Effects Of Weir Installation On Water Quality And Macroinvertebrate Community Structure Of Two Rivers In Lake Victoria Basin, Kenya

ABSTRACT

Riverine benthic communities play a crucial role in trophic relationships by providing major sources of energy to economically and ecologically important fishes. With the declining production of the Lake Victoria fisheries, the importance of surrounding rivers as sources of fish will continuously increase. There is therefore need to refocus the management of the riverine ecosystems in order to improve their fisheries production, particularly concerning their natural fish food production. Understanding the benthic invertebrate community dynamics and their responses to newly introduced management interventions such as weirs is key towards improving the riverine fisheries production. This study therefore aimed at assessing the benthic invertebrate composition, diversity and density in two small Lake Victoria Basin rivers (River Awach-seme and Kisian) before and after weir installation in order to establish the possible impacts of such management interventions on the benthic invertebrate community. Prior to weir installation, three sampling sites were selected along each river, representing the upper reaches (KS1 and AS1; KS- River Kisian, AS-River Awach-seme), middle (KS2 and AS2) and lower reaches (KS3 and AS3). Pre-weir sampling was carried fortnightly between August and October 2009 while post-weir sampling was carried out monthly between December 2009 and July 2010. Quantitative benthic invertebrates were sampled using a Hess sampler (100 μm). During every sampling, measurements of flow rate, water depth, temperature, dissolved oxygen, pH, conductivity, silicates, nitrates, phosphates and totals dissolved solutes were taken. Results showed that the two rivers to have similar physical and chemical characteristics, except for conductivity and silicates that differed significantly. Physico-chemical parameters, including dissolved oxygen, nitrates and turbidity, however differed significantly (p0.05) among sites within Kisian River. Installation of weirs had insignificant impact on the physico-chemical parameters except nitrates that decreased significantly at both rivers while turbidity decreasing only at Kisian River Site KS2. Benthic invertebrate community at all sites was largely dominated by chironomidae before and after weir installation followed by the Ephemeroptera, particularly the genus Baetis sp. 26 genera were obtained at River Awach-seme while 23 at River Kisian before weir installation, 25 and 17 were obtained after weir installation respectively. Macroinvertebrate densities were significantly higher at Awach-seme. Density increased steadily during pre-weir period in both rivers, but declined though not significantly after weir installation. There were no significant differences in density among sites at any of the two rivers. Macroinvertebrate Diversity (H') did not differ significantly among sites and within rivers before and after weir installation. PCA analysis showed TDS and conductivity to be closely related to macroivertebrate densities. Mean water depth at upper Awach-seme showed a significant negative correlation with density. Other parameters were not significantly related to density. Most physical and chemical variables as well as benthic invertebrate density and diversity were observed to be unaltered by weir installation. These results did not support the hypothesis that weirs induce an improvement in the quality of in-stream habitats and change in benthic invertebrate communities. This research therefore provides baseline data for further action.