An Abstract of the Thesis of

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During the summers of 1967 and 1968 the horizontal distribution and diel vertical migrations of zooplankton were studied within the unique environment of Crater Lake. Sampling of the horizontal distribution was done by towing plankton nets vertically in different locations. The vertical distribution was sampled by towing at different depths with a standard tow net and Miller samplers. During both summers, the greatest numbers of zooplankton were sampled in late August. Bosmina longispina was the most numerous zooplankter, while Daphnia e, insignificant in 1967, increased in abundance during 1968. The horizontal distribution of B. longispina was clumped, being consistently more abundant in some locations than others. D. pulex had a random, or near-uniform, distribution. Vertical migrations were not consistent and seem to occur only during certain times of the year. The depth of the maximum concentration of B. longispina was found to vary between distances of 12.5 and 25 m, and was located at depths of 75 to 50 m during the day and between 50 to 37.5 m at night. A few B. longispina, however, did migrate to the surface at night. On August 28 and 29, 1968, the entire adult population of D. pulex migrated from 62.5 m during the day to the surface at night. This migration appears to have reproductive advantages. Prior to this time, only a nocturnal scattering of a small portion of the total population of D. occurred. Various ideas are given as to why these different migrations occurred, but based on the information that is presently available, any direct relationships between the variations of vertical migrations and environmental factors are difficult to make.

A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science, June 1969