Evaluating the Effect of a Changing Climate on Thermocline Depth in Maine\u27s Great Ponds

Wind speed has been declining during the last few decades across terrestrial habitats in North America, and spring ice-out dates have become progressively earlier since the mid-19th century. These climatic changes are of interest because of their potential to alter the thermal structure of lakes. I used fossil diatom records to evaluate how these climatic changes have altered thermocline depths over time in large Maine lakes. Sedimentary diatom profiles spanning the last 3-5 centuries were determined and water-column sampling was conducted in three lakes chosen because of their large surface areas (\u3e500 ha) and their distribution across the state. Two diatom taxa, Discostella stelligera and Aulacoseira species, were used to reconstruct mixing depth using a diatom-based mixing index. These species were chosen because Discostella species have a higher relative abundance under shallower mixing conditions, whereas Aulacoseira species require deeper mixing. Diatom-inferred mixing depths became shallower in two of the three lakes, with strong declines between the mid-19th and 20th centuries. These inferred declines in mixing depth corresponded with either the onset of longer ice- free seasons or declines in wind speeds. In the third lake, seasonal phytoplankton dynamics revealed that Discostella taxa do not bloom during stratified conditions, suggesting the diatom inference model is not appropriate for this system. Consequently, diatom-inferred mixing depths showed no directional change in this lake over time. Results of this study further our understanding of climate- induced changes in lake ecosystems and highlight the importance of pairing paleological reconstructions with contemporary water column sampling