William M. Lewis, Jr.
Ph.D. Indiana University at Bloomington, 1973
Director, Center for Limnology
Professor, Ecology and Evolutionary Biology
Aquatic ecosystems, tropical fresh waters, biogeochemistry of inland waters, nutrient cycling, aquatic food chains.
Current Research Projects: Limnology 66 million years ago
Extinction of more than half of Earth’s plant and animal species, including all dinosaurs not capable of flight, occurred 66 million years ago, following the Chicxulub asteroid impact. The impact created a fierce infrared blast from hot particles distributed throughout the atmosphere and, subsequently, an impact winter caused by particles shading Earth’s surface. Mechanisms of extinction following the asteroid impact have been widely and hotly debated. Led by CIRES Fellow Emeritus Doug Robertson, a team including atmospheric scientist Brian Toon, with CU-Boulder’s Department of Atmospheric and Oceanic Sciences; paleontologist Peter Sheehan, with the Milwaukee Public Museum; and myself developed a set of ideas (forthcoming in two papers published by the Journal of Geophysical Research) that explain mechanisms of extinction. Robertson et al. propose that the firestorm immediately following impact and the subsequent impact winter were two separate mechanisms of extinction. The initial heat burst likely had severe effects on terrestrial organisms but not on aquatic organisms because of water’s capacity to intercept long-wave radiation. Clues supporting this contention include differentially low extinction of freshwater species and, among terrestrial species, selective survival of species that live primarily underground. An unresolved puzzle is differential extinction rates between marine taxa, which showed severe extinction, and freshwater taxa, which showed moderate extinction. Robertson et al. argue that marine species are less resilient physiologically and less likely to show dormancy than freshwater organisms. Freshwater organisms occupy habitats that are much less stable than the marine environment and, thus, are favored evolutionarily by adaptations, including dormancy, that allow them to survive environmental instability. In addition, freshwater organisms have access to a network of aquatic refugia provided by points of groundwater entry into drainage networks. These refugia would have been much warmer than surface waters at the lowest temperatures expected during impact winter. Furthermore, freshwater taxa that feed on nonliving (detrital) organic matter would have been favored by continuous supplies of new detrital organic matter entering freshwater ecosystems from terrestrial sources via surface or groundwater runoff, whereas marine organisms in the open ocean would not have been. These factors, taken together, explain why the impact winter was a much less potent source of extinction for freshwater organisms than for marine organisms (see figure).
Diagram of contrasts between freshwater and marine environments for factors potentially causing extinction in aquatic environments after the Chicxulub impact.
Lewis, W. M. Jr, and W. W. Wurtsbaugh. 2008. Control of Lacustrine Phytoplankton by Nutrients: Erosion of the Phosphorus Paradigm. Internat. Rev. Hydrobiol. 93: 446-465.
Lewis, W. M. Jr. 2002. Causes for the high frequency of nitrogen limitation in tropical lakes. Verh. Internat. Verein. Limnol. 28: 210-213.
Lewis, W. M. Jr. 2001. Wetlands Explained. Wetland Science, Policy, and Politics in America. Oxford University Press, New York, NY.
Lewis, W.M., Jr. 2002. Yield of nitrogen from minimally disturbed watersheds of the United States. Biogeochemistry 57/58: 375-385.
Lewis, W.M., Jr., S.K. Hamilton, M.A. Lasi, M. Rodriguez, and J.F. Saunders, III. 2001. Foodweb analysis of the Orinoco floodplain based on production estimates and stable isotope data. Journal of the North American Benthological Society 20: 241-254.
Lewis, W.M. Jr., S.K. Hamilton, M.A. Lasi, M. Rodriguez, and J.F. Saunders, III. 2000. Ecological determinism on the Orinoco floodplain. BioScience 50: 681-692.
Honors and Awards
- Past President, American Society of Limnology and Oceanography
- Member of the National Research Council's Water Science and Technology Board, 1993-1999
- Recipient of the Naumann-Thienemann Medal of the International Society of Pure and Applied Limnology, 1999
- Sustained Achievement Award of the Renewable Natural Resources Foundation, 1996
- Chair, National Research Council Committee on Endangered and Threatened Fishes in the Klamath River Basin (2001-2003).
Professor Lewis is a CIRES Fellow.