Ventilation of Lake Malawi / Nyasa

A tracer study was conducted on lake Malawi/Nyasa, one of the deepest and largest lakes in the world, in order to quantify the renewal rates of the deep water. For this purpose, concentrations of the anthropogenic trace gas chlorofluorocarbon-12 (CFC-12) were measured in water samples which were collected in glass ampoules and analyzed by a new gas chromatographic separation method. Based on measurements of stored duplicate samples, we conclude that the first-order degradation rate for CFC-12 in anoxic water of Lake Malawi/Nyasa lies in the range 0 to 0.012 yr−1. The tracer measurements are used in a 3-box mixing model from which average exchange times between the hypolimnion and the metalimnion of 18.5 years and 15.9 years are calculated for the cases of no degradation and maximum degradation in anoxic water, respectively. These exchange times are 2.7 to 2.2 times higher than have been estimated previously based on tritium measurements in 1976 by Gonfiantini and coworkers. The exchange times between the metalimnion and the epilimnion are calculated to be 3.7 years and 3.4 years, again for no degradation and maximum degradation, respectively. These exchange times are comparable to those estimated previously. Volumetrically averaged apparent CFC-12 ages of 8.9 and 21.1 years were calculated for the metalimnion and the hypolimnion, respectively, under the assumption of no degradation. Latitudinal gradients in the CFC-12 and dissolved oxygen concentrations on isopycnal surfaces suggests that the deep water originates predominantly in the southern part of the lake.