Chapter 17

RIVER AND STREAM ECOSYSTEMS OF NORTHWESTERN AFRICA

CHRISTIAN LBVEQUE

INTRODUCTION African rivers is still partly focused on taxonomic inventories, many species being unknown or still to West Africa lies between the Sahara and the Gulf be described, despite a real improvement in knowl- of Guinea, and includes the Chad basin on its edge (Durand and Lévêque, 1980-1981; Teugels eastern border. The southern limit is the Cross et al., 1988; Lévêque et al., l989,1991a,b). For econ- river, on the -Cameroun border. As de- omic reasons, most of the data collected deal with fined above, tropical West Africa covers a vast area hydrology or fisheries for use in management pro- (3 million km') with a distinct climatic gradient jects. This explains why the state of the knowledge of from north to south. river system is very poor compared with North The available information on West African river European or American countries. There is still a ecology is rather poor, often qualitative or descrip- great need for field data as well as for experimental tive, with the few quantitative data generally scat- manipulation (Lowe-McConnell, 1988). tered in research papers, obscure reports or unpub- lished theses. In fact, the hydrobiological investiga- tions were for a long time, as elsewhere in the world, GEOMORPHIC SETTING focused on lakes. Lake Chad, an endorheic lake which could partly be considered as an extension of Geomorphology and catchment characteristics the riverine environment, as well as the lower courses of the Chari and Logone rivers and their Most of the area consists of flat sedimentary associated flood plains, were well investigated dur- basins and upland plains ranging from 150 to 600 m ing the International Biological Programme (IBP) a.s.l., with patches of highlands above 1OOOm and and later on (Carmouze et al., 1983). The large distinct from one another. Among the most impor- interior also was investigated by many tant, the Fouta Djalon and the Guinea range along scientists because of its economic importance and its the western coast separate those rivers flowing di- peculiar situation at the limit of the desert (Gallais, rectly into the Atlantic Ocean from those flowing 1967; Grove, 1985). Short-term pre-impoundment northwards, which are the sources of the Niger, surveys often were conducted before the damming Sénégal and Gambia rivers. The rivers on the which created major man-made lakes such as the Atlantic side (in Guinea, Sierra Leone and Liberia) Akosombo on the Volta, and the Kainji on the are relatively short, and partly torrential in their Niger. Since 1974, information also has been col- upper courses, which often are punctuated by rapids lected on West African rivers to evaluate the impact and falls and flow through narrow valleys. The main of chemicals used in the Onchocerciasis Control headwaters of the Chari, Logone and Benue rivers Programme in West Africa (Lévêque, 1989; are on the Adamawa plateau in the east (Fig. 17.1). Yameogo et al., 1989). Although the northern fringe of the area is dry, Diversity of the fauna and flora is generally con- there is a dense river drainage. The Niger is the sidered to be greater in tropical than in temperate longest West African river. It rises in the Fouta river systems, and the research effort on West Djalon mountains in Guinea, and flows in a loop .i

CHRISTIAN Li\ ÊQVE

passing northeactiurds through the estensive in- I+ith :ì catchment Lirea greater than 10000km' ;ire terior central delta (20OOOkm'). then southwards given in Table 17. I. through the coastal delta (36000 km2)before enter- ing the Atlantic Ocem lts major tributrìry is the Benue river ivhich rises in the Adamawa m Cameroun. Roth rivers have fringing flood plains which cover more than 10000km2 at peak floods (\Velcomme. 1979. 1989). The Kainji dam. on the loiver Niger. forms one of the major impoundments in Africa. The Chari river Lind its major tributar>-. the Logone. drain large ureas of Central Africa sav;inn;i into the endorheic Lake Chad. lt includes an czten- si\e Rood-plain system of about 90 OOCI km'. includ- ing the 7000 hi2 Yaere in northern Cameroun. which is cnnnected to the Chari. Logone and Lake Chad. The Sinigal river originates at the confluence of the Bafing hvhich rises in the Foutri Djrilonl and the Bakoye rikerc. The middle coiirse of the SCnegal is tiow under control of two dams: hlanantali up- stream. and Diama in the estuary. The Voltiì is within the great meander of the . The river sy

1O0 1 1O0 50" O0 50" 4 1O0 80 80 80 40" 80 60 60 60 30' 60 40 40 40 20" 40 20 20 1 O' 20 20 O O O .. n JFMAMJJASOND JFMAMJJASOND JFMAMJJASOND

RAINFALL (mm)

> 3000 ...... 1500-3000 .. ..1000-1 500 500-1000 0

...... o- 250-500 II. 0 0 0 0. o o .-.....- YI .-. 0.0 R ("1 AD'OPODOUME 700

T 'C 1O0 50" 1O0 80 40" 80 60 30" 60 40 200 200 -p 40 20 I OD E 20 O Io 'FMAMJJASOND J C M A M J J A S O N D

Fig. 17.2. Distribution of rainfall over West Africa (distribution of isohyets from Cazenave and Valentin, 1990; climatic diagrams according to Walter and Lieth, 1960-1967). .

527 CHRISTIAN L F V 6 Q Li t

Climate the northern regions. near thc lQ(l-nim isohyet. thc rain) season is short (a few weeks in July ,August 1 The regular seasonal pattern of climate chnnge is and there i.; a very long dry season. Furthtrr south. a consequence of the seasonal migration of the near the 750-mm ihohyet, the rainy season extends intertropical front or intertropical convergence from June to September. follnwd by iì long dry se;ì- zone (ITCZI. This front is :i Lone of climatic instahil- son. Near the coastiil regions. there is a distinct four- ity sepnriiting two air masw: the soiitherlq mass of se:ison climatic regime. Ivith ;i ni;i.ior dry season moist niaritinle air. and thc northerly mass of hot (November to hlarch~.a mri-jor net seiison (April ta and dry continental air. From December through July). ;i minor dr>-se;ison (August- September). and hlarch. the ITCZ i5 located south. reaching 5 N: the ;I minor- wet seiison (October-Noveniberi. Iivrlherlj air mas< doniinntes. and thi\ is conse- 1 he yarly amount of rainfull nnd seasonal dis- quently the period of the major dry sason in the trihution pattern shw local variations. But the coastal forebt zmc. .A drj- 15 ind. the H;irmattan. .;triking feature is the large pxr-Ic,-ye;ir variation in blons northeast across the reginn. o totnl rainfall. and the existence of a drought period reaching the coast. and bringing ;i fine duzt from the through the 1970.; Lind 19SOs. relative to earlier Sahnru. Then the ITCZ migrates clo\vlj northu.ards records. :tnd rexhes 170 N b>- .Id)- through August. In September. the TTCZ once agnin begin.. its south- Vegetation \vard migration. Locally. the seasonal pattern and ;ìmount nfrain- The decreae in tot:ì1 r;iinf:ill :ind the length of'the fall (Fig. 17.3depend on the latitude and the position rainy season from the coii.;t to the north. result in ;i of ITCZ. The length of the rain) seiiwn and the more or less regular arrangement of climatic belts amnunt of rain increase from north to south. In and vegetation mnes i Fig. 17.31. In the south. some

a Semi-desert steppe Semi-desert grassland and shrubland a Drier savanna types rbloister savanna types a Lowland rain forest -A

RIVER AND STREAM ECOSYSTEMS OF NORTHWESTERN AFRICA 523

native rain-forest still exists, but most has been relative length of the two periods: the Sahelian extensively destroyed and replaced by industrial regime with brief sporadic floods during the rainy plantations (coffee, cacao and rubber). In Sierra season, and the transitional tropical regime with a Leone, for instance, traditionally included in the longer flood period and a less severe dry period. The rain-forest area, the savanna now extends almost to tropical regime (and variants) is widespread in the the coast. In Côte dlvoire as well, the rain-forest is so-called Sahelian and Sudanian zones in West more and more confined to areas where access is Africa, and can be observed in most of the large difficult, and the so-called Baoule “Gap” extends rivers: Sénégal, Gambia, upper and middle Niger, much further south. Volta, Chari, Ouémé, Mono and many coastal Further north is the Guinea-Sudanian transition rivers. zone (White, 1983), covered today with secondary The equatorial regime (Fig. 17.41, north of the grassland and wooded grassland. In the Sudanian equator, includes a long dry period from December region, the surviving stands of natural vegetation to April, a first flood period from May to July, a belong to various types of woodland, but in places second and short dry period from August to where agricultural activities are possible, the natural September, and a second flood period from October vegetation has been profoundly modified and to December. Such a regime occurs mainly in woodlands in various stages of destruction or regen- coastal rivers from Côte d’lvoire (Iltis and Lévêque, eration occur. Finally, the Sahel transition zone, on 1982) to Nigeria, and further south to Cameroun. the southern fringes of the Sahara desert, is covered The pattern of flood regime is in general compli- by wooded grassland in the south and semi-desert cated by river size and differences in surface relief. grassland in the north (Fig. 17.3). Long rivers such as the Niger, crossing different climatic zones, have mixed flood regimes. But, dur- Hydrology ing the past 30 years, the damming of several major rivers has resulted in a reduced and more stable flow The flood regime of rivers reflects the seasonal downriver. The lower courses of the Sénégal, Niger, distribution of rainfall. Rodier (1964) considered two Volta, Bandama, Sassandra and Mono have be- major flood types for African rivers based on the come transformed into what has been termed a seasonal flow pattern: the tropical regime type with reservoir type of river, The building of numerous a single high-water season and the equatorial regime small upstream reservoirs in some river basins also type with two periods of high water each year. can strongly modify the general flood regime. The typical tropical regime (Fig. 17.4) is charac- Hydrological monographs are available for the terized by a flood period from July to October and a main river systems: Sénégal (Rochette, 19741, Volta low-water period from December to June. This (Moniod et al., 19771, Niger (Brunet-Moret et al., regime could be further divided according to the 1986), and Chari (Billon et al., 1974)

4000 50 r 7 a h m 40 2 3000 E

o 0 30 m F 2000 2 c .-o .-g 20 ‘CI ‘CI 1 O00 10

n ...... O JFMAMJJASOND JFMAMJJASOND months months

Fig. 17.4. Typical hydrological regimes in West Africa. Tropical regime: the Chari at N’Djamena (Chad); equatorial regime: the Agneby at Agneby (Côte d’lvoire). Long-term climatic changes \t~ntercoursesin the Central Sahara. such ;is the Tilemsi. -i\zaouk. Dallol Bnsso. Ddlol hlanuri and Cnntinunl climatic changes occur in .-\fric:ì. and Tarku, flo\ved directly into the Niger river (see variations ha\e often hecn estreme. \tith differences Talbot. 19FO: LPvique. 1990). Linder present cli- in rainfall and evaporation resulting in esprrnsinn. matic conditions. the)- :tre relict features. P.I.c \ t ouret contraction. or even disappearance of river systems. et d. (19781 reported that thc large fresh-\v:ttcr The number and nature of these cliniatic events is discharge of the Niger ended abruptly around reasonably kvell established for the last 30 (100 )-ears. 4000 B.P. In the equatorial LVt-st African zone. ;I Between 30000 and 20000 B. P.. there \v;ts ;i \vet m:I.j?ior climge occurcd around 8500 B. P.: the sud- 1 clinintic period in northern tropic:il Africa (Servant den renppe:irance of the humid forest in the m;i and Scn ant-\;ild;iry. 1480: f'errot and Street Perrut. where it presently is found (kíalej-. 19%). 1981: hlaley. 19x0. 198h). After 10000 B.P. aridity There is stratigraphic and sedimentological evi- .- became general over West Africa :tnd nianj- rivers dence of chmges in thc nuture of ri1er.s from ceased to flow. The period around 1 S O00 B.P. \vas l100OB.P. to the present time in the Sahel the coldest period in relation to the last glacial (Sombroek and Znnneveld. 1971: Talbot. 19801. maximum in the northern regions ('l'an Zidderen From strongly seuson:iI and cphemeral. the rkers Bakker. 19S2I. One of the consequences n.ns the changed to sea sonal nitli hraidcd r; t re3ms d tiring reduction of the area of tropical rain-forests. ivhich the late Pleistocene. The flow regimc progrehsivelj could survive only in certain refugia. and the spread- became niore regular \t ith meandering channels ing of savannas and grasslands over vast regions. from 10 O00 to 8cl00 B. P. ;is a result of:t more regular After 11 O00 B.P.. humidity increazed and abundant rainfall regime more e\t.nl) dibtributed through the rainfdl :tillowed the existence of large riLers cros- \ear ~hlaley.1981 ). During the mid-Holocene per- sing the Sahara. Frnni 11001) to 8000B.P. (late iod ( 7500-4000 B.P.) the rainfall itas seasonal. and Pleistocene Lind earl) Holocene) the major braided streams of seasonrtl character prevailed in Sahelim rhcr systems probably assumed their the Sahel. Decline in rainfrlll in the late Holocene mrisimum development with permanent high water (4000-3500 B.P.) resulted in highly se:ison:il river (Talbot. 19801. The Niger and Sh&galrivers and regimes. Lake Chad had a much greater efiective catchment. Kncnvledge of the past histor? of ri\-ers i$ iniport- and discharge \\its probably at ;i maximum (hlichel. ant to understand the present composition of the 1973: Pastouret et al.. 1978).A number ofprnminent freshn-ater fauna. hlost fish species. for inst;inctr. ;ire

River Sénégal at Bake1 not able to survive in sevsonal rivers. and they \vas very lo\\ during the food periods (September to probably disappeared from :i number of river sys- K'ovemberl. hut after the recession of the high flow tems during the drought. 1 X 000 yr ago. Recoloniz- the rilgal populatinns recovered. and a maximum ation of those systems could occur miiinly during density was obserled by the end of the low-flow plu\ ¡:il periods. when connections were possible period. In fact. during the low-witer perind. the between river sy\tenis through n\erflo\v. riwr cap- rivers are a succession of riffles and pools. with :i Icw tures or marshes. According to Roberts i lL175).dur- discharge. The pools ;ìre therefore comparable to ing the Inst interpluvi;il. the Sin@ and Grimbia pond:, in which the plankton develops md then rivers ma>-ha\e been grcutly reduced or even ceased washes nut at the beginning of the flood season. to flo\v. and the present relatively full complement of E\tim;ites of mean biomass and chloroph) 11 ti ' Nilo-Sudanic fizhrs may he largely or cntirely the concentration (Tahic I 7.2) in rivers from CGte result of cc.)lonization during the last pluvial by d'Ivoire can hr ccunpared to data from other sys- cms4ng the Io~v-Ijing countrj- in het\vecn the lo\ver tenis. For instance. Bi5nras [ 196S1mentioned similar courbe5 of the river?. The great simil,iritl- of thc tish density \dues in the Yolta river hefore the damming fiuna of' the Nile. Chad. Niger. \-Olt;ì. Sintgal and at Aknsomho. ;is did n'hite I ILWtfor chlnrophj.11 ti various other basins

PRI\I 4R\ PRODUCERS

Very little information is iivuil;ible on ;tutntrn- phic bionia:,s and primary production in \$-est Afri- cnn riiers. md mly ;t fe\v paper5 ha\e been pub- li\hed on the riverine phl-toplanktnn communities (Gras et 4..lOh7: Eghorge. 1974: Iltis. 19S2a.b. 1983: Livique et al., 19831. In six rivers studied in Che d*l\oire (Iltis. 19S2u. 1 W).Chlnrophj ta and Euglenophyta \vere ivrll represented in the phj~~planl\ton.The density 53-6 CHRISTIAN LEVÊQUE liminary results obtained in the Niger river at Statzner et al. (1985a, b, 1987). Information on bio- Kainji, before damming, indicated that the mud- logy and population dynamics is available for surface (epipelic) algae were 50 times as numerous as Siiiitiliurii (Elouard, 1983, 1988) and Hydropsy- algae in the plankton (White, 1965). chidae (Statzner, 1982). In running waters, aquatic vascular plants are not Estimates of biomass of secondary consumers are generally abundant, with the exception of Tristicha scarce. Some data are available for fishes. In the trifLrria (Podostemaceae) growing on stones in Bandama river, before the closure of Kossou dam, a riffles. This plant can form dense mats, where a very biomass of 125kgha-' was observed in January rich invertebrate fauna develops. upstream of the dam, and fell to 50 kg ha-' in May The fringing flood-plain vegetation of the Kainji before flood. Downstream, the biomass was area was studied by Cook (1965,1968), that of the 177 kg ha-' in May. Another series of samples be- by Chevalier (1932), and Hall et al. low the expected dam site gave an ichthyomass of (1969) did a pre-impoundment survey of the Volta. 177 kg ha- ' in January and 113 kg ha- in August Different aquatic or semi-aquatic associations were (Daget et al., 1973). Conversely, in backwaters, the recognized (see for details Denny, 1985; John, 1986). fish biomass of 149kgha-' in March rose to For a long time, well-known weeds such as 305 kg ha- in June, probably as a result of immi- Eiclihoriiia crassipes and Salvinia rnolesta were not gration from the connected river system. Other fish recorded in West Africa. A few years ago they samples in the Bandama basin gave a mean biomass appeared in the southern Côte d'lvoire and started of 102kg ha-l in the Nzi (Lévêque et al., 1983) and to spread northward into the Buyo and Kossou 100 kgha-' in the Maraoué (Daget and Iltis, 1965). reservoirs, on the Bandania river. Similar values (100 k 144 kg ha- I) were obtained in the Sénégal river (Reizer, 1974). In the savanna river, Holden (1963) found a mean biomass SECONDARY PRODUCERS of 415 kg ha-' in residual ponds, but for the overall flood plain the mean biomass was much lower, Information on zooplankton in West African rangingfrom 12 to 17 kg ha-'. High biomass values, rivers is sparse. Flowing water being unfavourable 400 to 5600kgha-l, were also obtained in back- for their survival, zooplankton are scarce in the waters of the Chari (Loubens, 1969). main channels of rivers, although they can develop in quieter backwaters or in residual pools during the dry season. TRANSPORT OF ORGANIC MATTER In the Chari and Logone rivers, there is a clear seasonal cycle of abundance, with very low densities Although very few data are available, dissolved during the flood, and a peak from March to June organic carbon (DOC)concentrations ranging from during the dry season (Gras et al., 1967). The mean 2.0 to 6.5 mg!-' (Martins, 1982)have been reported annual zooplankton density in the Chari river at in the Niger river close to the Benue confluence. The N'Djamena is 1122ind.C -',with 87"; rotifers, 6% highest values were encountered in June-July, just copepods, and 704 Cladocera. In the Logone river, before the peak discharge, and the lowest values the mean annual value is much lower: 189 ind. I - '. corresponded with periods of low discharge. Ap- In the Sokoto river, Holden and Green (1960) found proximately 0.53 x lo6 tons DOC are transported only 16 200 ind. me2. annually. Complex carbon molecules are present Aquatic insects are well represented in the rivers, either dissolved in river water or absorbed onto fine but little information is available on their biomass, particulate silt. These are usually in the form of production and community composition. In the sugars, mainly glucose, fructose, galactose, man- Bandama basin, Dejoux et al. (1981) distinguished nose, xylose, arabinose, etc. (Martins, 1982). In the different species associations in relation to current Niger, dissolved carbohydrate concentrations fluc- speed and the nature of the substrate, but the results tuated seasonally around a mean of 0.36mgi-' obtained in a particular river system can hardly be (extremes, 0.1 to 1.6). For the Gambia river, the extrapolated to other systems. The relationship be- annual transport of DOC for the period July 1980 to tween drift and benthic densities was investigated by June 198 1 was 11 200 tons (Lesack et al., 1985).

I 4

RIVER AND STREAM ECOSYSTEMS OF NORTHWESTERN AFRICA 527

The proportion of organic carbon in the total face area, and suggested a relationship between suspended particulate load fluctuates between 2 and discharge and primary productivity of the drainage 970 in the Niger (Martins, 1983). This corresponds basin. Using an updated set of data for 39 rivers, to concentrations from 1 to 4.6 mg/ - l. In the Gam- mostly from West Africa, Hugueny (1989) was able bia river, similar values were obtained (1.2 to 891; of to show that species richness was positively related suspended particles) (Meybeck et al., 19871, and also to catchment area (Fig. 17.6), and a power function in the Sénégal river. For the 1983/1984 water year, with an exponent of 0.32 gave the best fit. Neverthe- the total load of the Gambia was 1.83 x lo3tons C, less, a multiple regression analysis, using data col- whereas it was 4.9 x IO3 tons in 1980/1981 (Lesack lected on 26 rivers, indicated that a model explain- et al., 1985). ing species richness as a function of mean annual discharge at the mouth, and catchment surface area, was a better predictor: BROAD ECOLOGICAL ASPECTS OF THE WEST In (species richness) = 0.245 In (discharge) AFRICAN RIVER ECOSYSTEMS + 0.1351n(area) + 1.504. Species richness of the rivers The model involving both discharge and catchment area probably gives a better approximation of the Insofar as river catchments are separated by bar- volume available for fishes, as well as for other riers impassable to fish, they could be considered as aspects of biological productivity. A sample of 11 biogeographic islands. Insular biogeography pre- tributaries shows a higher species richness than the dicts a positive relationship between island area and value expected for isolated basins of similar surface the number of species present. Freshwater fish com- and discharge. Such a result could be explained by munities in Africa have been analyzed in this context the absence (or difficulty) of recolonization in iso- and species-area relationships have been empha- lated basins, compared to tributaries connected sized. Daget and Iltis (1965) observed that the spe- with the major river, when one species disappears cies richness of a community increases in proportion (extinction). Thus, the lower species richness in to a power function of the surface area. They used small isolated basins could result from a higher catchment surface area in their study of 13 rivers, extinction rate. mostly in the Côte d’Ivoire. Welcomme (1979) de- scribed species-area relationships for 25 African Hydraulics as a determinant in aquatic ecology rivers and tributaries, using a power function. Livingstone et al. (1982) pointed out that discharge Statzner (1987) has suggested that the control of was a better predictor of species richness than sur- stream community systems is more physical than

2 3 4 5 6 7

Fig. 17.6. Relation between the fish species richness and the aren of the river catchment for different African river systems (from Hugueny. 1989). CHRISTIAN LÉVËQUE biological. If this is true. then the analysis of river is observed during the maximum rainfall. and is systems should focus on a better understanding of followed two months later by a flood resulting from the role of the physical variables and on a proper the overflow of the Logone into the Yaeres flood- description of b-ariations in the physical environ- plain depression (Gac. 1980). Such submersion by ment. The role of hydraulics is particularly iniport- rain-water saturates the soil and raises the ground- ant in affecting the behaviour and the distribution water level. so that relatively little of the overspill patterns of organisms (Statzner and Higler 1986; flood from the main channel. ivhich could occur Statzner et al.. 1988). Variations in discharge can much later. is absorbed into the ground. change the pattern of flow in a stream section. and The relationship between hydrology and fish bio- changes in species wemhlnge niay be correlated logy is quite well documented in West Africa. Many with changes in stream hydraulics. Similarlp. stream Sudanese fish species spawn at the beginning of the hydraulics may alsn determine the sequence of spe- flond. often at the end of ;i long-distance migration cies assemblages from source ti, mouth in a river of ripening adults to riverine breeding areas system. Until now. nevertheless. few studies have (Welcomme. 1979: Albaret. 1982: Benech and used a hydraulic approach. combining paranieters Quensière. 1985). like current telocity. depth. substrate roughness. etc. The reduction of flooding resulting from natural Some quantitative ininrmation is available on in- climatic variations. such as the severe Sahelian \-ertebrate distribution in rclation to current veln- drought during the years 1970 to 1977. has had city (Petr. 1970. 1986: Dejnus et al.. 1981: Elouard. important consequences on fish biology and tishe- 1983. 1987; Elouard and Gihon. 19851. ries of flood plains. The population of .-llesres In the Sudanian tropical zone. the wet season is burcmox decreased dramatically in the Chari river short. For rivers with a tropical hydrologicul re- nnd southern Lake Chad (Benech et al.. 1953) as a gime. \vuter-level changes associated with seasonal result of the greatly reduced flooding of the northern flooding appear to be the key factors in the biology Canieroun l'nere flood plains xhere the young of of species and system functioning. rather than the species spend their juvenile phase. After a changes in water temperature or dap length drought period from 1971 to 1978. the study of the (Welcomme. 1979: Lowe-McConnell. 19x5. 19881. composition of traditional fishery catches in the El Periodicity (or seasonality) of floods as well us Reid river. Lin effluent nf the Yaere draining toivards intensity must he considered. Lake Chad. showed a .;low recovery of the fish In low-order streams (rhitron). strongly in- species richness. but a successive and apparently fluenced by precipitation. floods are erratic. un- random appearance of abundant species Benech predictable and of short duration. They correspond and Quensiere. 1981. I US3b). Dansoko et al. 1976) to catastrnphic events. and bottoni materials. in- :ilso found that growth ofjuseniles of two species of cluding detritus. are swept downstream. In the dry HJI~TCJCJ'WISwas poor during two gears of bad tem niay become completely dry in floods. and Reizer 119741 discerned great differences the most extreme conditions. Usually such ri\ers in the growth of Cirhcrrinus cifliirrz!~in the Sknlgal. appear during the drj season as a succession of corresponding with the flood intensity in different isolated pools in which dissolved nsygen content is years. Lonp-duration floods allow a longer spawn- poor. and only a few species of animals are able to ing period. which results in the production of a survive. second cohort of juveniles by some species Benech The situatinn is more complex in the higher-order rind Quensilre. 19S3a). rivers (potamon). where floods result from the Thc relntionship betiveen flood intensity and dur- cumulative discharge of many tributaries. and are ation. and fisheries praduction. is ive11 known much smoother and more predictable. It generally is (Welcomme. 1979). In a detailed study of the assumed that the inundaticm of the associated flnod Logone flood plain. Benech iind Quensièrc (1983b) plains is initiated when rising waters or;erflow the found ;i positive correlation between fisheries pro- banks and invade the low-lying areas. But. in West duction and flond volunie. They also observed. fnr Africa. early flnoding of depressions in flood plains difkrent fish species. a significant correlation bc- bp rain-!vater has been observed in swxil syhtems. tiveen grnwth ofjuveniles iind the intensity of food. For instance. in the I-xre.II pre-flood ofthe outflow The fish catch in the Crocc river is also htronglj- -4

RIVER AND STREAM ECOSYSTEMS OF NORTHWESTERN AFRICA 529

correlated with the flood regime of the previous year (small Barbus spp., small Characidae, (Moses, 1987). Such observations could be used as Cyprinodontidae). In the long mid-course, three models to forecast the detrimental effect of suppress- Alestes species, Eutropius inelitah and Hjldrocynus ing floods after dam construction. forskalii are characteristic of the calm and deep reaches, whereas species adapted to turbulent con- Longitudinal zonation ditions prevail in the riffles (Amphilius, Mastacem- beltts, Naniioclrarax, etc.). The existence of a long Distribution of macroinvertebrates and/or fish mid-course with little physical change was also ob- along a longitudinal gradient has been studied in the served in the Ogun river (Sydenham, 1978). Such a Bandama basin (Merona, 1981; Lévêque et al., 1983; situation is probably the fate of rivers with gentle Gibon and Statzner. 1985), the Mono (Paugy and slope, and differs from results obtained in rivers of Benech, 1989), the Ogun river (Sydenham, 1977), the Zaïre (Malaisse, 1976) or Zambezi basins (Bal- and the upper Niger (Hugueny, 1990b). on, 1974), where a more distinct zonation pattern In the Bandama basin, three main zones were comparable to classifications developed in the distinguished (Lévêque et al., 1983): (1) headwaters North Temperate zone exists. A peculiar zona- and small tributaries, which are temporary streams tion pattern has been observed for a few species running for part of the year, but dry or with residual of fishes (Alestes loiigipinnis and Hepsetus odoe) pools during the dry season; (2) a long relatively which occur in the upper reaches, disappear in the uniform mid-reach, but with several successive al- mid-course zone where they are replaced by other ternations between slow-flowing and rapid reaches; species, and reappear in the estuarine zone. The and (3) a relatively short estuarine zone where saline homogeneity of the middle course also is demon- waters may penetrate a few tens of kilometres strated by a stable species richness along the longi- upstream. tudinal gradient. Based on a more detailed study of the taxonomy The study of community distribution along a and distribution of three insect groups [Hydropsy- longitudinal gradient was for a long time descriptive chidae and Philopotamidae (Trichoptera) and the in nature. The introduction of the River Continuum Siinuliuin damnosuin complex (Diptera)], Gibon and Concept (RCC) (Vannote et al., 1980) provided a Statzner (1985) provided evidence that the long conceptual functional framework, stressing changes mid-reaches are not a uniform faunistic stream zone. in trophic resources with increasing river size. African They found that, in general, from the furthest up- fish communities were not previously studied in that stream to the lowest downstream riffles, there ap- context. Nevertheless, Lowe-McConnell(l975) sug- peared to be no clear replacement and almost no gested some general tendencies: the abundance in loss of upstream species. Rather a steady increase of the upper course of surface-eating insectivores and species numbers was observed, because of the occur- omnivores consuming riparian allochthonous ma- rence of additional species. This odd zonation pat- terial, and the presence of herbivores and benthic tern, as compared to those found in other geo- detritivores in the lower course. Such a distribution graphical areas, is assumed to be typical for streams was verified in the Niandan river (Hugueny, 1990b) in this part of Africa of gentle slope and without a and the Mono river (Paugy and Benech, 1989), but well-defined source (Gibon and Statzner, 1985).The no relationship to changes in food availability has distribution of freshwater shrimps in the Bandama been demonstrated. river fits with this zonation pattern (Lévêque et al., The general emerging pattern is therefore the 1983). Among Atyidae, Atya spp. occur only in the existence of a clear longitudinal zonation in the estuarine zone, whereas Caridina colonizes mainly upper course in relation to river size, whereas in the upper reaches. Two Macrobrachiurn species (M. the middle course other environmental factors pre- felicinuin and M. vollenhoveni) are present in the dominate, with apparently no clear change in com- lower and mid-courses, and one in the upper course munity structure. At a small spatial scale, those (M. rariderts). factors could be the local characteristics of the For fishes (Merona, 1981), the low-order stations such as current speed or substrate, which streams are usually inhabited by a small number of may be independent of river size. At a larger spatial species with a small adult size and short life-span scale, the river can cross different climatic or vegeta- tion zones. such as s:ivanna or rain-forest. that can areas of the Senegal. Niger. Chari and Logone influence conimunitj- composition. rners. These land-water ecotones play a major role Most of the observations on fish zonation have in the i\ater budget. the chemical and physical been obtained during the dry season. when samp- erosion-sedimentation cjcle. the hiological di\ er- ling was possible in the river bed. It is known that sity. and the productiL ity of the iiscociated lotic many fish species migrate upstream at the beginning CJ stem\. of the flood se;ison and spawn in the .;maller tribu- taries or the inundation zones vihen thej- are pres- - Hydrology. The buffering capacity of large ent. Thus. in &'est Africa. the fish communities rire catchments associated with flood plains results in fairly different during the flood season. hut rather rather sninoth flood curws. But in the arid tropics. poorly documented. where evaporation is high and exceeds precipita- tion. the net outflow could bc \.er>-much loiver than The flood-plain "ecotones" the inflou.. Ofthe 7.1 1 \ II)'"ni-' entering the inland central delta of the Niger. only 3.82 Y 101"m.3. 4s defined by Holland (1988). "an ecotone is ;i emerges. giving ;i net loss of 4h",,( Welcomme. 1979). Lone of transition between adjacent ecological sys- In the Logone flood plain (l'aerei. which covers tems. having a set of' characteristics uniquel!- defined 8000km'. the river input is 3.17 Y I0'ni3. and the by space and time scales. and by the strength of the outflow 1.1 x 1(Iq m3. Rains contribute 8.5 s 10' m3. interactions between adjacent ecological systems". and evaporation 10.6 \ 10qni3to the water balance Such a definition applies well to the seasonally (Gac. 1980,. A similar situation occurs in the Chari inundated Rood-plain areas bordering tropical Rood plain (Gac. 1980). rivers. which are a transition zone between the lotic - Dissolved solids and water chemistry. In the system and the adjacent terrestrial system. where flood plains of the Chari hasin. 870000 tons of clay allogenic processes doniinate. is sediniented each year in the Yaere. 250 000 tons in In most Weht African savanna ri\ ers. large sen- the hlassenya depression dong the Chari. and sonal variations in rainfall result in great fluctu- 50000r)tons in the Ba-llli flood plain (Gac. 1980: Litions in water level. causing a seasonal cycle of Burgis and Symnens. 1487). In the Yaere flood plain, Rood and drought over the lateral plains and low- Y00000 tons of suspended material enter the system lying ;ireas. although some permanent wter does with the river ivater. Of this. 870000 tons sediment persist within the main river channels and the de- out each gear in the flnnd plain. while only pressions of the flood plain itself. Junk et al. (19891 27 000 tons (3",,)are exported through the outflow recognized that the principal driving force respon- ~Gac.1 !MI). For dissolved elements. 34 000 tons are sible for the existence and productivity in river lost in the Yaere of the 185000tons entering with flood-plain systems is the Rood pulse. The system flood water. During the transit of the water there is responds to the amplitude. duration. frequency and :ilcc7 chemical erosion. and the outflow waters are regularity of thc pulses. Short-duration. and often more concentrated in silica. bicarbonate. mag- unpredictable. pulses occur in low-order streams. nesium and calcium. whereah the concentration of Conversely. in ninst large rivers ivith large flood sodium and potassium doe) not change. Sedimenta- plains. regular pulses of long duration result in tion of iron associated with clay also occurs in the eAtensive but temporury lentic habitats. Organisms Yaere ( Lemoalle. 1973). inhabiting these types of systems have had to adapt - Fishes. Flood plains :ire a source of fish foods. to spatial and temporal fluctuations. But lateral of both plant and animal origin. tìnd the aquatic exchanges between Rood plain and ri\er channels vegetation provides cover for the young fish and -. have also more direct impact on biota thm the refugia from the many piscivores. In the river and its nutrient spiralling discuswd in the RCC (Junk et al.. flood plain. there is ;i large variety of habitats. 1989). ranging from small temporary pools to large perma- The role of flood plains in the functioning of river nent lagoons and swamps. which vary between the systems has been inlesrigated in West Africa. bvhere flood and dry-seawn phaws IHolden. 1 %3: Wel- isell-developed Rond plains ;ire present in nexly a11 comme. l Y 79: Lowe-hlcConne1. 1987: Welcomnie $aianna basins. The best studied nre the inundated md de hleronii. 14)iS). As :i result. nny attempt it

RIVER AND STREAM ECOSYSTEMS OF NORTHWESTERN AFRICA 53 l

to describe fish communities inhabiting the main Rains Dry season channel and associated floodplains can only nutrient input be superficial. From his own observations on the in aquatic systems Ouémé river (savanna-type system) during the dry explosive growth of grasses exposed by falling season, Welcomme (1979) suggested a general micro-organiT Flood 1water. grazed by animals or peak burned by fires scheme, linked to major trophic categories, to de- rapid growth aquatic plants start scribe the distribution of fish in flood plains. One of aquatic vegetation

group of fishes (“whitefish”-see Glossary) occupies main feeding, time fish increasingly vulnerable the main channel during the dry season and moves rapid growth lo\ to predation /rapidly expanding slowly contracting all aquatic animals into the flood plain to breed and feed during the aquatic environment aquatic environment confined to river flood. Some species adapted to extreme environ- channels and lagoons mental conditions, and belonging to genera such poor water oxygen conditions deoxygenation in pools oxygenation impmve - as Cithauinus, Distichodus and Labeo, enter the lowest water flood plain earlier and leave it later than more level sensitive fishes such as Alestes, Sckilbe or Synodon- 6 7 8 9 1011121 2 3 4 5 months (Northern Hemisphere) tis. Extensive upstream migrations can take place, Fish reproduction towards the headwaters, for breeding just before or gonad at the start of the floods. maturation spawning juvenile growth A second group of fish species c‘blackfish’) in- Fish movemenis habit the flood plain or the vegetation mats fringing up channels disperse on back to river restricted flood plains to dry the main channel. Movements are limited to some season refuges

lateral migrations, and species are adapted to resist Fish biomass adverse environmental conditions, principally production of rapid growth and heavy losses much reduced deoxygenation. Such species tend to be partial young increasing biomass from predation populations spawners, with a breeding season starting in the Fig. 17.7. Seasonal cycle of events in flood-plains (adapted from pre-flood period and persisting over the peak flood Lowe-McConnel, 1985). period. Most knowledge about flood plains of African come deoxygenated. Some of those water bodies rivers comes from studies carried out during the dry persist throughout the year, but tend to become season, in the residual water bodies. But there is very overgrown with vegetation and play the role of little information on the distribution of fishes during refugia. the flooded phase. During the flood, the whole system becomes dificult to study (Welcomme, 1988). MAN’S IMPACT AND BASIN MANAGEMENT The seasonal cycle of events in a flood plain has been summarized by Lowe-McConnell (1985) West African rivers were considered for a long (Fig. 17.7). The inundation of savanna is associated time as fairly natural systems, but economic needs with an enrichment of the water by nutrient salts have resulted in increasing pressure for river con- from the breakdown of organic matter, decaying trol. One of the most obvious and controversial vegetation, and animal dung (cattle or wildlife graz- aspects has been the damming of several of the ing in the flood plain). This leads to the rapid major flood rivers over the past 25 years development of bacteria, algae and zooplankton, (Table 17.3), either for electric power or for irriga- which support a rich fauna of invertebrates. During tion. This has produced beneficial and detrimental the same time, aquatic vegetation grows rapidly. consequences, up and downstream, on system There is an extremely rapid increase in the produc- functioning. tion and biomass of different kinds of fish foods. - Upstream. The damming of rivers brings After the peak, the level declines and water flows about the transformation of a lotic into a lentic back through numerous channels. Animals migrate environment. But reservoirs are to some extent to the main river bed, or are trapped in isolated different from lakes and subject to rapid succession small lakes, ponds and swamps, where waters be- of benthic and planktonic communities, which per- 531 CHRISTIAN LFV~VUE

TABLE 17.3 30 to 50km and the fishery experienced ;i slight boom (Hilton and Kow-Tsri. 1970). Subsequently. hiajor reservoirs in West Africa the steady dry season discharge of 500m.’~-~ Lahe Rker Closure .\rea (km:) pushed the salinity boundary downstream. within 10 km of the river mouth. The main clnm beds. and Akosombo Tolta l9h4 7700 fishery. shifted 20 hm downstream. K;o.;sou Bandsma 1071 16311 Disruption of the flood-plain ecosystem leads to ;i Kainji Niger I i)hX I 170 BUYO Sassmdra 1’)8(1 900 markedly adverse effect on fish populations down- Manuntali Stintigal 1YS7 447 stream. Declining fisheries following impound- Selingue Si.ntigal lWl MY ments have been observed in difierent pince< in West Tiga Kano 1974 17h Africa. Eelow the Kainji dam. catch fell by about Ayami. Bia I o:ii lhll Taabo Bandamci 19% XO 50”,,in three years (Lelek and EI Zarka. 1973). and changes in species composition were observed (Sagua. 1978). A similar pattern was observed in sist for many year.; after clnrure. Quite a number of other tropical rivers. Conversely. the reservoirs pro- studies have been published for West :Ifrican nian- vide opportunity to develop flourishing fisheries. made lakes (see Adams. 1985). the results being out Dam construction and resulting modifications in of the scope of the present chapter. Nevertheless. wter regime also have a significant adverse effect dams have a direct effect on migrating specieh. on the traditional prxticey of flood-recession apri- blocking upstream spawning migration or the culture in s:ivanna are:ìs (see Adanis. 19851. There- downstream drift ofjuveniles. In the Bandama river. fore irrigation schemes become necessary, often the large shrimp í~lCiL.rl)brtrL.hiii,)Ii~o//c~h)i~ct~i disap- kvith :ittendant soil problems. peared upstream of the liossou dam after closure (Leveque et al.. 1983). - Downstream. Below the dum. the flow varia- CONCLL’SION biliti- is reduced and the river nioves from a seasonal to a perennial regime. The cessation of the annual hlost theorieh on spatial succession and zonation floods eventually leads to reduction or disappenr- in flowing waters are hased primarily on low-order ance of dnvastreani flood plains. .As ;i result of the streams and ha\e not been ripplied very well to entrapment of sediment in the Like. the out-flowing describe the functioning of large rivers. which are \vater is relativelp silt-free and its nutrient le\el is complex systems. with interactions between flood- generalli lower. Two years after the closing of the plain and channel ( Welcomme. 1988). Davies and Akosombo dam, estensive stands of Por[rtwqcro,i Walker (1986)noticed that. with regard to the RCC. rind i’~i1lisncri~t\we observed in the bed ofthe lower there is :i need for cnnsider:ible modification for the Volta ri\er (Hall nnd Pople. 1968).where praiously large systems which ;ire continually “reset” b! topn- there had been little development of higher aquatic graphic feat ures rind anthropogenic influences. plants. Below the Tiga dam on the Kano river Compared to northern temperate countries. \Vest (Nigeria). the reduced flows \vere iiccompanied hy Africci still orers ;i unique npportunity to sitidy reduced sediment loads. changes in sediment de- ”natural” river sy\tems. But such ;i situation \vil1 not position patterns. and the stabilizstinn of the flood last very long. given that the influence of man is s plain by vegetstion (Olofin. unpublished. cited in increasing rapidly through such activities ;is dam- . Adams. 19S5). minp. chemicnl and industrial pollution. and Water The damming of the river also reduces the mem management. From :in ecological point of view. the -* annual discharge doivnstream. limits the ground- future of the West African rivers is ;ilso endangered water diccharge. rind could result in the migration of by the rapid deforestation of their catchments Lind the salt freshwater interface. The fishery of the clam the resulting consequences for aquatic biology. For ESltvk rti[iitrrti in the Voltri estuari- illustrates the instance. Sierra Leone. \vhich usually is considered influence of drims on estuarine ecosgstenis. During :is ;i forested areia. is noiv rtlmnst completely c1e:ired. the construction of the Akosombo dam. the l‘I<, and s:iv;inn;i specie5 occur far to the snuth. Another salinity zone. critical for >pawning.niosed in1:ind for dihquieting feature is the widespread use of man- 3

RIVER AND STREAM ECOSYSTEMS OF NORTHWESTERN AFRICA 533

made pesticides as poison for fishing in rivers. The Burgis, M.J. and Symoens, J.J., 1987. African Wétlands andSliallow practice is well known in countries such as IpUter Bodies. Directory. Travaux et Documents no. 211. and Côte d‘lvoire, where pesticides like lindane, ORSTOM, Paris. used for agricultural purposes, are easily available. Carmouze, J.P., Durand, J.R. and Lévêque, C., 1983. Lake Chad: Ecology aiid Productioity of a Shallow Fopical Ecosys- It is quite obvious that a better knowledge of lotic teni. Monographiae Biologicae, Vol. 53. W. Junk, The Hague. ecosystem dynamics in West Africa will improve Cazenave, A. and Valentin, C., 1990. Les états de surface de only if the scientific community is able to sustain an la zone sahélienne. Influence sur l’infiltration. Collections increasing research effort. The need for basic studies, didactiques. ORSTOM, Paris, 229pp. including taxonomic, has been frequently pointed Chevalier, A., 1932. Les associations végétales du lit du Moyen Niger. C.R. Somni. Séanc. Soc, Biogeogr., 9: 73-77. out, but the task is difficult as long as the national (compiler) Holland, M.M., 1988. SCOPWMAB technical consul- scientific expertise of West African countries im- tations on landscape boundaries: report of a SCOPE/MAB proves so slowly, and priorities are always given workshop on ecotones. Biol. bit., Spec. Issue, 17: 47-106. to short-term questions related to economic Cook, C.D.K., 1965. The aquatic and marsh plant communities of development. the reservoir site. In: E. White (Editor), The First Scientijc Report of the Kai?$ Biological Research Team. University of Liverpool, pp. 21-42. Cook, C.D.K., 1968. The vegetation of the Kainji reservoir site in REFERENCES northern Nigeria. Ikgetatio, 15 : 225-243. Daget, J. and Iltis, A., 1965. Poissons de C6te d’Ivoire (eaux douces et saumâtres). Mèm. IFAN, 74. 385pp. Adams, W.M., 1985. River control in West Africa. In: A.T. 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