Limnol. Oceanogr., 44(4), 1999, 1103–1113

The historical sediment record of Lake Sonachi (Kenya) was used to study the influence of lake depth and mixing regime on patterns of sedimentation in a small, fluctuating tropical soda lake. Lake Sonachi last desiccated completely in the early nineteenth century and has fluctuated between 3and 18-m lake depth over the past 115 yr. A freeze-core of offshore sediments describes recent lake history as a succession of meromictic episodes, represented by varved or subannually laminated muds, and holomictic episodes, represented by more coarsely layered muds. Two interbedded horizons of colloidal amorphous silica dated to a period of rising lake level after a prolonged lowstand were deposited by abiogenic, pH-driven precipitation from the water column and represent the instantaneous sequestering of an estimated 63 mg liter21 or possibly .50% of the lake’s dissolved-silica reservoir. Changes in offshore sedimentation and inferred bottom dynamics over time indicate that sediment resuspension and focusing in Lake Sonachi occur mostly during infrequent events of deep circulation between the average mud deposition boundary depth at ;2 m and the chemocline depth at 4–5 m; wind-driven sediment redistribution across the lake floor is important only at lake depths of #3 m. Dry sediment accumulation has varied between 86 and 620 g m22 yr21 over the past 175 yr, with no relationship to lake depth but, on average, lower rates during meromixis (199 6 90 g m22 yr21) than during holomixis (349 6 152 g m22 yr21). Net organic carbon accumulation offshore varied between 0 and 92 g m22 yr21, with no significant relationship to either lake depth or mixing regime at the time of deposition. Sedimentary organic carbon content (5.7–26.9%) is negatively correlated with bulk sediment accumulation; 73% of this variation is accounted for by the clastic dilution of sedimented planktonic algal production with low-organic littoral sediments redeposited offshore. Density stratification affects lake metabolism through its effects on oxygen distribution, hydrochemistry, nutrient cycling, benthic and planktonic production, and organic carbon mineralization (Hammer 1986; Kelts 1988; Talbot and Livingstone 1989; Miller et al. 1993). Consequently, the development and breakdown of density stratification accompanying lake-level fluctuation may be important drivers of long-term ecosystem dynamics in closed-basin lakes (Talling 1992; Jellison and Melack 1993). The interplay between mixing and stratification in saline lakes regulates carbon and nutrient cycling through its influence on deep-water oxygen renewal and on the areal distribution of sediment erosion, transport, and accumulation. Particularly in tropical saline lakes, combination of density stratification with high algal 1 Present address: Department of Biology, University of Gent, Ledeganckstraat 35, B-9000 Gent, Belgium.