The Horizontal Distribution and Vertical Migrations of the Limnetic Zooplankton in Crater Lake, Oregon by F. Owen Hoffman
DISCUSSION
The rapid increase in the numbers of zooplankton during late August of both summers appears to indicate a monocyclic seasonal variation that usually occurs during spring in most temperate lakes (Hutchinson, 1967). There seems to be some correlation between the warming of the surface water and the increase of zooplankton populations, but only the adult D. pulex and their contained juveniles show any movement into these warmer waters. It is not certain when the maximum population size of either species is reached, but it apparently peaks some time in the summer or fall after the last samples were taken.
Despite the dominance of D. pulex that has consistently been reported by past investigators, only during the latter part of the summer of 1968 were its numbers comparable to the numbers of B, longispina. The depths at which the maximum numbers of these two species occurred were also shallower than those reported in earlier studies. Seasonal succession through competition could be responsible for the variation in species composition, however, no decrease in the population of D. longispina was observed with the concurrent increase of D. pulex in 1968.
In general, B. longispina has a static clumped dispersal. High and low densities occurred in the same respective stations during both 1967 and 1968. The highest densities occurred in the central and eastern stations, and the lowest densities occurred in the more northerly and south-westerly stations that are less than one-half mile (800 m) from shore. D. pulex, however, does not have any definite dispersal pattern. A random or near uniform dispersal seems likely. Where B. longispina definitely seems to avoid an area like station 22 near Wizard Island there is no corresponding absence of D. pulex.
It is possible that the differences in the vertical migrations of the two species could account for their differences in horizontal distribution. A greater vertical movement ofD. pulex would expose it to a greater diversity of water currents both on the surface and below, dispersing the population in different directions. In contrast, B. longispinawhich has only a slight vertical movement, maintains an almost uniform depth. If upwelling occurs B. longispina could become locally concentrated, according to Ragotzkie and Bryson (1953). Strong upwelling was suspected by Kibby St. (1968) during their study of the surface temperatures and currents in Crater Lake.
Even though Crater Lake is exceptionally deep, clear, and unproductive, the observed vertical migrations were not as marked as those reviewed by Hutchinson (1967) in similar lake environments. Except for the migration of the entire adult population of D. pulex in late August 1968, die1 vertical migrations are limited to a partial upward scattering of both D. pulex and B. longispina during the night. Evidently true diel vertical migrations are not consistent and occur only during certain times of the year in Crater Lake. Seasonal and annual variations in the vertical distribution are apparent. Differences within the reported depths of the day maxima of zooplankton in previous studies by Kemmerer &a. (1923), Hasler (1938), and Brode (1938), indicate that these variations may be even more pronounced than those observed in this study. Of course, these observations may be biased by differences in sampling methods and equipment.
It is difficult to explain the vertical migration of zooplankton in Crater Lake based on presently considered theories for such phenomenon. Since the depth of the maximum zooplankton population by day is dependent on transparency (Kikuchi, 1930), high surface illumination must be the main environmental factor that determines the vertical distribution during the day. However, there are no obvious explanations for what determines the nocturnal distribution of zooplankton.
A direct relationship between the nocturnal distribution and environmental variations is not clear. Variations in weather conditions affected Secchi disc depths more than any variation in water transparency, so it is impossible to say anything about the relationship of water transparency and the nocturnal distribution based on Secchi disc information.
A relationship between the nocturnal distribution and water temperature is also difficult to make. Different thermal gradients occurred during each sampling of the die1 vertical distribution, yet B. longispina underwent a nocturnal upward scattering regardless of these differences. However, a full migration of adult D. pulex was observed to occur in August 1968 when there was the least change of temperature between the surface waters and the depths of the day maxima.
