Research Projects

The enrichment of oxygen 17 (17O) observed in photochemically produced nitrate is a powerful tracer of the deposition and transformation of atmospherically derived nitrogen (N) to watersheds. This novel approach was applied, on a small scale, to address the question of the importance of atmospheric N deposition to Lake Tahoe eutrophication. Measurements of 17 oxygen isotopes in nitrate aerosols collected in the Tahoe Basin showed similar enrichments as those analyzed from southern California with an average Ä17Oof 24‰. This atmospheric isotopic signal was assumed to be the same at Lake Tahoe based on comparative measurements, which was deposited to the lake surface generating a water column nitrate Ä17O average of 4‰. This means that ~ 20% of water column nitrate has retained its photochemical isotopic character and has not been recycled by biota. Using N deposition estimates from other Tahoe researchers, we derive a lake average nitrification rate of 1 mg/m2/yr. A significant number of atmospheric and lake nitrate samples were lost during the analytical process due to unexpected loading of organic material in the solutions. Consequently, a portion of the samples collected have been archived and will be analyzed and reported to the ARB once a more effective analytical method has been developed. Initial test results on this new analytical method suggest that it will be applicable by the end of 2007.