ECOLOGICAL INTERACTIONS AFFECTING DIATOM CLIMATE RECONSTRUCTIONS IN PRAIRIE SALINE LAKES OF THE NORTHERN GREAT PLAINS (USA)

ECOLOGICAL INTERACTIONS AFFECTING DIATOM CLIMATE

RECONSTRUCTIONS IN PRAIRIE SALINE LAKES OF

THE NORTHERN GREAT PLAINS (USA)

By Courtney R. Wigdahl

Thesis Advisor: Jasmine Saros

A Lay Abstract of the Thesis Presented

in Partial Fulfillment of the Requirements for the

Degree of Doctor of Philosophy

(in Ecology and Environmental Science)

December, 2012

Key words: drought; Great Plains; lakes; sediments; diatoms; salinity

Drought can have major economic impacts around the world, particularly in agricultural regions, by limiting food production and availability of water resources. Longer-term perspectives on drought frequency and intensity are needed to predict future patterns of drought and develop management plans in the face of global climate change. Paleolimnology can extend this context for previous drought severity beyond instrumental records, by using fossilized biological remains in lake sediments to reconstruct past drought conditions. In the northern Great Plains (USA), sediment records containing diatoms, a type of algae, from inland salt lakes are frequently used to reconstruct lakewater salinity as an indicator of drought. Salinity in these systems fluctuates in response to changing drought conditions, driving changes in diatom communities living in these systems. Though diatoms can serve as a proxy for past drought conditions, important disparities are evident across geographically-close sites in this region.

Here, I explore how within-lake ecological processes, such as physical changes in lake habitat and grazing by zooplankton, may affect the accuracy of diatom-based drought records. The accuracy of drought reconstructions was affected by site-specific characteristics that interfered with the representation of lakewater salinity signals within diatom records. Three-dimensional basin models showed that habitat changes can strongly influence diatom communities. Zooplankton community structure varied through time at each site, and the relationship between zooplankton species and the proportion of inedible diatoms also differed between lakes. This suggests that the influence of grazing on diatoms is likely unique at each lake, depending on the ecology of dominant zooplankton species and other within-lake ecological processes. New diatom-based salinity records, based on ecological subsets of diatoms rather than all diatom species that were present, improved the accuracy of drought reconstructions at 5 of the 6 sites that I examined. However, at some lakes, ecological features of diatom assemblages were more representative of drought than salinity signals. The integration of additional ecological information into interpretations of past climate records, particularly for biologically-based reconstructions, is important to improve the accuracy of climate reconstructions as well as our understanding of site-specific responses to regional environmental changes.