Date of Award:

1989

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Ecology

Advisor/Chair:

Wayne A. Wurtsbaugh

Abstract

Bear Lake, a large oligotrophic lake (282 km2), was studied from October 1986 to December 1987 to determine the temporal changes in the zooplankton assemblage at one site in the pelagic zone and one in the littoral area. In this study, species composition, abundance, biomass, and size frequency distribution were determined. Additionally, chlorophyll a, water transparency, and temperature and oxygen profiles were measured to help interpret zooplankton changes during the study.

The zooplankton species assemblage comprised eight species of cladocera, seven species of copepods, and five species of rotifers. The abundance and biomass of the zooplankton assemblage, particularly that of crustaceans, were very low and comparable with those of oligotrophic systems. Mean densities of crustaceans in the pelagic zone, excluding copepod nauplii, varied from 250 to 1,700 organisms/m3. The analysis of the size structure of the zooplankton indicated the dominance of small organisms and the scarcity of large organisms, particularly cladocera. The zooplankton assemblage in the littoral zone was similar in species composition, abundance, and size structure to that in the pelagic zone. Many results suggest the littoral zooplankton assemblage is an extension of the pelagic assemblage.

Low zooplankton food resources and interference of calcium carbonate particles in the feeding behavior of crustaceans are suggested as the primary factors controlling the low abundance and biomass of zooplankton in Bear Lake. Size-selective fish predation probably causes the assemblage to be dominated by small species.

The analysis of Epischura nevadensis, the dominant species in the system, indicated that this species is bivoltine in Bear Lake. In the spring and summer, adult E. nevadensis were more abundant in the littoral zone, whereas copepodites were more abundant offshore.

The low density, biomass, and small size structure of the zooplankton in Bear Lake limits its importance as a source of food for fishes.

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