MONITORING SEASONAL CHANGES IN WATER CHEMISTRY, PRIMARY PRODUCTIVITY, AND SEDIMENTATION IN A CARBONATE LAKE IN CENTRAL VERMONT

Monthly monitoring of water chemistry from November, 2013 to November, 2014 at Pecks Pond, Barre, VT, shows clear seasonal variations resulting from temperature-driven changes in primary productivity and precipitation-driven changes in the proportion of surface to groundwater flow. The correlation of seasonal climatic changes and water chemistry to sedimentation is more complex. During the winter-months, ice cover allowed the settling of clay-sized particles that were either introduced to the lake (via autogenic or allogenic processes) or resuspended during the previous spring, summer, and fall. Based on calculations of the ionic activity product for calcite (CaCO₃), Pecks Pond was at or above CaCO₃ saturation levels from early spring through the fall. An early increase in littoral CaCO₃ deposition in May was likely related to increased macrophytic productivity driven in-part, by an increase in phosphorous with spring meltwater. A temporary reduction in littoral CaCO₃ deposition in June, despite an increase in CaCO₃ saturation levels and continued productivity, may reflect the resuspension of sediments from wind-driven currents. A seasonal increase in CaCO₃ sedimentation did not occur in the deep-water until July, when phytoplankton primary productivity reached a high. These results suggest that at an annual-scale, increased CaCO₃ is correlated with increased primary productivity and supports the interpretation that increases in percentages of CaCO₃ in lake cores indicate prolonged periods of seasonal productivity. However, a direct correlation between increased summer productivity and increased lake temperature cannot be made because clearly, nutrient input and the trophic state of the lake are also important factors.

An additional and unintended finding of this study is the impact that the accumulation of road-salt within the watershed has had on Pecks Pond. As a result of a build-up of Na⁺ and Cl^- in the lake during the winter and the influx of dilute meltwater in the early spring, a vertical density gradient persisted and the lake only experienced a partial spring overturn. Without a full spring overturn the deeper parts of the lake remained enriched in dissolved ions, such as Ca²⁺, Mg²⁺, and HCO₃^- ,which supported phytoplankton growth and the development of eutropic conditions in July.