During the night of August 28 and 29, tremendous numbers of D. & were found at the surface (Figure 9). These numbers consisted primarily of large adult females whose brood pouches contained eggs or juveniles in the earliest molt. Those juveniles recorded at the surface were probably those just released from the brood pouches of the largestD. pulex females. Juveniles in the more advanced molt stages were not observed at the surface, and no changes were observed in their die1 vertical distribution.
The maximum concentration of adult D. &during the day was between 50 and 02.5 meanwhile almost the entire adult population migrated to the surface at night. If the presence of small numbers of adult D . pulex in samples taken from the lower depths can be attributed to contamination while towing up through the surface, then we may assume that almost no D. pulex adults were below the surface layers at 0230 hours.
Contamination
Estimates of contamination from depths above 125 m showed that most if not all of the organisms sampled at 125 m were contaminants (Table 4). This may also be true of the 100 m samples.
Environmental Studies
Most of the research on the Crater Lake environment during the period of this study was conducted by Larson (unpublished). Thermal and water transparency (Secchi disc) information was made available. In situ14c incubation was used to estimate the rates of primary productivity at various depths down to 140 m.
Table 4. Comparison of sample averages of zooplankton per horizontal tow at 125 m and sample averages from tows made to estimate contamination by zooplankton above 525 m.
