ABSTRACT
The vertical density structure in a lake depends on temperature and chemical gradients in the water column. The relative importance of dissolved constituents for water column stability often increases in deep hypolimnia, such as those of the deep rift-valley lakes of East Africa, where thermal stability is usually low.
In this paper, we quantify the water column stability in the deep basin of Lake Malawi as a function of in-situ conductivity, temperature and silicic acid concentration. The salinity–conductivity relationship and the coefficient of haline contraction for Lake Malawi water are determined on the basis of its physicochemical properties. The analysis of water column stability shows that: 1) temperature gradients are nearly adiabatic, implying thermal stability to be low in the deep hypolimnion (below 350 m); 2) dissolved solids contribute up to 10% to the stability throughout most of the hypolimnion; and 3) the non-ionic constituents (silicic acid and dissolved gases such as CO2, CH4 and H2S) cannot be neglected in stability calculations in the hypolimnion. During periods of low stratification in the austral winter, dissolved solids help maintain meromixis. We suggest that cool water occasionally introduced into the hypolimnion offsets geothermal warming and heat diffusion from above.
