Zooplankton

The crustacean zooplankton, protozoa, or insect larvae are dominant in the secondary food chain of many African Lakes. While studies on their identification continue to provide insights into ecosystem structure and species succession, these are also important to an understanding of ecosystem-wide trophic functioning. This is especially true where a whole fish species flock may have undergone a decline due to (i) overexploitation and where (ii) foreign introductions into natural complex ecosystems have caused adverse effects as in Lake Victoria, or through (iii) natural succession.

Predatory fish and other organisms cause changes to zooplankton diversity as evident from studies conducted in Lake Chad: here a microzooplankton of 80 mm is selectively consumed by Brachysynodontis batensoda although nauplii and rotifers are also variably preyed upon. Some zooplanktons like cladoc-eran Moina micrura are incapable of avoiding predation by fish owing to low mobility in contrast to diaptomids.

Where predation is through passive selection, mean prey size and species composition of zooplankton are altered. In Lake Naivasha, zooplankton Daphnia lae-vis grows to 2.0-2.5 mm, Diaphanosoma excism attains 1.0-1.6 mm while Tropodiaptomus neumanni reaches 2.6 mm; this is attributed to low predation pressure. By contrast, in Lake Tanganyika intense

|HTj Active catchments during the earlier Holocene

Figure 3 Maximum potential catchments of the Senegal and Niger-Benue river systems, and of the Chari and Chad basin. During periods of maximum humidity, such as the early Holocene, the whole of the catchments were probably active. Today, only part of the catchment (the shaded areas) contributes to the runoff.

predation by endemic clupeids Stolothrissa tanganyi-cae and Limnothrissa miodon is thought to have led to the absence of cladocerans. It is thought that inefficiency of a planktivore fish of the pelagic zone of

Lake Malawi is responsible for the occurrence of Bosmina longirostis, Diaphonosma excisum, and Daphnia lumhotzi. On the other hand, passive feeding behavior exhibited by the pelagic Engraulicypris sardella on Chaoborus during daytime offers little pressure on this zooplankton. Thus Chaoborus edulis larvae are known to dominate zooplankton of Lake Malawi, and are also found in other rift valley lakes of similar trophic structure. However, Chaoborus are absent from Lake Tanganyika, where instead protozoa like Strombidium cf. viride and their symbionts are prevalent. In Lake Victoria, swarms of emergent insects have in recent years increased owing to changes in niche structure as a result of a population crash experienced with native cichlids. In turn, this has resulted in the population explosion of the sand martin, Riparia riparia.

Further experimental evidence of predation pressure was obtained in Lake Kariba, where the introduction of Limonothrissa miodon in 1967-68 led to reduction of Ceriodaphnia, Diaphanosoma, and Diaptomus.

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