Punching above their weight: Wetlands moderate nutrient delivery to streams despite comprising just 5% of the conterminous US land area

Wetlands can mediate the transfer of materials between uplands and streams through interception of upland flows and connectivity to receiving streams. We used geospatial data to classify the connectivity of 6.7 million wetlands to 2.6 million stream segments across the conterminous US (CONUS). We defined a flowpath from each wetland to its receiving stream and classified wetland units based on stream proximity (i.e., riparian vs. non-riparian) and likely dominant flow and soil drainage types along these paths (i.e., surface vs. subsurface flow and poorly vs. well-drained soils). 71% of all wetland areas in the CONUS were classified as riparian, making them the dominant class. Further, we delineated each wetland basin to quantify the land area and upland N each receives. Maps of these basins reveal a profound spatial mismatch between the locations of upland N inputs and of intercepting wetlands across the CONUS. Despite this mismatch, wetlands, which comprise just 5% of the landscape, potentially intercept 25% of the total land area and associated upland N. Regression analysis of TN concentrations from 1,779 CONUS sample sites on wetland-intercepted and wetland-bypassed N inputs showed wetland-intercepted inputs (of any wetland class) did not contribute to instream TN concentrations, suggesting wetlands effectively removed these inputs. Instead, instream TN was associated with the 75% of N inputs that bypassed wetlands. The maps and model produced by this study suggest that wetlands are an underused, yet potentially effective mitigator of upland stressors on streams.

Impact/Purpose

Wetlands are a rare but important aquatic resource. Wetlands can help control the delivery of materials, such as nutrients, from landscapes to streams. They can do so by intercepting these materials and retaining them. The connective flowpaths of wetlands to streams can also mediate the delivery of materials. Wetland paths with slow or disconnected flow paths can impede the movement of these materials and in some cases, improve water quality. This study characterized the degree of connectivity of 6.7 million wetlands to 2.6 million streams across the US. Additionally, we calculated the total area and nitrogen each wetland intercepts from upland areas. We estimated that wetlands comprise just 5% of the US land area but potentially intercept 25% of upland nitrogen, such as from agricultural land. To our knowledge, this is the first time such values have been reported. The geospatial framework we used allowed us to pair these estimates with instream measurements of total nitrogen. By doing so, we show that instream total nitrogen (TN) concentrations are primarily driven by the 75% of upland nitrogen that bypasses wetlands. Upland nitrogen that is intercepted by wetlands does not contribute to instream concentrations of TN. The methods we used to characterize the connectivity and interception of wetlands was applied for the first time at the scale of the Nation. By doing this study at this scale, we provide first-of-their-kind statistics on the distribution of wetlands relative to their capability to intercept landscape nitrogen. Additionally, we produce maps that illustrate the spatial mismatch between where wetlands and where nitrogen application occur across the conterminous US. These results suggest that wetlands are a potentially important, yet underutilized tool for mitigating upland stressors on water quality.

Citation

Hill, Ryan A, S. Leibowitz, M. Weber, J. Compton, M. Rains, I. Creed, C. Jones, J. Christensen, R. Bellmore, E Henry Lee, H. Golden, AND C. Lane. Punching above their weight: Wetlands moderate nutrient delivery to streams despite comprising just 5% of the conterminous US land area. Annual Meeting of the Society for Freshwater Science, virtual, Virtual, May 23 - 27, 2021.