A HOLOCENE PALEOPRODUCTIVITY RECONSTRUCTION OF TWIN PONDS, VERMONT

Loss-on-ignition, macrofossil analysis, carbon and nitrogen isotopes, and radiocarbon dating of a sediment core from Twin Ponds, Brookfield, Vermont were used to reconstruct the paleo-productivity and ecology of the lake back to 10,000 year BP. An analysis of changes in the influx, size, and pigmentation of the resting eggs, or ephippia of the zooplankton, Daphnia sp., were the primary proxy by which productivity was reconstructed. The influx of D. ephippia shows several broad steps or shifts in equilibrium and an overall increase through time. The initial step occurs at 8200 yr BP, followed by a second step-up at 4100 yr BP, then 1700 yr BP, and finally a larger increase at 700 yr BP. These shifts to increasingly larger influxes of D. ephippia correspond with increases in the influx of organic carbon. Super-imposed on these millennial-scale shifts are centennial-scale peaks in both the influx of D. ephippia and organic carbon at 3200, 2400, 1800, 620, and 170 yr BP, while notable reductions in both proxies occurred between 6200-5950, 4400-4200, 2600-2900, and 1700-2100 yr BP. The observed increases in D. ephippia and organic carbon are interpreted as periods of increased lake productivity. Magnetic susceptibility data and lithologic observations suggest a shift in bottom anoxia at 4000 yr BP that marks the transition from an oligotrophic to a mesotrophic lake. The results from carbon and nitrogen isotopes will help confirm these interpretations. Data on the size and pigmentation of D. ephippia from 0-200 yr BP show a shift to smaller and darker ephippia that is consistent with evidence for increased fish predation. Prior to 4000 yr BP, the data on D. ephippia does not follow a predictable trend and implies that other environmental factors, such as temperature, water clarity, and overall productivity were more important controls on these characteristics. The step-like trends in productivity shown by this record suggest that the lake responded to large-scale climatic controls during the Holocene including, the collapse of the Laurentide Ice Sheet at 8200 yr BP and the decrease in seasonality after 4000 yr BP. Future work on developing an independent climate history from Twin Ponds will help determine the relationship between centennial-scale climate change, recent and historic land use, and changes in paleoproductivity.