Citation:

Brooks, J. Renee, J. Compton, J. Lin, A. Herlihy, A. Nahlik, W. Rugh, AND M. Weber. δ15N of Chironomidae: An index of nitrogen sources and processing within watersheds for national aquatic monitoring programs. SCIENCE OF THE TOTAL ENVIRONMENT. Elsevier BV, AMSTERDAM, Netherlands, 813:151867, (2022). https://doi.org/10.1016/j.scitotenv.2021.151867

Impact/Purpose:

Nitrogen (N) pollution represents a major threat to aquatic ecosystems globally. The EPA’s National Aquatic Resource surveys (NARS) indicate that nearly a quarter of the nation’s river and stream miles, and ~20% of the nation’s lakes could be improved by lowering the concentration of nitrogen. Indicators that can be used in NARS to help identify watersheds based on their ability to process N before it enters aquatic ecosystems would be useful. In addition, additional information on which nitrogen sources are entering aquatic systems could help with mitigation planning. In this work, we use the nitrogen isotope ratio (15N) of chironomids, a family of aquatic insects that is abundance and collected as part of NARS. We found that chironomid 15N values can be a valuable tool to assess integrated watershed-level N sources, input rates, and processing for water quality monitoring and assessment at large scales.

Description:

Nitrogen (N) removal along flowpaths to aquatic ecosystems is an important regulating ecosystem service that can help reduce N pollution in the nation's waterways, but can be challenging to measure at large spatial scales. Measurements that integrate N processing within watersheds would be particularly useful for assessing the magnitude of this vital service. Because most N removal processes cause isotopic fractionation, δ15N from basal food-chain organisms in aquatic ecosystems can provide information on both N sources and the degree of watershed N processing. As part of EPA's National Aquatic Resource Surveys (NARS), we measured δ15N of Chironomidae collected from over 2000 lakes, rivers and streams across the continental USA. Using information on N inputs to watersheds and summer total N concentrations ([TN]) in the water column, we assessed where elevated chironomid δ15N would indicate N removal rather than possible enriched sources of N. Chironomid δ15N values ranged from −4 to +20‰, and were higher in rivers and streams than in lakes, indicating that N in rivers and streams underwent more processing and cycling that preferentially removes 14N than N in lakes. Chironomid δ15N increased with watershed size, N inputs, and water chemical components, and decreased as precipitation increased. In rivers and streams with high watershed N inputs, we found lower [TN] in streams with higher chironomid δ15N values, suggesting high rates of gaseous N loss such as denitrification. At low watershed N inputs, the pattern reversed; streams with elevated chironomid δ15N had higher [TN] than streams with lower chironomid δ15N, possibly indicating unknown sources elevated in δ15N such as legacy N, or waste from animals or humans. Chironomid δ15N values can be a valuable tool to assess integrated watershed-level N sources, input rates, and processing for water quality monitoring and assessment at large scales.

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