Wetland Flowpaths Mediate Nitrogen and Phosphorus Concentrations across the Upper Mississippi River Basin

Abstract

Eutrophication, harmful algal blooms, and human health impacts are critical environmental challenges resulting from excess nitrogen and phosphorus in surface waters. Yet we have limited information regarding how wetland characteristics mediate water quality across watershed scales. We developed a large, novel set of spatial variables characterizing hydrological flowpaths from wetlands to streams, that is, “wetland hydrological transport variables,” to explore how wetlands statistically explain the variability in total nitrogen (TN) and total phosphorus (TP) concentrations across the Upper Mississippi River Basin (UMRB) in the United States. We found that wetland flowpath variables improved landscape‐to‐aquatic nutrient multilinear regression models (from R2 = 0.89 to 0.91 for TN; R2 = 0.53 to 0.84 for TP) and provided insights into potential processes governing how wetlands influence watershed‐scale TN and TP concentrations. Specifically, flowpath variables describing flow‐attenuating environments, for example, subsurface transport compared to overland flowpaths, were related to lower TN and TP concentrations. Frequent hydrological connections from wetlands to streams were also linked to low TP concentrations, which likely suggests a nutrient source limitation in some areas of the UMRB. Consideration of wetland flowpaths could inform management and conservation activities designed to reduce nutrient export to downstream waters.

Context is Everything: Interacting Inputs and Landscape Characteristics Control Stream Nitrogen

“…The negative association of wetland cover with stream DIN and TN concentration in high N input watersheds (Figure e) supports previous work on the strong N removal potential of wetlands if they are present and hydrologically connected. − Wetlands were also an important source of TON in high N input sites (Figure S6). Increasing wetland coverage was accompanied by decreasing agricultural land coverage in many regions, hence often coincided with the lower watershed N input rate.…”

“…For those watersheds, the location of wetlands in flow paths and in relation to the N input source regulate their ability to serve as removal barriers to excess nitrogen. 67 Proportion of Baseflow: An Indicator of Legacy Groundwater Contamination in High N Input Areas? The impact of baseflow contribution 70 on stream N concentrations also diverged at different input levels (Figure 3d,e).…”

“…We found that some NRSA watersheds with the highest N input rate (>100 kg N ha –1 yr –1 ) and high wetland percentages (>10%) had TN concentration < 2 mg N L –1 or even < 1 mg N L –1 , indicating the effectiveness of wetland N retention and potential conservation efforts in these areas. For those watersheds, the location of wetlands in flow paths and in relation to the N input source regulate their ability to serve as removal barriers to excess nitrogen …”

Comparing Drivers of Spatial Variability in U.S. Lake and Stream Phosphorus Concentrations

“…This in turn promotes the release of iron bound P in sediments from stream and lake beds (Del Giudice et al, 2018;Jensen & Andersen, 1992). Observations from the latter research and relationships demonstrated in this continental-wide study raise the prospects that projected increases in temperature throughout much of the United States may result in increased TP concentrations in surface water (Melillo, 2014) that will exacerbate eutrophication or contribute to the further loss of oligotrophic systems (Stoddard et al, 2016). The effect of temperature may be attenuated in deeper lakes (Figure 4), which had a strong negative relationship with lake TP concentrations.…”

Vulnerable Waters are Essential to Watershed Resilience

“…Like headwaters, non-floodplain wetlands are bioreactors (sensu Marton and others 2015 ) existing along a down-gradient connectivity continuum from highly connected to highly disconnected systems ( Cohen and others 2016 ; Mengistu and others 2020 ). The important biogeochemical functions performed by these vulnerable waters affecting watershed state and resilience characteristics are increasingly well supported in the literature ( Bernal and Mitsch 2013 ; Biggs and others 2017 ; Cheng and Basu 2017 ; Creed and others 2017 ; Lane and others 2018 ; Leibowitz and others 2018 ; Golden and others 2019) .…”