Kate A. Evans

Year of Award

2023

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Organismal Biology, Ecology, and Evolution

Other Degree Name/Area of Focus

Ecology and Evolution

Department or School/College

Division of Biological Sciences

Committee Chair

Matthew J. Church

Commitee Members

James J. Elser, Royce C. Engstrom, Robert O. Hall, Jr., Scott R. Miller

Keywords

biogeochemistry, microbial ecology, oligotrophic lake

Abstract

Planktonic microorganisms are the primary drivers of energy flow in aquatic ecosystems, and their ability to rapidly respond to changing environmental conditions makes them key sentinels of ecosystem variability. However, understanding how microorganisms respond to variations in their environment requires improved understanding of microbial physiology and metabolism. Microorganisms that live in fluid environments experience a dynamic world, where resources needed for growth can be scarce or plentiful. In some cases, such variation can be directly linked to the physical movement of water, for example, vertical mixing. Temperate lakes experience strong seasonality in stratification and mixing, so resolving the relationships between physical environmental change and biological activity requires sampling that is intensive in both time and space.

This dissertation examines three fundamental, but previously undescribed, aspects of plankton ecology in Flathead Lake, which is a large, oligotrophic lake in northwestern Montana. Chapter 1 uses 16S rRNA gene sequencing of microbial communities to describe time and depth variation in the phylogenetic composition of resident taxa. I found that the microbial community responds to variations in stratification and mixing at time scales ranging from episodic to seasonal, but that the impact of these physical processes varies between taxa. In Chapter 2, I use time-resolved sampling to quantify variability in the relationship between light and photosynthetic production at two depths in the lake. The emergent phytoplankton photophysiology exhibits robust seasonality, including periods where light limits production and other times when photosynthesis is saturated by light. In addition, lake mixing underlies seasonality in photophysiology. Chapter 3 describes lake metabolism using O2:Ar ratios, a previously untested method in lakes, to disentangle the role of physical and biological processes in modifying dissolved O2 concentrations within vertically-discrete layers of the lake. On annual time scales, Flathead Lake is heterotrophic, consuming more O2 and organic carbon than is produced in situ. Such respiratory demands must be sustained by external sources of organic material, highlighting the dependency of lake biology on energy supply from the surrounding watershed.

Together, these chapters provide unique and novel information on the microbial ecology of Flathead Lake and serve as a multi-faceted case study on the functioning of large, oligotrophic lakes.

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© Copyright 2023 Kate A. Evans