HR: 0830h
AN: B41C-0891    [PDF]
TI: Iroquoian and Canadian Disturbance to the Ecosystem of Crawford Lake, Canada
AU: * Ekdahl, E J
EM: eekdahl@umich.edu
AF: Department of Geological Sciences, University of Michigan 2534 C.C. Little Building, Ann Arbor, MI 48109-1063
AU: Teranes, J
EM: jteranes@ucsd.edu
AF: Scripps Institution of Oceanography, University of California - San Diego, La Jolla, CA 92093-0244
AU: Stoermer, E F
EM: stoermer@umich.edu
AF: School of Natural Resources, University of Michigan, Ann Arbor, MI 48109
AU: Guilderson, T
AF: Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA 94551
AU: Turton, C L
AF: Royal Ontario Museum, 100 Queens Park, Toronto, Ont M5S 2C6 Canada
AU: McAndrews, J H
AF: Departments of Botany and Geology, University of Toronto, Toronto, Ont M5S 3B2
AU: Wittkop, C
AF: Limnological Research Center, University of Minnesota, Minneapolis, MN 55455
AB: A suite of geochemical and ecological proxy show perturbations to the Crawford Lake, Ontario, ecosystem by both Iroquoian horticulturalists during the 13th century and Canadian farmers during the mid-1800s. Carbonate and Total Organic Carbon (TOC) accumulation rates change markedly following initial human disturbance, increasing from 1 to 13 mg/cm$^{2}$/year and from 1 to 5 mg/cm$^{2}$/year, respectively. Carbon isotopes of inorganic carbonate increase from -6 to -1.8 per mil. The sudden preservation of varves and changes in carbon cycling show that the Iroquoian People initiated permanent water anoxia through increased algal organic matter production, related to elevated nutrient inputs caused by changing land use. An age model combining varve counts and 23 AMS $^{14}$C dates allows unparalleled temporal resolution in investigating ecological response to changing nutrient levels. Iroquoian nutrient input, synchronous with the first appearance of {\it Zea} (maize) pollen in the sediment record, produces rapid changes in diatom species assemblages, where pre-disturbance meso-oligotrophic communities largely comprised of {\it Cyclotella bodanica} and {\it Cyclotella michiganiana} are replaced by species adapted for higher nutrient concentrations. The changes in species assemblage occur prior to alterations to the carbon cycle, and suggest that algal community structure may be a more sensitive indicator of ecological change than some geochemical pathways. As Iroquoian activity within the region declines, geochemical parameters drift towards pre-disturbance levels. However, diatoms remain in post-disturbance assemblages. Land clearance by Canadian farmers with plow-style agriculture begins in the 1800s, as shown by increasing {\it Ambrosia} (ragweed) pollen in the sediment. Diatom productivity, carbonate, and TOC accumulations subsequently reach their highest levels following Canadian land-clearance, yet produce no carbon isotopic shift. Lack of a carbon isotopic signal is due to increased oxidation of biologically produced methane, and is a direct result of permanent water anoxia caused by the Iroquoians. While diatom communities may fluctuate following Canadian land clearance, overall species composition remains similar to post-Iroquoian assemblages, suggesting that initial perturbations may be most important in altering ecologic communities from their baseline conditions.
DE: 4239 Limnology
DE: 4805 Biogeochemical cycles (1615)
DE: 4806 Carbon cycling
DE: 4857 Pollution
SC: Biogeosciences [B]
MN: 2003 Fall Meeting