An Ecopath model of the southern part of Lake Kivu, a deep African equatorial lake was constructed to quantitatively describe the possible impact of fish introductions in this ecosystem. This study is considered as an initial step in summarizing ecological and biological information, under a coherent framework, on this ecosystem. Fourteen compartments were considered. As a phytoplankton-based food web, it is observed that key food sources are not fully utilized as transfer efficiencies per trophic levels (TL) varies between 4.5 and 9.4%. Zooplankton plays a major role in transferring organic matter from TL1 to higher TLs due to the abundance of zooplanktivores. Shifts in food preferences and distribution of some of the fish functional groups were observed as a response to competition. The majority of fish biomass concentration is in TL3 (55%). The fishery is concentrated at TL3 and can, therefore, be consider as "immature", sensu Odum. The direct and indirect effects of predation between system components (i.e. fish, invertebrates, phytoplankton and detritus) are quantitatively described and the possible influence and role in the ecosystem's functioning of introduced fish species are discussed. (c) 2007 Elsevier B.V. All rights reserved.