Citation:

Brooks, Reneej, J. Compton, A. Herlihy, D. Sobota, A. Nahlik, AND M. Weber. delta15N of Chironomidae: an index of nitrogen sources and processing within watersheds as part of EPA’s National Aquatic Resource Surveys. American Geophysical Union Annual Meeting, Washington DC, Washington DC, December 10 - 14, 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. However, measurement of processes that remove N such as denitrification can be quite expensive and time consuming. Measures that integrate N processing within watersheds over time would be particularly useful for assessing the degree of this vital service. Nitrogen isotopes of aquatic insects from the basal part of the food chain integrate information from both sources and processing of nitrogen within the watershed. We combined this isotopic information with total nitrogen concentration in the water, and loading of synthetic nitrogen fertilizer on the watershed to help separate nitrogen sources from nitrogen processing, and used this information to classify watersheds to understand which watersheds are better at processing nitrogen than others and why. This information is of interest to our regional and state partners to help prioritize watersheds for restoration, and reducing non-point source pollution.

Description:

Nitrogen (N) removal in watersheds is an important regulating ecosystem service that can help reduce N pollution in the nation’s waterways. However, measurement of processes that remove N such as denitrification can be quite expensive and time consuming. 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, delta15N 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 delta15N 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 determined where elevated chironomid 15N would indicate N removal rather than possible enriched sources of N. Chironomid delta15N 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, delta15N increased with watershed-level agricultural land cover and N loading, and decreased as precipitation increased. In watersheds with high synthetic fertilizer N loading, we found lower N concentrations in streams with higher chironomid delta15N values, suggesting high rates of N removal. At low levels of synthetic fertilizer N loading, the pattern reversed; streams with enriched chironomid delta15N had higher N concentrations, suggesting enriched sources such as manure or sewage. Our results indicate that chironomid delta15N values can integrate watershed-level N sources, input rates, and processing for water quality monitoring and assessment at large scales.

Record Details: