Associations of Cladocera in the zooplankton of the lake sources of the White Nile



Thirty-one species of Cladocera are recorded from the lake sources of the White Nile. An account is given of the seasonal occurrence of the main planktonic species at eight stations in Lake Albert. The horizontal distribution of species in Lake Albert was studied by means of vertical hauls taken at intervals along the length of the lake, and along transects from the shore to the middle of the lake. It is possible to distinguish an inshore group, consisting of Moina micrura and the helmeted form of Daphnia lumholtzi, and an offshore group with Ceriodaphnia reticulata and the monacha form of Daphnia lumholtzi. The validity of these groups is demonstrated by analyses of percentage co-occurrence, correlation and regression. Diaphanosoma excisum and Ceriodaphnia cornuta overlap these two groups. These distributions can be explained by the combined operation of two major factors: the presence inshore of the zooplanktivorous fish Alestes baremose, and competition between cladoceran species of similar sizes. The overlapping of the inshore and the offshore groups by Diaphanosoma excisum and Ceriodaphnia cornuta can be explained by their sizes differing from the other main species sufficiently to reduce competition.

Similar transects on Lake Edward again reveal Moina micrura as an inshore form and the monacha form of Daphnia lumholtzi as an offshore form. Ceriodaphnia reticulata is replaced in Lake Edward by the similarly sized C. dubia. The helmeted form of Daphnia lumholtzi was not found in the samples from Lake Edward, but it is apparently replaced by Daphnia longispina, although this species also increases in abundance offshore. In Lake Victoria both the helmeted form of D. lumholtzi and D. longispina are found, but their distributions have not been studied. Lake George has D. barbata as the only species of the genus, and the samples from Lake Kyoga lacked the genus completely.

In most of the planktonic Cladocera from the lake sources of the White Nile the size of the eye is related to the size of the carapace, but the horned form of Ceriodaphnia cornuta from Lake Victoria has a relatively smaller eye than the unhorned form from Lake Albert. This may be caused by selection pressure exerted by the small planktivorous fish Engraulicypris argenteus.

Within one lake the numbers of Cladocera under one square metre increase with the depth of the station. This implies utilization by the Cladocera of an environmental resource related to total depth. This resource may simply be space. When the numbers per unit volume are calculated there is no apparent relationship to the depth of the station. With the exception of Lake Kyoga the lakes can be arranged in a series of increasing conductivity and increasing numbers of Cladocera per unit volume.