An outline of the physical limnology of Lake Malawi (Lake Nyasa)

Abstract

Lake Malawi, in tropical central Africa, is 560 by 75 km and consists of a single basin, maximum depth 695 m. Below 250 m the lake is homothermal at about 22.5°C and is anoxic. Above this level there is a seasonal cycle with the development of a marked thcrmocline. By May the upper 60 m is homothermal at about 27°C. During the dry, windy cool season the epilimnion cools but only in exceptional years approaches a homothermal condition. Complete mixing has never been observed. In the shallower southeast arm the surface temperature may fall to 21.0°C as the result of inshore chilling; this cool water flows northward as a profile‐bound density current and appears to maintain the stability of the stratification. Large‐scale oscillations of the level of the thermocline, apparently the result of internal waves and of wind‐induced upwelling, cause advections of water from the metalimnion into the southeast arm, site of the major fishery in the lake.