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ECOLOGY OF LAME CHAR
A TROPICAL SHALLOW hAK€
24 rue BAYARD, 75008 PARIS, FRANCE 11-2
1. THE LnC STRl
- 164 -
E ENVIRONMENT
L2z-e Chzd is situated b e e z ~ q 12' and 14'20 latitude north ad 13' and 15'20 2 longiP2e e s t . ?"ne lacustrine basin cccupies a large region of 25 O33 h w i t h ,few
&pressions, \+hich is no mre than 1/16 th of the surface it occupied bew- 5 , m
and 6 , 0 3 3 yaazirs B.P. (Servant, 1970). It is subject to the influence of a t r o p i d
clinate, ccíprising a hot, dry season f m March to June, a rainy season from June to Octskr, and a dry, cool, s-on from Noventer to February. The man annual rain- fs;ll on the l a k is 320 m. Insolation is high with a mnthly average of 275 to il0
?nurs z-.d Eie rean M l y radiation-is 553 d / m - /day. The lake is fed Tor the mst part by mL&s f m the dari a d Logone Rivers, which represent 95% of the river inflm.
5 'r&s zzpresznt only 10% of +& total input which averages 47 x 10 m /year. The h i n g of an erh-ic type has m exit therefore and 9 t o 95% of the iosses are
'
2
i
9 3
b e +a t he i r ïprkant evzpration (2.23 m p=r year) , the rest being lost by Fnfiltra- e o n (-uze, 1971).
%causs of the shallm &pth, estirrated a t an average 3 m for a water level altitu& of 281 m, the volwe of the lake is small and &e level presents -&nt znnual .md intermmal d a t i o n s , for the balance between hr?m and loss is rare- l y a t + d x C . %-!-! w a t s r s rerai? ín the lake m an average one and a half years (Car-
mwe, 1976). The inflam vary Fn relation to the characteristics of the p1uvion-e- try aid, since 1963, p l u v i m t r i c conditions havinc been unfamurable, ttre level fe l l mntimmusly unt i l 1975, leading to profound mdifications in the e n v i r o m t and _p3pl&do~?s. Tl-mz s ta tes of the lake m y be disthguished according to the levd of the water, the surface and the aspect of the lake (Tilho, 1928) :
- - G ~ a t Clad : water level alt i tude 283 m, surface estimted a t 25,003 Ian2. This
\ WS the s L k b of the lake betwen 1963 and 1965.
2 - N o x a Ckd : water level altitude 281-282 m, surface around 20,m h ._ This v i a s the sLdte of the lake between 1965 and 1971, the period during which rrost of the hy&obiological observations =re made.
- L i t t l e Cqad : water level alt i tude 280 m, with separation of the north and south bashs and exundation of the G r s a t Barrier. The north basin, no longer being f e d , dries up and the l&e is reduced to part of the south basin. These extrerre
J
axxiitions &ked in 1975 and have continued u n t i l 1979.
A - - - L >
B' . ~xtq the Qlad Nonml priod, b w s of 1zrd.sczpss are to bs s m :
,'<ne opln wate rs devoid of aquatic v q e t a t i o n , Lhe red-is lards which are isla-&
of v e p b t i o n (Papyrus, P h w k 8 ) an3 tk a r ù A p l a w C 3 n s i s t ~ of &xt ã
LFlcusand sandy islands mrrespading to the, àune crests of a setc&, part ly ixmr-
s e v e d
large natural - regions (fig. 1). !%ere is no rodcy or stony s&strate anywhere in lake and the bottan cxazsists exclusively of lmse s d i m n t s : mud, clay, smd zlnd
,Feu&-sand (Dcqrmt, 1970)
ged-erg. These various enviramental t y p s make it possible to dith-qush '
I
I + i t 14'
Lake C W
I
1
130
NIGER i. I N
&bolsa t
I 1 4 O 130
I I I ' I Z O -
. .
Figure 1 - Lake Chad : main types o f landscapes (water level a l t i t u d e : 282 m )
Tne -rature of the w a k r s fcllckrs a seasondl cycle in accordance w i t h the c l h t i c cycle, with a "um Fn Januzq (19' C) ard a nzud" Fn M a y J w . (31' C).
of sedinent due to the action of the wind, the abundance of phytoplank~n in cer
t a i n regions and river i n f l m s loaded w i t h s o l l d =ted.al. &e pH, slightly mre
T I " q is mt very great k a u s e of tt?e shallajr depth favouring sus-ion c
I
11-3
- 166 - ,
.'&fi 7 in the south basin, reaches 8.5 in the east arddplago and rises to 9 i n certain sms of the north-east arckipdago.
to a con&~ctivity of 50 to 1 1 " b s / a n .
Sa l in i ty ranges from 70 to 1 003 mg/l of dissolved s a l t s wh ich correspnds
F r m t h e p r e d " ' U y calcic free waters of the south, there is a change to a s d t p " h a n c e in the north. Alth0ugi-1 the lake is an accumulation basin where the s d t s h a d concatrate, due to intense evaporation, ye t the average sa l in i ty or' the wzters Fs seen to vary little from one year to the next. S u m balance seems then to e x i s t b e t i the îon input and loss and this salt regulation is ensured
by km of the waters along the north bank of the lake (63% of the ions
tyFes of rtechanim : nonselective evacuation of salts by inf i l t ra t ion deposited annually
in the lab) and selective sedFrrr3ntation of the rest of the inputs, principally Fn the' fom of.r~oformtions of mtites and prec ip i takm of calcite (Cmmuze, 1976).
2. POPULATIONS, BIOMASSES AND PRODUCTIVITY
' 2. I PHYTOPLANKTON A N D PRIMARY'PRODUCTION L
Over a thousand species a d Lm have been enu"ted in Lake Chad (Coqère,
1967, 1974-1978; Ilt3.s & Capere , 1974) . The algal florule is daninated qua l i t a t i - vely by tl-2 Esmidizcese ard Dia-, but the Cyanophyta play an wrtant mie - - fm the q u a n ~ t a t i v e pint of vieslr.
Dxing the chad pricd, there is a difference i n the ppula%ns ard @-~y-ttlark~Lanic dersities b e k m the south basin (c0nducLdvity of 50 to 250 prhcs, strong turjidity, influence of the Chari flood waters) and the north basin Wch is richer in plmkton (conGmtivity o'f for each of these b i n s , the al@ popul&ions of the open waters are distinct fm L h s s of the archipslagces. These four laye zones can be characterised by the pr&.nme of species and pups of species (Cmmuze e t aZ, 1972; Iltis, 1976-1977- (fig. 2) :
to 1 303 prrbs, s l ight turbidity). reov ver,
- Pie opzn waters of the north there the Desmldiaceae, CZosteYiwn acieuZare
,preù"te for long pcricds of the year. P e d i a s t m , Botryococcus and bucro- cystis. The mrth-eàstem archipelago where the Cywphyta, Anabaena and Mycrocys+is are p r e a c " t . C l o s t e r h , Psdiastrun and Botryococcus are still abundant.
are abundant and the Diatom, Melosira grmruzata is absent. -
. , A
o a; - 'Ire p n wate& of the south an6 south-east, directly influenced by the f l d
. of the C h a r i , where the Diatans, Felosira granttlata and SuAre l la muelleri ~n~'ii~3~e t h e bulk of the p u l a t i o n .
L - 167 - * I
- The ezstem and south-eastern ardhipelacps where the cq'ano_pyta, Hic~mrgstitâ m d
T .&"a a r e abundant, and oxasional ly SurireZla, Pe2iast-m and MeZosira. The neaïï b i m s is lcw i n the open waters of the south, the hi+st values
having been found i n the mrth basin (table 1) . The total phytoplanktonic bicrrass
for an elevation of 281 m of the water level, has been e s b t e d at 40 803 tans (fresh weight in b i & l m ) in February 1971 (Iltis, 1977).
by a rise in the aenSity of phytoplankton. The total bimss (expressed In tons of fresh weight) thus passed f m 76,803 in Jan- 1972 to 1 8 3 , m in A p r i l 1974
,
A drop in the water level , particularly noticeable after 1971 , is accapxied
and 244,033 in February 1975 (Iltis, 1976). At the s m tire, i3xmres wsrese=n
Fn t h pspu1akb.m : in the cpsn haters, where D i a m and Q ã ã w e r e p r e ã J z "
h 1971-1972, E u g l m y t a RE to in 1- proprtiO~S h >.Til 197'4, Zrd' th-en chlorophyta and Qlryso*yta (- ' ,1974). In khe xm.rth basin the phy-
tqlanktonic Fopulation changes to rese i le that of a natron p n d wiLh a prewn- C3Erzrce of centric diatms acmqzmied or follmsd >by the m i t i o n an3 &vels~ .mnt of Osc i lZabr ia platensis, a bïue-greee a l g a dnxackïnistic of ÜE na- waters f r m the mri;hem bo*r of ch&.
a l g a l dens i ty
dominant groups
seasonal variat ions
South bas in Aorth b a s i p
open vaters
0.03 - 0.22 p l / l
Diatoms , .
Cyanophyta
very & k e d
archipelagoes
1 . 4 p l / l
Chlorophyta
Qanophyta
average.
s p e n vatera
0.7 - 1.6 p I / l
Cblorophyta
Diatans '
a l ight
archi pe 1 :goes -
1.4 - 2 p l / 1
slight
Table 1 - C h a r a c t e r i s t i c s of ecological 'zones es tab l i shed f o r .phytoplankton. The a lga l dens i ty i s expressed i n b io- volume (from ILTIS, 1977)
i i
-
P r i m q ? pro3udcion has been masured over a period of s2veral years ia the south-east archipelago a t Bol and in the south hasin during the Nolmal Chad prid.
likely to be usedby the algae is given in the relation : 1'0 = 0.437 Io. 1'0 equals an animal average of 325 Kwl/m2/h between 11 h and 14 h.
of the waters had
The ã&y total radiation (Io) averages 5,410 Kcallm 2 and the part lof tk r a d i z k h n
The rreasured w i t h a secchi disc and the values obtained have been o3mrted into coefficient n l u e s of vertical ex-
1 1 4
! - 168
where D.S. is expressed in mters. A lcodel has been ewlved alloWing the esthnation of hourly production
a m d midday (E A] f m nurcerous series of &serrations (Lemalle, 1973) : S A = - tex Zi, %here Zi = 2.07 D.S. in water of clay tuxbidity
or Zi = 1.20 D.S . Fn water of organic turbidity, w i t h 2 T ~ . - i n '
an incubator, qressed in g z / m /h. The da i ly production I E A is abtained by ths qua t ion
/h, %t being the optimal b u r l y production masured in situ or ìn
A = 9.1 I: A w i t h TZ A in *,/m /day. ~n the eastem archipslap,
3 2
& 2 2 t& reai ~ l y prifiary produciion is 4.2 92/m /day i.e. 550 g/m /year of organic
carbon, in the N o d Chad prid. During the d.ry period the decreasing depth brought h u t an increase in turbidity and consqumtly a reduced transparency, part ly caq-ezz-
m i m unit . Thus h 1973 and 1974 , the rrmn annual W y act iv i ty was 7 . 4 $ymL/day
ii 1975 (Lemalle, 1979).
in the m a t e m archipkaga. On the other hand, it was no mre than 2.7 c+/m 2 /day
' P r h a r y prbfludion is slightly 1-r in the other regions of ~ south basin
thma in the east archiplacp. On the other hanà, it is himr in the mrthern of the lake.
2.2 MACROPHYTES
& a l , 1978) and a dozen plant associations are really hpx tan t on the lzke (Lemad, 1969 ; Carmue zt aZ, 1972). In par"Licular there is a rotable abundan- ce of Vossia cuspidata In the whole d e l b region of the Chari and of Cypersts Pcnyms in the south k i n . These species d i s w a r Kogressively in the niorth
b a s h , givFr..g way to mha australis.'Ihe PFz&tes crustraZis are well represented every&e-m.
The bicPrisses of the aerial parts of the reed beds has been-estimted, in dry
w e i & t , a t 31 t/ha 'for Phragmites, 28 yha for Cyperus papyrus, 17.5 yha for Vossia and. 15.5 t/ha for T h ~ h a , i.e. a total 7.2 x IO6- t for the *le lake. The total b i m s of rmts is assayed a t 13 x 10 6 t ãppr"tely (dry weight). An analysis
of the CiiSrnFcal capsit ian of the different species has enabled the detemination
63 x 10 t for C a , .38 x lo3 t for PQ, 10 x 10 t for Na and 2 340 x 10 t for Sioz,
EpZSenthg reSp=c"dVely 16.5%, 3.45, 4.5%, 0.4% and 68.8% of the dissolved lacus- kir2 s k k s ((2aìmW e t aZ, 1978).
of the quan t iues of salts stocked i n the mcrophytes : 2 a 3 x '10 3 t for X, 3 3 3
Co al m Co
L - 169 - 1 .
d . w i * keschinomene sP,aphoxyZon, ES w e l l as Vocsia o u p i c h t a and Ipomoer,
,' ,%qicaticcr (F'otiuu, 1974). Later Aeschynomene and T-ypha f fdstrazie d e v e k p d mr"k G f
the lake, whereis in the south, great extents of Xeschmomene were partly destroyed, giving way to Vossia and &perus p a p y r u s . P i s t i a stratiotes, P m h e a ard Cgperus
r,uZiccruZis w=re also s k to - develcp in the souiA (Fotius & Lertwlle, 1976) . -
2.3 ZOOPLANKTON
lbtifers are repA-esented in Lake Chad by s m .twenty species (Pourriot et aL, 1968). The planktonic crustacem populations "ãist for the mst _Dart of eight spcies of Cladocera ( m n g wkich M o i n a nÚcrruha, V h p h h o m a excidun, Bobmina f u n g i - Jtc~fid, D u p h d a barrba;ta, C ~ ~ ~ d ~ p k v L i a COWUAZ) an? fom species of - (T/rcpo&?;;
ZomU hcogyLi tL ld , The"Lmodiap2omud g d e b i , Thetvnocyelcp~ neglecAuJ znd Mesocyctops h ~ c h a w Z ) representing 99% of the brctnass (Ixzssõrk et QZS, 1966; %y & Saint-Jean, 1968, 1969).
&as et aL, 1967;
Thzee 1.zrg-e emlogical zones my bo, distiJlgUisM in the m d Chad _~"-riod,
according to e.2 a€. siL&on at the scale of the lake (fig. 3) :
zmplaa'kton density and the sezsord cycle of abmdance ( c . z " z e
1972) I for no itTportant variations have heen &erveil in the spcif ic CO--
- the opsn waters of the south-east are the prest (93 q / m 3 in dry Wei@t and i n
man annual density) ?he sesona l variations subject to the muence of the
flood w a t e r s of the frnm July to February
and a x r " n during the h t season (Gras c-t d., 1967) ;
tions are d l , . have a high man d.ersiw (315 q / m 3 in dry weight);
very mu& deepr. The seasonal d a t i o n s are small (RnbinSan, 1971)
are h-pxtant, w i t h a - -
- - t h e ,archipelagoes and reed-islands of the south h i n , where the seasonal varia-
- the north bash hzs a srraller density (216 q / m 3 ) than_ the south bas&, but is
The total zoaplanktmic bianass was estimated a t 12,203 tons (dry weight) i n
Fesrwq 1971. Its dlry w e i g h t averages 8.9 kg/ha in the north basin which contains 60% of tke zwplanktOnic stock, 6.9 kg/ha in the eastern archipelagos and 2.5'kg/ha
-
in thE? southern and south-eastern open waters. Wryonic develprent l z s t s about as long with the Cladocera as w i t h the -
---@a studied : ktc.reen 1 ard 1.5 days a t 30' C, 1.4 and 2 &YS at 25', 1.9
znd 2.3 ckys at 22' ( G r a e t Saint-Jean, 1969, 1976, 1978, 1979).
egg to egg cycle las t ing 1.2 and 2.3 days at 30" c (h40ina "a, IkphanobOma excibwn, Cettocfaphnia c o w ) , and amthex g m t q "prising three species of Daphnia w i t h a ITU& longer cycle : over 4.3 days a t 30' C.
longer than w i t h the Cladocera : at a tenperature of 30' C, it lasts zesp=ctively 26 days for T. -Ú~cognL& &d 11 days for T . g d e b i . With the CycLopids, U.
&garding the ClaciOœra, o= group has a rapid rpst-esr33xyonic develqpznt, the
With the -, the p0St-enbryoni.c developrent of tbe D h p t n r d d s is mch
P --_ - .. -
h ' - 170 - and T. nqtec/ttLs, it is no mre than 6 dap at 30' C. The increase in weight of these different species of Copem has been detennined by evaluating the individual w e i g h t s and the length of t h e stages at 30' G (Gras & Saint-Jean, 1978).
3.148
3,242
2,123
2,102
3,495
14,110
3 304
3,580
21,194
~~
Total Cladocera
190.8
117.9
71.7
, 75.6 .
151.3
113.3
23.2
63.5
63.7
Total Dlraptdds
Total Cyclopids
T o t a l zooplankton
- B
annual (mg/&
16.5
27.5
29.6.
27.8.
23.1-
124.5
151 .9
56.4.
332 .8
1
Table 2 - Mean biomass ( B ) , annual production and P / B r a t i o f o r the d i f f e r e n t gzoups o f zooplanktonic crustacea i n 1964 and 1965 in the eas te rn archipelago. The mean tempera- ture during the year was 26.2" C (from GRAS and SAINT- JEAN).
It has been pssible-to calculate zocplankton prcduction-in the "2 of an a-mud cyde in the eastem. archipelago (table 3) : it averages 21 - 2 g/m in ~IY .
wsi@t, b m thirds of t h i s prcdu3Aon being due to Cladocera. The annual P/B i s . 113 for Q-m, 23 for D i q t m d d s and 63 for CyCl$ids, i.e. a man 65 for
the *le of the _@ation (Gras & SaintJean, in przpantion).
In 1971, the annual prduction was estjliiated a t about 863.030 tons (dry wei+t) for the whole of the l&, i.-e. a man 474 kg/ha or 2.50 x.10 aal&.
3
-
6
2.4 BENTHIC FAUNA
s y s L ~ t i c inventories hve been made : oligochets (Lauzanne, 1968); ~dluscs (L&vScp~,~ 1968, 1972) ; insects (Dejoux, ,5968,. 1969, 1970, 1971, 1973) If the insects are represented by nmmus -cies ( a p p " t e l y 200) , only a &zen v i e s of m 0 ~ L c s and six -cies of o l t m t s are found.
- 171 -
The of b o t h s e h n t is one of the main factors intervening i n the
tiistribution .zii &un- of species of woms mlluscs. ~hallox dept)l~ d~ not to play any role in the N o d Chad pricd, oxygenation a t the b t t can always
baing adequate. The chemical c a p s i t i o n of the waters do kwzver exert mm inf lu- ence IDqnnt & Lévz?que, 1968; Dejoux, Iauzanne & L&Êque, 1971 ; Carrrouze et ai, 1972; Lévêap, 1972). A study of the Variations of abundance in se- statbns of the soul21 basin has revealed a seasonal cycle in the W D ~ and insects with a
density in the cm1 season (Lecabsr to Xxrd~) , corresp3nding dlso to the high water psriod of the lake, and a dni" at the end of the ml s a w n (-%y- P-t) (Dejoux, L a u z m , L&vêqe, 1969). Dese results proved valid in 1970 for other i-egions of th= lake ( C m w e t aZ., 1972) .
2entM.c a z " i t i e s =re sWdi& in 1970 wer the *le lake (Ginmue e t aZ.,
19-72)', a r ea& princim p u p extensive disLiribution mnes m e s t a b ~ ~ b e d acmr-
Amng the rmllu~scs, the MeZania tubercuZata xce nurrerically predrnninant in the north bash and th= CrZeopatru buZimoiàes in the es'- an%iplago (zone 7) . ~ 0 t h these species are also represented in the south-eastern orjen waters (zane 5) and the G r e a t Barrier (zone 4 ) . BeZZaqja un<coZor is abundant in the northern open waters, the northem axchiplago and the Great B d e r , were it u m s t i t u t e s bebeen 48 and
72 k s h a (dry l x ü y vdght , not including the shell). T?-E mlluscs dis== w a r d s
th2 -north of the lake (bimass of 0.2 kg/") and are also less abmidant to the east I
(20.6 kg/ha) - Finally, concerning the Chimdds , several ecologiczl zmtes have fomà,
hilt their boundaries VLZ w i t h the seasans (C-uze e t aZ,, 1972; Dejoux, 1976) Tnìs hstability my be &e ÙI ?he rapid d e v e l o p n t cycles and to the active d i sF r s ion capacity of * ì s e o.ganisw. On the whole, the bicaMss of insects is always small
in bnthos, particulwly in the south basin (table 6). The benthic bimass was estimated a t 71,030 tDns in 1970 (dry wight, not
including the shell for the mlluscs). Wlluscs represent nearly 90% w i t h an average 32.9 kT/ha for ,organic natter and 210 kgfha for shells. This great biamss of mlluscs is an original chaxacteristics of Lake Chad and is due tn the fact that the wble sup-
\ . t
II +I
- 172 - 9
Figure 2 - Main ecological zones 'of phytoplankton (from Carmouze e t al ,1972) .
Figure 3 - Main ecological zones o f zoopl ankton i n 1971 : densi ty and biomasses '
(from Carmouze e t a l ,1972)
Figure 4 - Main ecological zones of t he benthic fauna i n 1970 (see t a b l e 6) (from Carmouze e t al ,1972).
1 %
I .
38.6
11.8
10.6
face can be assir;lila”&d to a vast littoral zone, because of the s l - í í &pths.
f.re ea tem axhipelago (region of Bol) and fm 1968 to 1972 in the o”&r parts of the Ide. Ln general the density of the spcies w a s seen to diminish m s i à e r z b l y , pzrtly as a consequence of the fall ing level of the la3.2. In fact t h i s has caused. an increaed agita”don of the b t t c m waters which are effected by rather strong winds
throughout the year and diskurbed sedirrwt is unfavourable to rrolluscs ( C a r m u e ,
e t al., 1972).
ir Zie emlution’of malacological “ m i t i e s wzs folkwed frcnn 1967 to 1972 in
,
114
30
30
Zones
I
L
3
4
Total biomass (dry veight tons)
Hean biomass (%/ha)
Total production
Hean production . . .
worms 11nsecr I --
i--
1 .SI . o. O. O.
1 .
.I
Holluscs
63 280
i 279.090 dry veight
tons
145 kglhalar
~~
4 1,410
960
1,020
460
150
120
1 , I 1 6 ~ 1 0 ~ Kcal
580x103 Kcal/hafân.
Table 3 - Benthic fauna : mean biomass ( B , i n kg/ha,dry weight) i n worms, i n s e c t s and molluscs i n seven zones of Lake Chad ( f i g . 4 ) , and production o f molluscs.
Only the production of rmlluscs, wh ich constitute the bulk of the bsnthíc bicmìss, has ken studied. The growth rate of the ndin species was detexnined by ir s i t u breeding and theoretical growth curves hãve bsen calculated using- the von Eiertdanfly &e1 (Lev-, 1971) . Reprodtuction occurs throughout the year with the Prosjbrmchs (MeZwziaa BeZZamya and CZeopaDd and during the an1 season w i t h
CorXcuZa. P:kduction w a s calculated àuring an annual cycle i n different bioto-pss znd af fe ren t regions of the lake ( L é q U e , 1973) and the man annual P/B ratio wzs estimated a t 4 . 4 for Melania tubercuZata, 5.8 for BeZZrmTya W Z ~ C O ~ Q P , 2.6 for Cleopatra bulimoides and CorbimtZa africana and 2 for CaeZa$ura aegyptiaca.
( d r y weight) of organic matter in 1969-1970 and 1,883,m tons of sfr?lls, i.e. an
-
The annual prcduction of benthic mlluscs for Lake Chad wzs 279,033 tons
average respectively of 14.5 g/m 2 /year and 98 g/m 2 /yeax. Zxpressed in calories
-. 1 %
- 174 - 9 2
I' (Uvkpe, 1973), this production is 1116 x 10 K c a l , I .e. an average 58 Kcal/tn /Wo
The greater part of t h i s p e 'on is fiade up of Prosobranchs and abwe al1 B e Z Z q a .
"hese production studies were ccrnpleted by breathing TreaSUTem3-ntSr in order to esti- nate assimilation. ~nergy budgets have.been estabushed for natural pspulat.ic#Ls b b s e production had been studied in the murse of an mual cycle, me yield 02 - asshilatked eriergy -take for gztwth (P/N is' fairly high in the P m s d x "
(setwee?? 20 md 30 for Melaria, 13 and 20 for C¿eopatra, 32 for B e Z w c d and b I for -
'ch= Lzzïe1Mxand-L C. & f ~ - i ~ m t ~ b.t7heen 9 and 11%). (LévSp , 1973).
Fbs insects, the length of the larval cycle is 17 days et 26' C and 13 days at 30' C in the laboratory for Chi-8 plcher k j o u x , 1971) and 18 days be- 18' arid 23' C for T m y t a r s u s nigmcinctu8 h conditions close to natural onss
(Dejous, 1976)
in the cool se- In the mrth basin (Cejoux, 1976) For this l a s t species, the daily P/B 'has been esttmted 2rt 0.24
2.5 FISH
' The ich t€gdqLcal fauna of the le and i t s basin listea by Blache (1964) consists of about 140 s p d e s .
Tihe b i o l w and growth of a czrta311 ' n m k r of -cies has been studied :
AZastes bmmoze h r a r d , 1978; curard et ïkubens, 1969, 1970, 1971; Fbpson, 1968, 1972) , Lates niZoticus (kpsan, 1968, 1972; LrJuoens, 1974) , SchiZbe n ~ ~ s t u s artd SchiZbe urmxmmp~~ I-h.bk, 1974, 1935) , C i t h v h u s n ' t b (&ne&, -1974), Brachysy- t . n b G ¿ i s bazasoda (EE1Ilg&., 1975) , Sarot&ro&m goZiZaeus (hum, 1978) ; Micra- l z s tes acuCidms (L& 4 IL&, 1977) , Ichthyoborus && & Lek, 1978) . Pbst of these
- 175 - L. c .
s+p=cies r e p d u c z a t fhe Lkim of the river fl& a d carry out their &igrztions in order to spam ne= the flood plains. Tne m u l a t i a n ãymnics of
kleskes baremoze hzs been the subject of detailed studies, due to the b p r t m c e
of t h i s species in fisheries (Durmd, 1978).
01,
"
It has not been pssible to make any estimation of iCi7~ological
bianass in Lake Chad, with the exœpt im of s c m ichthyoto~c species in particular
envi romts (hubzns, 1969, 1970)
A t the bq inn ing of 1973, the level being mnsiderably laver, the lake broke up into three collections of water separated by shoals. This led, in csrtain
r q i o n s , to massive f ish mrtalities, w a t e r s , follming p r i m % of w i n d distubsnce bringing h u t a r e s u p x i o n of the
. s e n t (Ben& aZ., 1976) . The pred&ar s p i e s w e r e khcse ï rcs t af feckk ( I c t e s
z iZot icxs , Bydrocznus b r m i s , 32&c_unus ~ o F s ~ G Z ~ ~ ) , but saw s-pcies 5 e p t e d to be
rssistsnt to anoxia, Su& a s "Lhe T i Z q i c , &o fell victim to this s i b a t i o n .
By 1977 the n o e r bzsin, vhi.&. had hrid e in 1975, w a s no mre &&an a sort of t s i r a j r mr. The lake wzs -this reduœd to the south b ~ i n and in the q
wzL23rs mnnectea w i t h the river netwDljc the pgulaLLion diversifid and the sto&
of Alestes w a s reaexed.
to the terrfpraq demyyeration of the
W i t h the d r q in the 1-1 of the -lake, fishing intensified. In 1974 the nortlh
b s i n was so w e l l exploiL& tha t s t d u s =re pratically exhausted before t h i s region dried ur, in 1975. The f i s h e m mved mnsquently-tmards the south basin vi%& h a sime provided the bulk of the catches.
3. .TROPHIC RELATIONSHIPS
3.1 FEEDING OF PLANKTONIC CRUSTACEA
A study of the food diet of planktonic crustaea ( G r a s , I l e s & Saint-Jean, 1971) shms that the m a i n species of Cladoera ingest all planktanic algae, mnG- e l l u l a r or colonial, whose s i z e ranges f m h u t 4 p to 30 p. The Cy-yta, Anabaena ,fzos aquae harever, whidn represents m ixpr tan t part of the a l g d bimzss I
is not normally conswed by the C l a h c e r z . On L& other hand it cmstitutes an q-
preciaSle part of the fmd of the D i a p t m i d , Tr-pod iq ion ius incognitus (a p & c " n t species h the z q l a n k t o n b i m s ) and of the Cyclapid, TThermocycZops negZec-t;us. 2% consumption of Anabaena by both these spcies does not e a r to be regular
l a e v e r and could depend on the abundance of the algae and oc A the s t a t e i n which the cells chains are Ïcxnad.
-
--.i----̂ .- ---- ) * I? - 176 - .
m d t
If Cladoera and D i a p W d s are essentially phyta_pagous, bacteri@agous
and deixivorous, on the contrary the Cyclopids have a mre carnivorous diet. TFRrmo- cycZops negZectus has a mixed diet, just as the cGpepodit stages of MesocycZops leuckarti and !?Y"xyclops incisus, k , e d i e t is predcminantly phytqhagous during the f i r s t staqes and carniv0mu.s a t stage 5 . The adults of bo& " h e latter spscies are sser?t ia l ly ~ v o K 3 u s .
3.2 FEEDING OF FISH
, A stulry of f a diets (Lamanne, 1976) hzs qabled the classification of the min spcies into laqe groups of consurers, according to trophic levels. The first
. level mnsisting of "primitive" food soures (algae and detritus), three other levels
have bsm revelled :
- n-2e second level is f o n d of primary cx)flsurïers. h n g these we dist in-
gi i s f i ~e ~ y - t o p l a n k t c m - f ~ r s (Samtherohn gaZiZae) ad d e t r i t i m ( ~ d e o Bene-
gatensis, Citharinus cithartcs.. .). '&e essential differen- betweek these diets is due t o the different feeding behaviours. S m t h e r o d o n gaZiZae is a typical filter feeder selecting abme d l dqae , and even celrtain types of algae W a n n e & Iltis,
1975) .+,%ere= Labeo and Citharinus eat the bottan organic fi lm, mnsisting mainly
of sedirented algae. The pLimary 0=111surrers also camprise the &minant mcro@iytes- f e d r s , sudl as Alestes macroZepidotus.
'str ick ~ ~ a n k t O n - f e e d e r s , su& as AZestes baremoze or Hemisynodbntis membrcmacew or daminant OES su& as Brachysynodontis batcnsoda'and AZestes dente=, The ben--
f e d e r s su& as Synodontis schall, flyperopisus bebe,. Zleterotis niZoticus.. . f e d
cn hti-Lic. invertebrztes mnsunhcj the organic Z i l m a t the bottan (insect larvae, csL~zcDdsr mlluscs) (Ctliromminae and Tcmipodinae),
- The th i rd level consistes of seamdary consmrrers- These include the
The insect larvae m n s m are essential ly the Mronanids
Ephemra and Tridmptera.
- The fourth and l a s t level is font& by the b"l a r s i r ~ r s . C e r
M n are s t?z ic t fish-feeders : Lates nilotieus, Hydrocynus bretris. Diets of other sFcies are rare V a r i e d : Hychvcynus fo r ska l i i , Bagrus bayad, Eutropius n i b t h s , SchiZbe urcmoscopus... Besides fish, these spzcies eat sh-s and aquatic insects. It should be mked that B. bayad, S. uronoscopus and E. n i lo t icus 03nsm-e not only
;hole fis! but also laqe qumtities of f ish debris. In the gmup of the nm-strick
fish-feeikrs! these thres spcies are quite apart frcm the carnivores feeding essaì-
t i i d l y oil living prey. sudi as Hgdrocynus forskal i i .
'Y 4 l ' i7 - I . II-
,=/
I-
C O N S w a S S=°KD~Jzy ' @--J-J 1 0 9 ,
CONSUKERS
\ 1 DETRITUS
ORGANIC DEPOSITS M O D SLFPLY
l3OF'HIC LEVE= DETP.ITUS FOOD CFLAIN
QQ p m o r o IO
\ I / ZOOPLANKTON I
PHYTOPLW?TON
G?.AZISS FOOD CHAIN
F ig . 5 - Food cha ins of primary and secondary coy.smers i n Lake Chad open wa te r s . The arrows i n d i c a t e t h e d i r e c t i o n of i n p u t s and t h e numbers and vo lumet r i c pe rcen tages of t h e s e c o n t r i b u t i o n s t o t he food d i c t s . Numbers i n c e r c l e s correspond t o preys whose t r o p h i c p o s i t i o n i s unknown. S. s c : Synodantis schazz, B. be : Byperopistls bebe, H. n i : Beterotis n i l o t i e u , A. ba : Alestes baremoze, H. me : Beinis_wLodantis men&,Sicmacex.s, B. ba : Erachysynodantis ba tensoh , 4. à e : Alestes dentex, A. ma : AlesCes macroZepidottcs, T. ga : TiZap ìa galilaea, C. c i : Citharinus-cith-, C. d i : Citharinus dìstichodcddes, L. se : Labeo senegalensis, L: CO : Labeo coubìe, D. ro : Xstichodus mstratus, S. c l : Synodant i s cZarias (from, . I . . Lauzanne, 1976).
. . TfftRFSlRIkZ. INSECTS
Fig . 6 - Food webs of t e rmina l consumels i n Lzke Chad open waters. L.' n i : Lates niZoticus, H. fö : Hydrocynus f o r skaz i i , H. br : Bydroxynus brewis, E. n i : Eutropius niloticus, B. ba : Bagrms bqad, S. ur : SchiZbe urmoscopus (from Lauzanne, 1976).
12-1
- 178 - u1 * , The importance of the different groups of ~ n s ~ r s differs between the
arMpelago and the eastern orjen waters.
and above al1 zooplankton-feeders wki& represent 44% of the id-khymass, and the presence of a strong and stable planktonic biamass thmu#mut the year.
o p n waters are strongly d " t e d by the group gf teminai. consumers for- ming 64% of the fish bicnass. The maintenanœ of t h i s great b i m s seems to depend
indirectly on the fall of land insects and the abundance of ,sinall zqlankton-feeding prey f i sh , Micra7,estes acutidens ãnd PoZZimyrus i s idor ì whose prcduction is high. (fig. 5 and 6 ) .
The archipelago is oftea distinguishable by the abundanœ of plankton-feeders
To conclude, the food resources available i n lacustrine e n v h " k s during the Norma.1 mad @ase seem to have been relatively ,well m s u w d . The macrophytes
of &Lhe ard-dpalago beve r could probably feed a denser m u l a t i o n of herbivores.
TROPHIC LEVELS
TOP
SECONDARY CONSUMERS
PRIMARY
w P R O ~ U C E R S
PHYTOPLANKTON
Figure 7 - Energy accumulated ( i n c a l o r i e s ) by Lates n i l o t i c u s zccording t o the food chain used, s t a r t i n g from 100 c a l o r i e s f o r phytoplankton (from LAUZANNE,1976) e
kan the qclantitative point of vi& the focd wnwrsion rates havs b s s ~
detemired for three species, each correspnding to i? mwun-er level (Lauzanne, 1972, 1978). This rate (Xg) is the percentage b e h a the increase i n weight of the f ish
and the quantity of food ingested, e x p ~ ~ s e d in fresh &&t. I f kërgetic qdva len ts
are used, this ratio then ~ p - m e n t s the eneryic coefficient of first +er grwth
(K1) . Kg and I$ are resps t ive ly (&ytqhageus) , 8.8 and 44.8 in Alestes baremoze (zo@ankton-f&r) and 26.4 and 27. 3 in Lates n i l o t i c u s (piscivore)
,, II I,
to 3.1 and 18.9 Fn S a r o t h e m d m gaZiZaetce'
found for S. gaZiZaeus,,are fairly lckJ. Tnere':is in feet a p r use of tht
feed ingested i n 1. baremoze, Trie mversim rate is also lm (8. 8%) , but ths energy mzfficient is dzfkdtely bri&er. For L. n i t o t i m , a predator,-, the energetic c x f f i c i e n t is M h- &keen those. of the p- species.
ES is - gensrally the case w i t h ~ ~ ~ t n n - f ~ o
All these results form a pattern of &;he ttansfers of ar~rgy a m r d i q to tm p s i b l e c h z h s ending With L G ~ ~ s ni2oth t l s (fig. 7 ) . 3dLng idth 100 cdones p m
vi&d by the zlgae, L. niZotincs accucrulaLa 1. 7 in the case of t3Klo htemtdiq link and 5.2 Fn tl.R CilSe of a single that the glcbal e.r~=ryy outpst of the f ish y23pJla-lim of the ar&pel-o, f o d min-
ly of zplanktm-fe&ers, must be m~siderably higher than &&at of the pzpl=ätions of fhe o p a wate rs &ere "&mind C X - L S ~ T C , dcmina+ (Lauzanne, 1976)
(S. g z l i l m s ) . This p t t ~ r n &o shews
4. CBMChUSIONS
Due to &e mxxety of its b i o t c p s and to the i n p r t z m t nuber of species pre- sent, Lake Chad is a.ccmplex and hetercymus environn?=nt. Tne south- basin, submitted
to the action of the f l d w a t e r s of the Chari is r i b r ig fluviatile p u e s and
may be considered ta be an extension of the river system, in ~ a r i s m w i t h
north basin, mre stable but poorer in the nuber of species, yet w h i & mnstitutes the real lacustrine environnrent, s M & y spdc ing (Canmuze et a2 1972)
Y i x e a w r , because of the average shallcw Ckpfhs, Lake chad is an unstable eeosystgn, the surface of w i d may vary fran me to four tines wittrin a few years,
according to the m p n t of rainfall over the whole basin. These oscillations in the level do not w a r to have any well-defined periodicity and thRy d z in am&&- rable &angeF in Lhe envi-mment and the cdrma3itjes. Tnus, follcrtTing a rapid drop in the level OcCurrFng after 1972, the norkLh basin driep up and the maa@@es
azrran, a greater part of the south basin, the 1&e being redud to tlx eastem
cpen waters. Cbviausly the bianasses of organism and the prcductivìQ of the dif- ferent groups were greatly affected by these mdificaticms. The results abtdned ho- wever during the so-called "ormal Chad F-ecLod", i.e. for a water level bebeen 281
c? a
aï 282 m, may be considered repr~smtative of the averwe s t a b of the ldke. 12-2
J *__ I _ - _LI. - \-
- 180 - 4
-----
?hhytc p i B n k to n
F k c r o D h y +2 s ----
- 6
----- Zoo p l a n k t o n
Zen thos --- ---
Fish
,, Lake aad as a vihole, because of i t s l c f w average depth and its Well-OxygenatEd
water, may be assimilated to a vast littoral zone. Consequently the macrophytes ckvzlop mnsiikrably, producinq a very large biamass, and the benthic fauna is well- develq*H. With its average 37 kq/ha (dry might) Lake Chad is indeed cole of the r i&es t lakes frnn the benthic fauna pint of View. For canparison's sake, the rean 'benthic bi&s is only 7.4. kg/ha (dry weight) i n Wce George, another shallm African 1zke (Burgis e t aZ,, 1973) . It mt be e.i@asized that m l l u s c s largely dominate in Lle beqthos of Lzk Chad md this _@enmon w a s already &served in Lake G r é b j o u x , Lauzm-e e t Lév-, 1971) , as in other lakes of khe Sudan.
?he =an' zozplm-kton b i m s of 6.8 kq/ha is very close to that &served in L&e M q e (8.3 kgha dry might) by Burgis e t a2 ., (1973) . It should be noted at var;Ja+Aons in this bicmass are relatively unirrprtant ä d n g the year, where= j.n lakes of -rate regions, the xoplmkLan biomass can rea& nu& higher val-
&ring &&e suïrmz period, but decreases to a lwer level üuring a large par t of-the v e x .
Fisheries prcductdon ha.vever has k q es"ci?l&d a t about 100 kg/ha ( f m h Weight)
It has not been I - wssible to dekennine t?í~ bicmsse of fish in L&e Child.
& * h , s b u l d 031-~p_s.p3nd 53 a biamass 2 t o . 3 t i r r e s higher a t least.
Q
c: cc a
a l l u a d ZT estirration of the #~otosynthetic yield to be made a t abaut 0.25% of the
a d net pmàuction is roughly 10% of gross phytoplankton prduction. Proãuction- of rracroljnytes i s in the s m order of magnitude than gross plytoplanktan -production, but net pmàuction is much higher for macrophytes.
hc iòen t energy (annu& rean) Gross production is thus ar0a-d 5,403 KcaL/m 2 /year,
. I . - W zmplankton pr&ctim of 265 W m 2 / y e a r mrrespnds to 5% of the
gross production of #y-kqlanktan or 0.013% of the incident energy. As for the benthic pIDducLLion, it ss k r &telmined at 90 K c a l / m 2 /year, i.e. 0 , 0 3 5 % of the incident
energy. W t l y I fish prchction is mqhly'O,oc)2% of the incident energy.
Y
II
- 182 - I
BIBLIOGRAPHY
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DEJOUX (C.), 1968 - C o n t r i b u t i o n 2 l ' é t u d e des premiers é t a t s des Chirono- mides du Tchad ( ] è r e note). D e s c r i p t i o n de T m y t a r s u s nigrocinctus e t Chirw~oms puZchsr. EybbioZogia , . 31 (3-4) : 449-463.
DEJOZX (C.), 1969 - Les insectes aqua t iques du l a c Tchad. Aperçu systéma- t i q u e e t bio-Gcologique. Verh. Intemcrt. TeE in . L$moZ., 1 7 : 900-906.
DEJOUX (C.), 1969 - Cont r ibu t ion 2 l'étude des premiers états des Chirono- mides du Tchad (2ème n o t e ) . Desc r ip t ion de Tanypus fuscus T r m y p u ~ Zactcstfis. BUZZ. &As. nation. Bis t . n ~ , , 2e sér., 41 (5) : 1152-1163.
mides du Tchad (3ème n o t e ) . Desc r ip t ion cornparée des nymphes de NiZo&rus brevipalpis, N. brewibucca fl. f rmt i zobus . BuZI. Mus. naticm. Hist. ncrt., 42 ( 1 ) : 175-184.
mides du Tchad ( 4 è m e n o t e ) . D e s c r i p t i o n de Sticbchirunomus p d p m i s , Chimomus fonos ipennis , C. calipterus. Ca3L OMTOM, s&. Eydro%oZ., 4 (2) : 39-51.
pulcher. Canad. Entomol., 103 : 465-470.
n i d e s du Tchad (5ème n o t e ) . Desc r ip t ion de Ciaironomus (@tpbChirOnomuS) &"e, (Polypedilwn) f u s c i p e n n e . Cah. ORSTOH, sdr. B y d h b i o l . , 5 (2) :
DEJOUZ; (C.) , 19 - Le l a c Tchad e t les Chironomides de s a p a r t i e e s t .
DEJOUX (C.)s 1970 - C o n t r i b u t i o n 8 l ' é t u d e des premiers é ta ts des C h i r F o -
DEJOUX (C.), 1970 - C o n t r i b u t i o n à l ' é t u d e des premiers états des Chircmw
I)"tOUX (C.) , 1971 - Recherches s u r l e cyc le de développement de Cnirmomus
DEJOUX (C. )* 1971 - Cont r ibu t ion 2 I ' é t u d e des p r e m i e r s é t a t s des Chirono- . -
-+ i
?;,
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1 2 4
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LEK (S.), 1978 - Eco log ie e t b i o l o g i e d 'Ichtybbo-rus besse besse ( J o a n n i s , 1835) ( P i s c e s , C i c h a r i n i d a e ) du bassin du l a c Tchad. Cybim, 3ène sér . , no 4 : 65-96.
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Proceedings of the international scientific workshop on
USSR, Minsk-Pinsk-Tskhaltoubo,
July 12-26 1981
Centre of International Projects GKNT
Moscow 1982
3 % v) cn o
. .. "-rc:
