Some aspects of perturbation in the structure and biodiversity of the ecosystem of Lake Victoria (East Africa)

Abstract

Dramatic changes in the Lake Victoria (East Africa) environment were observed after the introduction of the Nile perch (Lates niloticus) in the 1950s. An extraordinary spectrum of endemic haplochromine fishes, a result of intralacustrine adaptive radiation was reduced by massive species extinctions (ca. 65%) due in part to predation by Nile Perch. Such an abrupt destruction of natural diversity has never before been documented by scientists. Lake Victoria's haplochromine species flock comprised upwards of 400 species (5% of the world's known freshwater fishes), encompassing a remarkably wide trophic spectrum and constituting 83% of the lake's total fish biomass. The lot evolved in an isolated part of the Nile system since the formation of the lake basin about 750 × 10³ years ago, but quite possibly as recently as 14 × 10³ years ago, when most of the lake dried up. More than 50% of the haplochromine species (by number) were phytoplankton-zooplankton-detritus consumers; 55% of their biomass were detritivores and 27% zooplanktivores. The piscivore Nile perch (Lates niloticus) was first introduced into Lake Victoria in 1954. It underwent rapid population expansion in the 1980s, accompanied by haplochromine decline. Consequently, phytoplankton and detritus consumption by fishes was reduced. The biomass of the endemic Cyprinidae Rastrineobola argentea increased, as did its fishery, and predation pressure on zooplankton was therefore intensified. The population of the prawn Caridina niloticus became very dense, mostly in deep waters. The fishery and fish industry were altered fundamentally. Limnological changes suggesting eutrophication have been observed since 1960: hypolimnetic anoxia increased and the period of extensive vertical mixing was restricted to about one month per year; phytoplankton productivity increased and shifts from diatom to blue-green dominance occurred. Increased inputs of N (from the 1920s) and P (from the 1950s), induced through precipitation and human activity in the catchment area (agricultural and urban developments, deforestation, etc.) and high water levels accompanied by decline of available silicon, have persisted. Both top-down (Nile perch piscivory) and bottom-up (nutrient changes) influences enhanced eutrophication. The concurrent system changes in nutrient dynamics may have contributed an additional impact to the extinctions of haplochromine fishes.