ALGAE FOR MONITORING RIVERS Epilithic diatoms as indicators in tropical African rivers (Lake Victoria catchment) Ludwig Triest • Henri Lung’ayia • George Ndiritu • Abebe Beyene Received: 30 December 2009 / Accepted: 9 May 2011 Ó Springer Science+Business Media B.V. 2012 Abstract We investigated epilithic diatoms in rivers draining to the Nyanza Bay in Lake Victoria (Kenya) with the aim of determining environmental gradients in the assemblages and testing the usefulness of diatom metrics from temperate regions. Spatial and temporal variations of assemblages were studied in 12 sites of three rivers. Kibos, Nyando, and Kisat rivers contained 224 diatom taxa collected on seven sam- pling occasions over 4 years. Species richness showed a marginal decrease downstream and was the lowest at sites with high conductivity and ammonia–nitrogen levels. Two-Way Indicator Species Analysis and Canonical Correspondence Analysis revealed two major groups of river sites. Conductivity, alkalinity, turbidity, dissolved oxygen, and silicate were the most important variables influencing species distribution. Ecological diatom metrics of temperate regions and the Specific Pollution sensitivity Index showed good relationships with environmental variables. Both dia- tom assemblages and averaged indicator values were strong in predicting sites of ecological deterioration and in differentiating an upstream site of better quality (drinking water supply of Kisumu), thereby confirm- ing epilithic diatoms as suitable water quality indica- tors in equatorial rivers. The use of metrics initially designed for temperate rivers, however, appears less valuable in good quality tropical rivers because potential indicators are not considered. Keywords Diatoms  Epilithon  Ecological quality  Streams  Pollution  Lake Victoria Introduction The Lake Victoria basin is endowed with numerous rivers and streams. Many of these streams traverse large areas of land which exposes them to various anthropogenic impacts. Destruction of catchments through deforestation, human settlement, and agricul- ture coupled with effluents from industrial establish- ments and urban centres contribute pollutant inputs directly or indirectly into these rivers. Consequently, river habitats are increasingly altered and are faced Guest editors: L. Ector, D. Hlu´bikova´ & L. Hoffmann / Proceedings of the 7th International Symposium ‘‘Use of Algae for Monitoring Rivers’’, Luxembourg, November 23–25, 2009 L. Triest (&) Plant Biology and Nature Management (APNA), Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium e-mail: ltriest@vub.ac.be H. Lung’ayia Kenya Marine and Fisheries Research Institute, Box 1881, Kisumu, Kenya G. Ndiritu Wetland Resources Department, National Museums Kenya, Box 40658, Nairobi, Kenya A. Beyene Department of Environmental Health Sciences, Jimma University, Box 378, Jimma, Ethiopia 123 Hydrobiologia DOI 10.1007/s10750-012-1201-2