1Kenya Marine and Fisheries Research Institute, Lake Turkana Research Station, P.O. Box 205, Lodwar, Kenya.
2Free University of Brussels (V.U.B), Laboratory for Ecology and Systematics, Pleinlaan 2, B-1050, Brussels, Belgium.
3Royal Belgian Institute for Natural Sciences, Vautierstraat 29 B-1000 Brussels, Belgium.

The zooplankton community structure of Lake Turkana, Kenya

Zooplankton distribution and abundance in Lake Turkana was studied from subsurface samples during the flooding of the Omo River between August and October 1999 at three sites in the western (Ferguson's Gulf), central (near Central Island) and eastern (Allia Bay /Sibiloi National Park) sectors of the lake. The aim was to provide information on the community structure and diversity at the study sites along an inshore-offshore direction. Standard environmental variables such as temperature, Secchi depth, conductivity, pH and dissolved oxygen were concurrently measured. Particulate Organic Carbon and phytoplankton chlorophyll a were also determined for all stations. When subjected to multivariate statistics, Canonical Correspondence Analysis (CCA) clustered all stations into those located in the Ferguson's Gulf and those in the rest of the study area. Results for environmental variables show higher concentration of dissolved oxygen, conductivity, chlorophyll a, and Particulate Organic Carbon (POC) in areas located within Ferguson's Gulf, compared to the rest of the study area. Zooplankton community analysis using TWINSPAN also showed the inner Ferguson's Gulf as a distinct community separate from the rest of the lake. The Cyclopoid copepodites and their nauplii, with nauplii accounting for over 80% numerical abundance in most gulf stations dominated zooplankton taxa. Calanoid copepod, Cladocera, Rotifera, and Protozoa were ubiquitous. A Monte Carlo permutation test (p < 0.05) showed the distribution of the taxa to be mainly affected in a decreasing order by temperature, Secchi depth, conductivity, and Particulate Organic Carbon. High carbon to chlorophyll a ratio suggested that the source of particulate matter might be allochthonous, mainly from the submerged decaying vegetation that once occupied the dry Ferguson's Gulf and Allia Bay areas, before the recent flooding caused by the El Nino rains. Diversity results show the Ferguson's Gulf to be the least diverse community, while stations influenced by the open lake waters but located in the Allia Bay/Sibiloi National Park as the most diverse. Potential influence of temperature, Secchi depth, conductivity, and Particulate Organic Carbon on the zooplankton community in these two areas is discussed.