Lakes & Reservoirs: Research and Management 2008 13: 145–154 © 2008 The Authors Doi: 10.1111/j.1440-1770.2008.00360.x Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd Blackwell Publishing Asia Case Reports Sediment-water fluxes of selected heavy metals in Lake Naivasha Anthropogenic and seasonal influences on the dynamics of selected heavy metals in Lake Naivasha, Kenya Joseph Nyingi Kamau, 1 * Anthony Gachanja, 2 Catherine Ngila, 3† Johnson Michael Kazungu 1 and Mingzhe Zhai 3 1 Kenya Marine and Fisheries Research Institute, PO Box 81651, Mombasa, Kenya, 2 Faculty of Science, Jomo Kenyatta University of Agriculture and Technology, PO Box 62000, Nairobi, Kenya, and 3 Faculty of Science, University of Botswana, Private Bag 00704, Gaborone, Botswana. Abstract Lake Naivasha is a freshwater lake in the Eastern Rift Valley of Kenya (0 °45′S and 36°20′E). It has no surface outlet and is perceived to be under anthropogenic stress. Being situated at the basin of the rift valley, the lake acts as a sink for wastes from the town of Naivasha and the surrounding horticultural industry. Flux experiments were conducted to investigate the dynamics of heavy metals between the sediment–water interface in Lake Naivasha. In situ benthic flux experiments were conducted at two sites, one near the municipal wastewater inflow to the lake (site SS), and one at the papyrus field near the horticultural farms (site SH). Sediment samples from the exposed riparian land were collected during the dry season after the lake has receded, and the fluxes of selected metals were determined in the laboratory under simulated conditions. Aluminium in situ benthic flux at site SS averaged 7 mmol m –2 h –1 , and was correlated positively with pH (Pearson correlation coefficient ( r) = 0.89). While the in situ benthic flux of aluminium at site SH averaged 1 mmol m –2 h –1 . In situ benthic fluxes of copper and manganese were predominantly positive at site SS, but not at site SH. The papyrus field at site SH played an important role in buffering of the lake in regard to the selected metals investigated in this study. Redox-sensitive metals were precipitated in the benthic flux experiment for this site. Key words aluminium, benthic flux, copper, freshwater, Lake Naivasha, manganese, sediment, zinc. INTRODUCTION Freshwater is absolutely essential for human survival. In addition to being one of the planet’s most valuable natural resources, freshwater is also its most vulnerable. Water- bodies and rivers bear major impacts of industrialization and population explosion. They can become choked with silt, domestic and industrial pollution, and also over- exploitation. One major example of environmental pollution is relatively high concentrations of heavy metals in water and aquatic sediments located in industrialized and densely populated areas (Rapin et al. 1983). One example of such pollution is illustrated by the work of Tarras-Wahlberg et al. (2002), who reported that the concentrations of iron (Fe), zinc (Zn), cadmium (Cd) and nickel (Ni) in Lake Naivasha sediments were elevated, compared to global averages. Sediments can act as both sources and sinks for certain trace elements (Petersen et al. 1995; Riedel et al. 1997). Most are adsorbed on suspended particulate matter (SPM), which can be transported to bottom sediments by flocculation and sedimentation processes. Enhanced mineralization processes can occur at the sediment–water interface, particularly within the oxic zone. Such microbially catalysed reactions can transform both the minerals to which the trace elements are bound, and their speciation, thereby also affecting the mobility of these trace elements (Ciceri et al. 1992). Diagenetic reactions therefore could cause a re-dissolution of a portion of the deposited materials in the sediment pore water, from where diffusive fluxes (resulting from the concentration gradient at the sediment–water interface) could remobilize them from the sediments to the overlying water column (Lyons & *Corresponding author. Email: jkamau@kmfri.co.ke † Present address: Howard College, University of KwaZulu Natal, Durban 4041, South Africa. Accepted for publication 8 December 2007.