High Turnover Rate in Portage Lake
Ann Arbor, MI — As lakes in the Northern U.S. warm throughout the late spring and summer, they tend to stratify, forming a warm surface layer over a colder and denser bottom layer. How often these layers mix and reform can strongly impact lake biota. This is because the lower layer in a stratified lake tends to be rich in nutrients that stimulate plankton growth when delivered to upper waters, where high light levels promote photosynthesis. Mixing and restratification are processes to which the plankton in Michigan's Portage Lake, which is connected to Lake Superior via navigation channels, are apparently well accustomed.
"What we've found in Portage Lake is a tendency for the lake to fully mix during cold air outbreaks and then restratify in the warm periods that follow," noted Jim Churchill, Research Specialist at the Woods Hole Oceanographic Institution in Woods Hole, MA.
Data collected in Portage Lake during the summer of 1999 showed four events of lake mixing and subsequent restratification. Analysis of the data by Churchill and colleague Charles Kerfoot of Michigan Technological University indicate that each time the lake restratifies, it then becomes more susceptible to overturn from surface heat loss during the next cold air outbreak. Their measurements also reveal that an ephemeral warm layer sometimes appears in the very near surface of the lake, forming during daylight hours and then dissolving overnight.
"The interaction between lake and atmosphere clearly has more than just physical implications," noted Churchill. "It is our hope that this study will be the basis of future work directed at understanding how resident biota in Portage Lake, and in similar lakes, may benefit or be harmed by the complex changes in lake stratification occurring in summer and early fall."
Original Publication Information
Results of this study "The Impact of Surface Heat Flux and Wind on Thermal Stratification in Portage Lake, Michigan," are reported by James H. Churchill and W. Charles Kerfoot in the latest issue (Volume 33, No. 1, pp. 143-155) of the Journal of Great Lakes Research, published by the International Association for Great Lakes Research, 2007.
Contacts
For more information about the study, contact Jim Churchill, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, jchurchill@whoi.edu, (508) 289-2807.
For information about the Journal of Great Lakes Research, contact Marlene Evans, Editor, National Water Research Institute, Environment Canada, 11 Innovation Boulevard, Saskatoon, SK, S7N 3H5, Canada; jglr@ec.gc.ca; (306) 975-5310.
Links
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