Connections Matter: National Classification Links Wetlands With Stream Water Quality
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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.