DEVELOPMENT OF POPULATION-STRUCTURE IN DAPHNIA CLONES IN A STRATIFIEDLAKE

The frequencies of single locus genotypes (clones) in a natural popula tion of Daphnia pulicaria in a Minnesota lake were estimated during fi ve months when the lake was thermally stratified in 1991. Further samp les were collected in early August 1990 and 1992. Daphnia pulicaria we re aggregated during daytime in two layers detected with high-frequenc y 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 w ere sampled at two locations in Long Lake on eight dates in 1991. Cell ulose acetate electrophoresis revealed four alleles at the phosphogluc ose isomerase (PGI) locus. Three clones occurred with sufficient frequ ency in 3077 individuals for statistical analysis. The proportion of h omozygous slow (SS) individuals increased with time in the deep layer at both locations, coincident with decreasing concentrations of dissol ved oxygen, but was nearly constant in the shallower layer, where oxyg en was near saturation. Analysis with log-linear statistics indicated that depth, date and the depth-date interaction were most important in explaining the variations in clone proportions during summer. Locatio n and the date-location interaction, although significant, were less i mportant. Data from early August 1990 and 1992 indicated that this pat tern may be an annual phenomenon.