Distribution, predation, physiology and behavior of clones of Daphnia pulicaria in a single basin freshwater lake

The summer distribution of clones of Daphnia pulicaria was investigated with high frequency sonar from August 1990 to August 1994. Clones (= single locus genotypes) were distinguished with cellulose acetate electrophoresis.;During the first three summers the zooplankton were aggregated in two distinct layers during daylight hours. One was in the metalimnion and the other deeper in the hypolimnion. Log-linear modeling indicated that by early August each year, the D. pulicaria population was genetically differentiated. One clone, homozygous slow at the PGI locus (PGISS), was more frequent in the hypolimnetic layer than the metalimnetic layer. More detailed observations in 1991 indicated that this increase of PGISS coincided with a progressive decline in dissolved oxygen in the hypolimnion throughout the summer.;Avoidance of visual predation in darker waters and physiological adaptations to low oxygen conditions were invoked to explain the genetic structure. Large adult D. pulicaria and pigmented individuals were found to be much more vulnerable to predation by rainbow trout. D. pulicaria that were homozygous slow at the PGI locus were 6 times more likely to be found in trout stomachs than in the plankton.;Concentrations of dissolved oxygen, clone type and, to a lesser extent, annual variation affected the proportion of D. pulicaria that contained elevated levels of hemoglobin.;When transplanted from the deep layer to the shallow layer, the PGISS clone was more likely to survive than the heterozygous (PGIFS) clone. A brief examination of clone-specific vertical migration indicated that the PGISS clone was more likely than other genotypes to migrate upward from the deep layer after sunset.;Following increases in the numbers of stocked rainbow trout and increases in dissolved oxygen in the hypolimnion, the density of the shallow layer of D. pulicaria declined in 1993 and it failed to develop in 1994. The deep layer failed to differentiate genetically. The frequency of hemoglobin pigmentation in D. pulicaria declined in 1993 and disappeared in 1994. Vertical migration from the deep layer was no longer dominated by the PGISS clone.