SUMMARY

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1 . Studies of the horizontal distribution and diel vertical migrations of zooplankton were initiated because of the unique environment created by the unusual optical and thermal properties of Crater Lake.

2. Plankton nets were towed vertically in different locations and horizontally at different depths in order to sample, respectively, the horizontal distribution and vertical migration of zooplankton during the summers of 1967 and 1968.

3. Bosmina longispina was numerically the most abundant zooplankter sampled during the course of the study. Daphnia pulex, insignificant in 1967, increased in abundance during 1968, and in some locations may have dominated the zooplankton biomass by August.

4. During both summers the greatest numbers of zooplankton were sampled in late August. However, it is not known when the zooplankton reach a maximum density.

5. A monocyclic seasonal variation seems to be indicated by the rapid increase of zooplankton numbers during late August of both 1967 and 1968,

6. Studies of the horizontal distribution indicated that B. longispina was clumped, and was consistently more abundant in some locations than others. D. pulex did not have any definite dispersal pattern, and a random or near uniform dispersal seems likely.

7. Upwelling as well as vertical migrations may have affected the horizontal distribution of both D. and B. longispina.

8. Seasonal and annual variations in the vertical distribution are apparent. Vertical migrations are not consistent and may only occur during certain times of the year.

9. Vertical migrations of B. longispina were only represented by a fraction of the entire population. The depth of their maximum concentration was found to vary only a distance of 12.5 to 25 m. No major differences were observed between the vertical distribution of adults and juveniles.

10. Vertical migrations of D. pulex were represented by the entire adult population in late August 1968. No apparent migration of juveniles was observed at this time. Prior to August 28 and 29, 1968, only a portion of the entire population underwent vertical migrations.

11. The range of the vertical distribution of zooplankton, contrary to previous investigations, was shallower than 125 m. Estimates of contamination of samples by zooplankton encountered in the shallower depths indicated that few zooplankters, if any, were present at 125 m.

12. It was not possible to make direct relationships between the different vertical migrations and the variations of water transparency, water temperature, and primary production.

13. An explanation of what causes the different vertical migrations in Crater Lake is difficult, but there appears to be a definite reproductive significance in the migration the entire adult population of D. pulex on August 28 and 29, 1968. A greater size and fecundity can be attained by remaining in cooler waters, but as food and temperatures increase in the surface waters, then migrations are advantageous.

14. It might also be possible that the August 1968 migrations of D. pulex are descendants of the few migrants in July whose reproductive rate was increased by the warmer surface waters.

15. The initial inhibition of vertical migrations could be explained by the concentration of primary production at depths coinciding with the depths of the diurnal zooplankton maxima during early summer. When primary production was concentrated at lower depths, any migration towards the surface would be away from the food source.