Development of population structure in Daphnia clones in a stratified lake

The frequencies of single locus genotypes (clones) in a natural population of Daphnia pulicaria in a Minnesota lake were estimated during five months when the lake was thermally stratified in 1991. Further samples were collected in early August 1990 and 1992. Daphnia pulicaria were aggregated during daytime in two layers detected with high-frequency sonar. One layer was in the metalimnion and upper hypolimnion (depth 10-15 m), the other deeper in the hypolimnion (18-22 m). The layers were sampled at two locations in Long Lake on eight dates in 1991. Cellulose acetate electrophoresis revealed four alleles at the phosphoglucose isomerase (PGI) locus. Three clones occurred with sufficient frequency in 3077 individuals for statistical analysis. The proportion of homozygous slow (SS) individuals increased with time in the deep layer at both locations, coincident with decreasing concentrations of dissolved oxygen, but was nearly constant in the shallower layer, where oxygen was near saturation. Analysis with log-linear statistics indicated that depth, date and the depth-date interaction were must important in explaining the variations in clone proportions during summer. Location and the date-location interaction, although significant, were ICES important. Data from early August 1990 and 1992 indicated that this pattern may be an annual phenomenon.