limnological observations on lake kariba during 1967 with emphasis

LIMNOLOGICAL
OBSERVATIONS
ON LAKE KARIBA
DURING 1967 WITH EMPHASIS ON SOME
SPECIAL FEATURES’
G. W. Begg
Lake Kariba
Fisheries
Research
Wild Life
Institute, Department
of National
Management,
Rhodesia
Parks and
ABSTRACTI’
Results of limnological
recordings
made during 1967 on Lake Kariba are described.
Kariba is monomictic,
mesotrophic,
and has five defined basins, each of which exhibits its
own individuality.
The two upstream basins are riverine; they are flushed out in May by
the Zambezi River floods and thereby assume turnover characteristics
earlier than the other
three basins, which are truly lacustrine with temperature-induced
turnover.
Great amplitude of variation in water chemistry exists relating to the basin locality, biotope (river, estuary, cleared area, open water), time of year (turnover winter/summer
), and
depth. Generally,
values of dissolved oxygen, conductivity,
alkalinity,
ancl pH fall from
surface to bottom.
When H2S is present, alkalinity
and conductivity
values increase.
A
Occasionally
density currents occur at
chemocline follows the profile of the thermocline.
depth, particularly
after the first seasonal floods. Transparency
decreases toward the head
of the lake and in all areas decreases after turnover.
The source of H&3 is the Sal&&z-infested
rivers. Dissolved oxygen in the hypolimnion
The hydrobiological
effects of Salvinia are disis depleted 4 to 5’ months after tumovcr.
cussed, and the influence of the bottom topography
is described.
INTRODUCITON
Lake Kariba lies in the geologically
leached out Gwembe Rift Valley on the
Zambezi River at an altitude of 484 m. It
was formed by construction of a dam, primarily desigpcd to produce electricity. The
dam wall, 128 m high, was begun in 1956
and completed in 1960; impoundment
of
water began in December 1958 and, in this
phase, it rose as fast as 5 m/24 hr. The
lake had filled to 487.8 m above sea level
by 1963. The discharge through flood gates
and turbines results in considerable variation in water level, which should become
less when additional turbines are put in
operation by 1973.
The lake has five well-defined
basins.
The axis of the lake lies along a SW-NE
line, between 16’28’ S and 18’6’ S lat and
26”40’ E and 29’3’ E long. The lake extends for 280 km, is 40 km across at its
widest part, and covers an area of 5,250
l This paper was presented at the Symposium
on Standing Waters of the Limnological
Society
of Southern Africa at Rhodes University,
Grahamstown, in July 1968.
sq km. The maximum depth is 120 m, but
30% of the arca is shallower than 17 m.
Previous hydrological
studies of Lake
Kariba have been made by the Joint Fisheries Organization from 1960-1964 under
D. Harding ( 1962, 1964). Only the work
of Cochc (1965) covered the total expanse
of the lake. According to these surveys, the
lake has exhibited a mono,mictic, stratified
nature since as early in its formation as
November 1959, with variation
of the
pattern only in the timing, depending on
and previous
meteorological
conditions
winter tcmpcratures ( Harding 1966). The
temperature of the hylDo,limnion dictates
the following
season’s turnover tempcrature. In fact the uniqueness of conditions
at Kariba lies in its novel hydrological conditions-the
changing nature along its axis
( Fig. 1). It is misleading to discuss conditions in “the lake,” as is done by Harding,
by referring to results obtained at only two
localities (Sampa Karuma and the vicinity
of Kariba Gorge ) , since conditions in these
areas are atypical.
Throughout
1967, limnological
observations were continued by the Lake Kariba
Fisheries Research Institute. I am sincerely
776
LIMNOLOGY
LAKE
OF LAKE
KARIBA
777
KARIBA
Legend:
B3
Basin
fj’)‘:>
Bush-cleared
a
II
DEKA
R
GiAAlR
;
,
European
Basin
areas
settlement
division
MLIBl21
II
“‘I,.~..... Old Zambezi
FIG. 1. M~IZI of Lake Kariba
desia, and general information.
showing
division
of lake
grateful for the valuable guidance of Chief
Fisheries Research Officer Mr. M. I. van
der Lingen and the criticism of Officer in
Charge Mr. F. J. R. Junor, and to Dr. J. F.
Talling for reviewing the manuscript.
I
would like to thank my colleagues in the
research team at Lake Kariba Fisheries
Research Institute for their interest, encouragement, and unfailing help. Also I would
like to, acknowledge the help of laboratory
assistants, Titus Matare and Geoffrey Marowa, Oswald Nyatitos2 ( field assistant ) ,
and Afiki Shayibu ( coxswain, MV Sampa).
I am indebted to the Power Station Superintcndent, Central Africa.n Po’wer Corporation, for meteorological
and hydrological
data.
METHODS
The following observations were carried
out at each station using standard methods.
2 Deceasccl.
number
into
basins,
main
affluent
rivers
river
from
bed
Rho-
Temperature
against depth-by
using a
bathythermograph
[Wallace and Tiernan
recorder, Model FA 190012, O-200 ft (O-61
m) range]. Light penetration-by
Secchi
disc (30 cm diam) and photometer (O.R.E.
Model 504). Dissolved oxygen-by
Winklcr determination.
Detection of H&3 by
smell. Conductivity
(expressed as pmhos/
cm at 2X)-by
portable meter (Beckman
K25 So\lubridge Type RBs with dip cell).
pH-electrometrically
(Beckman Model N2
meter). Total alkalinity (as ppm CaC03)by titration using BDH 4.5 indicator and
50 N H2S04. Water samples from any desired depth by means of a. messenger-closed
Ruttner bottle. Analyses were done in the
field.
OBSERVATIONS
Meteorology
Table 1 shows the monthly mean air
temperatures recorded at Kariba during
1967.
778
TABLE
from
G. W.
1.
Mean air temperntures
(“C) in 1967,
Central African Power Corporation
meteorological records, Ku&a
Township
~-__---
Month
2.
Thermocline
depth
basins l-5
Nov 19G7
Min
1
2
ii
5
19-32
15-30
16-25
13-18
7-12
Max
Min
31.17
31.75
31.70
22.47
22.78
22.22
34.6’9
35.0
34.24
21.11
21.11
20.0
31.97
22.96
34.44
18.78
29.14
27.67
25.96
29.09
18.58
17.45
15.15
17.32
30.81
29.79
29.14
32.38
16.16
15.56
11.45
13.58
32.22
35.45
35.40
32.2
20.25
24.54
24.69
22.22
37.78
41.17
43.34
37.42
16.36
21.16
21.21
19.75
Max
TABLE
Hasin
Extremes
Mean
REGG
(in meters)
in
Jan 1968
31-36
20-30
19-25
15-20
10-17
Of this capacity, 9 x lo6 m3 are lost by
evaporation each year, and 40 x 10° m3
pass through the flood gates and turbines,
depending on requirements.
Only a third
of the lake volume is replaced each year.
PHYSICOCHEMICAL
CONDI’SIONS
Figure 2 shows the thermal situation in
basin 5. Surface temperatures rose to 32X
The no,rmal rainfall
in Kariba
area in February, while hypolimnetic
tempera(basins 3, 4, and 5) is 610-800 mm.; in tures generally remained at 22C-reflecting
Binga area (basins 1 and 2) it is 400-610 the previous winter turnover temperatures.
mm. The bulk of rain falls in December
Clearly, except for July and August, the
and January. Rainfall for the 196&1967
lake remained thermally stratified. By Deseason was moderate, totaling 560 mm; the cember 1967 the thermocline
was well
1967-1968 seasonal rains were late and established and stable. As the year propoor, totaling 430 mm, follo’wed by an gressed, and the epilimnion cooled, it sank
unusually cold winter. During the rainy
to 3540 m, at the same time becoming
season, light rains may fall at night and sharper and more abrupt. Winter circulathunderstorms develop quickly during the tion occurred in July and oxygen pcneday-followed
by a rapid drop in tempcratrated to the bottom. As Table 2 suggests,
ture and rise in relative humidity.
The the thermocline occurs shallower in basin
winds generally blolw from the northcast,
5 than in basin 1, creating the progressive
although in basins 4 and 5 southwesterlies
phases of winter circulation
(referred to
blow frequently in the latter half of the below ) . Thermally, basins 3 and 4 behave
year.
rather like. basin 5.
Deviations from this normal situation ocHydrology
cur, such as the appearance of secondary
Lake level fluctuation over 1967 was bethermoclines, either at the surface during
twcen 482 and 484 m above sea level ( asl).
the hottest part of summer days, or deeper,
Coulter (1967) estimated the area oE
due to colder inflows fro,m rivers. Variation
Lake Kariba to be 1,718 sq miles (4,448
in the extent of the thermocline (or metasq km), but this could only be when the
limnion)
according to the thermal drop
lake was at 475 111:asl-9 m below the
per meter ranges from, 0.3-2.5 degrees C/
present level. At exactly 484 m asl, the
m, largely determined by the degree of
lake area is 5,250 sq km. At its peak height
exposure to wind action of the particular
in 1963 of 487 m asl,” the lake covered
locality.
5,500 sq km.
The thermal situation at Kariba Gorge
The capacity at 484 m as1 (mean retennear the dam wall differs significantly from
tion level.) is 160 X 1ctGm3.
the normal due to the flow through the
gates and turbines. The opening of the
3 The lake subsequently reached this level again
gates had the .transient, temporary effect
in 1969.
FIG.
2.
25
Oc
showing
3m
ZD-
-
o-
2
20
*C
F
25
30
changes in the relative
60
60 -
profiles,
30
60 Lc
temperature
20
40
I
40
20
Monthly
3m
0
position
N
during
7-
J
of the thermocline
bo
bo
20
1967 in the center of basin 5, Lake Kariba.
3-
A
3
~
780
G. W.
Jan
Feb
BEGG
Mar
0 I-
Gat’e
opened
on 20th
FIG. 3. Showing the effect on the isotherms in the vicinity
of Kariba Gorge
the suction created by the opening of a flood gate on 20 February 1967.
of drawing the thermocline
deeper, but
shattering it, so it is more gradual and
extensive. Figure 3 indicates that the opening of two flood gates on 20 February
coincided with the ascent of the 22.0, 22.5,
and 23.OC isotherms. This confirms Cache’s
(1968) observations in 1965. Turnover did
not occur in Kariba Gorge until the night
of 28 July, -shortly before surface temperatures began to rise in August. This delay
is related to the fact that the gorge is the
deepest part of the lake. Had turnover not
occurred by August, the stagnant water
of the hypolimnion would, in all probability, have remained at this locality for over
a year.4
* Since this paper was written
a phenomenon
arose in Kariba Gorge where the winter in 1969
was not sufficiently
severe to bring about turnover, so that this area was characterized
by a
double stratification,
and hydrogen sulfide would
have remained
in the hypolimnion
below
the
secondary thermocline
for an entire season. Cochc
( 1968) refers to areas in basin 3 where dcoxygenation persisted even after turnover.
(near
the dam wall)
of
During the early stages of restratification, a thermocline may be found within
the cleared areas and shallower zones osf
the lake, which are normally homothermal. During midwinter,
temperatures in
the shallow margins of the lake may fall
to 1lC at night.
Inflowing river water, which is generally
cooler than the lake water, is most apparent where the Zambezi enters the lake at
Devil’s Gorge, particularly
in July, when
the Zambezi River water has a temperature
of 17C. This is the coldest water to enter
the lake, and thus the annual thermal range
is greatest in basin 1.
From Fig. 4, it is apparent that there is
no conventional turnover in basins 1 and
2. As the Zambezi enters, the temperature
of the entire water mass changes in these
basins. At Devil’s Gorge in March, the
temperature of the entire water mass was
29C. In July the water mass temperature
was 17C-a difference of 12 degrees within
4 months. This indicates that when peak
LIMNOLOGY
OF
LAKE
KAIUBA
781
782
G. W.
flow of the Zambczi (resulting from the
Barotseland floods) occurs in Devil’s Gorge
in May, the entire water mass in basins
1 and 2 is flushed out, The process is
normally complete by the end of May.
This could bc interpreted as turnover because of the following characteristic consequences: chemical homogeneity from top
to bottom; abscncc of HZS; dccreasc in
normal physicochemical
values of surface
water, transparency, and plankton density.
Because these turnover characteristics
occurred in May, when lake sulfate tempcratures in basins 3, 4, and 5 wcrc at
2X’, it is clear they were not induced by
the sinking of cool surface lake water
during the coIdest month, but arose as a
result of a flushing-out process by the Zambczi River itself, which provides 70% of
the total inflow into the lake. During May,
“entirety shifts,” although most marked in
Devil’s Gorge, wcrc noticeable throughout
basin 1, through Sebungwe Narrows and
as far as Binga, at the Logola River entrance. This flushing process continued so
that by May, while the rest of the lake
was stratified, basins 1 and 2 assumed
turnover characteristics.
The noticeable rise in chemical values
after passing from basin 2, through Chctc
Gorge, into basin 3, is due to the fact that
the nutrient-poor Zambezi water is seldom
apparent beyond Chcte. Here it sinks and
may be found along the bottom of basin
3, as an underflow, tractable as far as the
Chisangasanga Strait.
The influx of relatively sterile Zambezi
water at peak Elow not only noticeably altered the tempera,turcs, but also the water
chemistry of basins 1 and 2. This was reflected by the low alkalinity, pH, and conductivity, the scarce plankton, the increase
in turbidity,
and the flushing out of the
S&in&choked
Zambezi system. While
temperatures fell in July, basin 3 tended
to set up a back-pressure in response to a
lcsscning of flow, so the above chemical
characteristics
of basins 1 and 2 rose
slightly and continued to rise. Subsequently
flow was so slight that a thermocline appeared in Devil’s Gorge by September and
BEGG
TABLE
3. Secchi disc depths (aueruged and in
cm) clt a-month intervals along axis of Lake Karibn
~___Basin
NO.
1
2
3
4
5
MU
May
84
286
470
550
325
131
240
4768
580
433
Jnl
Sep
Nov
Jan
248
300
412
524
407
320
325
426
506
410
317
462
486
533
455
145
385
520
593
424
became fully cstablishcd by November, by
which time basin 3 water completely modificd the surface of basins 1 and 2. Basins
1 and 2 altered from riverine conditions
to assume Iakclike characteristics-constant
hypolimnctic temperatures, stratificd water
body, and increased values of conductivity,
alkalinity, and pH. During this phase II$
appeared briefly from river sources, such
as the Senkwe and Sebungwe, and penctrated the deoxygenated hypolimnion. Thus
for 5 months of the year (September to
January) basins 1 and 2 are lacustrine,
while for the remaining months of the year
they become riverine in nature.
When lacustrine basins 3, 4, and 5 have
turned over in July, basins 1 and 2 have
rcstratified.
Although the surface water
over the lake is at its coldest, it is prcvented from sinking in basins 1 and 2, due
to the very cold Zambezi underflow.
The
back-prcssurc from basin 3 assists in causing it to remain at the surface, creating a
primary thermocline,
while the Zambczi
water flows beneath a decpcr secondary
thermocline.
Cache ( 1968) terms this a
“density current.”
Water transparency
Secchi disc transparency varied over the
lake from 10 cm to 10 m. In 1965 Cache
( 1968) observed Secchi values of 12 m.
Relative light intensity
( RLI ) measurements indicated that an RLI of 1% seldom
lies deeper than 20 m, and 50% of the light
absorption takes place within the first 3 m
of water. This confirms Cache’s ( 1968)
observations conduclcd in 1965.
LIMNOLOGY
TAIHX
4.
Secchi disc depths
DigGG;nJof
LL . I-
Jill1
Kariba sub-basin
( Rcdcliff )
Sanynti sub-basin
580
424
(averaged
FCh
593
417
OF LAKE
and in cm), recorded
Mar
Apr
May
JUll
593
325
640
407
806
433
880
533
Transparency
decreased in the areas
cleared of original bush due to the richer
planktonic life and sediments brought up
into suspension by wave and wind action.
Water transparency generally decreases
from basin 4 to basin 1. Basin 4 has the
clcarcst water ( see Table 3). Particularly
in basins 1 and 2, the cIcar;est water conditions coincide with decreased river flow.
The decline in transparency in bmasin 5 is
due to the greater productivity
of basin
5 and to inflow of the Sanyati River. Dccrease in transparency from basin 4 to, 1
is associated with change in water color:
from a deep blue-green, to green, to olivcgreen, to brown.
Estuarine and riverine localities have
due to silt, richer
lower transparency,
plankton, and suspended organic matter.
SaZvinia may tend to increase the transparcncy of river water owing to the flo’w
restriction
created by the mat (causing
suspcndcd matter to scttlc out), nutrient
extraction, and the inhibition
of pIankton
grolwth. Table 4 indicates the great differcnce in the optical pro’pcrties of the water
at the Kariba and Sanyati sub-basins (exccpt during turnover),
The former has
higher Secchi values because river inflow
is Iacking.
Most of the rivers carry heavy silt loads
at peak flo,w (e.g., the Sanyati in February
and March).
The Zambezi does not normally carry a heavy silt load unless its tributarics, such as the Deka (in January 1968)
or Gwaai (March 1967), arc in spate.
Water chemistry
The overall chemical nature of the lake
is related essentially to bottom topography
an d river inflow.
783
KARIBA
of basin
in two divisions
Jul
393
407
(Turnover)
Dissolved
Aug
scp
806
530
673
410
5 during
Ott
440
317
oxygen and hydrogen
2067
Nov
DW
473’
455
680
481
sulfide
Dissolved oxygen values of the surface
water in the open lake arc generally bctween 6 and 7 ppm, rising in the shallow
cleared areas, whcrc greater water movemcnt and increase. in photosynthetic activity by plankton and aquatic weeds may be
found. River water is usually low in oxygen as a result of the barrier created by
Sa~lvinda mats at the air-water interface,
the lack of photosynthesis beneath the mat,
and the uptake of oxygen by the suspended organic matter in the river water.
The highest dissolved olxygcn values rccorded were from the cold water of the
Zambczi in July, when these values rose
to 10 ppm, rcndcring the water supersaturated ( 115% saturation).
After turnover and circulation,
oxygen
penctratcd to the bottom, of the lake and
initially
was distributed
evenly from top
to bottom. With rcstratification,
oxygen
dcplction began in the hypolimnion
and
continued until a sharp oxyclinc dcvclopcd
at the thermocline. Cache (1968) remarked
on the same phenomenon
encountered
during his studies. In November, oxygen
depletion in the hypolimnion
is more advanced in basins 1 and 2 than in basin
5--another
indication
of the progressive
phases of rcstratification.
Table 5 clearly
indicates that surface oxygen values decrcasc at turnover as a result of mixing.
IIigh oxygen tensions rarely persist in the
hypolimnion
for more than 4 months.
This is related to the rate of restratification, whereby water exchang,es between
hypolimnion
and epilimnion arc rcduccd.
The bo,ttom water gradually bccomcs co;Mpletcly deoaygenated, finally bcco’ming utterly reduced and 112s bearing within a
784
G. W.
TAESLE 5.
Rate of oxygen depletion
1967
0
20
40
60
80
100
Thermocline
depth (In)
Jun
Jul
6.6
6.0
IIZS
H,S
HzS
Has
5.1
5.1
5.1
4.8
4.8
4.9
(turnover)
32
BEGG
in Kariba
Gorge
(concentrations
in ppm)
hs
Sep
Ott
Nov
Dee
1968
Jan
6.7
:::
6.6
5”*;
ii-2
4:9
4:2
4.2
4.2
7.4
5.8
5.0
3.1
3.6
2.9
7.5
3.7
3.6
2.9
2.0
1.8
6.9
3.6
3.5
3.3
1.4
1.4
6.7
2.6
2.2,
2.0
0.4
0.7
Nil
5
7
S-10
7-11
9-13
few months after commplete dcoxygenation.
In the S&&&-infested
river localities, the
water toward the bottom rapidly dcoxygcnates and here H$ first appears. The
worst Salvinin-infested rivers were the first
to have I-I& at the bottom of their old
channels. As if in contradiction, it is commonly found in the relatively weed-free
Sanyati River-where
it may seriously affect movements of the anadromous fish spetics (Cache 1968). Thus, while the lake
water is still oxygenated from the top to
bottom and although oxygen depletion is
in process, the river systems rapidly become deoxygenatcd and the majority of
them H$ bearing.
As oxygen depletion draws to completion
in the lake deeps, the rains arrive, the rivcrs begin to flow, and the dense H2S-laden
water is flushed into’ the lake where it scttles below the thermocline. The rivers now
become fully oxygenated water bodies,
while in the lake the reverse applies. The
II$ phase at any locality therefore varies,
depending on the time the rivers flow and
on the magnitude of the flood. Because of
the weak flow from the rivers after the
poor 1967-1968 rains, H2S did not penetrate
basin 5 farther than the sub-basin south
of the Long Island-Redcliff
ridge. Thus
H,S did not appear in the Kariba Gorge
vicinity in 1968, where the previous year
it was found for 7 months. This, in itself,
is a bottom topography influence on the
Harding ( 1966) stated
water chemistry.
that the H$ period was growing shorter as
the rotting vegetation on the lake floor
diminished; by 1963 this period in Kariba
Gorge existed only for 3 months. Cache
( 1968) reinforced this opinion. By the end
of Cache’s work in 1965, I&S could be detected at only three localities in the hypolimnion.
Today, the II&S period at any
locality is a variable phenomenon.
Some
rivers will carry it during one season and
not the next, and the period could vary
from 2-4 months, depending on the degree
of Sulvinia infestation and the arrival of
floods. Hydrogen sulfide periods farther
up the lake in basins 1 and 2 may also
be short-lived as a result of the flushing
process in May.
The presence of H& in the water has
several important effects. As a reducing
agent, it assists in mobilizing certain ions,
thus causing alkalinity and conductivity to
increase in II&S-laden water. An increase
in alkalinity and conductivity
renders this
water more dense; it therefore sinks and
remains within deep depressions on the
lake floor. Low pH values are often obtained where the I-1$-laden water is in
contact with the oxygenated epilimnion
(over the chemocline),
This could bc due
to the vertical distribution of carbon dioxide and bicarbonate o’r to the formation of
unstable sulfurous acid.
From a practical viewpoint, I-1$ renders
the chlorine injections in the turbine water
algicidally ineffective; those turbines drawing on HzS-laden water have overheating
problems due to algal accumulation.
Density
currents
Apart from those formed by the entry
of Zambezi water, density currents from
the Sanyati River were aIso apparent. As
Fig. 5 indicates, the water fro,m upstream
LIMNOLOGY
60
-
70
-
OF
enters the lake as a tongue or wedge at
depth- In JanuV 1968 such an oxygenated
body of water was noticed at 54 m. Above
this, at 30-36 m, lay an H&S-laden wedge,
In January 1967 an oxygenated wedge
temporarily appeared at 30 m, bearing the
silt-laden surface water characteristic
of
upstream areas. In the Gwaai River a deoxygenated wedge occurred at 10 m in
January 1968.
TL
influence
of bottom topography
Dissolved oxygen values decline in the
immediate vicinity
of affluent rivers, as
river water generally maintains lower dissolved oxygen values. The river water
tends to remain confined to the old river
channels beneath the lake and to follow
their original courses to some extent. This
is apparent when viewed from the air. In
basin 5, the isopleths indicating lomw surfact dissolved oxygen show no,ticeable intrusions opposite the inflow of the Gachc
Gache and Sanyati Rivers. The water in
the Kariba sub-basin (north of the Long
Island-Redcliff
ridge) clearly has different characteristics, relating particularly
to
transparency (see Table 4)) caused by the
lack of river inflows.
Maps of surface dissolved oxygen reveal
a pattern of isopleths that conform to the
LAKE
785
KA-RIISA
main features of bottom topography.
The
cleared areas of the Gache Gache, Sanyati
East, and Sanyati West normally have high
dissolved oxygen values and decreased water transparency because of their shallow
nature. Over the basin 5 deeps, surface
oxygen values decline. This pattern may
alter slightly during the year according to
the prevailing
wind direction and flow
from the affluent rivers.
Depth is always significant.
In basin 5,
should oxygen values at 30 m be 2.0 ppm
over deep water, at a locality only 30 m
deep (c.g., west of Sanyati River) the water
is completely deoxygenated at 30 m. The
nearness of the bottom increases the sharpness of the chemical gradients within the
water body, particularly
in the last few
meters above the lake floor. The shallow
cleared areas of basin 5 may even show
steep oxygen (7.8-U ppm,) and ~11 (9:07.2) gradients over 10 m. This is understandable while there is a thermocline in
the shallow margins of the lake, but it
d oes indicate a. high degree of biological
activity on and within the bottom muds.
. ..PH
The epilimnetic waters are normally distinctly alkaline, with pH between 7.8-8.5
and occasionally rising to 9.0. During advanced stages of stratification, the water of
the hypolimnion
( generally 7.3-7.0) may
become acid and pH may fall as low as
6.3. Surface water pH values during 1967
were considerably higher than 7.2-7.5 as
found by Harding ( 1966). These values
decrease as one passes from, basin 5 to
basin 1, but most noticeably after Chete
Gorge. These findings are in close agreement to those of Cache ( 1968)) indicating
a similarity between the situation in 1967
and that 2 years before.
Total alkalinity
Alkalinity of the surface water (as ppm
CaC03) in basin 5 in 1967 varied within
41-43 ppm. Again these results are closely
related to those of Cache ( 1968). In pro-,
ceeding from basin 5 to 1, alkalinity values,
786
G. W.
fall into the 20-30 ppm range, indicating
the poorness of the Zambczi water, which
complctcly modifies basins 1 and 2.
The highest alkalinity values ( lo& ppm )
were recorded from the Sanyati River in
flood in January 1968. From the same
locality, Cache recorded values of 196 ppm
in August 1965, when the river could not
have been flowing.
Generally alkalinity
falls with increase in depth-but
may rise
owing to the prescncc of Has-laden water
or the nearness of the bottom. In IIZSladen water, the alkalinity
may rise to
56 ppm.
In basins 1 and 2, alkalinity is lowest
in May ( 19 ppm ) during the period of
peak Zambezi flow. As these arcas assume lakelikc characteristics under the influence of less flow from the Zambczi and
modification
from the basin 3 back-pressure, alkalinity values rise to 35 ppm by
November.
Total hardness of the surface water near
the dam wall, as shown by Harding (1966>),
fell from 49 ppm CaCOR in 195S to 35 ppm
by 1964. Cache ( 1968) generally records
slightly lower values (32.0-34.6 ppm) for
the outflowing water in 1965.
Conductivity
Conductivity
values drop in proceeding
from basin 5 (9g105 pmhos/cm) to basin
1 (55-75 pmhos/cm).
Cochc (1968) noted
an increase in conductivity from basin 5 to
1 in January 1966. Harding (1966) recorded
values of specific conductivity
in 1964 of
76 pmhos/cm, having decreased steadily
from 121 pmhos/cm in 1958. These values
have increased to over 90 pmhos/cm by
1965 ( Cache 1968) in the surface waters of
basin 5, and further in 1967 to values betwccn 95 and 100 pmhos/cm. The lowest
value obtained was 49 pmhos/cm, from the
Zambczi at peak flow in May; .thc highcst, 240 pmhos/cm, was from the Mlibizi
River. Related to stratification, conductivity in the lake water normally decreases
with increase in depth but may rise near
the bottom, either because of the prcscnce
of I&S or bccausc of the nearness of the
bottom sediments.
BEGG
Mineral springs are common over the
lake floor, particularly
in basins 1 and 2
( Mauffc 1933). A sample taken from the
vicinity
of a mineral spring, in Devil’s
Gorge, had a conductivity
of well over
1,000 pmhos/cm and pH of 3.5. In the
Mulolo River (on the north bank of the
lake) Cache (1968) recorded a mineral
spring with water of a total hardness of
623 ppm CaCO:,.
The conductivity of the affluent water is
higher than that of the lake water due to
its silt- and nutrient-laden
state. Conductivity increases in rivers, such as the Sanyati, when they are flo,wing, so that values
may rise to 205 pmhos/cm as in January
1968. Cache has recorded values of more
than 300 pmhos/cm from the Sanyati River
in flood.
Salvinia
The area of Salvinia in June 1967 was
82,000 ha, covering 15% of the lake surface
( Mitchell, personal communication),
and
was restricted entirely to rivers, sheltered
bays, and uncleared areas. In such confines, a complex of plants grow as sudd on
top of the Snlviniu mats (mainly comprised
of scirpzls cubenasis) .
Recently Lemna and Pistia stratioltes
have appeared in the upper reaches of the
old Sanyati River area, together with a
dense Salvinia mat.
The Eern Sdvinia auriculuta indirectly
and directly plays an important role in
many hydrobiological
aspects of tho lake.
Nutrient absorption and subsequent rclcasc
during decomposition results in retention
of nutrients within the lake. Further, dcnsc
Salvinia tends to cause up to 25% dcoxygenation by decreasing photosynthetic
activity and preventing
direct atmospheric
reoxygenation of surface water. Records oE
water surface temperatures taken dircc tly
beneath Salvinin: mats indicate so,me degree
of insula .tion. A permanent stable mat may
lower surface temperatures appreciably in
certain localities, and, likewise, the water
tempcraturcs beneath Salviniu may remain
at a higher temperature than the. cooler
lake water that is free of Salviniu.
Salviniu mats increase water transparency by restricting water movement and
by preventing plankton development.
In
addition, exclusion of light by permanent
Salviniu prevents development of rooted
vascular plant cofmmunities ( Lagarosiphon,
Vallisneriu, Potamogeton, Ceratophyllum ) .
It seems that the production of H$ is
related to the quantity of rotting Salvinia
present in the old riverbeds, forming a
substrate on which I12S-forming bacteria
thrive, Naturally, there is still a certain
amount of H$ formation in the lake deeps,
caused by rotting and decay of submerged
trees-this
is hoswever no, longer the primary source of this gas.
Certain fish species (Tilapia mossambica,
Tilapia melanopleura,
Hemihaplochromis
philander, Al&es imberi, Distichodus mossambicus, Labeo coqpro,
Clarias gawkpinus, Gnuthonemus mucrokpidotus,
and
Cyphomyrus
discorhynchus)
have been
known to ingest Salviniu, cithcr inadvertently in quest of the periphyton or to feed
directly on various parts of the plant. In
completely dcaquaria, T. melanopleura
vour the roots and leaves of SaZvinia. As
indicated by 13.G. Donnelly (personal communication) , the protection
afforded by
shoreline Salvinia mats to juvenile cichlids
as primary nursery areas is of great importance. Equally important from the hydrobiological aspect is the community living
within Salvinia (Molluscs, Crustacea, Annelida, Nemertca, Protozoa, Hydrachnida,
Insecta, Algae, and diatoms ) .
Biologically, the presence of impcnetrable mats of Salvinia and sudd renders the
river systems less hospitable for occupation
by anadromous fish and possibly interferes
with the movements of adult fish on their
way upstream to spawn and juveniles rcturning to the lake before the rivers dry
up.
These rivers, as a result of the impounded water, have already been drastically reduced in extent, Should sudd and
SaEvinia enter the main body of the lake
as a result of river flow or wind action,
these masses of vegetation have a beneficial effect when blown up against the lake
shore where they become available as fod-
der and cover. Under this, due to wave
action, the water quality is not affected as
drastically
as in the permanently
suddchoked rivers (such as the Sengwa, Bumi,
Scnkwc, and Luzilukulu ),
DISCUSSION
Chemical analyses show variation in the
vicinity of each affluent river, and values
of dissolved salts tend to decline as one
approaches the head of the lake where the
Zambezi influence becomes most strongly
felt. The fall in chemical values rcflcct the
low concentrations
characteristic
of the
Zambczi water, and the influence of the
Zambezi causes turnover to) occur at the
head of the lake before winter. Turnover
is thus brought about by a flushing mechanism and is not temperature induced as in
basins 3, 4, and 5. It is interesting to foillow the increase in dissolved solids as the
lake filled ( Harding 1966). With the inundation of new land and the associated
leaching and decaying processes, dissolved
solids rose from 26 ppm in June 1950 to
65 ppm, in 1958. When the lake level stabilizcd in 1964, dissolved solids had fallen
to 42 ppm in the vicinity of the dam site.
Harding predicted that these figures would
continue to decline after 1964, until the
lake water took on the characteristics of
the inflows, But conductivity
(as an indcx of dissolved salts) has risen from 76
pmhos/cm in 1964 to 95-100 pmhos/cm
in 1967.
Kariba will never bccomc a water
body of uniform characteristics. There arc
marked changes in the vicinity of affluent
rivers, and in the Zambezi-modified
“river
basins” 1 and 2. In this respect, as indicated by Cache ( 1968), Kariba reflects the
characteristics of a typical reservoir.
The gradual dcclinc from 1960-1964 in
the period of deoxygenation (Harding 1966)
has not continued; the length of the period
is variable, depending on locality within the
lake and seasonal conditions.
IIydrogcn
sulfide was prcscnt for 7 months of 1967 at
Harding’s sampling locality and was totally
absent during 1968 (but see footnote 4,
p. 780).
788
G. W. BEGG
Harding
( 1966)) Coulter ( 1967)) and
Cache ( 1968) also realized that the affluent rivers had a marked influence on the
productivity
of the lake.
Seiche movements are slight and insignificant. Currents exist at localities where
different bodies of water flow as wedges at
depth, as well as in Kariba Gorge when the
flood gates are open. The Zambezi-modified water from basins 1 and 2 extends
along the bottom of basin 3 as a density
current.
As indicated by Harding ( 1966), hydrobiological investigations should start with
the process of filling any new impoundment. Cabora Bassa, another lake on the
Zambezi now under construction some 400
km below Kariba, will be longer than Kariba, of greater depth, narrower, and have
a steep, shelving shoreline. It will, therefore, in all probability have riverine characteristics such as those in basins 1 and 2
of Kariba. It is hoped that the lesson learnt
at Kariba, that preimpoundment
surveys
an d a co,ntinuity of limnological
research
arc necessary, will be heeded in this case.
REFERENCES
COCEIE,A. G.
1965. Limnological
research program for Lake Kariba, p, 63-65.
In Record
of the Kariba Res. Symp., Lake Kariba Fish.
Res. Inst., June 1965. ( Mimeographed.
)
1968. Description
of physico-chemical
-.
aspects of Lake Kariba, an impoundment
in
Fish. Res. Bull. Zambia
Zambia-Rhodesia.
5: 200-267.
1967.
What’s happening
at
COULTER,
G. w.
Kariba.
New Sci. 37: 750-753.
HARDING, D . 1962. Research of Kariba, p. 3240. In Joint Fish. Res. Organ. Annu. Rep.
10, 1960. Govt. Printer, Lusaka.
1964. Research on Lake Kariba, p. 25-.
50. In Joint Fish. Res. Organ. Annu. Rep.
11, 1961. Govt. Printer, Lusaka.
Lake Kariba.
The hydrology
-.
1966.
In R.
and development
of fisheries, p. 7-20.
I-1. Lowe-McConnell
[ea.], Man made lakes.
Symp. Inst. Biol. 15. Academic.
MAUJTE, H. B. 1933. A preliminary
report on
the mineral
springs of Southern
Rhodesia.
Geol. Surv. Bull. 23, p. 19-27.