Vertical Migrations
Before August 28 and 29, 1968, studies indicated that only a fraction of the entire populations of B. longispina and D. & underwent vertical migration. The maxima at most, moved only a distance of 12.5 to 25 m. Preliminary samples taken on July 28 and 29, 1967, showed the maxima of B. longispina to move between 75 m during the day and 50 m at night. A few organisms were found at 25 m during the night, but none were observed at this depth or above during the day.
Figure 5. The diel vertical distribution of Bosmina longispina on August 24 and 25, 1967 in Crater lake, Oregon. ( m = 100 organisms/m3)
On August 24 and 25, 1967, the vertical movement of B. longispina was still between 75 m during the day and 50 m, with a very few reaching the surface, at night (Figure 5). The initial upward movement of zooplankton began before sunset, and the downward movement happened after sunrise. Again, organisms were extremely rare in the depths sampled above 50 m during periods of intense illumination.
In 1968 the use of the Miller samplers increased the accuracy of sampling the vertical distribution of zooplankton. On July 24 and 25, 1968, the maxima of B. longispinaremained at 50 m, although a movement of individuals from 62.5 m during the day to 37.5 m at night was apparent (Figure 6). A few organisms occupied the surface waters at night and early morning, but by midday B. longispina was relatively absent from waters above 37.5 m. There were no major differences between the vertical distributions of adults and juveniles.
Figure 6. The diel vertical distribution of Bosmina longispina on July 24 and 25, 1968, in Crater Lake, Oregon. ( = 1’000 organisms/10 minute horizontal tow). * = 15 minute horizontal tow.
Vertical migrations of B, longispina were more pronounced on August 28 and 29, 1968 (Figure 7). The maximum concentration of organisms at 50 m during the day ascended to 37.5 m at night with large numbers present at 12.5 and 25 m. During this period a slight variation occurred between the vertical distribution of adults and juveniles. Not a s many juveniles were found in the 12.5 and 25 m depths at night; and while the maximum concentration of adults was at 37.5 at 2140 hours, the maximum concentration of juveniles was not at this depth until 0230 hours.
