Temporal trends of ion contents and nutrients in three Kenyan Rift Valley saline–alkaline lakes and their influence on phytoplankton biomass

This study, carried out between November 2003 and February 2005, aimed to investigate the temporal trends of conductivity, ions, nutrient concentrations and phytoplankton biomass expressed as chlorophyll a in the Kenyan Rift Valley saline-alkaline lakes namely Nakuru, Bogoria and Elmentaita. The influence of environmental variables on phytoplankton biomass has always been of much interest in understanding phytoplankton dynamics. Being shallow and endorheic, these lakes’ chemical, physical and biological properties were found to be strongly influenced by the hydrologic cycle within their catchment area. The lakes are characterised by high nutrient concentrations but with low Ntot : Ptot ratios. Significant differences between surface and near-bottom samples for water temperature, chlorophyll a and some nutrients were found in these lakes. A stepwise Discriminant Analysis with lakes as defined groups resulted in a significant model with SRP, nitrate–N, conductivity and light supply being of major importance. A significant correlation between specific conductivity and total alkalinity (Kendalls τ = 0.85, n = 132) was calculated. While L. Bogoria showed the least temporal variation in conductivity – (65–73 mS cm−1), larger variations were observed in L. Elmentaita (21–77 mS cm−1). Na+ and K+ form the main cations with Cl−, HCO3− and CO32− being the major anions in all the three lakes. Flouride was detected in high quantities (mean values L. Bogoria 72 meq l−1, L. Nakuru 17 meq l−1 and L. Elmentaita 71 meq l−1). A PCA followed by multiple regression analysis with chlorophyll a as dependent variable showed that nitrate-N, conductivity, phosphorus and light supply were the key variables influencing algal biomass in these lakes.