Connections Matter: National Classification Links Wetlands and Water Quality
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Wetland connections to or isolation from downstream waters are primary factors influencing watershed biogeochemistry and hydrology. However, no systematic approach for characterizing this connectivity exists. Our goals in this study were to classify wetland hydrologic connectivity (defined as the degree to which components of a system are connected and interact through various hydrologic transport mechanisms) across the conterminous US (CONUS), and to create a systematic description of how wetland hydrologic connectivity contributes to watershed function. Specifically, our system to classify hydrological connectivity between wetlands and downstream waters is based on the type of wetland 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 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. To evaluate the classification system for representing hydrologic connectivity, we related wetland class prevalence to a national dataset of stream acidification, browning, denitrification, and filtration. Results indicate that constituents associated with acidification (calcium, specific conductivity, magnesium, pH, and acid neutralizing capacity) become more positive with decreasing connectivity, while those associated with browning (color and dissolved organic carbon) become less positive with decreasing connectivity. In contrast, connectivity did not affect constituents associated with denitrification and filtration. This study demonstrates how our classification advances our understanding of wetland influences on water quality across the nation – with global potential.