Development of Some Lake Ecosystems in Tropical Africa, With Special Reference to the Invertebrates

Abstract

Summary 1. The flooding of a lake basin initiates a series of changes leading eventually to a more stable climax situation after some years. Sequential physical and chemical changes in the mud and water and related changes in the animal and plant populations of three types of tropical African lakes are considered. The giant man‐made lakes, Kariba and Volta, both several thousand square kilometres in area, provide the bulk of the material for this review. Two other kinds of tropical lake, the annual storage‐reservoirs like Jebel Aulia and Sennar in the Sudan, and natural lakes subject to periodic droughts, like Lake Chilwa in Central Africa, are also considered. Evidence is often patchy but suggests a number of generalizations regarding the course and causes of the developmental changes in these tropical ecosystems. 2. On the evidence available, the course of development appears to fall conveniently into two periods based on water level changes, the filling phase and the post‐filling phase. The former is characterized by sudden appearances of organisms and explosive growths of animal and plant populations. This unstable behaviour, especially characteristic of tropical lakes, is associated with the destruction of old habitats and the creation of new ones, the increasing area and volume of the new lake environment and the introduction of materials to the system at the advancing shore‐line. By the time that filling is complete, the situation has stabilized to a large extent and the ecosystem enters a new phase. 3. This post‐filling phase is characterized by the development and exploitation of existing habitats. Examples considered here are: the development of the mud habitat under the influence of processes like sedimentation and beach formation; the development of submerged woodland as a habitat for bottom dwelling animals; the spread of rooted aquatic plants and their effect on the mud; and the role of fluctuation in water‐level on the post‐filling phase. All four phenomena result in a modification of the substrata originally flooded, in a way that directly effects the associated fauna and flora. Apart from influencing the first three processes, water‐level fluctuations also result in an interaction between the aquatic and terrestrial ecosystems which brings about important changes in itself. 4. The relative importance of these two phases varies with the type of lake, filling‐phase phenomena obviously dominating in the annual storage reservoirs while post‐filling phase characteristics are fully expressed in the large lakes made by man. 5. In contrast to the development of temperate lake ecosystems, succession of species in these tropical examples is not so much interrupted by major annual temperature changes. In addition, both the course of succession and the climax communities achieved are different. This may be largely due to the more rapid decomposition of organic matter in the muds which has two main consequences. In the first place, extraneous organic material brought in to the system during flooding, is rapid...

The Environment of Lake Victoria (East Africa): Current Status and Historical Changes

“…These are typically eutrophic as a result of the sudden release of nutrients from the drowned terrain and exhibit algal blooms, the explosive growth of floating macrophytes, oxygen depletion and rapid changes in the populations of fish and invertebrates. These decrease over time as the lake matures until an ecological equilibrium is reached (MCLACHLAN, 1974). Lake Victoria may now also be reaching some kind of ecological equilibrium, in part because intense fishing of Nile perch has allowed species from lower trophic levels to increase (BALIRWA et al, 2003).…”

SPATIAL AND TEMPORAL VARIATIONS OF PLANKTONIC CRUSTACEA FROM WADI EL RAYAN LAKES, El FAYOUM WESTERN DESERT, EGYPT

“…It was recorded in the Nile River and its branches by Helal (1981) and Guerguess (1993). Mc Lachlan (1974) indicated that the development of the lake ecosystem in the newely created lakes falls into three phases. The first lake of Wadi El Rayan may be in the third phase (equilibrium phase) whereas the second lake is still in the first phase (filling phase) which is characterized by drastic changes-Therefore, the aquatic fauna and flora of the second lake are unstable and should be monitored carefully.…”

Some distinctive subject contributions from tropical Africa to fundamental science of inland waters

“…At Kariba there were wide-ranging studies of invertebrate colonisations by McLachlan, including the transient occupation of newly flooded and refilling areas by Chironomus transvaalensis (McLachlan 1970). Later McLachlan combined work here with that elsewhere in a broad survey of developing lake ecosystems in tropical Africa (McLachlan 1974). He subsequently analysed divergent strategies of survival and reoccupation of temporary rain pools by insects Cantrell 1980, McLachlan 1983).…”