Primary production was observed to have increased in the surface water, and has a similar curve during both July and August 1968, but the migrations of zooplankton tend to favor the conditions that existed in August. Vertical movements of B. longispina were more pronounced in August, but only the adult D. pulex migrated en masse to the surface. Juvenile D. pulex scattered upward like the adults in June, but no upward vertical movement of juvenile D. pulex was observed in August.
An explanation of the 1968 migration of adult D. pulex from 62.5 m during the day to the surface at night may be offered by McLaren’s (1963) theory on the adaptive value of vertical migration. McLaren concludes that migrations are favorable to reproduction and growth when a thermal gradient exists between the depths of the day and night zooplankton maxima. Assuming that all feeding is done at or near the warmer surface waters, the energy gained by “resting” in the colder lower depths during the day would result in a more efficient growth, larger size, and greater fecundity.
Since the migrants are mainly large adult D. pulex carrying juveniles and eggs, there appears to be a definite reproductive advantage in this migration. This migration was observed to take place when there is a thermal gradient and when food appears to be more abundant in the shallower depths. However, a marked thermal gradient and increased primary production at the surface occurred in July of 1968, but only a few adult D. pulex were found at the surface during the night. The only major difference between July and August 1968 is that D. pulex adults and juveniles were more abundant in August.
It could be possible that either the full migration of D. pulex does not occur until a maximum fecundity is reached, or that a new generation of die1 vertical migrants are being produced during this July 1968 period. McLaren states that both phenomenon are possible. As the surface temperature begins to warm and as food becomes abundant at the surface, those few organisms migrating to the surface will have a higher reproductive rate producing a greater number of offspring (Hutchinson, 1967 and Pennak, 1953). Eventually the greater proportion of the entire zooplankton population will be made up of descendants from those few July vertical migrants. The great increase in the density of juvenile D. pulex during August 1968 seems to support this idea (Figure 4).
Primary productivity studies (Larson, unpublished) along with phytoplankton studies (Utterback et al., 1942) seem to indicate that at certain times of the year the food source in Crater Lake is concentrated at lower depths coinciding with the depths of the day zooplankton maxima. This could explain the initial inhibition of migration as all upward movement would be away from the food source.
Although an increase in numbers at 12.5 and 25 m is observed with B. longispina at night in August 1968, the population does not generally experience much of a temperature change during its slight upward movement. However, full migrations of B. longispina may occur later in the season. Marked migrations of Bosmina sp, have been shown by Worthington (1931) who found the amplitudes of migration of B. coregoni to exceed that of Daphnia longispina in the Lake of Lucerne.
These ideas suggest that greater study is needed on the relation of the physical and biotic environment of Crater Lake to the horizontal and vertical distribution of zooplankton. Correlations of nutrient availability and phytoplankton density should be made with the difference in the abundance and diel vertical distribution of zooplankton. Primary productivity studies done in Crater Lake have just been estimates of photosynthesis and are not necessarily a true indicator of food density.
Similarities between current patterns and the horizontal distribution should be done to try to solve the problem of the apparent static clumped distribution of B. longispina. Optical properties should be studied with a light photometer in relation to vertical migrations. Finally, sampling earlier and later in the season should be done, especially when the maximum density of both D. pulex and B. longispina is reached. Unfortunately, the period of conventional access to Crater Lake is extremely limited.
Nowhere are the exact causes of vertical migrations fully understood. But, it is my opinion that Crater Lake would be an ideal environment in which to attempt to solve this problem. Here, the environmental variables are few, and the numbers of species to observe are limited to B. longispina and D. pulex. I hope that this study will inspire future investigations of Crater Lake zooplankton to attempt to answer the question of why there are diel vertical migrations.
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