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Hydrobiologia vol. 60, 3, pag. 243-259, 1978
THE COLONIZATION OF CABORA BASSA, MOC(AMBIQUE, A NEW MAN-MADE LAKE,
BY FLOATING AQUATIC MACROPHYTES
W. J. BOND* & M. G. ROBERTS**
Loxton, Hunting and Associates, P.O. Box 197, Gaberone, Botswana.
Current Address:
* Saasveld Forest Research Station, Private Bag 53I, George 6530. R.S.A.
** c/o Swakopmund High School, P.O. Box I87, Swakopmund 9160. SWA/Namibia
Received December 13, 1977
Keywords: Eichhornia crassipes, Salvinia molesta, aquatic macrophytes, man-made lakes
Abstract
Eichhornia crassipes and Salvinia molesta, both notorious
weeds, are present in the catchment of Cabora Bassa, a new man-
made lake on the Zambezi River, Mocambique.
Weed dispersal, controlled by wind and current (and indirectly
by rate of lake filling and lake morphology) culminated in the
distribution of mats in the eastern and western extremities with
very small cover in central lake areas. Eichhornia offset produc-
tion was initially very rapid later diminishing markedly with
many plants showing symptoms of nutrient deficiency. Eichhor-
nia completely dominated mat composition at the end of the
year whilst Salvinia cover was negligible.
Heavy drawdown in the middle of the year lead to destruction
of nearly 50% of the weed mats. Whilst pre-drawdown levels were
attained by the end of the year, there was no evidence for expected
explosive population growth and extensive weed colonization in
the filling phase.
Introduction
Cabora Bassa, a new man-made lake on the Zambezi
River, Mozambique, began to fill on 5 December 974.
Floating aquatic macrophytes are infamous for ex-
plosive population growth an example of which is Sal-
vinia molesta, D. S. Mitchell, at Lake Kariba, also on the
Zambezi River. This plant, a fern, showed spectacular
growth in the lake's formative years covering Iooo km2
Dr. W. Junk b.v. Publishers - The Hague, The Netherlands
in the fourth year after closure (Mitchell, I970, 1973). The
water hyacinth, Eichhornia crassipes, (Mart.) Solms, is a
widespread pest of waterways in the warmer parts of the
world and has been a serious problem in several African
impoundments including the Jebel Aulia Dam, north
Africa (Little, 966), Lake Mcllwaine on the Hunyani
River, a tributary to the Zambezi (Davies et al., 1975) and
Hartebeestpoort Dam, South Africa.
The water hyacinth, Salvinia molesta, Pistia stratiotes
L. and Azolla nilotica Decne ex Mett. were recorded in
the catchment of Cabora Bassa during pre-impound-
ment surveys (Macedo, 1974, Davis, Hall & Jackson,
1975) and were considered potential pests. Salvinia and
Pistia are present throughout the Zambezi system while
Eichhornia is present in both the Kafue River of Zambia
and the Hunyani (= Panhame) River of Rhodesia, both
of which are tributaries to the Zambezi above the new
dam.
Pre-impoundment assessments of the problem of
aquatic weed infestation, based largely on experience at
Kariba, predicted extensive and rapid colonization at
Cabora Bassa. Davies et al. estimated a probable macro-
phyte cover of 25-40% of the lake surface within two years
of closure followed by a gradual decrease as nutrient
levels stabilized. They predicted that Eichhornia would
be the major pest and would dominate Salvinia.
Extensive weed cover could interfere with hydro-
electric installations and water transport, seriously af-
fecting fishing, navigation and recreation (Little, 966;
243
Mitchell, 1974) with the additional danger of rapid spread
of disease vectors on floating weed mats (Mitchell, 1974;
Jackson & Davies, I976). Use of weed for soil improve-
ment or livestock feed (Davies et al., 1975; Oliviera, I972)
at Cabora Bassa is not, at the moment, a feasible propo-
sition because of prohibitive harvesting and transport
costs and the distance of agricultural settlements from
the lake.
Due to the possible disadvantages of these aquatic
plants and the likelihood of their explosive development,
a joint research and control project was started in the
initial filling phase of the lake.
The perennial input of floating macrophytes and the
unlikelihood of complete control in the source countries,
limits the possibility of overall weed control at Cabora
Bassa. Instead strategic weed control, which included the
use of floating barriers, herbicides, manual harvesting
and biological agents was employed but is not further
described here.
A study of invasion of the lake by floating macrophytes
was made and this paper records observations on aspects
of immigration and colonization in 1975, the first year of
the lake. Comparisons of weed ecology at Cabora Bassa
and Kariba, both on the same river system and in a
similar climate, may provide general insight into floating
macrophyte biology in the large African impoundments.
Morphology and Limnology
Cabora Bassa, situated on the Middle Zambezi River, is
smaller than lake Kariba which is some 500 kms up
stream (Table I). The lake has five basins, the Mucan-
gadze, Carinde, western and eastern Mucanha, Chicoa
and Gorge Basins, from west to east respectively (Davies,
Hall & Valente, 976; Bond et al., 1977), (Fig. I). The
western basins are shallow and, in the lake's first year,
showed riverine characteristics. The deeper eastern basins
rapidly developed lentic features (Bond et al., 1977). At
lake levels prevailing in 1975, basins were separated by
marked constrictions caused by islands or promontaries.
There is a marked difference in slope of northern and
southern shores, the former, particularly in the Chicoa
and Mucanha Basins, being generally steeply sloping
whereas southern shores have a very gentle gradient.
Since virtually no trees were cleared from the lake peri-
meter, the southern shores were choked with a broad
band of semi-submerged terrestial vegetation up to
Fig. i. Lake Cabora Bassa, Mogambique, at mean retention level of 326 m AMSL. Basins are numbered from east
to west respectively: -Gorge, 2 -Chicoa, 3a- Mucanha East, 3b - Mucanha West, 4 Carinde, 5- Mucangadze. Chicoa
and Mucanha Basins are occasionally referred to collectively as the central basins.
244
5 kms wide, whereas, on the north bank this was gener-
ally very narrow.
Climate
Meteorological data from Chicoa Velha, now sub-
merged by the lake, are recorded in Table 2. There are
three seasons: a dry-hot season from August to October,
a hot-humid season from November to April, and a cold-
dry season from May to July. Conditions generally ap-
peared favorable for optimal growth by both Salvinia and
Eichhornia although there seemed to be a possibility of
high temperatures limiting growth rate for a part of the
year.
Prevailing winds are of considerable importance to
weed dispersal being mostly south-easterly to southerly
becoming more easterly in the western basins (Table 3).
Wind direction is more variable in the summer months
from November to February, especially preceding and
during thunderstorms. During this period westerly and
south-westerly winds are more frequent in the eastern
TABLE 1
SOME COMPARATIVE DTA ON LAKES KARIBA AND CABORA BASSA
FEATURE KARIBA
Catchment
Geographical position:
Longitude
Latitude
(km2)
Direction of main axis
Direction of predominant wind
Number of Basins
Maximum depth (m)
Mean depth (m)
Maximum drawdown
Maximum length
Greatest width
Length of shoreline
Total area at capacity
Maximum capacity
Actual filling time
(m)
(km)
(km)
(km)
(km2)
(m3 )
years
Ratio annual inflow to volume
Important infestant aquatic
macrophytes present in the
system
409 600
260 40'E-293'E
16028'S-180°6'S
SW-NE
SE-NW
5
120
29,5
14
300
40
3155
5250
15,5x1010
4
1:3
1. Salvinia molesta
2. Pistia stratiotes
CABORA BASSA
200 000
30025' E-32044' E
15029' S-16000' S
SE-NW
5
151
26
36
250
38
1775
2739
7x1010
+/- 1
+/- 1:1
1. S. molesta
2. P. stratiotes
3. Azolla nilotica
4. Eichhornia
crassipes
245
TABLE 2
1kXETiSOOLOGICAL DATA, CHICOA VLHA, CABOitA BASSA
Chicoa Velha is now subnerged by the lake in the ChicoaBasin. Values are
given as means per month for each season.
Hot-Dry Hot-Humid
Aug-Oct Nov-April
Cold-Dry
hiay-July
Annual Mean
Rainfall (mm)
No. of rainy days
Potential
evapotranspiration (mm)
Temperature °C
Mean Max
Mean in
Mean
Relative humidity 
basins and northerly, westerly and north-westerly winds
in the western basins.
Lake Filling and Limnological Characteristics
Unlike Kariba, which took several years to fill, Cabora
Bassa was filled very rapidly with the lake level rising to
314 m AMSL, only 12 m below full supply level, by early
May, 1975. Lake level then dropped to 3 10 m by mid June
and then very rapidly to 305 m AMSL by the end of July
after which it continued to drop slowly and consistently
to 302 m by December, 1975 (Fig. 2).
For the greater part of the year lake waters remained
homiothermal and well mixed partly as a result of the
quantities of water released in drawdown. However,
TABLE 3
WIND DIRECTION - Mean Annual Percentage of Recordings (1960-1968)
N NE E SE S SW W NW Calm
Chicoa Velha+
Zumbo+ +
4 7 8 37 22 11 10 1
2 2 27 35 5 1 2 3 24
Data extracted from tables in Hidroteonica Portuguesa, 1973
ean of four years of complete data
Mean of six years of complete data
246
Season
Month hs
1
134
35,4
18,4
27,7
53
100
7
165
35,1
20,7
28,2
65
1
o97
97
31,1
14,4
23,6
605
43
1681
34,2
18,5
26,9
61
-Lke Lvtl
_ - - - - Ae.
J J A S 0 N D
DATE
stratification developed in the eastern basins from mid-
September becoming more pronounced and spreading
westwards as far as the central Mucanha Basin by the end
of the year. Hydrogen sulphide was present in the deeper
WO0 eastern basins by November. Chemical content of sur-
face waters showed little seasonal or spatial variation and
was, in general, comparable with pre-impoundment
,0o 0 levels (Hall et al., 1976). Some chemical and physical
1200 = parameters of surface, open waters in the Gorge Basin are
shown in Table 4. pH increased from 6,9 in July to 8,7 in
,000
November. Nitrates and ammonia also decreased as the
800 year progressed whereas there was some evidence for an
increase in sulnhates.
Fig. 2. Variation in Lake Level (elevation above sea level) and
Surface Area (square kilometres) at Cabora Bassa in 1975. Mean
retention level is 326 m AMSL.
TABLE 4
CHEMICAL AND PHYSICAL PARAMETERS OF SURFACE, OPEN WATERS, GORGE BASIN
Value Date 20/6 9/7 1/8 5/9 2/10 8/11 2/12
Ca mg 1-1
Mg mg 1-1
Na mg 1-1
K mg 1-1
NO3 mg 1-1
NO2 mg 1-1
NH3 mg 1-1
P04 mg 1-1
S04 mg 1
Fe mg 1-1
Yin mg 1-1
pH
Conductivity
inl S cm- 1
Temperature °C
Stratification
10,8
3,2
8,0
4,3
4,1 12,7
2,2
1,1
0,03
2,0
0,5
0
7,0
5,5
6,9
2,0
0,3
0
- 10,0 - 9,0
- 4,9 - 5,5
- 4,6 6,4 6,9
- 2,0 2,3 2,0
0,16 0,06 0,06 0,1
0 0 0
- 0,16 0,22 0,46 0,02 0,06 0
- 0
3,7
0,14
0
0,10 0,07 Trace Traoe 0,03
1,15
- <0,1
- 6,9
- 101
7,0
104
- 9,6
- ~0,1
- 0,5
7,7 8,3
115 106
- 9,6
8,7
108
24,1 24,3 23,2 23,3 24,3 26,3 30,4
0 0 0 0 + +9
Dashes = no data available 0 = not detectable
310
305
300
295
290
285
28O
275
270
F M A H
0
8,0
115
+
247
__ _
... _. __< o.r..._o.
.
I
I
II
i
I
/
Lake Colonization by Aquatic Macrophytes
Mats of floating macrophytes in pools and slow flowing
sections of the Zambezi River were displaced by annual
Zambezi floods and transported down stream by strong
currents. Plants from this source were first recorded at
the dam wall six weeks after closure (24 January, 1975)
when a mat of approximately 1500 m2 together with
scattered mats of less than loo m2 accumulated in the
vicinity of the barrage. Nine weeks after closure a nearly
continuous band of weed had collected in the canopies of
semi-submerged trees along the shores to a distance of 8
km upstream of the wall. At this stage very few plants
were observed in the Chicoa Basin which was the first of
the broad lake basins to fill.
By March movement of weed mats had diminished
and accretion by vegetative growth rather than addition
of plants from the old riverine source predominated in
areas east of Carinde.
Location and area of mats are indicated in Table 5.
Distribution and size of mats was recorded by aerial
survey, flying at approximately ooo ft above lake level,
and co-ordinating independent observations made by
two or three observers on I 0 Ioo ooo scale maps. As a
further check, the areas occupied by floating weed mats
were estimated from extensive boat surveys. Weed distri-
bution, in 1975, was discontinuous with fairly large popu-
lations at the eastern and western lake extremities but
only scattered mats in the central basins, mostly on the
north bank. Rapid drawdown in July led to the destruc-
tion of nearly 50% of the weed by stranding. Weed cover
increased rapidly and within two months was at pre-
drawdown levels. Offset production slowed from Sep-
tember until the end of the year and the weed area re-
mained static until the first floods in December carried
weed from riverine sources into the western basins.
Composition of the Weed Mat
Marked changes in relative species abundance in the
macrophyte communities took place during the course
of the year.
Subjective estimates of percentage cover of each
species were made from boats driven into mats usually 5
to 6 metres from the lakeward edge. This method proved
rapid and effective for smaller mats or narrow weed
bands. No attempt was made at quantitative estimates of
relative composition because of the severe difficulties of
penetrating mats, particularly those with abundant
TABLE 5
WEED AREA (HA) AND DISTRIBUTION BY BASIN.
The ratio of mat area on north to south banks is presented in parentheses. Rapid
drawdown in July depressed cover values while summer floods carried additional
weed into the western basins in December. Approximately 50 ha of weeds were des-
troyed by herbicides in the Gorge Basin. All weed in the Chiooa and Mucanha Basins
was destroyed by herbicides and/or manual harvesting.
GORGE CHICOA MUCANHA CARINDE
ha N S ha N S ha N S ha N S
MUCANGADZE TOTAL (ha)
ha N S
70 (3:2) 2 (9:1) 10 (4:1) 20 (4:1)
200 (3:2) 5 (9:1) 12 (4:1) 45 (4:1)
Drawdown150 (2:1) 1 (1:0) 4 (4:1) 12 (3l1)
ept 260 (2:1) <0,2 (1:0) 2 (9:1) 13 (3:1)
Dot 300 (2:1) 0,5 (1:0) 0,6 (1:1) 12 (3:1)
ov 300 (2:1) 0,5 (1:0) 0 10 (4:1)
ber 320 (7:3) 0 (1:0) O 12 (3:1)
April
June
Rapid
July
Aug/Se
Sept/C
Oct/Nc
Deceml
248
50 (3:2)
50 (3:2)
10 (3:1)
12 (3:1)
7 (3:1)
5 (4:1)
30 (3,2)
152
312
177
287
320
315
362
_> > __-
DT.+ 
Eichhornia, by boat. Fig. 3 indicates changes in composi-
tion expressed as % cover for weed mats in the eastern
Mucangadze and Carinde Basins where small mats pre-
dominated.
The striking increase of Eichhornia during the year, at
the expense of the other floating macrophytes, was
typical of weed colonies throughout the lake. The marked
decline of Salvinia is of particular interest as this is the first
recorded instance of the association of these two species
in a large man-made lake.
Composition of mats was first noted on 24 January,
I975, near the barrage. Eichhornia and Salvinia were
present in approximately equal proportions of 40% each,
w
o
.)
with Pistia stratiotes covering the remaining 20%, Azolla
nilotica was rare. The plants were associated with large
mats of floating Phragmites mauritianus Kunth, dis-
lodged from upstream river bank communities. These
mixed composition mats were typical of lotic popula-
tions. A successional sequence was recognized during the
course of the year. Salvinia was usually rapidly supressed
by explosive Pistia growth. Azolla remained competitive
for a longer period (in some cases reaching 65% cover
value) but it too was eventually supressed by Pistia.
Scattered Eichhornia plants expanded very rapidly in
stabilised mats and within a few months completely
dominated Pistia. Phragmites declined very gradually as
hornia
ia
igmites
inia
JUN (11) JUL (35) SEP (10) OCT (8) NOV (10)
DATE ( No. OF MATS)
Fig. 3. Changes in meanpercentage cover of Eichhornia crassipes, Pistia stratiotes, Phragmites mauritianus and Sal-
vinia molesta in macrophyte communities of the Carinde and Mucangadze Basins from June to November, 1975.
Cover was estimated subjectively (by the same observer throughout) as the vertical cover projection of each species
and, because of community stratification, usually exceeded 100oo% for total cover value. Bars in the histogram represent
mean % cover value whilst lines drawn through the bars represent the range. The numbers of mats used in each month-
ly estimate is presented in parentheses.
249
the dead plants became waterlogged and sank, removing
the growing platform of living plants.
Extensive Salvinia mats were first observed in May,
only in the Carinde Basin, behind a broad band of semi-
submerged trees. Twigs and branches appear to have
acted as a partial barrier to Eichhornia penetration. By
July, however, both Pistia and Eichhornia were found
to be growing vigorously in the area. A rapid drop in the
lake level eliminated most of the colony and no further
concentrations of Salvinia were seen.
By the end of the year Salvinia was very rare in the lake
and Eichhornia dominated all mats.
Post-immigration Processes in Lake Colonization
Processes of interest in lake colonization by floating
macrophytes following initial invasion by immigration
from external sources are: dispersal, growth (vegetative
reproduction and biomass increment and factors af-
fecting these) and mortality.
Dispersal
Dispersal controls the rate at which weeds spread around
the lake perimeter, and the final location of 'permanent'
floating weed mats. This process is important because
weed control efforts are simplified and far less costly if
the plants are geographically restricted.
Wind was the most important dispersal agent over the
greater part of the lake. South-easterly prevailing winds
(Table 3) caused a concentration of floating macrophytes
on the northern banks and limited eastward dispersal of
weeds through the physical obstruction of the Carinde
Narrows to the Mucanha Basin. Weed driven into the
northern banks of the Chicoa and Mucanha Basins was
rapidly destroyed by wave action except in very sheltered
bays and estuaries. The southern bank of the Chicoa and
Mucanha Basins, except for the extreme westerly section
of the latter, remained weed free. The very rare mats
driven into these shores, mostly by Zambezi currents in
the western Mucanha Basin, were eradicated by manual
removal to prevent their spread.
Mats remained remarkably restricted to a particular
area soon after lake filling when currents associated with
filling ceased and, except for vegetative reproduction,
mats were virtually static from June until mid Novem-
ber, 1975.
Wind direction is more variable during the summer
months, from November to February, especially preced-
ing and during thunderstorms. Weed that had remained
stable for months was torn loose by strong winds and
rough water preceding storms and, in the calms following
storms, drifted on open water from which it was fre-
quently dispersed to previously uncolonised areas.
Stormy weather in December was the most potent factor
in breaking up 'permanent' mats and this resulted in the
first major expansion of weed along the lake shore lines
since the lake levels had stabilised in July. This expansion
was particularly noticeable in the Gorge Basin. In most
cases mat fragments were driven back onto northern
shores.
Surface currents were important as a dispersal agent in
the early months during filling, causing a concentration
of weed in the eastern end of the lake. Observations in-
dicated that only a slight current is needed to carry weed
against an opposing wind. For example, a current of less
than i km/h in the Mucangadze Basin swept weed east-
wards against an opposing 18 km/h wind. From May
onwards, however, surface currents caused by the Zam-
bezi inflow could not be detected east of the Mucangadze
Basin and weed dispersal was subsequently controlled by
wind in the remaining basins.
Rainy season flooding of the Zambezi River may carry
weed beyond the Carinde Narrows into the lake but, as
flood waters are cooler than lake waters, the former will
probably sink beneath the latter and surface currents will
remain limited to the inflow area in the extreme west.
A seasonal cycle of dispersal in the rainy season (with
attendant dangers of infestation of weed-free southern
shores) followed by consolidation and stabilisation in
the dry season may be expected in the future.
Vegetative growth
Experience in several tropical lakes has indicated a poor
correlation between plant growth rates and nutrient
analyses of the water (Mitchell, 1973). In Cabora Bassa
high growth rates were recorded despite apparently low
levels of nitrate, nitrite, ammonia and phosphate (Table
6). Potential growth of macrophytes in both colonised
and weed free areas was recorded by observing growth
rates in floating wooden quadrats placed in the lake. The
plants studied were Eichhornia, Salvinia and Pistia, and
they are dealt with separately below. Growth rates were
measured as offset production for the waterhyacinth
and Pistia and in terms of leaf number for Salvinia, thus
discussion of growth is mostly limited to increase in
numbers.
250
TABLE 6
EICHHORNIA CRASSIPES - GROWTH RATES
Measured as Offset Production
DT(days)1 X2 Nt
3
Habitat
18/5 to 27/6 6,12
18/5 to 27/6 5,67
12/7 to 14/8 5,68
12/7 to 14/8 5,83
8/9 to 20/10 8,94
8/9 to 20/10 10,7
5/11 to 20/12 
5/11 to 20/12 
Muoanha 1/9 to 21/10
1/9 to 21/10
17/9 to 21/10
21/10 to 6/11
Muoangadse
and Carinde 4/7 to
4/7 to
4/7 to
30/8 to
30/8 to
24/9 to
24/9 to
16/11 to
16/11 to
13/10 to
13/10 to
30/8
30/8
26/7
2/12
2/12
16/11
16/11
2/12
2/12
10,8
12,2
9,5
27,4
14,9
12,0
20,2
21,6
26,0
19,5
20,7
ea
26/11 6,53
26/11 7,4-
1. D T = Doubling Time -= n2
RGR
2. X = daily factor of growth
3. Nt where t 90, No = 1
1,12
1,13
1,1297
1,1262
1,081
1,067
1,00
1.00
1,066
1,0583
1,076
1,0256
1,0474
1,0595
1,0347
1,0324
1,0268
1,0288
1,0344
1,00
1,00
1,1119
1,0981
26891
59849
58436
44197
1107
342
1
1
315
164
730
10
65
182
22
18
11
24
21
1
1
13993
4548
Deep water. Steep sloping shores
to W n It IT
I H II n II
Tn nT I It I
In II n II 
II n I I t
Shallow gently sloping shore
In I nI n
Shallow estuarine
1n fn
Sheltered sloping shore, 2,5m deep
of11 t to I
Exposed to waves, 10 m deep
Shallow estuarine water, 4 m deep
n n n n n
Gently sloping, 4 m deep
n it n 11
II It * n
I n i I
Sloping shore, 2,5 m deep
In Is If II
RGR l= n Nt - in No , where Nt = Number of plants at
t time t (in days) and No Number
of plants at initiation
inorement in Nt = No.Xt
251
Basin Date
Gorge
_ _ _ ___~~~~~~~~~MIV
Eichhornia crassipes: (i) Offset production.
One or two small Eichhornia plants (5-6 lamina) were
placed in each quadrat and the number of offsets record-
ed at regular intervals. Trials were terminated when off-
sets filled the quadrats to avoid distortion of rates due to
overcrowding influences. Growth rate was expressed as
X from Bock's (1969) formula Nt = N.Xt where No is
the number of plants at time t (days) and X is the daily
factor of growth increment. Relative growth rate and
doubling time (Mitchell, 1974) were also calculated to aid
comparison of Salvinia, Eichhornia and Pistia growth
rates.
Results are recorded in Table 6 and Fig. 4.
Initial growth rates (trials began in May, 1975) were
very high (Fig. 4) and were comparable with other areas
in the world where Eichhornia is a pest plant (Perkins,
1972). Growth rates remained exponential and offset
production was high until the end of August. During this
period Eichhornia increased from less than to ha in
January to 300 ha by the end of August in the Gorge
Basin, despite the loss of nearly oo ha of weed during the
drawdown in July.
From mid-September offset production declined
sharply in the quadrats. In trials inthe Gorge Basin
initiated in November no increment was recorded. The
weed colony in this Basin showed no perceptible increase
in area until the end of the year. From October deficiency
symptoms began to appear with yellow streaking of the
lamina, necrosis of lamina tips and eventually death of
the outer rosette of lamina and petioles.
Growth rates in the Mucanha Basin were very much
lower than the Gorge Basin though still sufficient to
allow rapid mat expansion. A marked decline of offset
production was recorded towards the end of October,
following the trend in the Gorge.
The Mucangadze and Carinde Basins showed very low
growth rates in trials, except in October and November.
These low rates were suprising since several quadrats had
been placed in comparatively fertile, shallow water.
These rates were confirmed by the fact that observations
on the weed mats in these basins showed that there was
no perceptible increase after drawdown in July. Before
July very rapid growth had taken place in a shallow water
section of the Carinde Basin, presumably at a time when
growth rates corresponded to those in the Gorge Basin.
Unfortunately no values for offset production are avail-
able for this period.
Increase in coverage of Eichhornia is primarily depen-
dent on offset production as distinct from dry or wet
U)
U-
U-
0L
LL
To
- 12/7-14/
- -8 /9-20110
- - 5/11-20/12
/ "
K
/
10 20 30 40
DAYS
Fig. 4. Changes in growth rates during the first year of filling,
measured as offset production, of Eichhornia crassipes in
floating 2 m x 2 m quadrats in the Gorge Basin. Curves fitted by
hand.
weight increases. Cessation, or a marked decrease in off-
set production was noted in growth trials when floating
quadrats became filled with plants. In one trial, for
example, 146 plants were recorded on 17 November, 213
offsets nine days later (an increase of 67) but only 218 off-
sets (an increase of 5) 5 days later when the quadrat had
filled with Eichhornia plants. Overcrowding thus
depressed offset production but not necessarily biomass
increase of individual plants.
Bands of weed occupying lake margins only produced
offsets on the open water fringes of the mats. Conse-
quently potential for aereal increase of mats, particularly
those anchored by semi-submerged trees, is limited to
plants near open water. Breaking up of large mats by
summer storms or, on a smaller scale, passage of boats
could result in a faster colonization rate of open water
because of the increased 'edge' available for offset produc-
tion. This would partially offset slow growth rates ob-
served in summer.
Plants in the dense, inner sections of mats ceased offset
production but continued to grow in bulk and height.
This increase in individual plant biomass predominated
252
-- -- --
in the latter half of the year while offset production virtu-
ally ceased. Consequently increase in area of mats was
minimal.
Eichhornia crassipes: (ii) Increase in individual plant
biomass.
Height above water level was used as an index of phyto-
mass. The percentage cover of Eichhornia in each of five
height classes was estimated subjectively in a number of
weed mats in the Carinde and Mucangadze Basins
between the end of August and the end of November (Fig.
5).
The proportion of tall plants in the population in-
creased markedly during this period, although the
estimated area of the mats remained the same or dimin-
ished. No migration of mats occurred.
By the end of the year mats were dominated by tall
plants, densely packed, with a very low potential for off-
set production and thus a diminished capacity for expan-
sion into suitable unoccupied habitats. Apparently differ-
ent factors control offset production and individual
phytomass increment (Bock, 1969), (the latter continued
several months after the former had ceased).
Eichhornia crassipes: (iii) Sexual reproduction.
Eichhornia plants with both long and mid-styled flowers
were present in the lake, the latter being by far the most
common. Seed was produced but no seedlings were ob-
served.
Salvinia molesta:
Salvinia growth rate was recorded as the increase in the
0
O
3018 (.4) 19/9 (6) 20/10 (5) 11/11 (5) 27/11 (5)
DATE I No. OF MATS)
Fig. 5. Size distribution (height) of Eichhornia crassipes populations in Carinde and Mucangadze Basins, 975. The
histograms show mean percentage cover of five height categories of Eichhornia estimated subjectively by the same
observer throughout. Lines drawn through histogram bars indicate the range of cover in each height category whilst
the number of mats used for each estimate is in parenthesis after the date.
253
number of leaf pairs of healthy primary or secondary
phase plants placed in floating quadrats in the lake.
Means of the initial inoculum and those of the final har-
vesting of plants were used to calculate doubling time.
Trials were of shorter duration (I-2 weeks) than Eichhor-
nia trials due to difficulties in retaining the plants in the
quadrats and consumption of the plants by fish (proba-
bly Distichodus spp.).
Salvinia growth was similar to results obtained at
Kariba (Mitchell and Turr, 1975) for mat-form plants
which are, however, slower growing than open water col-
onizing forms used at Cabora Bassa (Table 7). No decline
of growth rates comparable to Eichhornia was recorded
in the latter half of the year.
The fastest doubling time of 3,9 days was recorded in
shallow water in the Mucangadze Basin at the end of
November, while a doubling time of 4,95 days was
recorded in central Mucanha Basin in mid-October.
The decline of Salvinia in weed mats (Fig. 3) does not
appear to be due to infertile lake water or unfavorable
climatic conditions since the weed grew exceptionally
well at Kariba under similar conditions. It was probably
due to competitive exclusion by the larger Eichhornia
crassipes.
Pistia stratiotes:
Growth trials with Pistia were conducted from Septem-
ber onwards. Growth rates were invariably very slow.
Unfortunately no trials were made in the first half of the
year when the plant was an aggressive coloniser and a
major component of weed mats.
The highest recorded rate was 2 offsets in 39 days or a
doubling time of Io,9 days, in fertile waters in the Mucan-
ha Basin. Most trials had very slow growth rates and
several produced no offsets.
The poor capacity for offset production partially ac-
counts for Pistia's decline relative to Eichhornia.
Mortality
The main causes of mortality were wave action leading to
physical destruction of plants on exposed shores,
stranding and subsequent dessication following draw-
down, and herbivore predation. Drawdown was the most
potent mortality factor in the first year (Table 5). Rapid
drawdown from May until early August (Fig. 2) left quan-
tities of weed stranded on the banks. Small colonies in the
central basins, which might otherwise have acted as
potential foci for weed invasion, were destroyed. In the
Gorge Basin nearly half the weed population, and in the
Carinde Basin approximately 60 ha, were destroyed.
Lake Kariba experienced no appreciable drawdown
until five years after closure so that the critical stage of
initial invasion by Salvinia suffered no serious draw-
backs. Drawdown is the most potent weapon for weed
TABLE 7
Salvinia molest: MEiEUN DOUBLING TIME (LEAF N.BER) IN DAYS,
Gorge Basin, Cabora Bassa and Kariba+
Winter/Spring
Cabora Bassa1 13,6 (n=2)
Kariba2 14,3
Kariba3
Spring/Summer
11,0 (n=6)
13,2
Summer
7,4 (n=16)
8,2
13,8 (Jan-Feb)
+Kariba data from itchell and Tur, 1975
1. Plants in quadrats in open water
2. Tagged, mat-form plants
3. Plants in quadrats in open water.
254
control but may have many repercussions on other as-
pects of lake ecology, particularly as the lake matures
(Bowmaker, 1973).
Mitchell (1970, 1973) cites wave action as the major
factor limiting the extent of Salvinia at Kariba. The size
of waves is largely dependent on wind fetch which will, of
course, increase when the water reaches full supply level.
Even at low lakelevels, wave action destroyed a dense 50
m wide sudd of Phragmites and Eichhornia on an ex-
posed shore in a little over two months. Small mats and
isolated plants were destroyed within weeks of being
driven onto exposed northern shores. Offset production
was very slow in areas exposed to wave battering so that
expansion of colonies beyond sheltered creeks and estu-
aries was minimal on shores exposed to the prevailing
south-easterly and easterly winds.
Summer floods are likely to destroy large quantities of
weed occurring in estuaries of drowned rivers (Mitchell,
1973) and wash surviving plants into open water, where
most will be driven onto the exposed northern shores
resulting in further mortality from wave action.
Grazing of macrophytes by herbivors had very little
effect on the total weed population but this factor may
become more important in the future. Hippopotamus,
baboon, vervet monkey, warthog and kudu were all ob-
served eating Eichhornia, particularly in the dry season.
Water fowl, such as Egyptian geese, fed to a limited ex-
tent on stranded plants.
Fish, particularly Distichodus schenga Peters, D. mos-
sambicus Peters and possibly Sarotherodon mortimeri
and Tilapia rendalli Boulenger utilised weeds to a limited
extent. Loss of Salvinia by fish consumption curtailed
many growth trials.
Arthropods have been used for biological control of
both Eichhornia and Salvinia. Paulinia acuminata De
Geer (a grass hopper), Cyrtobagus singularis Hulst. (a
Curculionid weevil) and Samea multiplicalis Guenee (a
Pyralid moth) have all been released in the Zambezi Basin
for Salvinia control (Bennett, I974). Paulinia acuminata
was recovered in the Mucangadze Basin of Cabora Bassa
in June, 1975, and again in October. It was not collected
in the Gorge Basin where Salvinia was very scarce.
An unidentified moth, possibly Samea, was wide-
spread throughout the lake and helped check Salvinia
growth. It was first collected in May and remained com-
mon until the end of the year. The larvae cut leaves of
Salvinia and pupated in a 'sandwich' made of two leaf
portions.
The presence of both these insects at such an early stage
of Salvinia colonization bodes well for biological control
of the weeds. Whereas neither insect could be expected to
rapidly reduce a large existing weed population, their ef-
fects on the growth of a small, expanding weed popula-
tion may be considerable.
Eichhornia was attacked by caterpillars of a single
moth species. These fed on lamina and growth points of
the plant and caused appreciable damage in September
and October. They were, however, heavily parasitised by
wasps and dipteran larvae and very few adults emerged
from pupae. They are unlikely to be of importance in the
future. The Curculionid weevil, Neochetina eichhorniae
Warner and Orthogalumna terebrantis Wallwork, a
mite, have been released in the Kafue River (Zambia) and
in Rhodesia for Eichhornia control (Bennett in Mitchell,
1974). Ahmed (pers. comm. 1975) reports that no Neo-
chetina has been recovered in the Kafue but that the mite
is fairly common. Neither were seen at Cabora Bassa.
Discussion
Notable features of weed invasion at Cabora Bassa were
the disjunct geographical distribution of the plants and
the failure of expected explosive growth with cessation of
Eichhornia offset production in the floating commu-
nities and the very limited colonization by Salvinia.
Floating macrophytes were concentrated in the ex-
treme eastern and western parts of the lake (Table 5).
Rapid filling led to a strong west-east current in the early
months which carried weed to the dam wall. By the end
of March the rate of change of lake level diminished (Fig.
2), the current ceased east of western Mucanha, and
south-easterly winds blew weed onto the northern shores.
Here most was destroyed by wave action leaving only a
large concentration maintained in the west of the lake
behind the physical barrier of the Carinde narrows by
easterly winds. The effects of wind on wave action and
dispersal were magnified by the rapid development of
large stretches of open water due to high filling rates.
The unexpectedly small area occupied by floating
weeds at the end of I975 was due mostly to drawdown
and cessation of offset production by Eichhornia towards
the end of the year. Rapid drawdown (Fig. 2) ending in
July stranded small mats remaining in the Chicoa and
Mucanha Basins and curtailed weed colonization in
these basins. Weed in the Gorge was reduced by 50o% and
narrower bands near the Chicoa Basin were eradicated.
Even greater proportions were stranded in the shallow
255
western basins. Recovery of surviving mats was, initially,
considerable in the Gorge Basin but very limited in the
west. However, offset production virtually ceased in the
former area by the end of August and plant colonies re-
mained static until the end of the year with most Eichhor-
nia plants becoming streaked with yellow and later dying
back from the tips of the laminae.
No satisfactory explanation was found for this decline.
Stratification of the Gorge, reflected by the development
of thermoclines and oxyclines, began during the same
period and lasted until the end of the year (Bond et al., in
press, I977). Some chemical and physical parameters for
this period are listed in Table 4. Temperature is unlikely
to have limited growth except in shallow water in the
summer months. (Temperatures of over 33°C were re-
corded in sheltered waters in the Mucangadze Basin in
November and probably inhibited growth here (Pen-
found & Earle, 1948).)
Light was probably adequate for Eichhornia growth
but a shorter photoperiod in winter may account for
some of the variation in seasonal growth rates as faster
growth has been recorded in 8 hour daily photoperiods
relative to 16 hour photoperiods (Bock, I969).
pH ranges of 4 o 9 have been recorded by Bock (1966)
and optima between 6 and 8 by Pariji (1934) in trials
ranging from pH 4 to lo. pH exceeded 8 in surface waters
from August onwards and values of more than 9 were
recorded during the day in shallow, algae rich waters in
the western basins at the start of the rainy season. High
pH may therefore have had a limiting effect on Eichhor-
nia growth rates.
Overall nutritional requirements of Eichhornia crassi-
pes are very low but the plant shows a strong response to
NO 3 N in culture (Musil & Breen, 1977) and it seems
probable that NO3 and possibly PO4
3
- normally limit
growth under natural conditions. NO 3 showed a con-
tinuous decline in water collected in the Gorge Basin
(Table 4) and may be partly responsible for diminished
growth rates.
No significant variation in Ca++, Mg, Na+ or K+ was
found during the year. Excesses or shortages of trace
elements as suggested by yellowing of plant leaves, per-
haps associated with stratification, may be the cause of
decline in Eichhornia offset production but insufficient
analyses are available for confirmation.
Growth requirements of Salvinia molesta on Lake
Kariba have been studied by Mitchell & Tur (I975).
Growth rates are maximal at 30°C so that hot rainy
season temperatures may limit growth. Light condi-
tions, except under a canopy of taller plants such as Fich-
hornia would be optimal. Nutrient deficiencies, particu-
larly nitrogen and phosphorus, were considered the
greatest limitation on potential growth rates of Salvinia
at Kariba (Mitchell, 1973) and, since the water chemistry
is similar (Bond et al., in press), are likely to be so at
Cabora Bassa. If Eichhornia growth remains stagnant
and Salvinia is not controlled by insects or wave action,
Salvinia may yet prove to be a threat at Cabora Bassa.
The dominance of Eichhornia in weed mats at the end
of the year (Fig. 3) can be attributed to its initial growth
rates. Whilst Eichhornia and Salvinia may have com-
parable doubling times (Tables 6 and 7), the former has
a greater capacity for rapid water surface coverage by
virtue of the size of each offset. In addition exclusion of
light by the Eichhornia canopy (plants up toI m high
were recorded) probably limited Salvinia growth causing
its decline in relative importance. Salvinia growth was
also suppressed to some extent by arthropod and fish
herbivores.
Explosive growth of Salvinia in Lake Kariba has been
well documented (Mitchell, I970, I973, 1975). Com-
parisons of weed ecology in the two lakes may thus pro-
vide further insight into the problems of floating macro-
phytes in large impoundments.
Climate over the two lakes is essentially similar
(Harding, 1966, Begg, 1970, van der Lingen, 1973) al-
though Cabora Bassa is somewhat hotter. The species
composition of the problem macrophytes is similar
except for the presence of Eichhornia crassipes at Cabora
Bassa.
However, differences between the lakes are pronounced
both in their early history and their environmental set-
tings. Cabora Bassa filled very rapidly whilst Kariba filled
slowly with consequent effects on initial distribution of
the weeds. Winds on Kariba blow across the lake axis, but
on Cabora Bassa blow nearly parallel to it which, from
dispersal patterns in the first year, suggest disparate
patterns of invasion in the future. At Kariba very large
areas of bush were cleared and burnt before filling where-
as none was cleared at Cabora Bassa. This means that the
weed will suffer less from wave action and be less mobile
at Cabora Bassa.
There are indications that Cabora Bassa will be more
fertile than Kariba (Hall et al., 1976), but that the initial
phase of high nutrient availability will be much shorter
because of Cabora Bassa's rapid replacement time (Bond
et al., in press). Growth potential for macrophytes may
thus decline rapidly and Cabora Bassa may never reach
256
the high standing crop of Kariba. However, the potential
for weed growth in the maturing phase of the lake may be
far higher because of expected higher nutrient levels.
Competition between Eichhornia and Salvinia, both
aggressive weeds, did not occur in Kariba and may result
in a less vigorous weed population in Cabora Bassa
because of competition for limited resources.
Mortality factors in the two lakes have operated in
different ways. Kariba experienced its first major draw-
down five years after closure. Waterlevel fluctuations
were considerable in the first year at Cabora Bassa and
eliminated much of the weed colony. For reasons de-
scribed above invasion of weed from western riverine
sources into weed free central basins will probably be
slow at Cabora Bassa and thus the effects of early draw-
downs may be persistent. Cabora Bassa also has a larger
wind fetch which, in combination with a steep northern
shore line, may result in greater mortality from wave ac-
tion than occurs in Kariba, however, mortality may be
limited in part by the protective bands of semi-sub-
merged trees.
Paulinia acuminata was released in Kariba more than
a decade after lake closure. At Cabora Bassa Paulinia
and other grazers of Salvinia were present in the invading
weed population and are likely to limit its spread from
the start. There was no evidence of serious arthropod
infestation of Eichhornia but releases of insects in Zambia
and Rhodesia will have an early effect on the weed if these
animals do become established in the Kafue and Huny-
ani rivers.
Weed control was initiated at a very early stage at
Cabora Bassa. Whilst the quantity of weed destroyed was
relatively small, the removal of all plant mats in the
central basins which were potential foci for exponential
growth will doubtless have a significant effect on future
colonization of the lake. No comparable control pro-
gramme was carried out at Kariba in the early years of
Salvinia invasion.
A number of factors thus discourage comparisons of
weed performance in the two lakes, and will be of varying
importance at different stages of Cabora Bassa's history.
Conclusion
Experience at Cabora Bassa suggests that:
Predictions of early colonization and subsequent
growth of floating aquatic macrophytes in man-made
lakes still cannot be made satisfactorily at the present
time. In the case of Cabora Bassa there was no evidence
of the generally expected explosive growth and high cover
of floating macrophytes in the filling phase by the end of
1975.
Location of aquatic weed colonies is a function of
morphometry, wind speed and direction, and current.
Analysis of wind and current patterns in relation to
morphometry allows useful predictions of initial points
of invasion. The influence of current and wind is modified
by filling rates and lake morphology. Filling rates and
drawdown can, theoretically, be adjusted to suit the
needs of the biologist but, in practice, engineering con-
siderations are likely to predominate in the early years.
Weed mortality can be effected by control methods
and drawdown. Control methods will be least costly and
most effective in the early stages of colonization. Draw-
down can disrupt biological communities, particularly as
the lake matures (Bowmaker, 1973), but is possibly less
harmful in the initial phase of lake development.
Weed growth is effected by many variables, few of
which can be predicted with any confidence before the
lake fills. Mat composition will effect growth rates and
can be assessed from pre-impoundment surveys of the
lake catchment.
Chemical composition of lake waters is a result of
complex processes while the relevance of water analyses
to plant growth is incompletely understood. Offset pro-
duction is the main means of increase in water coverage
for Eichhornia and appears to be physiologically distinct
from increase in individual plant biomass. Laboratory
growth studies seldom differentiate between the two
processes. It thus seems likely that pre-impoundment
predictions of floating macrophyte growth rates in large
impoundments will remain 'rule of thumb' for some
time to come.
Summary
i. Two notorious floating aquatic weeds, Salvinia moles-
ta and Eichhornia crassipes, occur in the catchment of
Cabora Bassa, a new man-made lake on the Zambezi
River, Mocambique. Explosive population growth of
either or both of these species was predicted during the
initial filling phase of the lake.
2. Cabora Bassa, which is smaller than Lake Kariba, a
large man-made lake also on the Zambezi River, has five
basins seperated by islands and promontaries with steep
northern but gently sloping southern shores in the central
257
basins. Prevailing winds are easterly to south-easterly
blowing along the east-west axis except between Novem-
ber and February when northerly and westerly winds are
more common.
3. Lake filling (which commenced in December, 1974)
was very rapid and surface currents carried large quan-
tities of weed to the eastern outflow section of the lake.
When lake levels stabilised macrophytes were concen-
trated in the extreme east and west with scattered mats
on northern shores of the central basins.
4. A narrow section of the western lake (the Carinde
narrows) acted as a physical bottleneck to weed passage
eastwards, particularly after surface currents ceased and
easterly winds dominated plant dispersal. Mats remained
largely stable in position until variable winds preceding
thunderstorms at the end of the year lead to some re-
distribution. The surface area of the lake covered by
floating macrophytes never exceeded % in the first year
of filling.
5. Initially Salvinia and Eichhornia were present in
large quantities together with Pistia stratiotes. Salvinia
was rapidly supressed by Pistia and both species by Eich-
hornia a year after the dam closed.
6. Growth rates of three species was measured in
floating enclosures as increment in offsets or leaves.
Eichhornia grew very rapidly (doubling time ± 6 days)
until September when rates diminished and, in the Gorge
Basin, signs of nutrient deficiency developed. By the end
of the year no offsets were being produced by this species.
Increment in biomass, however, continued so that mats
became dominated by tall central plants. No definite
causes for the decline of offset production wereidentified.
7. Salvinia growth (increment in leaf number) was
comparable to rates at Lake Kariba where it showed ex-
plosive population growth. Its poor performance at
Cabora Bassa is attributed to competition with Eichhor-
nia in floating mats.
8. Rapid drawdown in July, I975, caused widespread
stranding and death of floating plants and was the major
mortality factor in the first year. Weed control by manu-
al harvesting and herbicide spraying eliminated weed in
the central basins and portions of the Gorge Basin. Pauli-
nia acuminata, a grasshopper, and a moth, possibly
Samea multiplicalis, were found on Salvinia molesta
and the presence of these insect control agents at such an
early stage may help control population increase.
9. Lake Kariba and Cabora Bassa, whilst in a similar
climate and with similar water chemistry, differ in the
events of lake filling, floating macrophyte species com-
position, and morphometry suggesting that colonization
by floating macrophytes will be dissimilar.
to. It is concluded that dispersal and location of floating
macrophytes at least in the early stages, can be predicted
fairly well by a study of wind, current, lake morphometry
and proposed filling events in man-made lakes but that
growth rates and quantities of weed at any particular
stage in a lakes history remains largely unpredictable.
Acknowledgements
We thank our colleagues N. Coe, G. Pitot and K. Rogers
for assistance in data collection, Dr. D. Mitchell and Dr.
Rose for advice and encouragement, the staff of Gabine-
to do Plano do Zambeze and the Instituto Investigacao
Agronomica de Moambique for arranging air flights,
and Loxton, Hunting and Associates for laboratory
analyses and their always helpful assistance. W. M. Bond
provided invaluable assistance at all stages of the project.
References
Begg, G. W. 1970. Limnological observations on Lake Kariba
during 1967 with emphasis on some special features. Limnol.
Oceanogr. 25: 776-788.
Bennett, F. D. 1974. Biological control, 99-106, in Aquatic Vege-
tation and its use and control edited by D. S. Mitchell, Unesco.
Bock, J. H. 1966. An ecological study of Eichhornia crassipes
with special emphasis on its reproductive biology. Ph. D.
Thesis, Univ. California, Berkeley.
Bock, J. H. 1969. Productivity of the Water Hyacinth, Eichhornia
crassipes (Mart) Solms. Ecology. 50: 461-4.
Bond, W. J., Coe, N., Jackson, P. N. B. & Rogers, K. H. (in press).
The limnology of Cabora Bassa, Moqambique, during its first
year. Freshwater Biology.
Bowmaker, A. P. 1973. Hydrophyte dynamics in Mwenda Bay,
Lake Kariba. Kariba Studies. 3: 42-59.
Davies, B. R., Hall, A. &Jackson, P. N. B. 1975. Some ecological
aspects of the Cabora Bassa Dam. Biol. Conserv. 8: 189-201.
Hall, A., Davies, B. R. & Valente, 1. 1976. Cabora Bassa: some
preliminary physico-chemical and zooplankton pre-im-
poundment results. Hydrobiologia (48) 1: 17-25.
Harding, D. 1966. Lake Kariba, The hydrology and development
of fisheries, in Man-Made Lakes, edited by R. H. Lowe-
McConnell, 7-20 Academic, London.
Hidrotecnica Portugesa 1973.lnstalacoes para navegacao na
Albufeira de Cabora Bassa. Tomo 4. Gabinete do Plano do
Zambeze, Lisboa.
Jackson, P. B. N. & Davies, B. R. 1976. Cabora Bassa in its first
year: some ecological aspects and comparisons. Rhodesia
Science News. Io, No 5: 128-133.
Lingen, M. 1. van der 1973. Lake Kariba: Early History and
South Shore 132-142, in Man-Made Lakes: Their Problems
258
and Environmental Effects edited by W. C. Ackermann, G. F.
White and E. B. Worthington. American Geophysical Union,
Washington, D.C.
Little, E. C. S. 1966. The invasion of man-made lakes by plants,
in Man-Made Lakes, edited by R. H. Lowe-McConnell, 75-86,
Academic, London.
Macedo, J. de Aguiar 1974. Vegetacao aquatica em Cabora Bas-
sa. Institute de investigacao agronomica de Moqambique.
Memorias No. 5.
Mitchell, D. S. 1970. Autoecological studies of Salvinia auricula-
ta Aubl., Ph. D. thesis, Univ. of London.
Mitchell, D. S. 1973. Aquatic weeds in Man-Made Lakes, 6o6-
61i, in Man-Made Lakes: Their Problems and Environmental
Effects, edited by W. C. Ackermann, G. F. White and E. B.
Worthington. American Geophysical Union, Washington,
D.C.
Mitchell, D. S. 1974. Editor, Aquatic Vegetation and its Use and
Control. Unesco, Paris.
Mitchell, D. S. & Tur, N. M. 1975. The rate of growth of Salvinia
molesta (S. auriculata Auct.), in Laboratory and natural con-
ditions. J. Appl. Ecol. 12: 213-225.
Musil, C. F. & Breen, C. M. 1977. The application of growth
kinetics to the control of Eichhornia crassipes through nutrient
removal by chemical harvesting. Hydrobiologia. 53, 2: 167-
171.
Oliveira, J. S. 1972. 'Zimofolia' Um novo alimento levedurizada.
An. Inst. Sup. Agron., Lisbon. 32: 21-36.
Parija, P. 1934. Physiological investigations on water hyacinth
(Eichhornia crassipes) in Orisa with notes on some other
aquatic weeds. Indian J. Agric. Science 4 (3): 399-429.
Penfound, T. T. & Earle, W. T. 1948. The biology of the water
hyacinth. Ecol. Monogr. 18: 447-72.
Perkins, B. D. 1972. Potential for Waterhyacinth Management
with biological agents. Procedings Annual Tall Timbers Con-
ference on Ecological Animal Control by Habitat Manage-
ment, 53-64.
259