ORIGIN
OF STRATIFICATION
IN AN AFRICAN
RIFT LAKE
J. F. Tailing
Freshwater Biological Association, Amblcside, England
ABSTRACT
Physical, chemical, and algal features of Lake Albert (East Africa) are described,
chicfly from three longitudinal surveys in 1980-61. The major ionic composition of the
water is largely determined by another lake (Lake Edward) higher in the drainage system,
but abiogenic sources are possibly responsible for a great enrichment of phosphate and
depletion of silica. Contrary to most earlier accounts, a well-marked thermal and chemical
stratification can develop in the lake. The thermal stratification is probably related to
profile-bound density currents of cooler water, which flow from shallow areas of the lake
and have varying effects in deeper water. Their interpretation is supported by examples
of chemical and algal stratification, -particularly of dissolved oxygen, silica, and a species
of the diatom Nitxschiu.
form of thermal stratification
in the lake,
and hence on the dependent distribution of
various chemical constituents and of planktonic algae. These physical, chemical, and
algological observations are described here,
chiefly in their relation to the conditions
of stratification; features otherwise distinctive of the lake are also summarized.
My thanks are due to the Director, the
late Dr V. D. van Somercn, and staff of the
East African Fisheries Research Organization, Jinja, for their co-operation and laboratory facilities, and to the Uganda Fish
and Game Department for making available a boat and crew. Mr Eric Hamblyn
gave much valuable assistance during the
cruises, as did my wife who was responsible
for the analyses of total phosphorus, phosphate, nitrate, silica, and sulphate. Spectrophotometric work was made possible by a
grant from the Royal Society. Other financial assistance was received from the Colonial Office, London. Dr J. W. G. Lund and
Mr R. S. A. Beauchamp gave helpful criticism on the manuscript.
INTRODUCTION
A pronounced stratification in many tropical lakes is controlled
by the density
changes associated with unusually small
differences of temperature.
This feature
is due particularly
to the rapid change of
water density with temperature,
which
exists at the high temperatures usual in
thcsc lakes. Ruttner
(1931, 1937) has
drawn attention to the unexpectedly high
stability of stratification
which can result.
However, if the water density alters rapidly
with temperature, it is likely to be unusually
responsive to small exchanges of heat. This
sensitivity is clearly seen in the superficial
stratification
which develops diurnally in
many tropical waters ( e.g., Talling 1957a),
and is usually ended by nocturnal cooling
and mixing. Apart from such superficial
changes there can be distinguished
(cf.
Worthington
and Beadle 1933) the more
persistent stratification
of some tropical
lakes, where a deeper discontinuity in temperature and density exists. The mode of
origin of the discontinuity,
and its relation
to the surface exchanges of heat, are little
known. This follows particularly from the
rarity of studies in tropical lakes on the
seasonal variation
of thermal structure, and
METHODS
Most
made during
of the lake,
between Butiaba-Ntoroko-Butiaba,
on 2427 November 1960, 16-18 March 1961, and
7-9 August 1961. Some additional
samples
of surface water, obtained
on 7 February
and 20 June 1961, were also analyzed.
Water samples from various depths were
three
of its horizontal
distribution
within the lake
b asin.
Thcsc aspects were studied in an East
African
Rift lake, L,ake Albert, on several
occasions during 1960-61. The results throw
some light on the origin of an unexpected
68
observations
cruises
along
were
the length
ORlGlN
OF STRATIFICATION
IN
AN
AFRICAN
RIFT
LAKE
69
was measured once during each cruise,
collected by a Ruttner or a van Darn
<1956) sampler, with capacities of 1 L and using a selenium photo-cell with violet,
5 L respectively.
Temperature was meas- blue, green, and red color filters. This optiured by a mercury-in-glass thermometer of cal work will be described more fully elsethe Ruttner sampler (read to 0.05”C) and, where; in connection with measurements
of photosynthesis by the phytoplankton.
jn more detail, by a thermistor thermometer
Counts of planktonic
algae were made
described
by
Mortimcr
(read to 0.02OC ) as
using iodine sedimentation and the inverted
and Moore ( 1953). Other analyses included
microscope (Lund, Kipling, and Le Cren
dissolved oxygen by the Winkler method,
without
modification
for reducing
sub- 1958 ) ,
stances; pII, calorimetrically
using thymol
GENERAL
FEATURES
AND MORPHOMETRY
blue indicator
and comparison
against
colored glass standards with the Lovibond
Lake Albert is the northernmost lake in
“Nessleriser”; silica, by reduction to molybthe west limb of the Great Rift Valley (Fig.
denum blue and spectrophotometric estima1). Apart from direct rainfall, the lake retion ( Mullin and Riley 1955) ; soluble phos- ceives water principally through the Semliki
phatc-phosphorus, by Deniges’ method folRiver, which flows from Lake Edward, and
lowed by visual estimation in the Lovibond
intermediate streams draining the Ruwen“Ncssleriser”; total phosphorus by the same zori massif. A number of small streams fall
method after oxidation with 20% perchloric
directly into the lake from the surrounding
acid, but with final spectrophotometric esti- Rift escarpment, and several hot springs
mation; nitrate-nitrogen
by reaction with
and salt wells near the shore probably conphenol disulphonic acid, decolorization with
tribute salts to the lake. The Victoria Nile
aluminum hydroxide,
and final spectroenters its northcast corner, but almost imphotometric estimation; alkalinity
due to mediately turns away bearing a variable
bicarbonate and carbonate, by titration to contribution
of lake effluent (IIurst 1925;
pH 4.5 with centinormal hydrochloric acid;
Beauchamp 1956; Tailing 1957b ) .
calcium and magnesium, by complcximetric
The lake lies at an altitude of 616 m
titration with versenate (Heron and Mac( 2,050’ ft ) , is 145 km long, 35-46 km broad,
kereth 1955 ) ; sodium and potassium, on has an area of about 5,600 km2, maximum
one sample only, by flame photometry;
depth of approximately 58 m and an averchloride, by titration with silver nitrate
age depth of approximately 25 m. The eastusing chromate indicator; sulphatc, by pre- ern shoreline shows a fine succession of
cipitation as barium sulphate in acid solu- curved spits, which may reflect wave and
tion and estimation of turbidity
with the current motion parallel to the shore generand electrical conducspectrophotometer;
ated by the predominantly southerly winds
tivity ( k20) by the portable “Dionic” con- (Worthington
1929a, p. 48; 192910,p. 118).
ductivity meter. Determinations
made in Deep sediments have been deposited in the
the field shortly after sampling included
lake basin since its origin; they include the
pH, phosphate-phosphorus, the evaporation
famous Kaiso fossil beds. Sedimentation
Cd loo-ml samples for later nitrate analysis,
near the Semliki discharge has led to a
and silica estimation by Atkins’ modification
shallow underwater bar projecting into the
cd Dienert and Wandenbulcke’s
m&hod.
lake ( Fig. 1 ), dividing the main deep-water
The values finally used for silica were those area near the western escarpment from a
obtained spectrophotometrically
after stor- subsidiary deep groove or zcanyon” near
age of the samples in the dark for l-5 days. the eastern (Uganda)
shore. The lake
All chemical determinations were calibrated
water is often rather turbid with fine, silty
using standard solutions, and, except for material in suspension, besides varying denoxygen measurements, all samples were col- sities of phytoplankton;
the euphotic (and
lected in polyethylene bottles. Light pene- photosynthetic)
zone was measured as 6tration at an arca midway along the lake 11 m deep. Suspended silt was often con-
J. F. TALLING
b-IG.
1.
Bathymetric
insct shows its relation
map of Lake Albert,
to other African lakes.
with
spicuous in the lowermost 5-10 m of the
water column at the deeper offshore stations, suggesting that water movements
were appreciable in this deep water.
Further sources of relevant limnological
information include the surveys of Worthington ( 1929a, b, 1930), and by a Belgian
expedition
published
partly in Verbeke
(1957) and van der Ben (1959). A general
compilation is given by van Meel ( 1953),
but contains errors in two tables of chemikal constituents (pp. 131, 234).
WATER
CHIEMISTRY
The highly distinctive chemistry of the
lake water is shown in Table 1, which sum-
isobaths
in mctcrs,
from
Vcrbckc
(19rj7).
The
marizes the most complete analyses available. The data of Elskens have not yet been
fully published and are taken from the
works of Verbeke ( 1957) and van der Ben
( 1959). Less detailed analyses have been
given by Worthington
( 1929a, 1930), Fish
( 1952a, 1952b, 1953)) and Talling ( 1957b).
The moderately high salt content in Lake
Albert is reflected in the values for conductivity and alkalinity. These indicated no
pronounced variation with depth or along
most of the lake’s axis, though small inflow
streams caused local dilution, as at Ntoroko.
The high proportion of potassium to sodium,
and of magnesium to calcium, are unusual
in African and indeed in most lake waters;
ORIGIN
TrADLE
1.
Chemical
OF
STRATIFICATION
composition
IN
AN
AFRICAN
of the sur(ace
water
RIFT
71
LAKE
of lakes AZbert and Edward
Lake
Elskcns”
Fcb ‘53
-
Conductivity
( k20, pmho )
p 11
Na (mg/L)
:a
[m$k{
* Station
No.
Talling
Nav
740
9.12
97
9.3
66
Mg (Zg,L)
31.5
HCOa + CO, (meq/L)
7.8
Cl
(mg/L)
32
SOi (mg/L)
25
SO2 (mg/L)
3.4
l%*P
(pg/L)
130
Total P ( ,ug/L )
N0a.N
b-M4
9
Sum of cations (mcq/L)
8.95
Sum of anions ( meq/L ) 9.22
1149
in Lake
Albert,
Lake
Albert
‘60
Feb
‘61
Mar
June
----
‘61
Aug
‘61
735
91
9.8
65
735
9.0
10.8
-
780
9.9
-
730
8.9
9.0
-
31.2
7.30
32.9
27
0.1-0.8
120-170
4.5-7.5
31.2
7.33
33
32
0.09-0.9
200
8.74
8.93
32.1
7.27
33.7
43
0.04-1.1
31.9
7.30
38.5
34
0.33
168
9.09
31.5
7.25
45
o-4-0.9
120-150
133-164
10-33
-
No.
683
in Lake
Edward.
they also occur in other lakes of the western
Rift such as Tanganyika, Kivu, and Edward.
Comparison with the analyses for Lake Edward suggests that the major ionic composition of Lake Albert water is determined by
inflow from the region of the upper lake,
with a slight dilution, mainly due to run-off
from the Ruwenzori mountain mass ( Beauchamp 1956). Beauchamp also believed
that the sulphate content of Lake Albert
water varied considerably in relation to corresponding changes in the surface water of
Llake Edward. Though the published data
are scanty, this view gains some slight support from the apparent variation of sulphatc
shown by the present analyses from Lake
Albert.
Features more peculiar to Lake Albert
a:re a remarkably high concentration
of
phosphate (120-200 pug PO4 P/L) and a
generally low concentration of dissolved silicon (here expressed as silica ), The source
of the phosphate is not established, but hot
springs near the lake and beside the Scmliki
River inflow may be suspected, or possibly
changes in other forms of phosphorus prcsent in Lake Edward water ( cf. Table 1).
The total phosphorus content usually agrees,
within experimental error, with the soluble
p hosphatc-phosphorus,
so that any uptake
by phytoplankton is obscured by the large
l
Tailing
Elskcns*
‘61
720
8.9
10.0
-
8.79
Edward
120-170
13
9.12
Conductivity
corrcctcd
from
J;m ‘54
DCX
‘60 June
‘61
935
8.89
112
9.7
79
880
100
12.5
82
925
9.1
110
12.4
90
44.5
10.1
27
35
2.0
30
23
11.0
11.6
50.5
9.22
35
32
5.7
-
47.8
9.85
38
31
6.5
18
127
24
11.6
11.5
values
1G
10.9
at 25°C.
excess of fret phosphate.
The same is
largely true of the release of phosphate from
the bottom deposits. Concentrations
of
silica are more variable, both with respect
of depth ( discussed later) and to distance
along the axis of the lake. The very low
values often found (Figs. 2-4) differ so
much from those expected from the Semliki
inflow, where 3.8 mg/L was found by
Elskens (van dcr Ben 1959, p. SO), that
some active mechanism for silica removal
must exist in the lake. The lowest values,
often less than 0.5 mg/L, are usually found
at stations most distant from the Semliki inflow, and values above 1 mg/L (including
at least one by Elskens) are from the area
adjacent to that inflow.
The planktonic
diatoms common in the lake must contribute
to silica depletion, but the large quantities
which are probably involved suggest the
occurrence of some non-biological precipitation or polymerization.
Such a process
would be expcctcd to be favored by the
high pH of the water, and possibly also by
the presence of calcium which may be
readily precipitated.
TIIERMAL
STRATIPICATION
In the most detailed accounts of the lake,
by Worthington
( 1929a, 1930), Verbeke
( 1957), and van der Ben ( 1957)) thermal
72
J. F. TALLING
stratification is said to be slight, with windinduced mixing usually extending to all the
depths sampled. However, Fish ( 1952b)
found that an appreciable depletion of dissolved oxygen could develop in the deeper
water. Elsewhere (1956) he suggestedthough without direct evidence-that
a discrete and deoxygenated bottom layer might
originate from the inflow of colder Semliki
River water, and cause the mass mortalities
of Nile Perch ( Lutes nlbertianus ) which arc
occasionally reported. The present work
shows that a discrete bottom layer of colder
water can develop, but an alternative explanation of its origin appears more likely.
Figure 2 shows depth profiles and longitudinal sections of thermal structure in the
lake during November 1960. Thermal discontinuities with depth appear partly in relation to a superficial diurnal heating, and
partly bounding a lowermost layer of water
below 27.1”C. The latter discontinuity
is
most obvious at stations 5, 6, and 8, but the
chemical evidence ( discussed later) shows
the 27.1” isotherm to mark an equivalent
boundary to vertical circulation at stations 0
and 10. Water below 27.l”C is also present
in the shallow southern end of the lake, so
that this isotherm, with others, tilts strongly
along the axis of the lake. Similar tilting
of the isotherms is obvious in the March
and August 1961 data (Figs. 2 and 4)) but
there is little indication of a deep thermal
discontinuity
in March. In August it has
reappeared about the 26.O”C isotherm in
the deeper area adjacent to the southern
shallows, but does not extend to the more
fall
northerly stations. The tempcraturc
about the deep discontinuity is always small,
not exceeding 0.7”C and usually much less.
Three possible explanations of these conditions can be considered.
1) A direct
origin of the lower layer from cold Semliki
River water is improbable, as a perceptible
change with depth of the major ionic constituents was not found at any station. This
applied even to the quantities most precisely
measured such as alkalinity, I+ l%, calcium
plus magnesium, I+ I%, and conductivity, +
3%. The composition of the Semliki inflow
water, however, shows comparatively large
seasonal variations. Beauchamp ( 1956, Fig,
1) recorded changes of conductivity,
from
a station near the lake, between 360 and 810
pmho. 2) The stress of southwest winds on
the lake surface might lead to an upwelling
of deeper and cooler water at the windward
end, in the manner illustrated by Mortimer
( 1952). Although some of the higher isotherms shown here may be tilted in this
way, it can scarcely account for the close
connection between the lower colder stratum and the basin profile. 3) Given this
connection and an intralacustrine
origin,
the bottom layer is most easily seen as a
profile-bound
density current descending
from the shallow end of the lake, where surface heat exchange has produced an area
of slightly colder and denser water. Nocturnal cooling, for example, can be very
pronounced
in shallow African
waters
(Talling 1957a). At 0830 on 8 August 1961
the O-5 m water in the shallow Buhuka Bay
was at a temperature (25.75”C) lower than
any measured above 20 m at the adjacent
station of the offshore transect line. A convective descent of cooler water from such
shallows can therefore be visualized. Since
the largest shallow area is in the southern
end of the basin, a longitudinal
section of
the lake would show the observed asymmetry of thermal stratification.
Other and
smaller areas of shallow water may also
contribute to the effect.
This suggestion can be tested further by
reference to examples of stratification
in
chemical constituents and phytoplankton.
CHEMICAL
STRATIFICATXON
Chemical stratification is shown in measurements of dissolved oxygen, nitrate, silica,
pH, phosphate, total manganese, and total
iron. The first three of these are the most
useful indicators. The pH changes are small
due to the high buffering of the lake, and
phosphate changes are obscured by the
large excess of this ion. The distribution of
total manganese and iron is insufficiently
known; small amounts, such as 15-30 pg/L
of manganese and 45-80 pg/L of iron, were
mcasurcd in the deeper water.
Three types of oxygen-depth distribution
ORIGIN
OF STRATLZ1‘ICATION
/’
IN
AN
,&
B
AFRICAN
(station
RIFT
nos.)
73
LAKE
,/
3
_..-
TEMPERATURE
PR( 3FILES
DEPTH
-I
27
0
Ii
I.5
28 27L
28’
..
29’
27I
28,
29,
zp
27
2,8
- 27
28
29
270
-27
28
--
27
28
tbT-----l
2.3
29
Oc
.
I
.*
\
.i
made during 2427 Novcmbcr
1960, at stations indicated on the map above.
-. FIG. 2. Observations
‘l’cmpcrature
measurements
arc shown on a longitudinal
section and in individual
depth profiles;
chemical and algal data arc given as depth profiles at the stations numbered.
74
J.
17. TALLING
KILOME TRES
7
27
28
27
’
20
’
29
’
;7L;Q
27
..
1
216
1
0
FIG.
3.
Observations
made during
16-18
March
1961, shown
i”
20
as in Figure
40
2.
100
500
ORIGIN
I
I
OF S’IltATWICATION
I
I
I
I
IN
I
I1
AN
AFRICAN
120
L3
;
y
LAKE
I
0
O
RIFT
KILOME
TRE 5
_L
-7
I2
PROFILES
I
30
45
24
25
LY
25
0
26
2r-Tb
2zpy,
25
26
25
~zpzi--
25
-
26
27
26
27
26
d7
. . .
.
1
r
I3
IS
..
"K"
!*
.
30
.*
d
2s
-
2276
PO,-P
I
45
L0
I 0
(I@)
200
7
..
8
9
.a
.
.
J
”
FIG. 4.
i
13
.
Observations made during ‘7-9 August 1961, shown as in Figure 2.
IO
76
J. F. TALLING
were found ( Figs. 2-4). In November 1960
the water below the deep thermal discontinuity was poor in oxygen. Concentrations
increased rapidly across the discontinuity
and values close to saturation (about 7.2
mg/L) were found between 0 and 30 m.
The concentrations in the cooler water below
27.l”C were highest in the shallows
(station 3), less in the adjacent deep water
( station 6)) and lowest in the more distant
northern stations (0 and lo). This sequence
is in agreement with the expected “aging”
of the postulated downflow of cooler water.
In March 1961 the oxygen concentrations
decreased rather continuously with depth at
all stations. The absence of a sharp discontinuity in the profiles accords with the
non-development of the deep thermal discontinuity.
However, the oxygen deficit in
the deeper water columns is still large, implying active consumption at least in the
lower layers.
During August 1961 the oxygen stratification is practically abolished. Though this
condition corresponds with the more nearly
isothermal state of most of the lake, it also
extends to the shallow southern end, where
a bottorn layer of colder water is recorded
from stations along an “underwater valley.”
This layer must therefore be of local and
recent origin, Its spread to the more distant
sampled arcas of the lake floor is apparently prevented by more intense vertical
mixing. As before, a substantial oxygen
deficit exists in the deeper water columns,
and even the surface water is appreciably
undersaturated.
The vertical
distribution
of nitratenitrogen suggests a production in the lower
layers or mud surface, and consumption in
the upper layers. A strong vertical mixing
in August is indicated, when the highest
surface concentrations were recorded.
Similar sites of production and depletion
are likely from the depth profiles for
dissolved silica. In November 1960 the
widespread bottom layer of cooler water
contains high concentrations, sharply distinguishcd
from the low values above.
More gradual vertical changes exist in
March 1961, as in the corresponding tcm-
perature and oxygen profiles; in August
1961 the vertical differences are almost
eliminated. In these features silica enrichment generally follows oxygen depletion,
and a similar hydrological background can
be invoked. However, an additional source
of dissolved silica probably exists in the
Semliki River inflow, as mentioned above
in relation to the higher silica concentrations usually found in the shallow southern
end of the lake. If water from this area
descends to northerly stations, a part of the
deeper silica accumulation there may be rccruitcd from the southern shallows.
PEIYTOPLANKTON
Two diatoms, a Nitzschia sp. (probably
N. bacata Hust. ) and Stephanodiscus astraea
( Ehr. ) Grun. with its variety minutula
(Kiitz.) Grun., normally compose the main
part of the phytoplankton.
Besides the
1960-61 observations, one or both species
were probably also noted in 1952 by Fish
( 1952b) and Ross (1955), and in May 1954
by Dr G. A. Prowse and myself. Bachmann
( 1933) recorded S. astraeu as the dominant
alga of net samples from offshore water
collected in 1928. These diatoms usually
occurred throughout a wide range of depths
(Figs. 2-4), and densities of the Nitxschiu
sp. greater than 1,000 cells/ml were occasionally recorded, as in February and
March 1961. Certain other algae were less
regular components, but sometimes reached
large densities in the uppermost ( O-10 m )
layer; these included the blue-green alga
Anabaena flos-aquae ( Lyngb. ) B&b. in
November 1960 (Fig. 2)) a species of the
flagellate Gymnodinium
in August 1961
(Fig, 4), and two unidentified
flagellates
( one a cryptomonad ) in March 1961. Various algae, and particularly
Anabaenopsis
tanganyikae (G. S. West) Wolosz. et Miller
and Lyngbya limnetica Lemm., arc frequent in shallow or inshore water.
The stratification of the Nitzschia poplation could be clearly traced on the three
main cruises, and is closely related to the
varying thermal stratification.
In November 1960 ( Fig. 2) a sharp drop in cell numbers often occurred at the deep thermal dis-
ORIGIN
OF STRATIFICATION
continuity.
A more gradual decline with
depth was present in March 1961 ( Fig. 3),
when there was no obvious discontinuity in
the deeper water. The situation in August
1961 (Fig. 4) is particularly interesting, as
thlp higher cell numbers were then found
in the shallow southern area ( stations 3 and
10) and in the colder bottom layer of water
at the nearby deeper stations. EIere the
Nitzschia concentration may act as a biological “tag” to the water in the supposed
profile-bound density current. The survival
of apparently healthy cells in this water is
another indication
of its recent surface
or.igin during August, although sedimentation of cells from above cannot be excluded.
DISCUSSION
From this work three distinctive features
of limnological interest emerge. Physically,
there exists a peculiarly mobile and asymmetric form of stratification; chemically, the
high enrichment in phosphate combined
with an often extreme impoverishment
in
silica are remarkable; and algologically, the
phytoplankton
is of a singularly restricted
composition centered on the diatom genera
Nitzschia and Stephanodiscus.
None of
th#ese three conditions seems to be known
from the other large African lakes. However, if the convective theory of the origin
of stratification
in Lake Albert is valid,
similar processes are likely to be widespread in tropical lakes which have small
vertical differences of temperature,
More
surveys of the horizontal variation of thermal stratification
in such lakes would be
valuable. The most detailed yet published
are of Lake Victoria by Fish (1957) and
Newell ( 1960). Here the enormous lake
area and complex shoreline make an interpr’etation more difficult. An annual cycle of
stratification was followed during 1960-61
and will be described elsewhere.
In non-tropical lakes profile-bound
density currents have been occasionally
discussed, particularly in relation to a thermal
(convective) origin under ice ( Hutchinson
1957, p. 476; cf. Mortimer and Ma&e&h
1958) and to a solute-enrichment
origin
during later stratification (\ Hutchinson 1957,
IN
AN
AFRICAN
RIFT
LAKE
77
p. 477). The significance of such solute additions in tropical lakes is conjectural, but
tropical conditions would seem to favor a
thermal origin of density currents. In such
examples as Lake Albert these currents may
possibly come to dominate the thermal subdivision of the lake. A full description of
the annual sequence of thermal stratification in Lake Albert is lacking, but Verbeke
( 1957, Fig. 14) and van der Ben ( 1959, Fig.
9) have published a series of temperatures
at three depths, 1, 20, and 40 m, taken
throughout 1953 at a location probably near
Kasenyi.
Here the annual temperature
range at 2040 m is only 26.6-27.4”C, and
most of the vertical differences of temperature between 1 and 20 m are probably controlled by diurnal heating and cooling.
These results, and the present work, suggest
that the lake possibly approaches the condition of a constant temperature bath more
closely than any other large tropical lake so
far investigated in sufficient detail. This
equable background,
and the shallows
placed asymmetrically in the lake, may allow
the convective consequences of local cooling to be expressed in an unusually striking
way.
REXERENCE3
H. 1933. Phytoplankton
von Victoria Nyanza-, Albert Nyanza- und Kiogasee.
Bcr. Schweiz. Bot. Ges., 42: 705-717.
BEAUCHAMP,
R. S. A. 1956. The electrical conductivity of the headwaters of the White Nile,
Nature, 178: 616-619.
BEN,
D. VAN DER.
1959. La &g&ation
dcs rives
dcs lacs Kivu, gdouard
et Albert.
Explor.
Hydrobiol.
des Lacs Kivu, adouard
et Albcrt
(1952-54),
Vol.
4 (l),
191 pp.
Bruxcllcs.
DOIIN, W. G. VAN.
1956. Large volume water
samplers.
Trans.
Amer.
Geophys.
Union,
BACHMANN,
37: 682-684.
FISH,
-.
-.
G. R. 1952a. Appendix
A (5).
Zn East
African
l?isheries Research Organization
Annual Report for 1951, p. 33. East African
High Commission.
Nairobi.
1952b.
Appendix
A, Chemical
Analyses, Lake Albert.
Zn East African Fisheries
Rcscarch
Organization
Annual
Report
for
1952, p. 27.
1954. Appendix
A2, Lake Albert.
Zn
East African Fisheries Research Organization
Annual Report for 1953, p. 27.
J.
-,
I?.
1956. Some aspects of the respiration
of six species of fish from Uganda.
J. Exp.
Biol., 33: 186-195.
-.
1957. A seiche movcmcnt and its cffcct
on the hydrology
of Lake Victoria.
Fish.
Publ., Lond., 10, 68 pp.
HERON, J., AND F. J, MACKERETII.
1955. The
estimation of calcium and magncsiui~1 in nattural waters with particular
reference to those
of low alkalinity.
Mitt. Intern. Vcr. Limnol.,
5, 7 PP.
IIUItST, H. E. 1925. The Lake Plateau Basin of
the Nile.
Ministry
of Public Works, Egypt.
Physical
Dept. Paper No. 21, Govt. Press,
Cairo. (Section 13: Salt content of the water
of the Lake Plateau, pp. 67-73. )
HUTCHINSON, G. E. 1957. A treatise on limnology. Vol. I. Geography,
physics and chcmistry.
New York. pp. xiv + 1015.
LUND, J. W. G., C. KIPLING, AND E. D. LE CHEN.
1958. The inverted
microscope
method of
estimating
algal numbers and the statistical
basis of estimations by counting.
Hydrobiol.,
11: 143-170.
MEEL, L. VAN. 1954. Lc Phytoplancton.
ExLac Tanganika
1946-47,
plor. Hydrobiol.
Vol. 4( 1) : 681 pp. Bruxelles.
MORTIMER, C. II.
1952. Water movements
in
lakes during summer stratification;
evidence
from the distribution
of temperature
in Windermere.
Phil. Trans. B, 236: 355-304.
AND F. J. MACKERETEI. 1958. Convectidn and its conscqucnccs in ice-covered lakes.
Vcr. Intern. Ver. Limnol., 13: 923-932.
-,
AND W. H. MOORE.
1953. The use of
thermistors for the measurcmcnt of lake temperatures, Mitt. Intern, Ver. Limnol., 2: l-42.
TALLING
MULLIN, J. B., AND J. P. RILEY.
1955. The colorimetric determination
of silicate with special
rcfcrencc to sea and natural waters.
Analyt.
Chim. Actn, 12: 162-176.
NEWELL, B. S. 1960. The hydrology
of Lake
Victoria.
Hydrobiol.,
15 : 363-383.
Ross, R. 1955. The algae of the East African
Great Lakes.
Vcr. Intern. Vcr. Limnol., 12:
320-326.
RUTTNER, F. 1931, Hydrographische
und hydrochcmische Bcobachtungcn
auf Java, Sumatra und Bali.
Arch. Hydrobiol.
(Suppl. ), 8:
197-454.
-.
1937. Stabilitgt
und Umschichtung
in
tropischen und tempericrten
Seen. Arch. Hydrobiol.
( Suppl. ), 15: 178-186.
TALLING, J. F. 1957a. Diurnal changes of stratification
and photosynthesis
in some tropical
African
waters.
Proc. Roy. Sot. B, 14’7:
-.
57-83.
1957b.
The longitudinal
succession of
water characteristics
in the White Nile.
Hydrobiol., 11: 73-89.
VERBEKE, J. 1957. Rechcrchcs
&cologiques
sur
la faune dcs grands lacs de l’est du Congo
Beige.
Explor. Hydrobiol.
Lacs Kivu, Bdouard et Albert ( 1952-54),
Vol. 3( 1) : 177 pp.
Bruxclles.
WORTIXINGTON, E. B. 1929a. A report of the
fishery
survey of lakes Albert
and Kioga.
H.M.S.O. London. 136 pp.
1929b.
The lift
of Lake Albert
and
-.
Lake Kioga. Gcograph. J., 74: 109-132.
1930. Observations on the temperature,
-.
hydrogen-ion
concentration,
and other physical
conditions of the Victoria and Albert Nyanzas.
Int. Rev. Hyclrobiol.,
24: 328-357.
-,
AND L. C. BEADLE. 1933. Thcrmoclines
Nature, 129 : 55-56.
in tropical lakes.