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Using d15N of Chironomidae to help assess lake condition and possible stressors in EPA?s National Lakes Assessment.
Citation:
Brooks, Reneej, J. Compton, A. Herlihy, D. Sobota, J. Stoddard, AND M. Weber. Using d15N of Chironomidae to help assess lake condition and possible stressors in EPA?s National Lakes Assessment. Presented at ESA 2012, Portland, OR, August 05 - 10, 2012.
Impact/Purpose:
We are exploring the utility of stable isotopes from basal food chain organisms in providing information about nitrogen sources and processing at large scales within EPA?s water quality monitoring program.
Description:
Background/Questions/Methods As interest in continental-scale ecology increases to address large-scale ecological problems, ecologists need indicators of complex processes that can be collected quickly at many sites across large areas. We are exploring the utility of stable isotopes from basal food chain organisms in providing information about nitrogen sources and processing at large scales within EPA?s water quality monitoring program. EPA has implemented the National Aquatic Resource Surveys, which use a probabilistic survey design to monitor 1000-2000 sites across the nation per water body type (Lake, River/Stream, Estuary, Wetland). While EPA measures many parameters during the one-day site visits, data on complex processes such as denitrification cannot be measured with such limited visit times. We are exploring the potential for d15N measured in a family of insects, Chironomidae, which occupies several functional feeding groups in aquatic ecosystems to help classify lakes based on likely sources of or processes that affect nitrogen (N). In 2007, EPA conducted the National Lakes Assessment (NLA) on 1000 lakes, collecting composite benthic invertebrate samples from the littoral zone at 10 locations around each lake shoreline. After samples were counted and identified for biodiversity assessments, chironomids were separated, and then analyzed for d15N. Results/Conclusions Chironomid d15N values varied from -2 to 20 ? with a mean of 5.7 ? and were significantly higher in lakes with high nutrient concentrations. Since d15N values can vary from both changes in N source (e.g. fertilizer vs. manure) and from N processing such as denitrification, we cannot determine a unique cause of d15N enrichment. Instead, we used a decision tree approach to categorize lakes for likely N sources and whether denitrification is an important process in watershed N dynamics. Lakes with relatively high fertilizer loading (>10 kg/ha/yr) in the watershed generally had higher ch