HISTORIC CHANGES IN SEDIMENTATION AND PRODUCTIVITY AT TWIN PONDS, VERMONT

Watersheds in Vermont have seen extensive historic land use change which resulted in altered erosion rates and aquatic ecosystems. This study examined short-sediment cores (70-120cm length) from Twin Ponds located in Brookfield, VT to identify the impacts of historic land use change on sedimentation and lake productivity. Two shallow cores taken from a littoral bench dominated by the macroalgae, Chara sp. and a deep core taken from the center of lake were analyzed for: organic and inorganic carbon using loss-on-ignition, sand-sized grains, and macrofossils of Chara sp. and a planktonic crustacean, Daphina sp. Both taxa are influenced by the abundance of algae which is closely tied to the trophic state of the lake and are thus used as indicator species for historical productivity.

The LOI analyses show an increase in percentages of organic carbon in all cores starting at ca. 70 cm. For the shallow cores which are distal to the inlet and rich in CaCO₃, organic carbon increases from 6-9% at the bottom of the cores to 13-22% and then returns to lower levels (7-11%) at the top of the cores. In the deep-water core, organic carbon increases from ca. 40% at the core base up to 60% and then declines again towards the top of the core to ca. 40%. The results of the macrofossil analysis on a shallow core show an inverse relationship between concentrations of Chara oospores and Daphnia ephippia. Chara oospores decline steadily from the base of the core to 30cm depth where the lowest concentrations of the record (10.1/mL) are seen. Concentrations of Daphnia ephippia begin high, decline and then steadily increase to a 2.5/mL peak at 25 cm. This sequence is then followed by a dramatic increase in Chara oospores up to 160/mL and a congruent decline in Daphnia ephippia to ca. 0.2/mL. A similar trend in Daphnia ephippia concentrations is seen in the deep water core. Lower concentrations of Chara and higher concentrations of Daphnia fossils immediately follow the increase in organic carbon evident throughout the lake and suggest a temporary increase in algae abundance and a shift towards more eutrophic conditions. While this event has yet to be dated at Twin Ponds, a similar increase in organic carbon at nearby Knob Hill Pond has been correlated with European settlement. Further macrofossil and grain-size analyses and Pb-210 dating will test this interpretation.