| | | 20.2 Hydrography, Malawi, Victoria, Tanganyika |
As indicated above Lake Victoria is much shallower than Lake Malawi and Tanganyika, which have an anoxic deep hypolimnion, hence the monimolimnia6 of these two latter lakes are N sinks. Very little ammonia regenerated in the monimolimnia reaches the euphotic zone (Figure 3), because it is oxidized to nitrate when it mixes with oxic waters [53]. Lake Victoria in total is currently experiencing eutrophication and its many once common species are no longer found there and maybe extinct. This is not yet true for Lake Malawi and Tanganyika except locally. This demonstrates that the biodiversity vulnerability is not only a function of eutrophication, pollution etc, but also depends on the hydrology of the water body.
The Nile outflow from Lake Victoria is almost twice greater than the Shire flow from Lake Malawi (nearly proportional to the difference in catchment area of the two lakes). Hence the flushing rate of Victoria is much smaller (Table 1). As a consequence the concentration of introduced pollutants can rise much more quickly in Lake Victoria, but it will also recover much more quickly if their supply would cease (Figure 2). On the Great Lakes scale, Lake Erie in North America is the best-known case of substantial recovery from pollution [451]. Lake Erie is like Victoria, large and shallow, but its flushing rate is on the order of five years, allowing it to flush out excess dissolved nutrients and contaminants relatively rapidly [53]. It would take many years for Lake Malawi to recover if it ever experienced pollution concentrations comparable to those that occurred in Lake Erie and what is currently being experienced in Lake Victoria. As such the ability to predict the effects and decisions on how to prevent or mitigate the effect of pollution requires an understanding of the specific limnology of each aquatic system. For Lake Malawi and Tanganyika prevention is the only realistic and affordable policy for maintaining the lakes in healthy condition and insuring their continued beneficial use by the people.
| | | 20.2 Hydrography, Malawi, Victoria, Tanganyika |