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

Biol. Rev. ( 1974), 49. PP- 365-397 365 BRC PAH 49-10 DEVELOPMENT OF SOME LAKE ECOSYSTEMS IN TROPICAL AFRICA, WITH SPECIAL REFERENCE TO THE INVERTEBRATES By A. J. M cL A C H L A N Zoology Department, University of Newcastle upon Tyne NE 1 7RU, Britain {Received 22 January 1974) CONTENTS I n t r o d u c t i o n .......................................................................................................... 365 Man made lakes ........... 369 Lakes Kariba and Volta . 371 The pre-impoundment catchment . .371 The filling p h a s e .................................................................................................372 (а) Physical and chemical characteristics ...... 372 (i) Turbidity .......... 372 (ii) Nutrient salts .......... 372 (iii) O x y g e n .................................................................................................373 (iv) Organic matter ......... 373 (б) Animals and p la n t s ............................................................................. 374 (i) Explosive population growth ....... 374 (ii) Distribution p a t t e r n s ....................................................................375 (c) Conclusions ........... 378 The post-filling phase .......... 378 (a) The role of submerged woodland ....... 380 (b) Development of the mud habitat ....... 382 (c) Rooted aquatic plants as a habitat for sedentary animals . 384 (d) Fluctuations in lake level ........ 385 (e) Conclusions ........... 388 Equilibrium p h a s e .................................................................................................388 Annual storage-reservoirs.......................................................................................388 Semi-permanent natural waters ......... 389 Summary ............. 391 Acknowledgements . .392 References....................................................................................................................393 INTRODUCTION The concept of an ecosystem, first used by Tansley in 1935, has become one of the few unifying ideas in ecological thought. It is defined by Odum (1959) as “ an area of nature that includes living organisms and non-living substances interacting to produce an exchange of materials between the living and non-living parts” . Tradi tional examples of an ecosystem, like a forest or a lake, are somewhat misleading as these are rarely self-contained entities within definable boundaries as is often imagined (see Forbes, 1887). The exchange of materials and energy usually takes place over a 24-2 366 A. J. M cL achlan much wider area than the lake or forest under consideration. The size of the area involved is demonstrated for example, by Hutchinson (1969) in connexion with lake pollution in North America. In my examination of developing lake ecosystems, therefore, I will be concerned not only with the lakes themselves but with entire lake catchments, and it is often necessary to consider more widespread occurrences like planetary trade-wind belts and continental climate patterns. Ecosystems are generally thought of as being self-regulating or cybernetic systems. In fact, there is a tendency among many authors to liken the ecosystem to the organism, with the proviso, of course, that an ecosystem is not self-replicating although it is self-sustaining (Margalef, 1968). Situations are continually arising in which existing ecosystems are destroyed. This may be due to natural causes, like floods, wind erosion or volcanic action, or to artificial events like the ploughing of a field or damming of a river. It has been recognized for many years that the development of a new eco system follows an orderly course of events. This leads eventually to a relatively stable situation, in dynamic equilibrium with local climate, which is known as the ‘ climax’ (Clements, 1916). These developmental or successional changes may be regarded as constituting the period of ecosystem ‘embryology’ and essentially involve the re arrangement of existing species. This is in contrast to long-term evolutionary changes which involve the appearance of new species. In a newly created lake the successional changes occur over a period of the order of ten to twenty years (Rzoska, 1966). Quite different information is available from the ancient African lakes like Tanganyika and Malawi which have been in existence for millions of years. Because of their age and isolation, these latter lakes are an excellent source of material for the study of the mechanisms of evolution in inland waters; a number of interesting accounts may be found for example in Worthington (1937), Trewavas (1933), Greenwood (1951), Fryer (1959), Fryer & lies (1972) and in the recent book by Beadle (1974). However, since these lakes had appeared before the end of the Pleistocene era we must look elsewhere in order to study the ‘ embryology’ of tropical lakes in Africa. Man-made lakes provide the opportunity for such a study and they are becoming increasingly common features of the landscape. In addition, man-made lakes are valuable sources of information for the construc tion of ecological theory. Ecosystems are extremely complex and one way of approach ing the study of such systems is to interfere; to quote Margalef (1968) “ to give the black box of the ecosystem a shake” . But ‘shaking’ a large ecosystem usually proves to be prohibitively costly as a research exercise. It is therefore important not to lose the opportunity offered by the creation of impoundments which provide the requisite situation. Reliable predictions regarding the chemical composition of the water and sustained yield of fish are needed and these require a fundamental understanding of the way these systems work. While they are the commonest examples, artificial lakes are not the only situations in which lake embryology can be examined. Very occasionally natural lakes dry out and re-fill, and a study of the filling phase provides valuable information of a similar kind. There seems to be general agreement that the development of lake ecosystems Lake ecosystems in tropical Africa 367 throughout the world falls into three phases: an initial brief productive period during filling which is followed by a trough and then a slow rebuilding (Dussart et al., 1972; Zhadin & Gerd, 1961). Work on the large Russian reservoirs (as well as the work on African reservoirs discussed below) has led to the conclusion that the initial peak in activity is due to the flooding of terrestrial material including trees, grass and soil. This phase is therefore dependent upon extraneous organic matter, that is, plant material originating outside the lake itself. During the second phase, decomposition of this organic matter depletes the extraneous reserves, and releases, among other things, plant growth materials to the lake waters. The release is accompanied by an increase in the quantity of aquatic plants. This is the third phase and can be regarded as the ‘ autochthonous ’ phase where organic matter is being manufactured within the lake. The change over from extraneous to autochthonous systems accounts for the depression in the second phase (Morduchai-Boltovskoi, 1961, 1963). Tropical and temperate systems seem to differ fundamentally in at least one important respect. A characteristic of tropical lakes is the massive development of plant communities which starts immediately filling commences (see Rzoska & Tailing, 1966; Leentvaar, 1966; Mitchell, 1969), presumably because of more rapid decomposition at the higher water temperatures (Russell, 1961). As a result, the three distinct phases of the temper ate systems become obscured. Partly for this reason, but also because of the kind of data I have on the African situation, lake development is considered here to fall into three phases dependent largely upon water-level changes. These phases are: the filling phase, post-filling and equilibrium phases. Each phase corresponds only very roughly to the extraneous, change-over, and autochthonous phases of Zhadin & Gerd ( i961)- The kinds of African lakes available for examination fall conveniently into three categories: (a) Man-made lakes. I am using this category to include only the large artificial lakes like Volta, Kariba, Nasser and Kainji, all several thousand square kilometres in area (Fig. ib, a, c and d). Lakes of this type result from the damming of a river, and once they are full, annual fluctuations in level due to flood control measures are a conspicuous feature. There is more information available on this category than any other and most of the space in the following pages is devoted to it. Nasser is of interest since it lies near the northern extreme of the tropics and in arid desert, but there are virtually no published data on this reservoir. Kariba, near the southern tropic, and Volta close to the equator, are better represented in the literature. Further details concerning the main lakes discussed here are given below. (b) Annual storage-reservoirs. Here are included the much smaller schemes in which the lake fills and returns to original river conditions every year. Because of their ephemeral existence as lakes, the biological regime is characteristic. Lakes of this type that will be referred to are Sennar and Jebel Aulia on the Blue and White Niles respectively (Fig. 1 / and g). (c) Periodic natural waters. As a result of the high evaporation rate and variable rain-fall in parts of the tropics, many shallow lakes dry out periodically. Examples are the Makarikari Pan in southern Africa, Lake Stephanie in north-east Africa,

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