Half a century after the Tanganyika sardine (Limnothrissa miodon Boulenger) was introduced into Lake Kivu, several aspects of metazooplankton ecology were investigated from January 2002 to June 2005 in the pelagic zone of the lake. As in other large lakes of the region, zooplankton in Lake Kivu is species-poor, and is dominated by copepods. In addition to three cyclopoid species, four cladoceran and 12 rotifer taxa were recorded. Zooplankton showed marked seasonal variation. Total crustacean abundance increased to a distinct dry season maximum (August-September), following the rise of phytoplankton production associated with deep vertical mixing. The three copepods and the most important cladoceran species exhibited different patterns of vertical migration, depending on their feeding habits, life stages and body size at the adult stage. The relatively small Tropocyclops confinis Kiefer was permanently present in the euphotic layer, while the largest copepod species, Thermocyclops consimilis Kiefer and Mesocyclops aequatorialis Kiefer, as well as the cladoceran Diaphanosoma excisum Sars, exhibited a typical diel vertical migration, with some differences among life stages. Total biomass of copepods in Lake Kivu (Mean= 0.3 g C m-2) is lower than in lakes Tanganyika and Malawi. Similarly, mean annual total production (8.3 g C m-2 y-1) is about three times as low as in lakes Malawi and Tanganyika. The ratio between phytoplankton production and zooplankton production is low (about 1.6 %), suggesting a low transfer efficiency at this food web level. As primary production is in the range found in the other large lakes of the region, it is suggested that the low total zooplankton biomass in Lake Kivu is related to the disappearance of Daphnia curvirostris Eylmann, which was likely the key grazer before the planktivorous fish introduction. The absence in Lake Kivu of a calanoid copepod species, which can more efficiently exploit phytoplankton production, may be another reason why carbon transfer efficiency between phytoplankton and zooplankton is low. Data analysis using multivariate methods showed that seasonal variations of the ratio mixed layer depth: euphotic depth (Zm:Zeu) is the key driving force influencing plankton dynamics via its influence on quantity and quality of zooplankton food resources. This suggests that mesozooplankton dynamics in Lake Kivu is essentially bottom-up controlled. Presently, the sole indication of a significant impact of L. miodon predation on zooplankton is the decrease of average body size of the cladoceran Diaphanosoma over time.
|Date of Award||29 Sep 2006|
|Supervisor||Jean-Claude MICHA (Supervisor), Jean-Pierre DESCY (Co-Supervisor), Henri DUMONT (Jury), Kenneth IRVINE (Jury) & Mwenyemali B. Kaningini (Jury)|