Citation:

Brooks, J. Renee, J. Compton, A. Herlihy, D. Sobota, A. Nahlik, AND M. Weber. d15N of Chironomidae: An index of nitrogen sources and processing within watersheds as part of EPA’s National Aquatic Resource Surveys. Sixth Interagency Conference on Research in the Watersheds, Shepherdstown, West Virginia, July 23 - 26, 2018.

Impact/Purpose:

Nitrogen (N) removal in watersheds is an important regulating ecosystem service that can help reduce N pollution in the nation’s waterways. Having measurements of this important service at the national scale would be highly useful in understanding watershed integrity across the USA. However, processes that remove N are difficult to measure. We are working on simple to measure indicators of nitrogen removal that could be incorporated into EPA's National Aquatic Resource Surveys using biological material that is already collected as part of the surveys. Our results indicate that chironomid nitrogen isotope values can integrate watershed-level N sources, input rates, and processing for water quality monitoring and assessment at large scales.

Description:

Nitrogen (N) removal in watersheds is an important regulating ecosystem service that can help reduce N pollution in the nation’s waterways. However, processes that remove N such as denitrification occur at defined points in space and time. Measures that integrate N processing within watersheds over time would be particularly useful for assessing the degree of this vital service. Because most N removal processes isotopically enrich the remaining N, 15N from basal food-chain organisms in aquatic ecosystems can provide information on watershed N processing. As part of EPA’s National Aquatic Resource Surveys (NARS), we measured d15N of Chironomidae collected from thousands of lotic and lentic ecosystems across the continental U.S.; these larval aquatic insects were found in abundance in almost every lake, river, and stream surveyed. Using information on N loading to the watershed and summer total N concentrations in the water column, we assessed where elevated chironomid d15N would indicate N removal rather than possible enriched sources of N. Chironomid d15N values ranged from -4 to +20 ‰, and were higher in rivers and streams than in lakes (median = 7.6 ‰ vs. 4.8 ‰, respectively), indicating that lotic chironomids acquired N that was processed to a greater degree than lentic chironomids. For both lotic and lentic chironomids, 15N increased with watershed-level agricultural land cover and N loading, and decreased as precipitation increased. In rivers and streams with high synthetic fertilizer N loading, we found lower N concentrations in streams with higher chironomid d15N values, suggesting high rates of N removal. At low levels of synthetic fertilizer N loading, the pattern reversed; streams with enriched chironomid d15N had higher N concentrations, suggesting enriched sources such as manure or sewage. Our results indicate that chironomid d15N values can integrate watershed-level N sources, input rates, and processing for water quality monitoring and assessment at large scales.

Record Details: