Habitat heterogeneity, environmental change, and the clonal structure of Daphnia populations

1. Organisms capable of asexual reproduction often maintain substantial clonal diversity within their populations. We ask whether such diversity in Daphnia pulicaria populations is maintained by temporal and spatial variation in selection. 2. We monitored temporal and spatial variation in the relative abundance of electrophoretically determined clonal groups in three lake populations of D. pulicaria from April to October 1991. 3. Clonal composition differed among lakes, but within each population we observed strong temporal changes in the relative abundance of different clonal groups. The timing and rate of this clonal succession was associated with the rate of loss of deep water habitat from seasonal anoxia. 4. Within each lake, clonal groups also segregated vertically into shallow and deep water assemblages. The magnitude of this habitat segregation varied temporally and was associated with the abundance of a shallow water competitor species, D. galeata. 5. We suggest that strong associations of clonal succession and clonal habitat segregation with environmental changes (habitat availability and competitor abundance) indicate selection operating on the clonal composition of these populations.