A PALEOLIMNOLOGICAL STUDY OF HOLOCENE SEDIMENTS IN MISSISQUOI BAY, LAKE CHAMPLAIN (USA-CANADA)

This project applies multi-proxy analysis of sediment cores from Missisquoi Bay (MB) to make interpretations about environmental changes within the bay and surrounding watershed during the last 9,000 years. Located at the end of the Northeast arm of Lake Champlain, MB is a large, shallow, well-mixed and unstratified embayment (mean depth = 2.8 m, surface area = 77.5 km²) that has recently experienced an increase in external nutrient loading (mostly due to agricultural runoff), leading to a state of eutrophication. High levels of internal nutrient loading appear to be maintaining a high level of productivity within the bay, despite efforts to reduce external nutrient sources.

In the winter of 2010, two sediment cores (275 and 230 cm in length) were retrieved from the ice covered MB using a piston coring device that captured a longer record than previous paleolimnology studies in MB. Core samples were analyzed for %C_org, %N, C/N ratio, biogenic silica, water content, grain size and magnetic susceptibility in order to make interpretations about the amount, type and distribution of organic matter in the MB sediment record. Five plant fragments found throughout the cores were radiocarbon dated.

We found an older period (~8,000-9,000 cal yr BP) of high aquatic productivity in MB that is distinctly different from the modern period of anthropogenic driven eutrophication. This older period is characterized by elevated %C_org, C/N and biogenic Silica values, and decreased magnetic susceptibility, and is thought to be related to the bay’s response to climatic variability. Warm and dry conditions during this part of the Holocene could have lowered lake level enough to lead to an expansion of rooted aquatic vegetation within the bay. This period of high productivity ended rather abruptly, and is followed by an extended period of low %C_org values. This transition may indicate a change in climate (to colder and/or wetter conditions) that limited productivity. Internal nutrient cycling or a lack thereof, may be an important driver of these changes within the bay on both the long and short-term time scale. A better understanding of the past lake environment may aid in our interpretation of the current situation in MB and in making predictions of future responses to climate change.