Date of Award

12-2012

Level of Access

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ecology and Environmental Sciences

Advisor

Jasmine Saros

Second Committee Member

Sherilyn C. Fritz

Third Committee Member

George Jacobson

Abstract

Sedimentary diatom profiles from saline lakes are frequently used to reconstruct lakewater salinity as an indicator of drought. However, diatom-inferred salinity reconstructions (DI-salinity) from geographically-close sites in the Great Plains (USA) have yielded disparate results. Here, I explore how within-lake ecological processes, such as physical changes in lake habitat and zooplankton grazing pressure, may affect the accuracy of diatom-based salinity reconstructions. I examined how relationships differed among drought, lake-level change, and diatom community structure over the last century by developing three-dimensional models of planktic:benthic habitat (P:B) relationships with lake level change. I explored the potential for zooplankton grazing influence by comparing fossil zooplankton records with diatom inedibility for two prominent sites. I also examined ecological characteristics for diatom communities over the past century at six sites in the northern Great Plains, and developed new DI-salinity transfer functions based on ecological subsets of diatoms rather than full communities. I compared these reconstructions and characteristics with instrumental drought records to understand drivers of change in diatom community structure and how drought conditions were recorded in fossil diatom assemblages on a site-by-site basis.

The accuracy of drought reconstructions were affected by site-specific ecological characteristics that interfered with the representation of lakewater salinity signals within diatom records. Three-dimensional basin models showed that P:B habitat changes can strongly influence diatom communities, and in simple basins, P:B of sedimentary diatoms may be a preferable proxy for drought over DI-salinity. Zooplankton community structure varied through time at each site, and the relationship between zooplankton taxa and diatom inedibility also differed between lakes. This suggests that influence of grazing pressure on diatoms is likely unique at each lake, depending on the ecology of dominant zooplankton taxa and other within-lake ecological processes. New DI-salinity reconstructions, based on ecological subsets of diatoms, improved correlations with instrumental drought records at 5 of the 6 sites, though at some lakes ecological features of diatom assemblages were more representative of drought than DI-salinity reconstructions. The integration of additional ecological characteristics into interpretations of paleoclimate records, particularly for biologically-based reconstructions, is important to improve our understanding of site-specific responses to regional environmental changes.

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