Past, present, and potential landscape nutrient interception by wetlands across CONUS

Wetlands provide numerous ecosystem functions within their connected landscapes that improve water quality, including nutrient removal. Due to growing recognition of the role of wetlands in maintaining downstream water quality, there is an increasing emphasis on identifying existing and potential wetlands for prioritized conservation efforts (e.g., preservation and construction) as a strategy to buffer nonpoint source nutrient pollution within watersheds and improve water quality. We investigated the capacity of wetlands to intercept nutrients from anthropogenic runoff across the conterminous United States (CONUS) by identifying existing and potential wetlands lying on surface flow paths draining agricultural and urban lands and developing a dataset of their delineated basins. We did this for wetlands within the National Land Cover Database every 5 years from 1987-2017. We also applied the National Nutrient Inventory to the wetland basins to quantify the total landscape inputs and the % of these inputs that must pass through a wetland before arriving at a stream. We found that from 1987-2017, the total wetland area decrease <1% nationally, while agricultural area decreased by 5% and developed land increased by 25%. Meanwhile, landscape nitrogen increased from agricultural from developed inputs, including 28% from agriculture and 45% from developed land cover in wetland basins. However, the percentage of nutrients being hydrologically intercepted by wetlands held steady during this period. This presentation will also discuss future work to relate instream water quality to nutrient inputs and the percent of inputs hydrologically intercepted by wetlands.    The presentation is part of the upcoming seminar July 2, 2025: Applying EPA’s National Nutrient Inventory to quantify landscape nutrient inputs and interception to support state and local water quality objectives.   Harmful Algal Blooms, Hypoxia, and Nutrients Research Webinar Series | US EPA

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 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 prioritization among existing and potential wetland areas (PWA) to support state led conservation efforts of nutrient management. In this talk, we describe development of a database containing delineated catchments of existing and potential wetlands that lay on surface flow paths of agricultural nonpoint source pollution entering water resources. We examined catchment characteristics (e.g., land cover) to quantify the amount of nutrients existing wetlands intercept within watershed landscapes and to identify PWA that intersects with high nutrient runoff and serve as candidates for conversion to meet water quality strategies across the conterminous United States. This dataset will provide critical insights for the prioritization of wetland areas 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. Past, present, and potential landscape nutrient interception by wetlands across CONUS. Harmful Algal Blooms, Hypoxia, and Nutrients Research Webinar Series, Corvallis (online), OR, July 02, 2025.