ENVIRONMENTAL HISTORIES OF TWO WISCONSIN LAKES RECONSTRUCTED FROM GEOCHEMICAL AND GEOLOGICAL PROXIES

Freshwater systems are important natural resources, yet they face multiple environmental threats from human activity. Preservation and restoration of freshwater systems is, therefore, vitally important to the health of natural ecosystems and the humans that depend on them. Efforts to restore freshwater ecosystems can be aided by environmental histories created from the geological record that shed light on the major anthropogenic impacts on the system, as well as provide a natural baseline. Here, we describe the use of geochemical proxies to create environmental histories of two Wisconsin Lakes. Lake Monona in Madison represents a highly impacted, urban lake and Shadow Lake in Waupaca represents a remediated lake. Magnetic susceptibility and Pb-210 activities suggest sedimentation rates of approximately 2mm/yr in each lake. Heavy metal concentrations in the cores were measured using X-ray fluorescence; increases in tin, zinc, arsenic, copper, and lead were noted in the upper 50 cm of cores from both lakes, although these metals were much more abundant in Lake Monona. These increases reflect human impacts, including the widespread use of chemical herbicides and the growing importance of leaded gasoline at the beginning of the 20^th century. Carbon, oxygen, and nitrogen isotopic ratios of core sediments from each of the lakes were also used to reconstruct environmental histories. Stable oxygen and carbon isotopes from Lake Monona reflect the large size of the lake, and are fairly constant through time. However, shifts in the d¹³C of Lake Monona carbonates towards the top of the core hint at a change in the primary carbon source as human impact increased. Shadow Lake, which is much smaller, displays opposite trends. Pre-European settlement, oxygen isotopic ratios display high variability, but with increasing impact, variability decreases. Carbon isotopic ratios show an increasingly positive trend, indicating higher aquatic primary productivity. However, in the upper 8 cm of the core, d¹³C values begin trending again toward the negative, reflecting remediation efforts to decrease nutrient load. Environmental histories from geochemical proxies indicate that human activities have impacted these lakes disrupting ecosystem function and warranting management and remediation efforts to restore full ecoystem function.