SEDIMENT CARBON DYNAMICS IN NORTHERN WISCONSIN LAKES IN RELATION TO DEFORESTATION, DROUGHT AND LAKE LANDSCAPE POSITION

Northern temperate kettle lakes have sequestered a large pool of carbon in their sediments over the past ~10,000 years and researchers are interested in predicting the role of these lakes as carbon 'sources' or 'sinks' in the context of changing climate. Historical rates of sediment carbon accumulation have varied over time and between lakes but this variability is not well-quantified nor incorporated into carbon budgets for use in modeling future scenarios. In an effort to characterize the extent and magnitude of variation in sediment carbon accumulation in relation to deforestation, drought, and lake landscape position, we collected ²¹⁰Pb-dated sediment records of total organic carbon content, algal pigment concentrations, and δ¹³C stable isotope signatures over the past ~200 years from several lakes in northern Wisconsin.

Preliminary results from two lakes show increasing concentrations of pigments indicative of cyanobacteria (echinone) and siliceous algae (diatoxanthin) over a period of clear-cut logging activities (1880-1910 CE). These trends may reflect increases in the availability of catchment-derived ions and nutrients to aquatic primary producers due to decreased uptake by terrestrial vegetation. Post-logging (1910-1930 CE) trends in carbon accumulation rates (gC/cm²/yr) and overall productivity (indicated by %TOC, δ¹³C, beta-carotene nmol/g) differ between the two lakes. Productivity levels appear to return to pre-logging conditions in Allequash but not in Crystal. Differences between the two lakes could be influenced by lake landscape position. Allequash, a flow-through drainage lake, may have rebounded to pre-logging conditions more easily than Crystal, a precipitation-fed seepage lake with a longer residence time.