The zooplankton community in Crater Lake was comprised of 11 rotifer species and 2 species of cladocerans. Most zooplankton taxa were distributed in winter and spring from the lake surface to a depth of about 200 m, the maximum depth of mixing of lake waters by wind energy. The distribution of zooplankton species was partitioned in the water column to a depth of 200 m during summer and fall, which corresponded to the period when the lake was thermally stratified. At that time of year, zooplankton density in die upper 20 m of the water column was very low, whereas highest densities were found in the depth interval between 80 and 120 m. Closely related or competing species were found in different portions of the water column. Daphnia pulicaria the largest cladoceran species, was cyclic in abundance, and its density corresponded to patterns of lake productivity and fish predation. When D. pulicaria was abundant, abundances of rotifers and Bosmina longirostris declined and changes in the vertical distribution of Bosmina were observed. Relationships between variations in Secchi disk clarity and zooplankton abundances in the upper 40 m of the water column were not obvious.
