"Isotopic analysis of sediments as a proxy for environmental quality in temperate lakes of Wisconsin"

John Dannehl, Thesis 2017

Abstract: Anthropogenic changes to lacustrine ecosystems in the last century, such as deforestation, eutrophication, and water pollution, must be addressed to improve lake water quality conditions in the United States. Geochemical tools like stable isotopes help us better understand how humans affect lacustrine environments, as well as facilitate management strategies for restoring biodiversity and essential ecosystem services lost due to human activity in the biosphere. Stable isotopes preserved in lacustrine sediments are one such way to reconstruct the environmental history of a region. Here, we use stable isotope analysis from temperate lakes in Wisconsin to highlight paleolimnological changes in response to increasing stress due to human activity. Cores from Lake Monona and Shadow Lake provide a record of lacustrine responses to increased nutrient loading after human settlement began in the mid-1800s. Several changes in the biogeochemical cycle in these lakes have occurred due to cultural eutrophication, such as decreased oxygen and increased heavy metal concentrations (McNelly and Turyk 2012). I measured 13C/12C and 18O/16O ratios of organic and inorganic C (calcite) in the core sediments to infer decadal trends in nutrient loading and primary production at these sites. My preliminary results indicate that d13C values from Lake Monona sediments 50-140cm below the sediment water interface oscillated in a natural pattern before decreasing sharply between 50cm and the surface of the lakebed. d18O values between 50-140cm vary in a similar pattern but bunch together from 50cm to the interface. I hypothesize that increased nutrient loading and eutrophication from agricultural and urban development of the area drove changes in d13C values. Changes in d18O values point to disruption of thermal stratification in the lakes and suggests a link to sudden climatic changes in the last century. These changes indicate that ecological stresses of anthropogenic origin warrant renewed management strategies that can return lacustrine environments to their natural state.

Full thesis available. Contact the Geology Department at 540-458-8800.