Date of Award

2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Environmental Engineering

Abstract

Over the past 2000 years, the hydroclimatic setting of Pretty Lake, a 16,000-year-old, hydrologically closed kettle lake in northeastern Indiana, has shifted from cool and dry to warm and wet. The lake has experienced marked periods of increased and decreased average temperatures during the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA), respectively. Major shifts in the dominant diatom taxa within the lake system correspond well with hydroclimatic records concerning the MCA and LIA in Europe and are interpreted as responses to changes in stratification and nutrient cycling. Recent trends in anthropogenically forced climate change and human land use are reflected in the diatom assemblage as increasing thermal stratification in summer and increasing phosphorus inputs to the lake system. The Pacific-North American teleconnection (PNA), an upper atmospheric phenomenon that strongly affects the Midwest during winter months, may cause shifts in the diatom record. It is characterized by the following changes in the environment of Pretty Lake: negative PNA being more associated with periods of increased lake level and stronger summer stratification and positive PNA being more related to periods of decreased lake level and increased ice cover duration. During the last 150 years of the record, the effects of human development around Pretty Lake dominate the diatom record and obscure connections to the PNA. Ultimately, however, shifts in the diatoms of Pretty Lake are most strongly forced by changes in nutrient cycling and the strength of stratification.

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