HR: 10:20h
AN: H52B-01 INVITED     [Abstracts]
TI: Stable Isotope Constraints on N Deposition and Cycling in Lake Tahoe
AU: * Michalski, G
EM: gmichalski@ucsd.edu
AF: University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0356 United States
AU: Young, R
EM: burritofile@hotmail.com
AF: University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0356 United States
AU: Thiemens, M
EM: mht@chem.ucsd.edu
AF: University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0356 United States
AB: Oligotrophic Lake Tahoe has seen a decrease in opacity over the past 5 decades, which has been attributed to particulate matter (shoreline development) and algal growth (nutrients). The lake has also seen a shift from being nitrogen limited to phosphorous limited in the same time frame. Identifying the source of the increased nitrogen loading is essential for mitigation strategies to keep Lake Tahoe Blue - a 10 billion dollar watershed restoration campaign. Atmospheric deposition of nitric acid and nitrate aerosols (NO$_{3}^{-}_{atm}$) is thought to be a significant source of new N to the lake surface. Quantifying the flux and fate of NO$_{3}^{-}_{atm}$ is limited by modeling estimates of deposition, utilization and re nitrification of organic N. Stable isotope tracers can help resolve these limitations. $\Delta$$^{17}$O measurements, the $\delta$$^{17}$O enrichment over the expected .52 $\delta$$^{18}$O enrichment, have been shown to be a sensitive tracer of NO$_{3}^{-}_{atm}$. Oxygen isotopic analysis of NO$_{3}^{-}_{atm}$ from the basin have shown $\Delta$$^{17}$O values of ~ 22$\permil$. Lake water nitrate have $\Delta$$^{17}$O values of 1-4$\permil$ depending on depth and season, indicating that up to 20% of the lake nitrate is retained from the atmosphere. The $\delta$$^{18}$O values (-2.0 to 12$\permil$) cannot be used to estimate of the NO$_{3}^{-}_{atm}$ loading because of the wide range of $\delta$$^{18}$O values associated with nitrification. Variations of $\Delta$$^{17}$O with season can provide estimates of the flux of the nitrification of organic N. Balancing the isotopic budget with $\delta$$^{15}$N and $\delta$$^{18}$O measurements further constrains the N cycling dynamic within the lake. From these data a nutrient flux/utilization model can be developed.
DE: 4815 Ecosystems, structure and dynamics
DE: 1871 Surface water quality
DE: 1040 Isotopic composition/chemistry
DE: 1615 Biogeochemical processes (4805)
DE: 0330 Geochemical cycles
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting