Recent Changes in the Zooplankton Assemblage of Waldo Lake, Oregon

ABSTRACT

Since 1969, two years after introductions of Mysis relicta, the species composition and total density of zooplankton in ultraoligotrophic Waldo Lake have undergone significant changes. During our study, spanning 1969–1998, taxa richness and zooplankter body size decreased. Zooplankton density increased by three orders of magnitude, from <1 m⁻³ to greater than 600 m⁻³. Hesperodiaptomus persisted from 1969 through 1998, and appears to be a major determinant in zooplankton dynamics. Rotifers, except Collotheca, were held to extremely low numbers, and cyclopoid copepods were eliminated. Bosminal longirostris, the only cladoceran which is presently common in the limnetic zone of this high altitude lake, exhibits daytime avoidance of the surface waters possibly due to high light intensities.

Key Words:

• long-term limnological research
• Waldo Lake
• Cascades
• Bosmina
• Hesperodiaptomus
• Mysis