Wetland Connectivity Influences Water Quality of Downstream Waters

Hydrologic connections between wetlands and downstream waters moderate surface water quality across watersheds. However, large-scale studies elucidating this phenomenon are lacking – approaches to classify hydrologic connectivity of wetlands for large spatial extents do not exist. Here we developed a hydrologic connectivity classification system for 6.7 million wetlands to 2.6 million downstream stream segments across the conterminous US (CONUS). This first of its kind study found that wetlands occupy 5.4% of the CONUS but “filter” 24.6% of the area. Yet there is a spatial mismatch between where nitrogen inputs and wetlands that can filter nitrogen occur. Wetlands in the watershed with low connectivity generally reduced stream nitrogen concentrations, and those with high connectivity increased stream carbon concentrations. High connectivity riparian wetlands, however, increased nitrogen and reduced carbon stream concentrations. Quantifying where and how wetland connectivity occurs at watershed scales is important for safeguarding surface water quality – both across the CONUS and globally. 

Impact/Purpose

Wetlands contribute to the structure and function of downstream waters by altering material fluxes to receiving waters. Studies have shown that the degree of this contribution is mediated by the connectivity of wetlands to downslope streams, but these studies have been limited to empirical analyses of limited geographic scope or modeling approaches. We developed a classification system of wetland connectivity with receiving rivers and streams and applied this system to map the connectivity among 6.7 million wetlands with 2.6 million stream reaches across the conterminous US (CONUS). We evaluated this classification system with modeling that showed that wetlands and wetland connectivity influence instream measures of total nitrogen (TN) and dissolved organic carbon (DOC).  This first-of-its-kind, CONUS-scale study showed that wetlands only occupy 5.4% of the land, but intercept (“filter”) 1.8 million km2, or 24.6% of its area. However, there is a mismatch over where TN input and wetland filtration occur throughout the CONUS. Modeling of instream nutrients showed that wetland area (as a total or as a percent of the watershed) is an important factor that influences instream TN and DOC; two constituents that are often managed for by states. In the case of TN, we showed that wetlands and wetlands of specific connectivity types can be very effective filters of landscape TN. Our approach for quantifying the inputs of TN to wetlands highlights the role that wetlands and their hydrologic connectivity to downstream waters can play in mediating instream responses to landscape stressors. Such information could be useful in weighing trade-offs between wetlands and non-wetlands or between wetlands with different connectivity characteristics to achieve water quality goals as part of watershed management plans.

Citation

Leibowitz, S., Ryan A Hill, I. Creed, J. Compton, H. Golden, M. Weber, M. Rains, C. Jones, E Henry Lee, J. Christensen, R. Bellmore, AND C. Lane. Wetland Connectivity Influences Water Quality of Downstream Waters. 11th Intecol International Wetlands Conference, virtual, N/A, October 14, 2021.