Citation:

Brooks, J. Renee. What can one sample tell us? Stable isotopes can assess complex processes in national assessments of lakes, rivers and streams. Washington State University Science Seminar, Vancouver, WA, February 29, 2016.

Impact/Purpose:

Stable isotopes can be very useful in large-scale monitoring programs because samples for isotopic analysis are easy to collect, and isotopes integrate information about complex processes such as evaporation from water isotopes and denitrification from nitrogen isotopes. In this presentation at Washington State University, I will share results on a pilot program to incorporate stable isotopes into EPA’s National Aquatic Resource Surveys, which are spatially extensive surveys designed to assess the condition of the nation’s waters for congress. I use stable isotopes of water to assess hydrologic indicators for lakes, and stable isotopes of aquatic insects to help assess nitrogen dynamics within lakes, rivers and streams.

Description:

Stable isotopes can be very useful in large-scale monitoring programs because samples for isotopic analysis are easy to collect, and isotopes integrate information about complex processes such as evaporation from water isotopes and denitrification from nitrogen isotopes. Traditional techniques to acquire such information require significant time and effort beyond what is possible in a one-day assessment. Thus, measurements of these important processes are often excluded from spatially extensive monitoring programs such as EPA’s National Aquatic Resource Surveys. We used the stable isotopes of water (2H and 18O) and chironomids (15N) analyzed from samples collected from 1000+ lakes across the country in EPA’s 2007 National Lakes Assessment, and 1000+ Rivers and Streams in 2008-2009 National Rivers and Streams to derive information on evaporation and denitrification. Using water isotopes, we estimated evaporation as a percentage of inflow (E:I) for the nation’s lakes in 2007, and found that for 50% of lakes, evaporation was , 25% of inflow, with values ranging up to 113%. Chironomid delta15N values represent an index of nitrogen cycling and removal processes. Chironomid delta 15N 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 N was processed to a greater degree in lotic chironomids than in lentic ones. Chironomid delta 15N values increased with watershed-level agricultural land cover and N loading, and decreased as precipitation increased, and were useful to indicate both nitrogen sources and nitrogen removal processes within lakes, rivers and streams. Our results indicate that stable isotope analysis can provide valuable information about complex processes for national water quality monitoring efforts.

Record Details: