existing conditions but markedly less than widely cited estimates. Our lower wetland efficacy estimates are partly attributed to improved representation of processes not considered by preceding empirical studies - namely the potential for nitrate to bypass wetlands (i.e., via subsurface tile drainage) and be stored or transformed within the river network itself. Our novel findings reveal that wetlands mitigate surplus nitrogen basin-wide, yet they may not be as universally effective in tiled landscapes and because of river network processing.