Spatially explicit estimation of wetland nutrient inputs and their relationship with water quality

Wetlands are effective in reducing nutrient loads in surface waters, but there is a need to optimize allocation of conservation efforts to efficiently maximize the likelihood of water quality improvements. This presentation describes efforts to quantify nutrients flowing through wetlands and identify existing and potential wetlands that maximize attenuation of excess landscape nutrients across the conterminous United States (CONUS). We then combined our datasets with harmonized national water quality data to model regional and seasonal variability in the relationship between wetland nutrient interception and in-stream nitrate across CONUS. We found that there was a consistent negative relationship between in-stream nitrate concentrations and wetland interception of landscape nutrients in the Upper Midwest, Temperate Plains, and SE Coastal Plains regions of the US with strongest relationships during Spring, illustrating the role of wetlands in reducing landscape nitrogen transport to streams. Drier regions of the western US exhibited either neutral or positive relationships of wetland interception and stream nitrate, depending on the season, indicating that wetlands may accumulate and then serve as a stream nitrate source in these regions under certain conditions. We are currently developing methods to apply these relationships to potential wetland areas across CONUS to estimate potential improvement of water quality through wetland restoration. These analyses and data will provide an optimized CONUS-wide wetland conservation framework to aid local communities better tailor their nutrient reduction strategies.

Impact/Purpose

Wetlands provide numerous critical functions, including nutrition cycling and water quality improvement, that have made them a critical component of many state and local stewardship programs. Broad-scale wetland loss derived from variability in long-term climate patters and human activities has reduced the capacity of wetlands to buffer nutrient inputs in watersheds, making downstream water resources more susceptible to eutrophication. Wetland construction (e.g., either restoration of previous wetlands or creation of new wetlands) is a potential strategy for nutrient reduction to improve water quality within watersheds. However, implementation of water quality strategies including wetlands are often resource limited, creating the need for targeted prioritization among existing and potential wetland areas (PWA) to maximize effectiveness and efficiency of these efforts. In this talk, we describe development of a database containing delineated catchments of existing and potential wetlands and quantification of major landscape nutrient inputs within those basins derived from EPA’s National Nutrient Inventory. We accumulated this data up to individual stream catchments across the conterminous US (CONUS) to examine regional variation in landscape nutrients intercepted by wetlands and which regions have the most potential to increase nutrient interception through wetland construction. We then describe development of modeling efforts to link wetland interception of nutrient  inputs to instream water quality data using the ChemLotUS dataset derived from EPA Water Quality Portal data. Our results indicate that there is regional and seasonal variation in the relationships between wetland nutrient interception and in-stream nitrate concentrations, with a negative relationship between wetland interception and in-stream nitrate concentrations, indicating overall benefit of wetlands for water quality. We then describe efforts to apply these relationships to potential wetland data to improve efficiency and impact of wetland management strategies for water quality. These data will provide critical insights for the optimization of wetland conservation strategies to help state and local managers and stakeholders meet their water quality goals. The results of this work will support efforts to meet the goals of the Clean Water Act and contributes to StRAP Subproduct SSWR.405.4.1.1  – National mapping of nutrient interception by existing and potentially-restorable wetlands to prioritize restoration with additional connections to work under StRAP Product SSWR.405.1.1 – Novel applications of the nutrient Inventory (N, P, or both).

Citation

Alford, S. Spatially explicit estimation of wetland nutrient inputs and their relationship with water quality. Invited Webinar - National Association of Wetland Managers, Corvallis, OR, September 24, 2025.