Effects of some abiotic and biotic factors on the zooplankton community in Lake Baringo, Kenya

Abstract

Lake Baringo is a turbid lake that lies in a closed drainage basin of the Kenyan East African Rift Valley. The lake’s water quality has deteriorated in the recent past mainly due to sedimentation from its catchment arising from poor agricultural practices, deforestion and overgrazing. Its fishery has also declined since the 1980s and frequent lake closures to fishing activities have not alleviated the problem. This shows that there may be other critical ecological and environmental factors affecting the ecosystem. To understand problems facing the lake, there is need for well-coordinated and comprehensive ecological investigations considering the complexity of the ecosystem. Zooplankton is important in energy transfer from primary producers and constitute a significant component of the diets of the juveniles and some adults of many fish species. The objective of the present study was to determine the effect of some physical, chemical and biological factors on the spatial and temporal distribution, abundance and biomass of zooplankton in Lake Baringo. Stratified random design was used to allow for statistical comparison between zooplankton abundance and biomass at different stations and months with environmental factors using Analysis of Variance (ANOVA). The relatively stable environmental factors across the sampling stations in the lake were attributed to its small size, shallowness and the daily mixing by wind action. A total of 39 species of zooplankton belonging to Rotifera, Cladocera and Copepoda groups were recorded. The results indicate that distribution, diversity, abundance and biomass of zooplankton were influenced by environmental factors especially depth, conductivity and turbidity. Diel vertical distribution of zooplankton was the reverse of what is reported from clear lakes with organisms congregating to the surface during the day and descending to the bottom at night. Investigations into the diet of three main fish species in the lake showed that Oreochromis niloticus baringoensis mostly depended on algae, Clarias gariepinus depended on fish while Protopterus aethiopicus thrives on molluscs as their dominant food. The growth performance of the once dominant endemic fish, O. niloticus baringoensis could be affected by the high turbidity, which reduces primary production. Moreover, reduced clarity hampers the feeding success of this visual feeding fish and has decreased macrophytes to near extinction. There is urgent need for rehabilitation of Lake Baringo and the study recommend afforestation and reduction of livestock numbers in the catchment as some of the ways of reducing soil erosion and sediment input in the lake. The results of the study may be used as an important tool for the detection of stability and trophic levels of the ecosystem and to provide data for models on maximal resource production of the lake.