Studies were conducted in Lake Baringo between May 1994 and April 1995 to map
temporal changes in productivity (phytoplankton standing stock) and attempt to relate
these changes to environmental parameters in this ecosystem. Fourteen phytoplankton
genera were recorded and described. Blue green algae (Cyanophyta) were the most
dominant contributing 90.89% of the total biomass followed by green algaeChlorophyta
(9.08%) and finally by diatoms - Bacillariophyta (1.23%). Seasonal changes were observed for the three major plant nutrients. Soluble reactive phosphorous (P04-P) ranged from 3.75 to 1l21lgP/I. Nitrate nitrogen (NOJ-N) ranged from 1- 110 Ilg Nil and finally the values for dissolved silica (SiOz) ranged from 2.1 - 20.5 mg SiOl!. Secchi disc depth ranged from 0.04-0.06 m. The study further observed that phytoplankton productivity in this ecosystem is limited by the high level of suspended silt, except for the dominant phytoplankton Microcystis aeruginosa (Kutz.) Kutz., which regulates its buoyancy by forming vacuoles, while other algal species sink out of the euphotic zone. Results obtained in this study show that the lake is undergoing drastic eutrophication resulting in deterioration of the water quality, frequent algal blooms, decreased fish yields and subsequently reduced income to the local communities. The eutrophication ofthe lake is further aggravated by unchecked damming and diversion of rivers flowing into the lake coupled with long spells of drought being experienced in the area and an increasing human and livestock population.