Investigations on the zooplankton in Lake Kariba were carried out as part of a broader programme investigating components of the sardine, Limnothrissa miodon, food chain and factors affecting their productivity. This report deals mainly with the crustacean zooplankton, Bosmina longirostris and Mesocyclops leuckarti, which are the most important species in the sardine's diet. Factors which influence the number and distribution of the zooplankton are discussed. The relationship between the zooplankton and Limnothrissa are also briefly discussed

Marshall, B.E.

English

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JSPEQTS THL COLOY OF THE CRUSTACLT
ZÛOPLC LT TL TYiI BASIN, L2CT KJRIBA
L1KE iCIB. EIS ERiE RESEJ«H INSTITUTE
PROJECT REPORT NO. 35
B. E. ASiL..LL
JThTE 1980
iPJBTNT OF NATITL PARKS AND %LDLiT
ZIMBABWE

INTRODU CTI ON
Since its introduction into Lake Kariba in 1967, the Lake
tangan)rika clupeid Limnothrissa rnìodn (Boulenger) has become the most
important commercial species in the Lake (Marshall and Langerman 1979
IvIarshall l979) It occupies the open waters of the Lake and feeds
principally on the crustacean zooplankton, of which Bsia longirostris
(o.. luller) and iiesocyclops louckarti (Claus) are the most important
(Begg 1974 a : Coc}irane 1978). These two species make up over 8O
of their diet whilst others such as Ceriodaphnia dubia Sars and
Diaphanosoma excisum Sars make up most of the remaindor This report
deals almost entirely with Bosmina and lIesocyclops as the other species
wore raro,, and is part of a broader programme investigating components
of the sardine food chain and factors affecting their productivity.
Previous work on zooplankton in Kariba has been centred on the
Mwenda river estuary and tho anyati basin. Bowmaker (1973) and
Mills (1977) produced general surveys of plankton in the Mwonda area,
whilst more detailed studios there havo dealt with jellyfish (Millo
1973) and Chaoborus (MItchell 1974 Mills 1976). Tho abündance of
zooplankton in relation to thermal stratification in the Sanyati basin
was describodbyBogg (1974 b) who also investigated therolationship
botwoon the diurnal movements of zooplankton and Limnothrissa, (Begg
1976). Cochrane (1978) also studied zooplankton in relation to
sardines in the Sanyati basin.
THODS
The programme was carried out in the Sanyati basin (Fig. i) and
two main investigations took place. In the first a weekly sample
was taken at the sampling station nearést Kariba dam wall (Fig. i).
The sample was integrated from subsamples taken at 5m depth intervals
in order to give an indication of the total zooplankton abundance and
to overcome the effects of diurnal movements. The ampios for each
2írtonth nero combined to giv mean monthly abundance and the rrogrammo
lascd from January - December 1979.
The second part of the prograrìrno was an attempt to aeoue the
horizontal distribution of the zooplankton and the off.. cte on them
of the tributary rivra, Integrated amples from the whole water
column were taken from 15 stations (Fig. i) at four different times.
The first was in uguat 1979 whc.n the lake was isothermal whilst river
flows ware low, whereas in Fbrùnry 1980 river flows had increased
slightly. The l::st samples ware taken in April 1980 when the tributary
rivers sere flooding strongly after a prolonged period of havy rain.
Zooplankton earspl se;re taken with 25mm di:meber hose attached
to an sLctric punp with am epproximeto pumping r:to of 81 aia Th
hose wam lowered to a amximum of 6Oei nd ma:lpl2 s were t .kan at every 5rs
to the surface. Fo-;a,r samples wro taken at shallow sttions but the
toLl quantity of untar sampled was nover lees than 751. The water
was pumped into a vertical column and pasamd through a bolting silk
sereon of 7,8 macham (64mm between parallel threads). The screen
was removed, after sampling and stored in l0> f orualin.
In the laboratory the screens were wamhed and the sample reduced
to 10 or 15ml. Counts were mndc in a lai sodgewick - Rafter cell and
numbers nero converted to no. a' x l0 and expressed as such
throughout this report.
RE81JLTd
Zooplankton abundance was principally influenced by the thermal
regime cf the lok5 and by the inflowing rivers (Fig. 2). Rainfall
from five stations n th., banyat catchaient was used as an mccx of
river flow as flow data were not evakiable. surface and bottom
temperatures are ehem to indicato turnover which occurred from June
september.
-3-
cìi:±e umbers were particularly influenced by turnover and they
were most abundant during the period June - August when the water was
coldest, Phytoplankton numbers also increased during this period
(I.H. Beattie, pers. comm.) followin nutrien release from the
hypolimnion and both and IIeecareloas responded to this.
arn also showed a peak in the rainy season which was probably
caused by nutrients brought in by the mf lowing rivers. The water
was very much warmer during this period and this may be the reason why
Bosmina numbers showed no increase.
Nauplii wore abundant in the latter part of the rainy season
(January - Maxch) but in tho winter and early rains (October
Decmbor) the post-nauplii were nIere numerous. This may be a
result of predation pressure fro
steadily from January - August (Fig. 3). Pom August - December
sardina numbers in the open water decreased id predation on the
zopla ton was reduced. This probably the survival of
the post-nauplii stages of Lthooyclops.
Paw other crustacean zooplankton were found and in all o
their populations wore very low (usually < 25m3). In Jonuc a few
Diab:nosoma were recordad, and a small unidentified cladocoran
occarred in Mach, May, Juno and July.
The horizontaÏ distribution of Bcsmina and iosoc cl is shown
in Figs. 4 - 6. Th only period in which Bosrnina was abundant was
in August and their numbers wore lowest in February (Fig. 4). They
wore generally most common in the river mouths, particularly the
Naodza, Sanyati and Gacho C-ache, and least common in the shallow
arcas oat and west of thu Sanyati and in the arca between Antelope
Island and the mainland.
cocr10 nauplii wore niost abundant in the cautern part of
the basin in August and November but those areas were less productive
othrissa which increased
but there vías coma variation in
in February and April (Fig. 5).. The central and south c-rn. arc-as
of the basin became niora important during these periods. Thus in
August thu eatcot denoity vías in the Gacho Gacho boy in shallow water
but by April they were more numerous in the dee-p water north of the
oanyati mouth.
The poot-naupliar otages of sooyo1ops wore uouilly commonest
in deeper water (Fig. 6). In August, November and February numbers
viere highest in the centre of the basin from the Sanyati mouth to the
darn wall area and in the Naodza - Gacho Gacho arc-a. In April,
however, the southern hail' of the basin vias generally unproductive
except off the Sanyati. Contres of high density occurred in opon
water south of the dam wall and in the Naodza estuary whilst the
shallow water in the Antelope Island - Charara mouth area was
unproductive.
Once again few other species were recorded in tins proßTcU1'rne.
In some Diaphonocoma were found to the west of the Naodza
(1872m°) whilst a small population of Laphnia (89m°) occurred in
C-ache Gacho bay, In November cornu Diaphanosoma wore recorded at
the Charara river mouth but in February no other spccies viere taken.
Small populations cl' Diaphanosome. were recorded at thron stations in
ApriL and they wore moot abundant at the anyati mouth (1370m3).
DICUSeION
Thc basic seasonal pattern found in this study is similar to that
noted by Begg (1974 b) and Cochrane (1978). Both workers found that
were most abundant in the winter months from June to August,
ocio s abundance, Bugg found
that both neuplii end post-nauplii ac-re abundant during the. latte-r p
of the, 1970/71 rainy season (January - March 1971). There was,
howovr, no increase in the early part of the 1971/72 rainy season.
Co chr one dud the rainy season peak f1e :vc' a. but found no
-5-
lara.. increase in numbers in winter.
Thu horizontal dietribution skewed no definite patterns although
there voie a tendency for zeoplankton to bu more abundant in the
riverine arcar. 'This is cintilar to the pattern shown in the Lwenda
estuary (Bowienker 1973 yifls 1977). In general the eastern portion
of the basin appeared to bu most productive but the reasons for this
aro not clear. It nay bu that the nutrient lv1 in rivers such as
Charara, Harden and Gaelic Gacho are high or else this i,s an effect of
currents and wind patterns.
The relationship between the zeopleeikton and is of
±ntor;st. Sardine catches follow the cycle of plankton abuiadance to
soue dogreu in that they are highest soon after overturn. However,
they aro low duing the rainy season when lil000yc1op is most numerous
and this appears to bu related to the breeding cycie of the fish.
Cochreaic (1978) shrev:ocl that they breed from September to February in
shallow :arginal areas and that there is a migration into deeper
waters as tacy groe; in siso. This accounts for the stuaclily increasing
catches from January to iugust. Idaturo fish (i.e. approx 1 yr. old)
are thus in the cm. n waters at overturn and are able to take advantage
of the plankton increase that occurs. Following this there is a
period of f co when there io a e ly considerable mortality
amongst the fish. However, the-re must also. be a migration back to
shallow water to breed and these two factors account for the drop in
çatch that occurs at this time.
Soie evidence of the impact of predation on the zooplankton is
available. Data front the open water station at the liwenda estuary,
collected in 1967/68 before the sardine had become etab1ishod, ara
gvori by Bownaker (1973). He found that the zooplankton was
d"inatod by larger animale, princii ily Ceriodaphnia (26),
'ntsiids , end Dinii sema (12/), Thu post-naupliar stages
were only ì
- : Collinge, Puto Jokonya Cnaxles Makoni and the crew of
"Polican' assisted with field sampling, whilst Mark Collinge and Peter
Jokonya also undortook a large proportion of the counting required.
-6-
ade up 1 .' the total whilst nauplii an1
5 resctively,
Bogg, 0.11. (1974 b) Investigations into the biolor and status
the Tanganyika sardine Limnothrissa miodon (Boulengor) in Lake
Kariba. Fi e Insti 19:
l-151 (cyclostyle).
Bcgg, o.w. (1974 b) Tha influence of thermal and çrgen
stratification on the vertical distribution of plankton at the
mouth of anyati gage, Laka Kariba. Lha tudies (4) : 60-67.
Begg, 0.W. (1976) The relationship between the diurnal movements
of sorno of tho zeoplankton and the sardine Limnoth miodon
*a Iake Kariba. o1 Ço:eo. 21 : 529-539.
Bcwmaker .P. (1973) 4ydrohi mi theI\Iviond a rn
By contrast this study found that Ceriodaphnia. Diaphensca
the diaptomids had been almost completely climi
..J;c» and
nauplil now made up 32 of the total planl,ton res1:-r'ively.
Size selective predation on zooplankton by fish has been shown to
eliminate the larger cistacoa (Brooks and Dodson 1965) and
inothrieea appears to have had this effect in Lake Kariba,
quantitativo changos are loss easy to dtoct as Bowmakors d
eprossod as dry weight and he gives no indication of numbers.
No figìres edst for I(ariba to enable the present data to be
converted to dry weight. However, the biomass of zooplai'_kton is
likely to be the sara although mortality and production rates m be
considerably higher.
and i t s ir'- Ph.D. thesis, TJ:j .ty of Viitwatorsrand.
rooks, J.L. A Dodson, S.l. (1965) Prodation, body 5±ZÜ and
ccmpoition uf plankton. Scionca 26-35.
Cochrans, K.L. (1978) Ssaocnal fluctuations in th catohus of
Limnothrjssaaj.Thn (Boulung-or) in Lako Kariba. Lak Kariba Fisuauïic
Laico Kariba,
Marshall, B.E. (1979) Fish populuticno and thu fishorius potuntial
7
M.Phil, thosis, Univursity of London.
Marshall, B.E. & Langurman, J.D. (1979) Thc Tanganyika sardino in
Lako Kariba, ithod. dci. Nuns 104-105.
halls, M.L. (1973) Th 'oxplosivo occurrondo of thc frushvatcr
medusa Limnocaida targ-an icac untcr in Lako Kariba during 1972.
6 (14) l-6.
valls, M.L. (1976) Thu Chaoborus (Diptora Chauboridas) ccmponont
of thu invurt-ubrato drift of ti-ìs hAcenda rivor, Laico Kariba, Contrai
Africa. Hyrobio1oa 247-250.
Mills, ILL. (1977) A pruliminary ruport cn thu planktonic micro
crustacea cf thu knonda bay arca, Lalcu Kariba. Trans. Rhod. Scie
28-37.
latohull, d.A. (1975) The bi contra
onO thoir distribution over soluctod a o
A I. Sci, 485-486.
Rescarch Instithto, 29 l-163 (cyclostyld
Cs -
.
l_JÇ. i I..i.
s-)
:* \.pI
_-.._/_' 5,i__t I
s'
r.. s'
Lo ng Is. ----
s'
IW
I \ I
I
I "s'
Fotri[1 Is. Is.
s'
Ss'
-
.
Sanyati R.
- - _5f - - I¡ s'----
.5 -- s
Gachegache R
Fig. i Wain features of the anyati Basin, Lake Kariba. The webtern boundary
of the basin is Fotheril1 Is. - Long I. - Rhino Is. - Sampakarurna Is.
The broken line is the appio;dmate 20u depth contour and the 15 samp1in
stations are hovn thus . The weekly eanpling station i shown
thus
Sampakaruma Is.o'
;cJI oa 'i ., II
Rhino Js.
10km I
XE
o
4-
29
21
mm 125
loo
75
50
25
2-
BOSMINA
MESOCYCLOPS
RAI N FALL
WATER TEMPERATURE
Post-nauptii
s
J
250
J FMAMJ JASOND
Fig. 2 : The abundance of Bosmira and Mesocyclops in relation to
water temperature and rainfall in 1979.
lo
8
6-
CD
CD
CD
2-
Fie. 3 : The re1ationhip between sardine catches and zooplankton
abundance in 1979 , zoop1ankton abundance Q , sardine
catches.
FMAMJ JASONO
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Aspects of the ecology of the crustacean zooplankton in the Sanyati Basin, Lake Kariba

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