National hydrologic connectivity classification links wetlands with stream water quality

Wetland hydrologic connections to downstream waters influence stream water quality. However, no systematic approach for characterizing this connectivity exists. Using physical principles, we categorized conterminous US (CONUS) freshwater wetlands into four hydrologic connectivity classes based on stream contact and flowpath depth to the nearest stream: riparian, non-riparian shallow, non-riparian mid-depth, and non-riparian deep. These classes were heterogeneously distributed over the CONUS; e.g., riparian dominated the southeastern and Gulf coasts, while non-riparian deep dominated the Upper Midwest and High Plains. Analysis of a national stream dataset indicated acidification and organic matter brownification increased with connectivity. Eutrophication and sedimentation decreased with wetland area but did not respond to connectivity. This classification advances our mechanistic understanding of wetland influences on water quality nationally and could be applied globally.

Impact/Purpose

1) Wetlands are critical watershed components contributing valuable services such as improving water quality. Connectivity between wetlands and downstream waters provides pathways for energy and material transport, influencing the structure, function, and dynamics of watersheds. Yet there is little research classifying wetland connectivity in regional landscapes or at national scales. A wetland connectivity classification would help determine whether connectivity influences water quality and should also be considered when prioritizing wetland restoration and protection. Here we develop a nationwide wetland connectivity classification; map and characterize the distribution of wetland connectivity throughout the conterminous US (CONUS); and use this classification to illustrate the potential effects of wetland connectivity. 2) We found that 8 of 11 water quality constituents, related to acidification and brownification, had strong relationships with connectivity; three constituents associated with eutrophication and sedimentation did not, but were related to wetland area. The strong relationships with acidification and brownification suggest that managers should target both wetlands and their connections for restoration or protection of stream water quality. In contrast, managers should target all wetlands – without considering connectivity – for eutrophication and sedimentation.3) Until now, no standardized approach existed to characterize hydrologic connectivity between wetlands and downstream waters at the CONUS-scale and determining regional and nationwide effects of wetland connectivity on water quality endpoints was impossible. Our approach to classifying wetland connectivity using geospatial analysis provides important insights on how wetlands affect water quality at the CONUS-scale. The flexibility of the approach in handling diverse data could make it useful at different scales and across the globe.4) Regulators and watershed managers can use our classification to help target both wetlands and their connections for restoration or protection of watershed water quality. It can also be used by scientists to determine relationships between wetlands and water quality constituents.

Citation

Leibowitz, S., R. Hill, I. Creed, J. Compton, H. Golden, M. Weber, M. Rains, C. Jones, Jr, E. Lee, J. Christensen, R. Bellmore, AND C. Lane. National hydrologic connectivity classification links wetlands with stream water quality. Springer Nature, LONDON, UK, 1:370-380, (2023). [DOI: 10.1038/s44221-023-00057-w]

DOI: National hydrologic connectivity classification links wetlands with stream water quality