First Advisor

Yangdong Pan

Term of Graduation

Summer 2025

Date of Publication

8-20-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.) in Earth, Environment, & Society

Department

Earth, Environment, & Society

Language

English

Subjects

Daphnia, Ecology, Lakes

Physical Description

1 online resource (xiv, 209 pages)

Abstract

In the modern era, commonly referred to as the Anthropocene, human-caused changes to the environment are prevalent and complex. The study of impacts to biodiversity and ecosystem functioning requires multi-faceted approaches that encompass scales of organization from genes to ecosystems. It is often difficult to tease apart whether species responses to environmental change are due to phenotypic plasticity or genotypic variation. Therefore, it is important to try to understand where variation originated, that is, to link a phenotypic response to a genetic change or to gauge whether the response is from the environment. In addition, neutral genetic variation arising from gene flow or genetic drift structures populations and can be influenced by environmental change. Understanding how phenotypic and genotypic variation originate and are maintained is necessary to better grasp the drivers of and responses to environmental change. In my dissertation, I focused on Daphnia, a cladoceran zooplankter with a diverse life history, genome, and mechanisms of dispersal. My goals were to collect genetic and phenotypic measurements from Daphnia across a diverse and geographically widespread group of lakes and multiple time periods to learn how ecological drivers might structure variation in the face of environmental change.

Lakes possess an island-like nature with well-defined boundaries and are therefore amenable to the study of local individual and population variation, as well as ecological interactions. Additionally, lakes are connected across landscapes to other water bodies, the terrestrial landscape, and the atmosphere. Lakes are also repositories of information over time through accumulation and preservation of physical, chemical, and biological materials in the sediment. In addition, lakes are often home to organisms that have been transported across the intervening landscape, such as plants and zooplankton, through vectors such as wind, water, animals, or even humans.

My study lakes were in the arid Channeled Scablands region of southeast Washington, east of the Cascade Mountain range, including some of the lakes and canal systems of the Columbia Basin Project (CBP). Other study lakes outside of the CBP, while not subject to water construction projects, were also subjected to multiple stressors.

I provide background information about the relevance of lakes and zooplankton, particularly Daphnia to the overall scope of the study in Chapter 1. In Chapter 2, I used population genetic analysis combined with environmental data collected from the study lakes and information on dispersal vectors in this region to model drivers of genetic variation over a broad geographic range. I found both evidence of Daphnia gene flow between two lakes across a highly trafficked interstate corridor and relationships in some lakes between genetic variation and bird dispersal, indicating that this species was dispersed by human and bird vectors. Next, in Chapter 3, I researched changes to Daphnia abundance and size over time in a single lake that had been subjected to known historical anthropogenic changes and contemporary recovery. Daphnia produce resting eggs that are preserved in the sediment and can persist for decades to centuries. In Chapter 3, I compared lake sediment nutrient data to that from preserved resting eggs over approximately 150 years from sediment cores. I found a positive relationship between sediment nutrients and resting egg abundance, allowing a reconstruction of possible environmental variables that highlight trends of a long-term lake restoration effort. Finally, in Chapter 4, I explored Daphnia resting egg size as a proxy for adult female body size from multiple lakes between historical and contemporary time periods. I also evaluated hatching success of resting eggs between time periods. I evaluated relationships between abiotic and biotic variables, as well as hatching success, with resting egg size. I found evidence of shifts in trait richness, which is the range of trait values (here, size), within populations between the contemporary and historical time periods. I also found differences in hatching success between time periods, with higher hatching success in the historical time period.

In this dissertation, I found several trends that inform research into the ecology of lakes, especially how ecological drivers impact variation in the face of environmental change. First, I found patterns in population genetic variation showing that Daphnia is dispersed through diverse modes and had gene flow between lakes, indicating at least some ability to overcome the properties of cyclic parthenogenetic Daphnia populations that can lead to genetic structure from founder effects and local adaptation. My study showed that human-caused dispersal could have a disproportionate impact on Daphnia genetic variation across space. Additionally, I found a positive relationship between Daphnia abundance and nutrients over time, illustrating the sensitivity of populations to changes in environmental conditions. The study of Medical Lake was unique because the Daphnia population response tracked a restoration effort through time but also uncovered eutrophication patterns through the use of paleolimnological methods. Finally, I observed trends in body size over an environmental gradient and time, illustrating that Daphnia individuals respond to environmental changes and that these responses can be monitored over an informative temporal scale by studying sediment cores. I investigated intraspecific trait variation, specifically trait richness of body size, across a large geographic range and across time to understand Daphnia phenotypic variation. I also uncovered novel information on hatching processes of Daphnia, which could have critical impacts on ecological interactions and biodiversity in lakes.

Overall, I pursued several avenues of research that would be useful for management of species and ecosystems in the face of environmental change. Using genetic data to investigate patterns in organisms with diverse dispersal modes can inform how dispersal vectors are structuring populations across space. Research on threatened and endangered species, as well as non-native species could benefit from a closer look at genetic variation in order to see patterns of gene flow or genetic drift. Few lakes have long-term monitoring data, so therefore tracking environmental changes over time is difficult, and is especially useful for understanding baseline conditions for restoration. I found value in reconstructing disturbance and restoration with paleolimnological techniques. Finally, in my last chapter, I evaluated how individual variation in an important functional trait could inform drivers of environmental change. Several global lake management plans incorporate zooplankton as indicator species and this research indicates that those plans should continue and expand. By evaluating Daphnia at multiple scales of biological organization and across time and space, this dissertation uncovered ecological patterns and processes necessary to better understand the impacts anthropogenic change.

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Persistent Identifier

https://archives.pdx.edu/ds/psu/44116

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