A field study measuring depth distribution of chlorophyll and light was carried out in Lake Malawi over 2 years: from these data, modelled estimates of primary production were obtained. It is well known that, in this lake, the rate of nutrient loading to the trophogenic zone from the deeper, nutrient-rich water is dictated by the degree of vertical mixing of water layers. In the present study, estimated values for primary production were strongly correlated with periods when this mixing was assumed to be highest. Primary production in Lake Malawi thus seems driven by physical, climatic factors, notably ambient temperature and wind regimes.
This conclusion was supported by the large differences between estimated production values for 1992 and 1993. The higher production values seen in 1993 were due to the greater degree of seiching activity in that year, which increased the extent of nutrient loading in the upper water layers. The simplicity of the modelling approach used here makes it an effective means of obtaining primary production data, and for Lake Malawi the low spatial variation in algal biomass suggests that whole-lake production could be estimated through extrapolation of data collected from only a small number of sampling locations.
There is a paucity of comparative data on primary productivity in tropical lakes, especially
ancient lakes Malawi, Tanganyika and Victoria. Monitoring of whole-lake primary production
in these lakes would potentially be a useful management tool where environmental perturbations,
first evinced as increased production, need to be monitored and, where possible, controlled.