Meromixis in an equatorial African soda lake1

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Abstract
Lake Sonachi, a small volcanic crater lake in central Kenya, was chemically stratified on all of 17 sampling dates spanning 8 years. Values of chemical stability, the amount of work required to mix a stratified lake to a uniform concentration, were low (700–200,300 ergs·cm−2). Factors that contribute to the maintenance of meromixis are basin morphometry, the diurnal periodicity of the winds and of thermal stratification, biological decomposition, and seasonal and yearly changes in rainfall. Lake Sonachi is sheltered from wind by crater walls 30–115 m above its surface. Wind speeds have a diurnal pattern and typically were maximal when the lake was thermally stratified. Higher values of hydrogen sulfide, soluble reactive phosphate, and ammonia in the deeper waters, as well as a lower pH value, suggest that biological processes contributed to the meromixis. Rainfall during 1971–1976 totaled less than average, and chemical stability declined to 700 ergs·cm−2. When the lake was again sampled after 2 years of above average and 1 year of average rainfall, the level had risen 3 m and the stability had increased to 200,300 ergs·cm−2. Freshening of the surface waters by rain contributed to the increased stability, and the conductivity and the volume of water below the chemocline had increased substantially. We suggest that chemical stratification is enhanced by ground‐waters that dissolve evaporites in the lake sediments and seep into the lake.