FIGURE 1.1. An example of a network of environmental abiotic characteristics that determine the quality of a lacustrine system. The proximal variables (bottom line of the figure) have a more direct action on the composition of communities.

The role of physico-chemical variables: temperature and salinity

1The water temperature of rivers and shallow lakes are normally close to the mean air temperature, in contrast with deeper lakes where temperature is unevenly distributed over depth in the water column. The diurnal and seasonal solar irradiance cycle variations result in delayed cycles of temperature in the water column and alternation of stratification and mixing.

2The main factors that determine the temperature of water are latitude and altitude, with a local influence from atmospheric circulation. The latter explains why climate is often more variable than expected in equatorial regions of Africa where seasonality is expected to be minimal. The temperatures observed in different shallow African aquatic environments show that seasonality increases with latitude (figure 1.2). In the entire inter-tropical Africa, the average temperatures are high, most often above 20° C, thereby favouring the rate of chemical and biological reactions at different trophic levels, such as bacterial decomposition, photosynthesis or metabolic reactions (Lemoalle, 1981).

3The effect of altitude on the temperature of a series of shallow tropical lakes (figure 1.3) also applies to rivers: it is a major factor of species distribution in inter-tropical regions.

FIGURE 1.2. Annual variations of surface water temperature of some shallow African lakes (from Talling, 1992).

4From a physicochemical point of view, shallow lakes offer a great variety of conditions owing to the relative importance of evaporation in comparison to the water depth. The water salinity ranges from that of rain water (with a conductivity of about 10 µS. cm^-1) in very dilute environments or on bedrock, to far above that of sea water as observed for the lake of Latir (175 g L^-1 or 100,000 µS. cm^-1) in Chad located in a highly evaporative environment. Salt water lakes in Africa include Lakes Magadi, Nakuru, Natron in Kenya, the Kanem lakes in Chad and Pink Lake in Senegal (Burgis & Symoens, 1987). Regardless of the quantitative aspect, the chemical composition of water is also variable, with different relative proportions in anions and major cations (figure 1.5 and table 1.I). In tropical zones, specific soil formation processes result in the production of laterite. During this process, high concentrations of ions are released, notably sodium, bicarbonate and silicate which are characteristic of tropical waters.

FIGURE 1.3. Decrease of bottom water temperature according to altitude in some shallow African lakes (from Talling, 1992).

5Whereas high salinity can be shown to be a limiting factor for the diversity of the fish community, a selective effect of the relative ionic composition does not appear to influence fish communities in diluted waters.

FIGURE 1.4. Mean opposite rainfall and water temperature cycles in Northern and Southern Africa. In Northern Africa (Tunis) the rains occur during the cold season (boreal winter) whereas in Southern Africa (Germiston), they occur during the warm season (southern summer). From Griffiths, 1972).

TABLE 1.I. Physicochemical characteristics of some African lakes and rivers

FIGURE 1.5. Concentrations of major anions and cations as a function of conductivity for a series of African lakes in East and Central Africa (from Talling & Talling, 1965)

The lakes and rivers of Africa

6Africa is an ancient shield with a consolidated continental base dating at least 600 million years. This base is composed of gneiss, shale and slab and is grooved in a criss-cross manner resulting in basins with raised edges (moles) levelled by erosion. With the exception of the Atlas to the north and the Cape Mountains to the south, this general relief does not seem to have been modified by the folding which is responsible for the creation of other mountains in the world.

7Five large basins occupy the centre of the continent (figure 1.6). The flow of most of the rivers towards the ocean occurs via gorges and cataracts which have a major influence on the distribution of aquatic species in the Congo, Zambezi, Nile or Niger basins (Lévêque, 1997a). Some basins have no outlet to the sea (endorheic basins). A good illustration is the Chad basin which during the Pleistocene enclosed an inland sea larger than any of the present lakes. Another example is the Kalahari and Okavango which, although drained by the Zambezi River, lose a major portion of their water in the Okavango and Makarikari marshes. Finally, to the north of the continent, almost all the tributaries south of the Atlas drain into chotts and seasonal saline pools in regions of semi-arid climate.

FIGURE 1.6. General topography of Africa showing the principal hydrographic basins, the altitude zones above 1,000 m, and the approximate division between lower and higher Africa (redrawn from Beadle, 1981).

8The ancient repartition of the main basins of pre-Miocene age was altered in the eastern part of the continent by the uprising of a wide band of 500 to 800 km width, oriented north-south from Eritrea to Zambezi. The trough, approximately 1,000 m deep, is due to plate tectonics and was formed about 20 million years ago. It is the origin of the creation of the vast depressions of the Rift Valley oriented in NE/SW direction and in which most of the Great African Lakes were formed. Lake Victoria, which covers an area of 65,000 km², occupies a depression located at the centre of the two branches of the Rift Valley. The volcanic activity associated with these different rifts resulted in the creation of hundreds of crater lakes, mainly along the Western Rift.

9The African continent is thus divided schematically into two large regions:

• to the East, the highland Africa, with an altitude generally above 1,000 m, where the Rift Valley and the great lakes are located;
• to theWest, an Africa characterized by large sedimentary basins, with average altitudes less than 500 m, encircled by eroded plateaus which isolate these basins from each other and from the sea (figure 1.6).

10It is in this geomorphologic context that the current aquatic environments and their biological communities have developed progressively.

The main types of aquatic environments

Lotic systems

11The African hydrographic system is well developed despite the presence of vast barren zones (figure 1.7 and table 1.II). Some water courses originating from rainy upper basins flow through arid regions, such as sections of the Senegal, Niger, and especially the Nile River, which seems contradictory given the dryness of their surrounding landscapes.

12A distinction may be made between the large and the smaller river basins. The hydrological regime of the large rivers of Africa (Congo, Nile, Niger, Volta, Senegal, Chari, Zambezi), results from a diversity of rainfall regimes on different parts of the basin, with different landscapes and biotopes. The smaller coastal basins, hydrologically isolated from one another, are situated in small homogenous climatic zones. They are more directly in phase with the local climate, and therefore likely to experience sharper variations in their discharge.

13In most water courses a progressive change occurs in altitude, slope, flow rate and section (width and depth) of the river, from the source to the estuary.

FIGURE 1.7. Distribution of river systems in Africa.

14These conditions determine a variety of successive biotopes, from small rocky mountain streams to wider silted meandering rivers. Each corresponds to equally differentiated communities.

15Most of the main river basins have been the subject of monographic studies: the Nile (Rzoska, 1976; Said, 1993; Dumont, 2009) and Jonglei Canal (Howell et al., 1988), Niger (Grove, 1985), Bandama (Lévêque et al., 1983), Volta (Petr, 1986; Lemoalle & de Condappa, 2009) and Zambezi (Davies, 1986).

Floodplains and marshes

16The most remarkable influence of the seasonality of rains and water flow regimes concerns the floodplains, ox-bow lakes and temporary pools. These systems are similar to shallow lakes and are located in plains and landscapes of low relief.

17The term ‘temporary pools’ groups together shallow water bodies that are not necessarily linked to a hydrographic system. Their water input is via local runoff and by direct rainfall on their surface (the term ‘rain-pool’ would be more explicit). In savannahs, they are completely dry during part of the year but are flooded annually. Their number and total area within the savannah is however often underestimated. In arid zones, their flooding is more irregular. Their principal characteristic, however, regards their aquatic vegetation and fauna. These organisms are adapted to a pronounced metabolic resting period (aestivation) during some stages of their life or may be actively or passively migratory (transported by other organisms). As regards fish, the genus Protopterus demonstrates two complementary modes of adaptation to these environments which also provide habitat to Nothobranchius (Cyprinodontidae) which accomplishes its entire biological cycle within a few months.

18Alluvial floodplains are often composed of depressions, including ox-bow lakes which are permanent water systems seasonally linked to the river and which form an active, biological and chemical transition zone between the terrestrial and aquatic environments. Several classifications of these interface environments have been proposed, according to their size and the chronology of their link with the main river, some being fed via intermediary pools or depressions connected to the river (Junk, 1982). In floodplains, the ratio of the area covered by permanent water to the total surface liable to flooding is also an important ecological variable. The Sudd (Nile basin) and Okavango (Botswana) have a permanent/seasonal ratio of 1:1.7. These are primarily vast marshes with variable water levels during the year; the recruitment of their fish communities does not rely heavily on the fluvial stocks. A ratio of 1:6 has been estimated for the Niger floodplains (Niger Inner Delta, in Mali) or the Senegal River before the Manantali and Diama dams were constructed. Much higher ratios (1:20 to 1:100) apply for the Chari and Logone floodplains (Chad and Cameroon) and in particular to the Grand Yaéré in North Cameroon (figure 1.8). In this case the fish community of the flooded zone is composed mainly of juvenile fish originating from the spawning of the river adults at the beginning of the flood.

19In tropical regions, the inundation of floodplains is initiated directly by rainfall while the flooding from the river occurs later. These environments are primarily colonized by herbaceous plants. In equatorial regions however, rain exceeds evaporation most of the time and the river water level is less variable. The alluvial plains are therefore permanently covered by a flooded forest. A good example is the great flooded forest of the Congo basin (about 200,000 km²). Coastal alluvial environments are characterized by a gradient from fresh to saline water in the inter-tidal zone which favours the establishment of mangroves, such as those on the coast of the Guinean Gulf. In both cases, fallen leaves from the trees form the principal organic source of the aquatic trophic web.

FIGURE 1.8. Location of the main wetlands of Africa (modified from White, 1983 and Denny, 1993).

20The permanent marshes (Sudd, Okavango, shallow annexes of Central African lakes, banks of Lakes Chad, Chilwa, Bangweulu or Kyoga) are covered with a dense vegetation of helophytes such as Cyperus papyrus, Phragmites spp., Vossia cuspidata, Typha spp. and sometimes Echinochloa pyramidalis. These macrophytes significantly modify the oxygenation and chemical composition of the water via the assimilation of nutrient and dissolved ions (Denny, 1985). Some plants such as the leguminous Aeschynomene elaphroxylon, which can reach 8 to 10 metres in height, are indicators of recent past arid periods because their seeds can germinate only in exposed sediments. Although the life span of one of these trees lasts no more than four to six years, a forest may last more than twenty years due to lateral re-growth (Howard-Williams 1975, 1979). The conductivity of water or sediment is also a factor in species selection: C. papyrus is limited to fresh water (400 μS.cm^-1 being its highest limit in Lake Chad), while Phragmites, Typha and Cyperus laevigatus are tolerant to higher salinities (up to 20,000 μS.cm^-1 in the Kanem small lakes) (Iltis & Lemoalle, 1983).

21The herbaceous riverside floodplains are characterized by currents that favour the oxygenation of the water. In addition, the low density of the vegetation allows the penetration of light into the water where the stems support an epiphytic algae (with a net production of oxygen) and an epibiotic community which is eaten by the fish, mostly juveniles. Finally, the herbaceous biomass is mineralized only after the water has receded, which is favourable for the preservation of oxygen in the water and later promotes local soil fertility.

22In marshes with dense permanent vegetation, a large segment of the primary production is aerial but the bulk of decomposition occurs in the water. Little light penetrates into the water and oxygen exchange at the water-atmosphere interface is limited, resulting in a lack of dissolved oxygen in the water. The fish must get their oxygen close to the water surface or in shallow areas. Some fish have however developed lungs that allow them to use oxygen directly from the air.

23Whereas micro-epiphytes constitute the first link in the trophic foodweb in herbaceous floodplains, the base of the web in marshes originates from decomposing debris from the aerial parts of vegetation and their bacterial associated fauna (the detritic food chain). The former environment is temporary and harbours mostly an eco-phase of various riverine or lacustrine fish species while the latter provides habitats for adapted species during their whole life cycle with a predominance of juveniles on the more oxygenated edges.

The role of underground water

24A main component of a river system is the related aquifer groundwater. Between the river and the underground layers, a permanent process of water transfer occurs. It has been estimated that the transfer of water between a river flowing over a pervious substrate and the underground is as large as the river flow itself. During the river flood, the water table is fed by the surface network. Conversely, during low flow, the stored water is returned to the surficial river network and maintains the surface flow in the rivers. In general, groundwater systems are more permanent and less variable than surface systems because the transfers are slower and the hydrologic surface variations are damped.

FIGURE 1.9. a: monthly discharge at low flows (Ubangi); b: evolution of the recession coefficient of rivers in the Sahelo-Sudanian part of Africa) (from Olivry et al., 1993).

25In the recent context of rain deficit over inter-tropical Africa, the river flow of the Sahelo-Sudanian rivers has decreased significantly during the last forty years. The cumulated impact of weak rainfall has resulted, with a delay, in a continuous and significant decrease of the river discharge. Whereas the decrease in total discharge was only 7% in the 1970s, it reached 16% in the 80s although rainfall had partially recovered (Olivry et al., 1993). This decrease is accompanied by frequent very low water levels and accelerated receding of the water (figure 1.9). The decline of underground storage has impacted the surface river flows with a lag of about a decade, before a possible future climatic improvement.

Shallow lakes

26Unlike marshes, the largest part of the area of shallow lakes is not colonized by vegetation. However, when the lake lies in a flat landscape, its shores are often composed of a vegetation belt with the same species. In Africa, shallow lakes are numerous and very different in size, from those with dimensions of a pool to Lake Chad which in the 1960s covered about to 22,000 km² (table 1.III). Some lakes such as Chad and Chilwa are endorheic (with no surface outlet) and all the water input from the rain and tributaries is lost through evaporation and seepage to the underground water table.

27Shallow lakes are polymictic, meaning that the water column is homogenous virtually every morning after the night cooling. Depending on the season and wind regime they may become stratified again during the day. The high frequency of the thermal cycle assures that the whole column of water is well oxygenated and that the regeneration of nutriments is continuous, promoting an abundant phytoplankton community. Most of these lakes are eutrophic (average chlorophyll concentrations above 25 mg/m³) and are susceptible to climatic accidents. If it happens that the stratification may last several days due to the lack of wind or no cooling during the night, anoxic conditions may develop in the water mass and thus result in massive fish mortalities. The turbulence created by strong storms may also stir the organic sediments leading to similar anoxia.

28Several large African lakes have been the subject of monographs: Lake Chad (Carmouze et al., 1983), Lake Turkana in Kenya (Hopson, 1982), Lake Bangweulu in Zambia (Toews, 1975). Smaller lakes have also been studied: Lake George in Uganda (Burgis et al., 1973; Ganf & Viner, 1973; Talling, 1992), Lake Chilwa in Malawi (Kalk et al., 1979), Lake Guiers in Senegal (Cogels 1984), Lake Naivasha (Litterick et al., 1979) and Lake Nakuru in Kenya (Vareschi, 1978, 1979). Detailed information on these environments is available in Serruya & Pollingher (1983) and Burgis & Symoens (1987).

Deep lakes

29The great lakes of East Africa are characterized by unusual dimensions in terms of volume (Tanganyika, 18,900 km³), surface area (Victoria, 68,800 km²; Tanganyika, 32,900 km²; Malawi, 30,800 km²) or depth (Tanganyika, 1,435 m max; Malawi, 758 m max.) (Table 1.IV). Pelagic processes are dominant in the Rift Valley lakes owing to the reduced littoral zone and immense depth linked with the high local relief. A synthesis of their characteristics is available in Johnson & Odada (1996).

30Deep lakes, large or moderate in size, are generally characterized by seasonal or permanent stratification. The water of the upper layers is warmer and less dense than in the deeper layers. The lake is thus constituted by two superimposed layers that hardly mix. Schematically, the upper layer is well illuminated and oxygenated whereas decomposition processes dominate in the deeper layer which is less oxygenated. This character can also be found in lakes with low surface area but sheltered from the wind such as crater lakes or small forest basins such as Lake Opi in Nigeria (Hare & Crankcase, 1984).

31Hundreds of deep crater lakes are found in volcanic regions of Africa, notably in East Africa and Cameroon. These include Lake Bunyoni (Uganda), and lakes Nyos (with a maximum depth of 208 m and surface area of 1.58 km²) and Tizong (48 m and 0.08 km²) in Cameroon (Kling, 1988). These are usually small lakes with poor native fauna while others such as the Barombi Mbo in Cameroon accommodate several endemic fish species (Trewavas et al., 1972). Lake Kivu, which was formed after a volcanic eruption blocking the course of a river, is permanently stratified with a deep anoxic zone saturated with methane.

32In a stratified lake, the fish live mainly in the upper layer where they feed and breed. In such cases the shores are therefore important for species that are not strictly pelagic.

33The accumulation of substances in most deep African lakes is slow and involves, though rarely, gases likely to accelerate the onset of de-stratification (a notable exception being Lake Kivu with methane). Climatic variations, cooling or wind may sometimes initiate mixing in the transition zone between the surface and deep layers thereby enriching surface waters with nutrients. It is however less frequent than in shallow polymictic lakes where this enrichment occurs daily.

Reservoirs

34In recent decades, numerous dams have been constructed across rivers in Africa chiefly for hydroelectric power, irrigation and/or urban water supply. The largest and most well-known of these are Lake Nasser on the Nile (the Aswan Great Dam), Lake Volta on the Volta, Lake Kariba and Lake Cabora Bassa on the Zambezi (Balon & Notch, 1974), Lake Kainji on the Niger and Lake Kossou on the Bandama (figure 1.10 and table 1.V). These reservoirs have created new aquatic environments, promoting the development of autochthonous or introduced species adapted to lentic conditions, but in addition, have disrupted the cycle of species more dependent on the seasonal flow of the river.

35Filling of large reservoirs with water often occurs without prior removal of the previously existing vegetation. Decomposition of this organic matter consumes oxygen and liberates nutrients into the water. This results into a phase of high phytoplankton production favourable for phytoplanktivorous fish, a situation that may last a while before stabilization to lower production levels. The consumption of oxygen is however more durable and its effects may continue for over twenty years (e.g. Lake Volta and Lake Mcllwaine).

36As regards fish, two environmental variables are important in a reservoir: the residence time of the water and the fluctuations of the water level during the year. Although rapid and substantial changes in water level may largely prevent the development of a littoral community important for the recruitment of juveniles (as occurs in Lake Nasser), it has been shown that these changes are often related with a higher fish catch (Kolding & van Zwieten, 2012) The stratification of the lake and the anoxia of deep zones are directly dependent on the horizontal flow of the water mass which in turn is evaluated by its residence time.

TABLE 1.V. The main reservoirs of inter-tropical Africa (source: Van Den Bossche & Bernacsek, 1990) (*: average depth)

37Below reservoirs, the river regime is often modified with smaller floods and, in some cases, counter season floods, leading to narrower flood plains and changes of the environmental conditions necessary for fish reproduction.

38Reservoirs constructed close to urban centres or highly populated regions experience very high nitrogen and phosphorus inputs which induce a proliferation of phytoplankton in the surficial layers and anoxia in deeper layers. When such reservoirs are intended for human supply in cities, eutrophication impinges on the treatment costs (filtration and the elimination of taste and toxins produced by some algae) as in Lake Mcllwaine in Zimbabwe (Thornton, 1982). The introduction of phytoplanktivorous fish, such as Hypophtalmichthys molitrix, may be used to fight against the proliferation of the phytoplankton, as in Lake Sidi Mohamed Ben Abdallah, which supplies water to one third of the urban population of Morocco (Bouloud, 1982).

FIGURE 1.10. The main manmade lakes in Africa

39Thousands of small dams have also been constructed on seasonal rivers in Africa, notably in the sub-arid zones in order to store water supplies for livestock, agriculture and domestic use. Numerous trials of fish culture have been developed in these small reservoirs, as fishing is another means of optimizing their use (Baijot et al., 1994).

Conclusions

40The characteristics that may be used to describe an aquatic system are multiple and of several orders. These result in the large variety of environments observed in Africa and other continents. Within each aquatic system, a variety of biotopes coexist that may be best described by means of a comparative approach.

41The diversity of the biotopes may rely on water quality and its relation to the metabolism of fish. Salinity and temperature are thus important and selective environmental variables. For example, Mormyridae do not occur in waters of conductivity above 500 µS.cm^-1 while Oreochromis spp. do not tolerate temperatures below 10° C. Few species are likely to survive in water containing less than 2 mgL^-1 of oxygen, which is the respiratory threshold for species strictly dependent on dissolved oxygen for their respiration (Bénech & Lek, 1981).

42Trophic relationships may also be considered as the main characteristics of a biotope. Although most fish are opportunistic, it is necessary for their survival that each eco-phase is able to fulfil its nutritional requirements at the right time. This is the point at which the primary productivity (whether autochthonous or allochthonous, benthic or pelagic), the position of the species in the trophic web, competition for resources and predatory relationships play important roles.

43Finally, an environment must be capable of supporting a complete biological cycle. Although the scale is different for migratory fish species, in all cases it is necessary that eggs and larval stages benefit from a suitable environment, as well as a protection from predation. The variety of environments and of biotopes in a given system is one of the conditions for the diversity of species.