Zooplankton of Lake Kivu, East Africa, half a century after the Tanganyika sardine introduction

The introduction into Lake Kivu of the planktivorous fish Limnothrissamiodon at the end of the 1950s raised major concern about thefate of mesozooplankton, but few data were available to assessthe impact of predation. In this study, we followed zooplanktonvariations in Lake Kivu for 3.5 years. Present Lake Kivu mesozooplanktonis dominated by cyclopoid copepods (Thermocyclops consimilis,Mesocyclops aequatorialis and Tropocyclops confinis), but cladoceransand rotifers are also present. Each year, total crustacean abundancein the plankton increased to a distinct seasonal maximum followinga rise of phytoplankton production associated with a deep epilimneticmixing in the dry season (August–September). This dependenceon phytoplankton resource suggests that mesozooplankton dynamicsin Lake Kivu is mainly bottom-up controlled, contrary to expectationsfrom the food web structure. However, measurements of body sizeindicate that sardine predation affects the cladoceran Diaphanosomaexcisum, whereas the larger copepods may efficiently escapepredation by migration in the deep mixolimnion at daytime. Totalbiomass of mesozooplankton in Lake Kivu is lower than in lakesTanganyika and Malawi. This may be related to the disappearanceof a large grazer that existed before the sardine introduction,whereas the large lakes of the same region have a more complexpelagic food web, with piscivorous fish, and with calanoid copepods,which can more efficiently exploit phytoplankton production.