DEVELOPMENT OF SOME LAKE ECOSYSTEMS IN TROPICAL AFRICA, WITH SPECIAL REFERENCE TO THE INVERTEBRATES

Abstract

Summary1. 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 rapidly broken down. Early filling stages, therefore, are associated with anoxic conditions and development of conspicuous communities of algae and large aquatic plants. The result is an overlapping of what in temperate systems are two distinct episodes: the early extraneous and later self‐contained systems. Secondly, the replacement of chironomids by an oligochaete climax in the mud as seen in temperate lakes does not normally occur, apparently because of the lack of accumulated organic material. The presence in Africa of the may‐fly Povilla adusta, on the other hand, provides an extra stage in the colonization of submerged woodland, replacing the earlier chironomid communities.