Connections Matter: National Classification Links Wetlands and Water Quality.

Wetland connections to or isolation from downstream waters are primary factors influencing water quality. However, no systematic approach for characterizing this connectivity exists. Our goals in this study were to classify wetland hydrologic connectivity across the conterminous US (CONUS), and to create a systematic description of how wetland hydrologic connectivity contributes to watershed function. We classify hydrological connectivity between wetlands and downstream waters based on wetland type and the properties of the soil flowpath between the wetland and the downstream water. We applied this system across the CONUS to identify the following four classes with decreasing hydrologic connectivity: riparian, non-riparian with shallow subsurface flowpaths, non-riparian with mid-depth subsurface flowpaths, and non-riparian wetlands with deep subsurface flowpaths. Riparian wetlands comprised >70% of all wetlands by area, while the other three classes represent ~10% each. The four wetland types are heterogeneously distributed over the CONUS: for example, riparian wetlands dominate in the southeast and Gulf coasts, while non-riparian deep occurs in the prairie pothole and playa lake regions. To evaluate the classification system for representing hydrologic connectivity, we related wetland class prevalence to a national dataset of stream acidification, brownification, eutrophication, and sedimentation. Results indicate that constituents associated with acidification and brownification become more positive with increasing connectivity, while those associated with basic constituents decrease. In contrast, connectivity did not affect constituents associated with eutrophication and sedimentation. This study demonstrates how our classification advances our understanding of wetland influences on water quality across the nation – with global potential.  

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 hydrologic connectivity that could be implemented at the scale of the conterminous US (CONUS) and contained four classes: riparian, non-riparian with shallow subsurface flowpaths, non-riparian with mid-depth subsurface flowpaths, and Non-Riparian wetlands with Fill and Spill surface flow and Deep subsurface flow (NRFSD). Riparian wetlands comprised >70% of all wetlands by area, while the other three classes represent ~10% each. The four wetland types are heterogeneously distributed over the CONUS: for example, riparian wetlands dominate in the southeast and Gulf coasts, while NRFSD wetlands occur in the prairie pothole and playa lake regions. In a firstof- its-kind, CONUS-scale study, we applied this classification system to map the connectivity among 6.7 million wetlands with 2.6 million stream reaches across the CONUS. Results showed that constituents associated with acidification become more positive with decreasing connectivity, while those associated with browning become less positive with decreasing connectivity. In contrast, connectivity did not affect constituents associated with denitrification and filtration. This study advances our scientific understanding of the importance of wetlands and their hydrologic connectivity on water quality across the nation – with global potential.

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. Connections Matter: National Classification Links Wetlands and Water Quality. Joint Aquatic Sciences Meeting (JASM), Grand Rapids, Michigan, May 14 - 20, 2022.