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The inland water fishes of Africa

 | 
Didier Paugy
, 
Christian Levêque
, 
Olga Otero

The diversity of aquatic environments

Jacques Lemoalle

Résumé

The long-term survival of a fish species in an aquatic system depends on whether that system provides conditions that favour, among other requirements, the species’ growth and reproduction. These conditions are grouped into two categories:
• the physico-chemical (or abiotic) environment deriving from the geological context and the climatic conditions as well as the aquatic landscape as perceived by the fish;
• the trophic (biotic) environment defined by the interactions between species, notably the predator-prey relationships during the successive stages of the fish development.
These habitat characteristics must be considered in both their spatial and temporal dynamics: the survival of a cohort or population depends on the synchronization between the needs of an eco-phase and the characteristics of its environment (Lévêque, 1995a).
The physico-chemical quality of both lotic and lentic aquatic systems is indirectly but rather precisely regulated by their geographic location. This takes place through complex interactions of different environmental variables (figure 1.1). The abiotic conditions (seasonality and spatial distribution of turbulence, oxygen, temperature, salinity and transparency, habitat structure) that have a direct influence on fish communities depend on two constraints: the local climatic conditions and the geology of the basin. In general, an aquatic system is a component of a landscape, which itself depends on the same two constraints. It is within these interactions between terrestrial landscapes and the aquatic environment that we will discuss, in brief, the variety of African environments, first by dividing Africa into two large regions, and then reviewing the main characteristics of the aquatic systems encountered in each region.

Texte intégral

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.

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).

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).

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).

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).

Rain or temperature?
The triggering factor of some phases of the biological cycle of many organisms is often related with climatic variables.
Sometimes the factor is clearly identified, such as a flood resulting from local rain.
In other cases, it may result from a season change, with no possibility to determine which individual factor is involved as many climatic factors change synchronously.
In Northern Africa, the rainy season starts in autumn when the temperature starts decreasing.
Which factor – temperature or rain – triggers the reproductive migrations of some fish populations?
It may be useful here to compare Northern Africa with Southern Africa where the warm season occurs respectively in the dry season in the north and in the wet season in the south (figure 1.4).

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

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

Na, K, Ca, Mg, HCO3, Cl, SO4 expressed in mg/L; conductivity expressed in µS.cm-1
Pmoy: mean depth in meters
* : HCO3 + CO3; ** : meq/L
References: B & S., 1987: Burgis & Symoens, 1987; H & H, 1992: Hughes & Hughes, 1992; VB & B, 1990: Van den Bossche & Bernacsek, 1990; W, 1972: Welcomme, 1972a.

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)

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).

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.

FIGURE 1.7. Distribution of river systems in Africa.

TABLE 1.II. Physical characteristics of the principal rivers of Africa (* volume: km3 s-1)

Rivers

Length (km)

Basin area (km2)

Annual mean flood (m3. s-1)

Congo

4 700

3 457 000

40 487

Nile

6 695

3 349 000

2 640

Zambezi

2 574

1 300 000

7 070

Niger

4 200

1 125 000

6 100

Orange

2 300

650 000

Chari

950

600 000

1 100

Senegal

1 640

441 000

687

Volta

1 270

398 371

1 260

Limpopo

1 680

358 000

5,33*

Ogowe

920

205 000

4 758

Rufiji

300

177 000

1 133

Ruvuma

640

165 760

Kafue

945

154 200

10,3*

Sanaga

890

135 000

2 060

Malagarasi

470

131 572

Cuanza

960

121 470

58

Okavango

2 560

115 000

254

Bandama

1 050

97 000

392

Save/Sabi

715

88 395

5*

Great Ruaha

750

84 000

103

Cunene

975

83 000

6,77*

Comoé

1 160

78 000

206

Gambia

1 120

77 000

170

Cross

485

75 000

569

Sassandra

840

75 000

513

Logone

550

73 700

1 500

Kouilou

605

60 000

700

Ouémé

700

50 000

220

Wami

490

46 361

2*

Incomati

714

46 246

2,3*

Tana

800

42 217

151

Ntem

460

31 000

348

Pongolo

565

29 800

2,8*

Pagani

395

29 526

0,85*

Cavally

700

28 850

384

Ruzi (Buzi)

360

28 800

1,45*

Nyong

690

27 800

443

Corubal

600

23 200

Pra

445

22 710

238

Mono

360

22 000

104

Ruvu

270

18 389

2*

Moa

425

17 900

Tano

625

16 000

129

Sewa

385

14 200

Kaba

280

12 900

Rokel

280

10 600

Bia

300

9 500

83

Oshun

267

9 014

Geba

255

8 000

Awash

815

7 700

40

Jong

249

7 500

Bengo

300

7 370

47

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).

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).

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).

TABLE 1.III. The main shallow lakes of inter-tropical Africa (source: Van Den Bossche & Bernacsek, 1990)

Lakes

Countries

Area (km2)

Average depth (m)

Chilwa

Malawi, Mozambique

750-1 000

2

Guiers

Senegal

170-300

2.5

Naivasha

Kenya

115-150

11

Bam

Burkina Faso

12-20

0.8

Chad “normal”

Cameroon, Niger, Nigeria, Chad

18 000

3.9

Mweru

DRC, Zambia

4 650

3-10

Tana

Ethiopia

3 500

8

Rukwa

Tanzania

2 300

< 6

Mai-Ndombe

DRC

2 300

5

Kyoga

Uganda

1 822

2.3

Bangweulu

Zambia

1 721

4.1

Natron

Kenya

900

0.5

Tumba

DRC

765

2.4

Upemba

DRC

530

1.7

Malombe

Malawi

390

4

George

Uganda

250

2.4

Ngami

Botswana

200

1

Chiuta

Malawi, Mozambique

200

5

Baringo

Kenya

130

5.6

Awasa

Ethiopia

130

10.7

Magadi

Kenya

108

0.6

Ihema

Rwanda

86

4.8

Nakuru

Kenya

52

05-4.5

Lere

Chad

40.5

4.5

Nabugabo

Uganda

30

< 5

Dilolo

Angola

18.9

From lake to marsh: the case of Lake Chad
During the 1950-1970 period, the surface area of Lake Chad was about 20,000 km2 with a volume of 50 km3, and thus a mean depth of 2.5 m. Owing to its water turbidity and wind induced turbulence, the riparian vegetation was restricted to a narrow band. Most of the lake area was covered with open water, and the fish diversity corresponded well with this lacustrine environment (Bénech & Quensière, 1989). As a result of the decrease in rainfall that started around 1970, a large part of the lake dried out between 1973 and 1975 (Lemoalle, 1991). The seeds present in the dried sediment could then sprout, and many plants developed (among which Cyperus papyrus and Aeschynomeme elaphroxylon) and continued to grow when the water came back. In the southern basin of the lake, with the same water level (280.5 m asl) the open water area which was about 6,000 km2 before 1972 has decreased to around 1,700 km2 afterwards. A wide marsh covered most of the inundated area, with impacts on the fishing techniques not only because of this new environment, but also because the fish species had changed from a lacustrine to a marsh community with different behaviours (Neiland & Verinumbe, 1991).

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).

TABLE 1.IV. The main large inter-tropical deep lakes of Africa (source: Van Den Bossche & Bernacsek, 1990)

Lakes

Countries

Area (km2)

Average depth (m)

Maximum depth (m)

Victoria

Kenya, Tanzania, Uganda

68 800

40

84

Tanganyika

Burundi, Tanzania, DRC, Zambia

32 900

700

1 435

Malawi

Malawi, Mozambique, Tanzania

30 800

426

758

Turkana

Ethiopia, Kenya

7 570

29.7

73

Albert

Uganda, DRC

5 270

25

58

Kivu

Rwanda, DRC

2 370

240

489

Edward

Uganda, DRC

2 300

34

117

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.

Lake Victoria: an example of rapid evolution
Measurements performed in the centre of Lake Victoria in 1990-91 have been compared with results from 1960-61 surveys (Hecky et al., 1994). The deep, poorly oxygenated layer has increased: it started at a 40 m depth in 1990-91 compared with more than 50 m in 1960, and its oxygen concentration has decreased.
This change points to the eutrophication of the lake that results from several causes. A slight climate change with an increase in temperature may have contributed (Diaz & Graham, 1996).
But the main cause is related to human activities in the watershed, with increased inputs of nutrients to the lake, that induce a change in the phytoplankton and in the distribution of oxygen.
During the same time, the small endemic cichlids, either pelagic or demersal, have partly disappeared as a result of the introduction of new fish species, such as Lates, andalso because of the new distribution of dissolved oxygen in the lake which has significantly reduced their biotope.
In a span of 30 years, the trophic web in the lake has been radically modified. The process is likely to be accelerating and the further changes in the aquatic environment may lead to important modifications in the fish communities (see the chapter Fish communities in East African rift lakes).

Nyos, an extreme example
On August 21, 1986, 1,700 inhabitants in a village close to Lake Nyos, in Cameroon, were poisoned in their sleep by a cloud of carbon dioxide emitted by the lake. In this lake, as in other crater lakes of the region, CO2 accumulates in the deep water layer. The gas originates partly from the decomposition of settling organic matter, and mostly from telluric activity.
Large quantities of gas may accumulate due to the high pressure existing at the bottom of the lake (208 m deep), up to 10 or 20 times more than in surface waters.
Similar to many deep African lakes, Lake Nyos, being protected from the wind by high crater sides and submitted to
a very stable climate, is (almost) permanently stratified.
What triggered the overturn (destratification) of the lake in August 1986 has not been clearly identified. Some authors have put forward a stronger than usual atmospheric cooling (Kling, 1987; Kling et al., 1989); others have assumed that a telluric microseism was the trigger. The resulting mechanism and its effects are however unambiguous: a first volume of deep water was uplifted, its dissolved CO2 thus became oversaturated at a lower pressure (depth) and was converted into gas bubbles. The bubbles amplified the upward stream, as when a giant soda water bottle is opened.

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)

TABLE 1.V. The main reservoirs of inter-tropical Africa (source: Van Den Bossche &amp; 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

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.

Table des illustrations

Titre 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.
URL http://books.openedition.org/irdeditions/docannexe/image/25160/img-1.jpg
Fichier image/jpeg, 162k
Titre FIGURE 1.2. Annual variations of surface water temperature of some shallow African lakes (from Talling, 1992).
URL http://books.openedition.org/irdeditions/docannexe/image/25160/img-2.jpg
Fichier image/jpeg, 189k
Titre FIGURE 1.3. Decrease of bottom water temperature according to altitude in some shallow African lakes (from Talling, 1992).
URL http://books.openedition.org/irdeditions/docannexe/image/25160/img-3.jpg
Fichier image/jpeg, 107k
Titre 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).
URL http://books.openedition.org/irdeditions/docannexe/image/25160/img-4.jpg
Fichier image/jpeg, 148k
Titre TABLE 1.I. Physicochemical characteristics of some African lakes and rivers
Légende Na, K, Ca, Mg, HCO3, Cl, SO4 expressed in mg/L; conductivity expressed in µS.cm-1Pmoy: mean depth in meters* : HCO3 + CO3; ** : meq/LReferences: B & S., 1987: Burgis & Symoens, 1987; H & H, 1992: Hughes & Hughes, 1992; VB & B, 1990: Van den Bossche & Bernacsek, 1990; W, 1972: Welcomme, 1972a.
URL http://books.openedition.org/irdeditions/docannexe/image/25160/img-5.jpg
Fichier image/jpeg, 360k
Titre 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)
URL http://books.openedition.org/irdeditions/docannexe/image/25160/img-6.jpg
Fichier image/jpeg, 152k
Titre 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).
URL http://books.openedition.org/irdeditions/docannexe/image/25160/img-7.jpg
Fichier image/jpeg, 279k
Titre FIGURE 1.7. Distribution of river systems in Africa.
URL http://books.openedition.org/irdeditions/docannexe/image/25160/img-8.jpg
Fichier image/jpeg, 396k
Titre FIGURE 1.8. Location of the main wetlands of Africa (modified from White, 1983 and Denny, 1993).
URL http://books.openedition.org/irdeditions/docannexe/image/25160/img-9.jpg
Fichier image/jpeg, 338k
Titre 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).
URL http://books.openedition.org/irdeditions/docannexe/image/25160/img-10.jpg
Fichier image/jpeg, 100k
Titre TABLE 1.V. The main reservoirs of inter-tropical Africa (source: Van Den Bossche & Bernacsek, 1990) (*: average depth)
URL http://books.openedition.org/irdeditions/docannexe/image/25160/img-11.jpg
Fichier image/jpeg, 183k
Titre FIGURE 1.10. The main manmade lakes in Africa
URL http://books.openedition.org/irdeditions/docannexe/image/25160/img-12.jpg
Fichier image/jpeg, 385k

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