IT'S NOT OUR FAULT: EVIDENCE OF EUTROPHICATION FROM SEDIMENT STABLE ISOTOPES OF LAKE GEORGE, NY

Lake George (LG), NY is a headwater lake with Class AA Special water quality rating, surrounded by forever wild forest, and is a valuable tourist destination. Lake monitoring reveals increasing anthropogenic impact over the past 30 years (Boylen et al., 2014). We conducted a multiproxy study using microfossil assemblages (testate amoebae, ostracods, diatoms, pollen, and non-pollen palynomorphs) and bulk sediment stable isotopes (N, C) from 8 cores and 13 Ekman samples to assess the modern lake bed and reconstruct anthropogenic influence (e.g., salt loading, eutrophication, pollution, temperature) from pre-instrumental records preserved in the lake sediments. Sedimentation rates are based on ragweed horizons and will be corroborated by radiocarbon dates.

δ¹⁵N values of bulk sediment experience a 2.0‰ shift upcore, indicating a change in source toward lacustrine algal (+8‰; Peterson et al., 1985) from a vascular, land plant signal (+1‰; Peterson et al., 1985). This 2.0‰ shift may also indicate a decrease in lake level (Meyers et al., 1998). The highest δ¹⁵N (6.2‰) value is in a surface sediment sample near the village of Lake George and may show an alternate source, such as fertilizer or human waste influx. Upcore decreases in δ¹³C values of bulk organic matter are consistent with industrial era fossil fuel burning (Suess Effect; Keeling, 1979).

Larger grain size and higher C/N ratio suggest terrestrial source and C₃ land plants, with an upcore shift in C/N, δ¹⁵N, and %C values likely indicating an increase in productivity in the lake (relatively more lacustrine algae and plankton; Meyers and Lallier-Vergès, 1999). Unique C/N ratios at Warner Bay may be attributed to the Big Blowdown of 1950, pending radiocarbon date confirmation. Shifts to higher productivity upcore suggested by stable isotope values are also illustrated by testate amoebae and NPP assemblages (future work will incorporate diatom assemblages). This trend toward higher relative lake productivity and eutrophication occurs prior to human habitation of the surrounding watersheds, leading us to interpret LG shift towards eutrophication to be predominantly non-anthropogenic. Sediment stable isotopes support our interpretations of microfossil assemblages, particularly the utility of testate amoebae as an eutrophication proxy.