In North America, land use practices of the last two centuries have strongly influenced aquatic communities and freshwater quality, but the impacts of prehistoric land use on freshwater resources remain poorly documented. Here we investigate the influence of prehistoric and historical land use on Horseshoe Lake, Illinois, USA, an oxbow lake in a floodplain of the Mississippi River that is adjacent to Cahokia, the largest prehistoric indigenous population center north of Mexico. Diatom assemblages from Horseshoe Lake’s sedimentary record track shifts in aquatic environmental conditions over the last ca. 1600 years. During the period of prehistoric population growth and agricultural intensification associated with Cahokia’s emergence (ca. 600–1200 CE), the relative abundance of Aulacoseira granulata—a planktonic diatom associated with shallow eutrophic lakes—increased. Following the abandonment of Cahokia in the 14th century CE, the diatom flora of the lake shifted from planktonic Aulacoseira taxa to the epiphytic taxa Cocconeis and Gomphonema. This shift in diatom assemblages is consistent with a reduction of nutrient inputs to the lake and/or reduced fishing pressure as prehistoric populations abandoned the area. Following the intensification of historic settlement after 1800 CE, diatom assemblages shift to epipelic species of small Staurosira and Fragilaria, indicating a reduction in aquatic macrophytes and increased turbidity. Our results document prehistoric indigenous impacts on a freshwater system beginning nearly 1000 years before European colonization of the Americas and demonstrate the antiquity of human impacts on freshwater resources in North America.
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Acknowledgements We thank Jim Gowen of the Illinois Department of Natural Resources for lake access, and gratefully acknowledge Jessica Blois, Brigitta Rongstad, Grace Schellinger, Jack Williams, and Chad Zirbel for field and laboratory assistance, and Kristina Brady, Amy Myrbo, Anders Noren, and Ryan O’Grady of the National Lacustrine Core Facility (LacCore) for assistance with initial core description. We also thank Stephen Juggins of Newcastle University for help in evaluating total phosphorus reconstructions with our data, and Julie Zimmerman of Southern Illinois University at Edwardsville for discussions about Cahokia Archaeology. This work was supported by the National Geographic Society (Grant Number YEG-9008-11) and the National Science Foundation (Grant Number BCS-1333070).
© 2018, The Author(s).
Copyright 2018 Elsevier B.V., All rights reserved.
- Horseshoe lake
- Oxbow lake
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