Date of Award

Summer 8-1-2026

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Earth & Environmental Systems

First Advisor

Jeffery Stone

Second Advisor

Jennifer Latimer

Third Advisor

Karlyn Westover

Abstract

Copepods are one of the most abundant types of zooplankton, and their populations are often a vital part of the food chain, but their past populations cannot be estimated from lake sediments because their exoskeleton usually does not fossilize. Gull Lake is a glacial lake located in the subalpine region of the Sierra Nevada, California, which stands out due to the presence of the diatom Fragilaria cyclopum, which is rarer in diatom assemblages. Fragilaria cyclopum exclusively grows on copepods in lakes. Diatoms have an opaline silica skeleton that often fossilizes in lake sediments, so we can use this relationship with Fragilaria cyclopum to infer the abundance of copepods in the lake even though this diatom species is often present in low relative abundance in lake sediments. To determine the abundance of Fragilaria cyclopum, light microscopy analyses were conducted on lake sediments to determine the relative abundances in three cores and from a collection of surface samples. X-ray fluorescence analysis of lake sediment samples was used to provide some evidence of environmental conditions preferred by Fragilaria cyclopum and copepods. The diatom abundance and XRF data were compared to the abundance of other diatoms in the lake system and to the timing of fish introductions in the lake system to determine the effect of fish stocking on copepod abundance. The timing of variation in Fragilaria cyclopum abundance nutrient changes suggest that trout introductions may have had a significant impact on the overall lake ecosystem through nutrient cycling changes and stratification and likely had an impact on the abundance of Fragilaria cyclopum and copepods in Gull Lake.

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