River impoundments transform lotic aquatic systems to lentic with changes in physical and chemical properties, biotic assemblage and productivity. Chebara reservoir is located at 3605E and 022S and situated within Keiyo-Marakwet County. The reservoir was formed as a result of damming the Moiben River to supply water to Eldoret town. A study was conducted on the physic-chemical properties as well as on community structure and primary productivity of phytoplankton in the reservoir from December 2007 to April 2008. Sampling was done every month at six stations distributed over the reservoir; one station at inlet one station at the outlet, two at minor inlets and one within the reservoir. Secchi depth visibility was measured by vertically immersing a 25cm diameter Secchi disk to disappearance. Phytoplankton were collected using a 28m diameter plankton net immersed vertically below the photic depth. Phytoplankton were identified and enumerated using a compound. Primary production and biomass were determined by chemical analysis of chlorophyll-a concentration and biological oxygen demand (BOD). Phytoplankton diversity indices were determined according to Shannon-Wiener’s index. Data on spatial and temporal phytoplankton abundance, temperature, pH, conductivity, dissolved oxygen, BOD and nutrients were analysed using One-Way Analysis of Variance (ANOVA), followed by Duncan’s Multiple Range Test (DMRT). All data was analysed at 95% level of confidence. Interrelationships between spatial and temporal physico-chemical factors and phytoplankton abundance were analysed using Pearson’s Correlation Matrix and Correspondence Analysis (CA). The relationship between phytoplankton species composition, abundance, sampling stations and physico-chemical was carried out using Canonical Correspondence Analysis (CCA). All statistical analyses were performed with STATIGRAPHIC 2.1 Plus and STATISTICA 6.0 procedures. There were no significant differences in the spatial or temporal physico-chemical parameters. The reservoir was homogenously oligotrophic and alkaline with only very slight variations. The productivity of Chebara reservoir was low (approximately 0.8 μgmillilitre-1) as estimated by chlorophyll a, suggesting oligotrophy. Six phytoplankton classes were identified including Cyanophyceae (22 genera) Bacillariophycae (25 genera), Chlorophyceae (55 genera), Euglenophyceae (3 genera), Pyraphyceae (6 genera) and Crysophyceae (8 genera) and similar to observations made in tropical oligotrophic lakes. The order of abundance was Pyraphyceae > Cyanophyceae > Chlorophyceae> Bacillariophyceae > Crysophyceae > Euglenophyceae> Rhodophyceae. Members of Chlorophyceae showed lower species abundance. There were strong relationships between the various phytoplankton genera and physical and chemical conditions, except for biological oxygen demand which had a weak effect. The study also indicates that phytoplankton growth in the reservoir is more likely to be limited by availability of P than N. The results obtained from this study can be useful for tracking the effects of changing activities in the drainage basin and the tributaries that contribute water directly to the reservoir. Calcium concentrations were consistently low, but the high abundance of pyraphytes in this reservoir could suggest a need to monitor management practices in the reservoir catchment that maintain calcium concentrations and populations of pyraphytes low in order to reduce the water treatment costs. This research further recommends that a research be carried out on macro invertebrates in order to accumulate sufficient knowledge which will be useful for watershed best management practices aimed at ensuring long term protection for water supply.

Mpiga chapa
University of Eldoret


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