Effects of pH and salinity on the sedimentation of clay particles in turbid lakes of the rift valley of Ethiopia and Kenya

Abstract

Some lakes in the Rift Valley of East Africa are among the most productive in the world, while others are dystrophic, possessing a stable, opaque suspension of fine silt or clay. Understanding the causes of flocculation and sedimentation of this material could provide insight into why these lakes are so turbid. We collected samples of suspended clay from three turbid lakes (Langano and Abaya in Ethiopia, and Baringo in Kenya), measured the size-distribution and organic content of their suspended sediment, and performed laboratory studies of sedimentation rate as a function of pH and salinity. Most of the turbidity in the two Ethiopian lakes was due to particles between 0.5 and 1.5 μm, and the suspended material had a low organic content and C/N ratio in all the three lakes. Sedimentation velocity of the material increased with both salinity and pH, with velocities varying from near 0 at neutral pH and low salinity, to 3 cm h^-1 as pH approached 12 or salinity approached 25 ppt in the case of Lake Abaya. These results may explain why it is only freshwater lakes that demonstrate these turbid clay suspensions, and suggest mechanisms through which their turbidity might be altered.