TEMPORAL VARIATION IN THE STABLE ISOTOPIC COMPOSITION OF SENECA LAKE (NY) WATERS

Seneca Lake is a large (lake surface area = 175.4 km²), monomictic, glacial, hardwater lake located in western New York. Weekly measurements of the isotopic composition of Seneca Lake water between March and October during 2009 through 2012 showed similar epilimnetic (-6.99 ‰) and hypolimnetic (-7.10 ‰) δ¹⁸O averages each year. Epilimnetic and hypolimnetic were very similar (within 0.02 to 0.1 ‰) March through late July. The δ¹⁸O values deviated during late July through October when the lake was thermally stratified. Epilimnetic waters were isotopically lighter than hypolimnetic waters by up to ~0.4 ‰. Hypolimnetic waters showed the least variability during the year.

Records of isotopic composition of precipitation in the region are limited. Streams in the Finger Lakes region are mainly fed by recent precipitation events through direct channel precipitation, surface runoff, or rapid flow through the shallow subsurface. Therefore, discontinuous measurements of headwater tributaries feeding Seneca Lake were also measured for their isotopic composition. These data provided the basis for establishing a local meteoric line. The δ¹⁸O of tributaries varies from -12.3 to -4.8 ‰. The δ¹⁸O and δD of the tributaries and lake water lie close to the GMWL. Variations in δ¹⁸O of the tributaries along the LMWL may be attributed to changes in the air mass origin feeding the precipitation, snowmelt, and/or temperature. The slope of the local meteoric water line (LMWL) is ~6.2, not too different from predicted values for a mid-latitude location. Seneca Lake waters plot below the LMWL, suggesting lake waters may have undergone evaporation and the isotopic composition of the residual lake water became progressively more enriched. The intersection of the δ¹⁸O of lake water with the LMWL is -8.3 ‰, which likely corresponds to the average isotopic composition of the water entering the lake. The δ¹⁸O of average epilimnetic lake waters is -7.1‰, suggesting input of isotopically lighter water during summertime contributes to the observed variation in δ¹⁸O.

Seneca Lake appears to buffer most seasonal variation in the δ¹⁸O of inputs from tributaries, likely due to its large volume (15,540 m³) and long residence time (~17 years). Average δ¹⁸O of lake waters may reflect the integrated seasonal variation in the δ¹⁸O of inputs.