THE BIONOMICS O F SOME EAST AFRICAN SWAMPS
136
Scientific results of the Cambridge Expedition to tho East African Lakes,
1 9 3 0 - 1.4. Observations on the bionomics of some East African Swamps.
By L. C. BEADLE. (Communicated by Dr. E. B. WORTHINQTON, F.L.S.)
(With 6 Text-figures)
INTRODUCTION.
Tho bionomics of a tropical swamp were investigated in some detail in the
Paraguayan Chaco of South America in 1926-7 (Cartor and Beadle, I, 11,
and 111). Certain chemical and physical conditions of the water were examined
txq being possible limiting factors in the distribution of the fauna.
I n comparing the swamps themselves with certain open and unshadod rainpools i t was found that the latter could support a t all depths a large number
of planktonic organisms, while the swamps were practically devoid of phytoFlankton, and the zooplankton wm by comparison scarce, and was living
only in the surface-layer of water. I n a study of the chemical and physical
conditions of these two environments this difference could only be attributed
to the much lower oxygen content of the swamp water. During the hot months
as little m 0 . 2 4 3 c.cm. of oxygen per litre, and very often none at all, was
found in samples taken from its near the surface of the swamp as possible,
while a t a depth of about 4 inches none was rver detected during this pcriod.
This condition was not only responsible for the scnrcity of the zooplankton,
but was also reflected in a number of adaptations to aerial respiration among
the fauna. No satisfactory explanation was found for the lack of phytoplankton in the swamp water.
The conclusions drawn from this work were that the condition of low oxygen
content of the water is probably typical of stagnant tropical watem and that
the development of the air-breathing habit in fishes, which must also have
taken place during the evolution of the terrestrial vertebrates, was a result
of this condition and did not, M generally supposed, originate from the habit
of migrating out of the water, which a t least some of these fishes undoubtedly do.
It was obviously of importance that further evidence should be obtained
from similar waters in the tropics of another continent. An opportunity
for this was offered in 1930-1 during the course of the ' Cambridge Expedition
to the East African h k e s ' (Idby Dr. E. B. Worthington).
Thc main objcct of this expedition was a biological survey of certain largo
lakes, and this work dcmandcd most of the available time and necessitated
moving about from place to place. It was, therefore, not possiblc to make
a continuous and thorough examination of any one swamp. The few scattered
observations recorded in this paper were made in three different places in East
136
MR. L. C. BEADLE ON THE
Africa, one in RwampR bordering TAakeNaivasha in Kenya, and two in swamps
in the Kazinga Channel which connects Lakes Edward and George in Uganda.
The value of a comparison between these three places is enhanced by the fact
t,hitt the first (Lake Naivasha), though in gencral a swamp of similar appearance
to thc other two and situated neiir the Equator, is, as the meteorological
ob.wrvations clearly Rhow, not subjected to t d y tropical conditions, being
a t an altitude of 6,200 fect. The two Uganda swamps are 3000 feet above
sea-lcvcl, tind the climate is therefore more truly tropical, though the temperatures were not as high aa those of the Ptbraguayttn Chaco (altitude under
100 fcct).
A general idea of the nature and richness of the fauna and flora of these
waters will also bo given. It was, however, impossible, owing to the ciitsculty
of collecting with a hand-net in water in which dcnso vegetation ww growing,
to get any but t i rough idea of the relative density of the plankton in different
plnccs. Rut the differences were often so striking that the roughest collecting
methods were sutlicicnt to show them.
More detailed dewriptiona of these lakes, together with complete records
of the observations, w i l l be published in mother paper, p. 157 of this volume.
Only those points which are relevant to the present problem will be
recorded in this paper.
METHODS.
The following determinations were made :(1) Temperature.
(2) Alkalinity.
(3) Hydrogen-ion concentration.
(4) Phosphate content.
( 5 ) Oxygen content.
(6) Amount of Iodine absorbed.
The methods used, except for no. 6, were identical with those employed by
Carter and Beadle (I, p. 217).
The amount of iodine absorlmd l)y a given volume of water-sample was not
determined irr the Paraguayan Chaco. It wa3 done on the U p n d a hike-waters
as an indication of the amount of ~ulphide(including free H,S) present. The
method used was a modification of that described by Thresh and Beale (1925,
P. 313). The quantity of iodine fibsorbed from a standard RolUtioil by a known
volume of the acidified Rample is equivalent to thc amount of sulphidc present.
Exce~sof N/100 iodine was tulded to 100 c.cm. of sample and the residual
(unabsorbed) iodine was estimated by titrcition with standard sodium thiosulphrite, using starch as an indicator. During the course of the work it became
evident thnt, since the Winkler method of oxygcn dcterminrition entails the
cstinuition of iodine which is produced in an amount equivalent to the oxygen
content of the sample, the figure obtained by this method would be subject
to error if the watcr contsincd a significant amount of sulpliide, a condition
BIONOMICS OF SOME EAST AFRICAN SWAMPS
137
which might be expected in stagnant swamp waters. The water might in
reality be richer in oxygen than would be supposed from the Winkler determination.
This complication was, unfortunately, not realized until the end of the
expedition, and only one swamp was examined with this in view. Alskrberg
(1926) has developed a modification of the Winkler method to ohviatc this
source of error, which I did not discover until after the expedition had returned.
It was, however, found that the water of the swamp at Kitoma contained an
amount of sulphide equivalent to 0-56 c.em. of oxygen per litre. This waa a
swamp in which decay appeared to be as active &fi in any swamp so far investigated, and, owing to the volcanic nature of the surrounding country where
sulphur springs are common, it is reasonable to assume that the sulphide
content of its water was probably higher than that of the Chaco swamp waters.
It is interesting to note that Ruttner, working on lakes in .Java, compand
figures of oxygen content obtained by the Winkler method with and without.
Alaterberg's modification. Using water from Ranu Lamongan,which is a volcanic
crater lake, he found that the greatest error incurred by using the unmodified
method amounted to 1.00 c.cm. of oxygen per litre (Kuttner, 1931, p. 206).
T t therefore seems that the errors of the oxygen estimations by the Winkler
method in these and similar swamps will always be less than 1.0 c.cm. of oxygen
per litre.
Tn the following results (and in the results of the work on Paraguayan swamps)
some correction should be applied to the oxygen estimation. Thus a zero
estimation should be regarded as one in which the oxygen present is certainly
less than 1.00 c.em. per litre, and in all probability does not exceed 0.56 c.c~n.
per litre as was found for the Kitoma swamp.
LAKENAIVASHA
(0"45' S., 36" 24' E.). (Pl. 5. Map 4.)
This is one of the Kenya Rift Valley lakes. It is situated, as h a been said,
a t an altitude of 6,200 feet. The work described here was done in h'ovember 1930,
February, March, and Map 1931. Meteorological observations taken on the
shore in November 1930 showed an average maximum shade temperature
of 25-5"C., and an average minimum of 9.6"C. I n March 1931 the average
maximum was 26.9" C. and the average minimum 11.7" C. Thc latter figures
represent the temperature range of the hot season.
In 1930-1 most of the east and north-east shores of the lake were fringed
with Papyrus swamps of varying width. The point a t which this work
was done was opposite Crescent Island on the east shore, where the swamp
was from 70 to 80 yards wide. A diagrammat.ic cross-section of the swamp
a t this point, showing thc positions a t which ob.wrvstions were made, is given
in text-fig. 1. At A the water was about ti inches deep, it was nearly covered
with water-lilies and Hydroeotyle, and water-weeds such as Potnmogetm, h'aias,
and Utriculuria were to be found under the surface. This area merged into
a belt of thick Papyrus about 20 yards wide which effectually shaded the water.
139
THE BIONOMICS OF SOXE EAST AFRICAN SWAMPS
Here floating and submerged vegetation ww scarce. Position B waa selected
inside the Papyrus where the water was about 4 feet deep. Beyond this belt
was position C in a zone of water-lilies about 20 yards wide which complctcly
covered the surface of thewater, which was from 5 to 6feet deep. Here also
the submerged vegetation was scarce. At the outer edge of this zone was
the open water of the lake, the bottom of which, when the depth did not exceed
about 15 fect, was carpeted with a dense growth of Ceratophyllum and Potumogeton. A t position D in this open region the water waa about 12 feet in
depth.
Temperature measurements.
The diurnal maximum and minimum temperatures a t the surface and bottom
were taken over a periodof 5 days in November 1931 a t positions B and Dthat is to say, inside the Papyrus zone and outside the swamp in the open water.
The figures obtained each day were very similar, and Teblo I gives those for
one day, which are typical. It will be seen, firstly, that the water reaches
T A B LI.~
Maximum.
Position B
....................
{ Surface
Bottom (4 ft.) .............
Position D
................... 22.8
{ Surface
llottoni (12 ft.). ............ 20.0
Shade Tomperaturoa, 20. xi. 3 0 . .
........
0
250C.
204
27.1
Minimum.
0
17.2
19.0
18.2
18.5
13.0
a higher temperature inside the swamp than outside, probably because the
former position is more shaded from the disturbing effects of winds, and,
secondly, that in both places there is complete reversal of the temperature
gradient at night. At first sight it may seem surprising that a reversed gradient
of 144°C. should occur in water only 4 ft. deep. But experiments done in
Cambridge after the return of the expedition showed that, when the surface
of water in a glass cylinder is cooled by ice, a reversed gradient of moro than
2" C. can be produced in a column of water 18" high.
Alkalinity,pH, and phosphate eontent.
Measurements of these characteristics of the water a t three different dates
ere recorded in Table 11. The small increase of alkalinity at D between
November end Fcbruary, and the decrease in May are probably due to periods
of lower and higher rainfall respectively. The decrease of pH from that of the
main lake water to that of the edge of tho swamp nearest the shore illustrates
the effect of t h e increased intensity of decaying processes inside the swamp,
and of greater photosynthesis due to wbmerged vegetation in the open water.
LINN. JOUl$N.-ZOOLOQY,
VOL. XXXVIII
11
140
MB. L. 0 . BEADLE ON THE
The absence of any phosphato measurable by tho %nighs method, except
within a few yards of the shore, wm a condition which wae found in November
1930 and in February and March 1931, and was verified a number of times.
The reason for this condition will be discusacd in a paper dealing with the lake
itself. In the present connection i t will be sufficient to point out that in January
TDLE 11.
Position A , .
22. xi. 30., 10 a.m.
pH.
PI06.
alk.
0.0026N. 8.3 1.26 mg. p. 1.
....
c ......
-
-
-
n ......
0.0028
9.4
nil
B.....
.
-
25. ii. 31., 1 p.m.
alk. pH. P,O,.
-
4. v. 31.. 10a.m.
alk. pH. P,O,.
- 7.5 0.23
7.4
1.075
8.0
nil
-
7.8
8.1
nil
-
8-2 0.10
0.003 9.3
nil
0.0029 9.0
0.09
0.11
and February, when the growth of the submerged vegchtion waa a t a maximum,
and the phosphate content of the outer part of the swamp and of the main
lakc was not detectable, there was always a considerable amount in the shallow
water a t the edge. In May, after a rainy period had occurred and the submerged
vegetation had to a great extent d i d out and decayed, some phosphate was to
be found in all positions, but lesR than that previously present a t the edge.
This WUY probably the result not only of weed decay in the lake, but also of
phosphate being washed in from the surrounding land by rain (cf. Carter and
Beadle, I, p- 244).
Oxygen Cantent.
It is obvious from Table I11 that oxygen was abundant in the water at all
positions. Outside the swamp at D the oxygen content in the surfscc-water
a t mid-day reached 94 per cent. saturation. Inside the swamp it was considerably lower (43 per cent. saturation a t B), but still very high compared
TABLEI11 (6. iii. 31, midday).
0, C.C.
....... depth 1'
B ......... 1'
Poeition A
c
D
.......
...... i
,*
1'
tt
18'
.,)
1'
7feet
por litre.
Total depth of
Water.
2.86
6 ins.
2.61
4 feet.
2.88
1.56
5-6 feet.
8.60
12 feet.
4.95
141
BIONOMICS OF SOME EAST AFaICAN SWAMPS
with the figures obtained from the Paraguayan swamps (Carter and Beadle, I,
p. 246), and, as the estimation a t position C shows, the water at a depth of
IS inches was still well oxygenated. It appears that in these swamps oxygen
is not a biologically limiting factor.
Corrections should be applied to Table 111 to allow for the error due to the
presence of sulphide (as described on p. 136). The oxygen content is probably
slightly higher than is indicated in the Table.
Remarks on Fauna and Flora.
Pond-net collections were made at positions A, C, and D in March 1931.
The plankton a t A was extremely rich and included the following ;Crustacea (Cyclops,Diaptmus, Cladocera, and Ostracods).
Oligochaeta (Naididae).
Hirudinea.
Platyhelminthes (Mesostomurn and Microstomum).
Rotifera (colonial and solitary).
Nematoda.
Hydrachnida.
Insecta (Ephemorid larvae).
Protozoa ( Volvox).
Algae (Chlorophyceae).
A t C fewer species were found :Crustacea (mostly Cladocera, a few Cyclops, D h p t m u s , and Ostracods).
Insecta (Ephemerid larvae).
Diatomaceae (Melasira sp. abundant).
The plankton a t D was less abundant than a t either A or C, and included :Crustacea (Cladoccra, Cyclops, Diaptmius, and Ostracoda).
Hydrachnida.
Rotifera (solitary).
Diatomaceae (Melosira sp.).
Algae (Cyanophyceae).
It was quite evident that the swamp supports a greater density of aquatic
organisms than does the main lake. This is especially true of the region near
the shore, where the great abundance of plankton may perhaps be ascribed
in part to the higher content of phosphate and organic food-stuffs in the water.
The lake contains no indigenous fish except the small Haplochilichthys antinorii,
which was only found in the open waters. But Tilupia n@ra, which has been
recently introduced and is now exceedingly common, ma always to be found
in great numbers in the swamps. This fish haa a truly aquatic respiration.
KATUNGURU
(0" 10' S., 30" 0' E., 3,000 f t . above sea-level). (PI. 4. Map 2.)
A diagram of the swamps at the mouth of the Chambura River, which flows
into the Kazinga Channel $ mile cast of Katunguru in Uganda, is given in
11*
142
MR. L. U . BEADLE ON "HE
text-fig. 2. The poRitions selocted for observation are .marked as E, F, G,
and H.
E iu in one of a wries of opcn pools about 1foot deep at the edgc of the swamp,
which are well shaded by the trees on the bank. F is about 10 yards inside
the swamp, which is composed solely of P q y r u s . The water is almost entirely
coverod with a carpet of dew1 and dccaying Papyrus stems, nearly compitct
enough to support the weight of a man. I n a fcw pl;tcm, however, the water
is sufficiently uncovered to enttble sampling bottle^ to be inserted. A photograph of a typical I'npyru~~
swamp of this neighbourhood is given as text-fig. 3.
TEXT-FIG.
2.
Both the positions are well protected from winds by the stirrounding vcgctation.
Position G is situnted in the open coursc of the river inside the swamp, and
H in the open water of the Channcl. The Kazinge Channel, in spite of its
appearance, does not contain flowing water. It merely connects Lakes Edward
and George, which are a t the same level.
Observations on them swamps were m d o in May and June 1031. Readings
of shnde temperature made on tho shore8 of tho channel over a period of 1 0 chys
in June and July 1931 show a n average maximum of 30°C. rmtl an averago
minimum of 18.8" C.
143
BIOXOMICS OF SOME EAST AFRICAN SWAMPS
Temperature, p H , and Oxygen Content.
From Table IV i t can be seen that in tho early afternoon in positions E and F
tlicrc was a liisge temperature gradient desccnding from the surface, and this
WiLs more markcd amongst the Z'apyrus a t F than in the pool E. In the early
morning t,his gradient disappeared, the temperature being almost identical
a t top ttnd bottom. A very slight rcversal o[ the gradient occurred on the
morning of 31.v. 31, which was cooler than thc preceding morning, but this
was small in comparison with the reversal found in the Naivasha swamps.
Tho pH measurements indicate that, owing no doubt to a greater intensity
of processes of decay, the water of the Papyrus region (F) was more acid
than that of thc pool E. The figurcs showing a reduction of the pH a t E from
6.2 to 6.1 between 2.15 p.m. and 7.0 a.m. cannot be taken as trustworthy,
sinco tho samplcs, being collected a t different times, could not be compared
together after addition of the indicator. Photosynthesis, which a diurnal
TABLEIV.
Date.
Time.
29. v. 31.
2.30 p.m.
Temperatures, O C.
r--h
>undersurface, pH,
Air,
E.
.'!I
0,1' c.om. p. 1. surfme.
Akahity.
F. E. F.
E.
F.
shade. Top. l f t . Top. l f t . E.
29.8 26.4 22.6 26.7 21.4
0.00075 0.00075
30. v. 31.
7.0 a.m.
20.1
20.3 20.3 20.6 20.7 trace*
2.16 p.m.
29.8
27.5 23.5 26.7 21.6
,,
trace
7.0 a.m.
18.9
19.8 20.1 20.5 20.6
,,
,,
31. v. 31.
-
-
nil
-
-
-- 6.2 6.9
6.1 6.9
-
-
lowering of pH would indicate, must, howover, have been occurring in tho pool
E because a small quantity of plant-life was present there (see remarks on
fauna and flora).
I n view of the absence of photosynthesis in the Papyua region, it is not
surprising that its watcr contained no detectable oxygen within an inch
of the surface even in the afternoon, but that the oxygen content of the pool
watm a t E, in spitc of the photosynthesis occurring there, should also be undetectable certainly would not be expected. It seems that, owing to the absence
of more thau a very slight temperature gradient reversal and to the thorough
protection from winds afforded by the surrounding vegetation, the extra
oxygen produced by photosynthesis must have been removed as a result of the
high intensity of organic decomposition. This point will be dealt with more
fully in the discussion.
* The
per litre.
minimum amount of 0,measurable by the Winkler method is about 0.02 cxm.
la4
Mn. L. C. BEADLE ON THE
Alka,linity and phosphate &nt.
The alkalinity of the Rwamp wiltcr at E and F (0.00075 N.) and that of the
Chambura River a t G (040094 N.) wits considerably lowor than that of the
main channel at H (0.00196 N.).
The phosphatc content of tho two swamp positions (JC, 0-21 ; F,0.18 ;
G, 0.205 mg. per litre) was higkor than that of the channel watcr (H, 0.11 mg.
per lit're).
Both these determinations (done on thc same day, 27. v. 31) again indicato
TEXT-FIG.
3.
that rain-water running in from the surrounding land, to which the low alkalinity
a t E, F, and G is presumably due, brings a relativcly large amount of phosphate
with it.
Remarks on Fauna and P h a .
The phytoplankton of the open channel was so abundant that thc watcr
was quite green. A blue-green alga (d-licrocystis jlomzquze) was mainly
BIONOMICS OF SOME EAST AFRICAN SWAMPS
145
responsible for this coloration. The phytoplankton of the pool E was by
comparison very scarco, but a few filamentous a l g a and diatoms were found.
The swamp water a t F wm not entirely devoid of plant-life, but contained
only a very small number of diatoms.
Owing to the difficulties of collecting in the swamp, i t was impossible to make
a comparison of the densities of the zooplankton in the channel with that
in the swamp. That of the pool E, howcver, which included Cladocera, copepods,
and ephcmcrid l a ~ a o was
, very much more abundant than that of the swamp
a t F, where only a very few Cladoccra and copepods could be found.
The many fish which inhabited the open channel will be recorded elsewhere.
Observations were insufficient to establish definitely which species were to be
found in the swamps and which were incapable of living in them, but no fish
were ever seen a t positions E and F, except the air-breathing Spirobranchm
sp., which was very common, especially at E. Amongst a number of fish
with purely aquatic respiration living in the channol WM Tilcvpia niloticu,
which was never found inside the swamp. In contrast, i t will be remembered
that Tila/pia nigra was a common inhabitant of the Naivasha swamp. Clam'm
lazera and Protopkrw aethiopicw, both air-breathing fish, wero very common
in the channel water, where they seemed to live in preference to the swamp.
The oligochaete Alma emini * was found in great numbers amongst the
rotting Papyrus stems floating on the swamp surfaco and in the mud a t the edge
of the swamp. It presented a most striking similarity both in iB appearance
and in its habits to tho Drilocriua sp. found in the Paraguayan swamps (Carter
and Beadle, 111, pp. 380-5). Without exception, all the peculiarities of behaviour
from which it was concluded that the latter was able to breathe atmospheric
oxygen (exposure of a dorsal groove a t the surfaco of the water above the
mud &c.) were also observed in the behaviour of Alma emini.
KITOMA.
About a mile from the Lake George mouth of the Kazinga Channel on the
south side is a narrow arm of water, about thrco-quarters of a mile long and
100 yards wide, completely choked with Papyrw swamp oxcept a t ih mouth.
A plan of this area is given in text-fig. 4. The positions selected for observation (J,K, and L) were situated 20 yards outside the swamp and 10 yards
and half a milo inside the swamp respectively. This work was done a t the
beginning of July 1931.
s t o m a is within eight miles of Katunguru and a t the same altitude. Both,
therefore, are subjected to the Rame temperature conditions.
The swamp was composed entirely of dense Papyrms, and the water of the
less dense regions a t the outer edge a t K was covered by the floating cabbagelike Pistia. Position L WM in an open pool, well shaded from both sunlight
I am indebted to Lt.-Col. J. Stephenson, F.R.S.,for the identification of thie apeciee.
146
MH. L. C . BEADLE ON THE
and winds by Papyrus, and by trees on tho bank. The surface W ~ L Hpartly
covcrecl by Lemnn. The water here was quita clear, while that of the channel
was green in colour due to blue-green algae.
Alkalinity, p H , phosphate, o x y p n , and iorline absorption.
These are recorded in Table V. Ternperature mcasuremenk were not made.
The much grrakr alkalinity a t L can perhaps be explained by the fact t,hat
there are at least two very ulkttlinc crater lakes less than two miles inland
TPJXT-FIQ.
4.
KAZINGACHANNE 1
from this swamp (L. Ragusa 0.23N. and L. MaHeche 0.71 N. alkalinity), and
t,hc existence of these lakes suggests that the water of this swamp may
gain soda from some twbtarrctnectn source.
I n Hpite of iB higher alkalinity the water a t L was considcrably more acid
(lower pH) than t h a t at J and K. ThiR shows how great must be the intensity
of decay in the swamp and of photosynthesis in the channel. Thc phosphate
content of thc swtt~~ip,
howevcr, showod only a Amall increme over that of the
channel.
The differences between tho oxygen content of the water a t the three positions
was very marked. Owing to the great intensity of photosynthesis at J the water
a t mid-day contained oxygen to the extent of 154 per cent. saturation, while
141
BtOKOMICS OF SOME EAST AlrHICAS SWAXPS
a t L the water was devoid of a measurable amount of oxygen. The relatively
high oxygen content a t K w&s no doubt duc partly to a certain degree of photosynthesis, and partly to invasion of water from J.
The measurements of iodine absorbed indicate that the sulphide contcnt
inside was greater than that outside the swamp. I n the present connection,
however, the main significance of this is that, though the estimation by the
Winkler method showed no oxygen a t L, there is no direct evidence against
the existence of any amount up to 0.66 c.cm. of oxygen per litre, which is the
equivalent of 1 c.cm. of N/lW iodine per 100 c.cm., and which could not,
therefore, have been detected by the Winkler method.
TABLEV.
c.crn. N/100 I.
Position.
Alkalinity.
0,c.cm. p. 1. Abeorbed by
100 c.cm.
Mg.p. 1.
’li’
2” under surface.
0.00207N
9.2
8.6
0.388
0.15
K...
0.00207
8.6
6.0
0.495
-
L
0.00716
6.7
nil
1.000
0.19
J
............
.........
...........
It is unfortunate that more work was not done on the quation of sulphide
content of swamp waters so that generalizations could be made more safely.
I n this case, however, even if there was as much as 0.56 c.cm. per litre (there
may have been less) in the surface-watcr a t L, the oxygen content would still
fall a long way below that of the Naivasha swamps, and would be of the same
order as that found in the swamps of the Paraguayan Chaco. It would,
therefore, perhaps be safe to conclude that, though the oxygen content maybo
higher than was originally thought, it is still a biologically limiting factor
in theso two Uganda swamps and in those of the Chaco.
Remarks 012 fauna; andjbra.
The plankton of tho channel was, as at Katunguru, extremely rich, especially
in blue-green algae. That of the swamp a t L was entirely devoid of phytoplankton, and appeared to consist only of a few Cladocera and copepods,
which were certainly scarcer than in the Katunguru swamp a t F.
The oligochaete Alma eniini was found in the mud a t the swamp edge.
DIscixsIox.
From a faunistic point of view the three tropical swamps described in this
paper present two quite distinct types. The first (L. Naivasha in Kenya
a t an altitude of 6,200 feet) supports a large and vaned phyto- and zooplankton,
the second (Katunguru and Kitoma in Uganda, altitude 3,000 feet), to which
type must also belong the swamps of the Paraguayan Chaco in S. America
148
Mn. L. c. BEADLE ON THE
(altitude less than 100 feet), iR relatively very poor in plankton of every kind,
both in density andin number of speciq. So far as the zooplankton is conccrnud,
them appears to be no limiting factor which Reparates the two typcs other than
the oxygen content of the water. This in the s w a mp a t the higher altitudc
is abundant (e.g. 1.56-5-0 c.cm. per litre a t Neivasha), while in those a t the
lower altitude it is scarce even in the surface-waters (nil-0.56 c.cm. per litre),
and falls below thc minimum tension which most aerobic aquatic aninici1.r
are able to survive under experimental conditions (Carter and Beadle, 1,
pp. 247-8).
The following factors must be involved in influencing the oxygen content
of such waters :I n c r w i n g the Oxygen.
Diffusion from tho air.
1 2. Photosynthesie.
(1.
Day.. . $
i
i1. Diffusion from tho air.
2. Lessening or reversal of temporaturn
Night
.
1i
\
gradiont end thus more oxygen
gained by convection.
Decreasing the Oxygen.
1. Absenco of wind inking.
2. Tompenrture grdiont acting orgsinRt
convection.
3. Docay end respiration (increased by
high temperaturo).
1. Absence of wind mixirig.
3. Non-rovorstrl of temporaturo g r d ent.
3. PhotosynLhesia stopped.
4. Docay and mpiration (but docreeeod by lower temporaturo).
I n the swamps at thc lower altitude i t appears that the environment is
dominated by those factor8 which tend to decrease the oxygen content. At
highor altitudes this is not the cam, and oxygen is abundant in the waters.
The remon for the difference in oxygen content between the two t p of
Rwamp is probably to be found in the differencu of temperature conditions,
which is a. necessary consequence of thc different altitudes a t which the swamps
lie. The average temperatures of air and swamp water in the Chaco, Katunguru, and Naivasha have been put together in Table VI.
So far as sir temperature is concerned, Katunguru is intarmediate betweun
the hotter Chaco and the cooler Naivaeha. These temperature differences would,
in the first place, result in different ratas of bacterial decomposition in tho
waters of different altitudes. It can be found from Table V I that the average
top and bottom temperatures of the Chaco water are greater than those of the
Naivasha water by 5"-6"C. Amuming the temperature coefficient (Ql0) of
bacterial decomposition to be in the region of 2.0, it could be said that the retc
of decomposition in waters a t the lower altitude of the Chaco is about 1.5 times
as great as the rate of decomposition occurring in the Naivasha waters a t u
. higher altitude. It would thus be reasonable to expect that in this way the
higher temperature of the lower altitude is responsiblo for the lower oxygen
content of the water as compared with waters a t a higher altitude where they
are subjected to lower temperatures.
scarce.
abundant.
Naivasha 4feet
(8,200 feet).
scarco.
E(etunguru 1 foot
(3.000 feet).
( <100 feet).
ChaCO
Paraguayan 6 ins.
Swamp.
*-f-
Air temperatures, O C .
-
A.
Water temperatures, "C.
Nov. 1930.
5days.
2 days,
May 1931.
11.2
15.9
9.6
25.5
12.0
18.8
23.0
30-0
15 days, 35.0
Nov. 1926.
20.2
19.5
20.9
18.1
23.2
22.0
23.0
28.0
20.05
22.0
27.0
35.0
2.3
4.0
7.0
- 1.4
-0.15
0
z
E
0
2
rn
z
q
c
&cj
k!
E
2
s
- l S
8
Differenre.
Top.
Bottom.
Period of
Differences.
,- ---A,-'Top-bottoni.
A Topbottom.
min.
mas.
Depth. Zooplankton. observation. av. max. av. min. av. max.-av. min. av. max. av. min. av. max. av. mm.
-
TABLEVI.
180
MK. L. C. BEADLE O N TIIE!
It is also clear from Tablo V I that the differencebetween the average maxirnrrm
ant1 civeragc minimum air temperatures in the Chaco (I%O"C., daily rnoan
temp. 29"C . ) , and at K a t u n y r u (11.2" C., daily mean temp. 24.4" C.) LS less
t l i t t r i that i b t NaiviLsha (15.9" C., daily mean temp. 18" C.). This would mean
that thc suI.fucc-temperatl1l.uof tho water at Naivasha would bo lowered to
D greater extent at night, and reversal of the tcmperaturo gradient w o ~ l t l
themby be morc liktdy to occur then in the emo of thc Chaco or Katuriguru.
That this is the actual state of affairsis shown clearly by thc differences betwwn
the surfacc and hottom minima. The revcrae gradient, which is very marked
in the h'aivasha swamps, does not occur in tho Chaco water8 except under
un~isualclimatic conditions in rare periods of cooler weather. A ruversal
of temperature gradient a t night would rewlt in oxygcnation of the water
from tho air by convection currents which could not h a p p n if, w in tho case
of the Chnco swamps, theru is no tumperature gradient reversal.
-
TEXT-FIG.
6.
0
Chaco
1
Y
u
QI
L
E 2
Narvasha
I
10
20
30
Temperature
40 Oc
There is probably a second reamn for thc teinpcraturc gradient reversal
at the higher and for none a t thc lower altitucte. The temperature gradient
a t mid-day in the water of the latter is very much steeper than that of the
former. This is shown in text-fig. 5, in which the surface and bottom maximum
tempcratures in the Chaco and a t Naivasha are plotted against depth. What
i N the cxplanation of the occurrence of a, much steeper mid-day tcmpcratrtrc
gradient in warmer waters a t a lower altitude than in cooler waters at a higher
altitude ? The answer may be found in the fact that, although the maximum
temperatures of the surface-waters in the three phceR (Table VI) are widely
divergent, the differences between the maximum temperatures at the bottom
are smaller, and the bottom minimum tcmperatures approach one another
still more closely (22-0, 20.2, and 19'6C.). It appears, in fact, as though
BIONOMICY OF SOME EAST AFRICAN SWAMPS
151
the bottom mud werc acting to a certain extent a9 a thermostat in rostraining
the temperature changes of the bottom watcr, in spite of very different degrees
of surface-heating during the day a t the M e r e n t altitudes. I n other words,
the surface of the water in the hotter climate is warmed up to a greater extent
than is that in the cooler climate, the temperature of the bottom water, however,
does not rise proportionately.*
The result is a much steeper tempcraturc gradient a t mid-day in the warmer
place a t the lower altitude. This being so, it is evident that, even with the
same range of air temperaturc, the surface and bottom temperatures in the
cooler and higher placo would require less time to become equalized a t night,
and thus to cause a temperature gradient reversal, than would be the case
in the warmer and lower place.
An obvious criticism of the conclusions which have been drawn from thc
figures given in Table VI is that the total depth of the water is different in
each cam. I n the water of the Chaco swamp a lower oxygen content woultl
be expected, since there was only a small quantity of water (6 inches) subjected
to the decomposition of the mud. I n the Naivasha swamp the water wa.9
4 feet deep, and for this reason alone would be deprived of less oxygen
per unit volume than would thc shallower water of the Chaco. Can we then
justify the concluqion from the figures in Table VI, which werc obtained from
waters of different depths, that the occurrence or non-occurrence of a temperature gradient reversal is largely responsible for the presence or absence
of oxygen in the water ?
This criticism can be met in two ways. Firstly, oxygen measurements
were made in the decpcr parts of the Chaco swamps wherc the water was
3 feet deep. The results, which werc not published in detail, showed
that even in water of this depth the oxygen content of the surface-layers did
not exceed 0.18 c.cm. per litre undcr normal conditions, :tnd a t a depth of 1 foot
was of the order of 0.02 c.cm. per litre, both figures being subject to correction
for sulphides. These figures arc thereforc comparable with those obtained
from water 4 feet deep a t Naivasha, where the oxygen content of the surfacewater was as high as 2.5 c.cm. per litre.
Secondly, it was found that on a few occasions oxygen was detectable in thc
lower layers of Chaco swamp water, wherc it was normally undetectable. This
is shown by text-fig. 6 (extracted from Carter and Beadle, I). The four periods
in which an increase of oxygen content of the lower water took place (with
maxima on October 18th, November 20th, December Int, and December 18th)
corresponded to falls in the minimum air temperature. The greatest increases
of oxygen content, which occurred from October 13th to 18th and from
* In this connection it is of interest to noto that in tho swamps of the Paraguayan Chaco
tho temporature of the bottom mud at a depth o f 6 inches was remarkably c o n h n t both
from one day to tho next arid from morning to afternoon in spito of great changes in thr
temperature of the surfaro-water. On no occasion waa thoro found a dirirnal temperature
change in tho mud of more than 1" C. (Carterand Beadle, I, p. 226).
162
XR. L. 0. BEADLE ON THE
December 5th to 23rd, were contemporary with a high maximum in addition
to a low minimum air temperature. It will be seen from the rainfall rccords
that them two periods were particularly dry, and it way generally found that
the rainfall had little effect on the oxygen content of the lower water. Thus
a fall in tho minimum air temperature, by causing a reversal of the ternperaturc
gradient a t night, way primarily responsible for the rise in the oxygen content
of the water. This WM particularly effective when the difference between
the maximum and minimum air temperatures was relatively high and, in fact,
of the same order as that found at the higher altitude of Lake Naivasha.
It would appear from theso two examples that the occurrence or nonoccurrence of convection currenh plays the major part in determining the
presence or absence of oxygen in the wuter.
0
2.0
CCb
p litre
O X Y G E NCONTENT
10-
A
loo
RAINFALL
MmS
m
I
I
It seems then that the difference of temperature conditions only, by dctcrmining both the rate of bacterial decomposition and the occurrence of convection currents, is ultimately responsible for the lack of oxygen in the wiitors
a t a low altitude and for its abundance in those at a high altitude.
There is no obvious reason why the swamps at a higher altitude should be
able to support a much richer phytoplankton than those a t a lower altitude,
since, of all the factors investigated, the temperature conditions and the oxygen
content only appear to be consistently different in the waters of theso two
environments. The waters of tho swamps a t low altitudes contain ay much
bicarbonate and phosphate as do tboso of tho open lakes with which they are
BIONOMICS OF SOME EAST AFRICAN SWAMPS
153
connected and in which phytoplankton is abundant. It therefore seems
probable that the water contains sufficient quantity of dissolved substances
necessary for plant-growth. It is perhaps possible that, owing to the higher
temperature of the swamp w a h r a t the lower altitude, a greater quantity
of some substances toxic to phytoplankton are produced as a result of a higher
rate of bacterial decomposition than in swamps at a higher altitude. This
question, however, needs further investigation.
In considering the distribution and adaptations of the fauna, it must be
remembered that the three African swamps discussed here were in open communication with the well-oxygenated water of open and deep lakes. This was not
the case in the Chaco. It is, therefore, not unexpected that the numbcr of
adaptations to aerial respiration was smaller in the African swamps. It may
secm surprising that the commonest air-breathing fish of these African waters
(Prorotoptert~and Clarias) were found mostly in the open lake water. It is
reasonable to assume that the aerial respiratory organs of these fish were not
evolved in such an environment. Perhaps the geological history of the country
may give us a clue to the manner in which air-breathing has been evolved
in some fish of these swamps. The Great Lakes of Central Africa have arisen,
since Mesozoic times, as a result of the formation of the two Great Rift Valleys
in the floor of which they all lie with the exception of Lake Victoria, and the
depression which the latter occupies waa formed by the same earth-movements
which gave rise to the Rift Valleys (Gregory, 1921, p. 359 ; Wayland, 1931,
p. 40). Before the uprising of the earth’s surface, which preceded the formation
of these valleys, the waters of Central Africa were in all probability slowflowing rivers and shallow lakes. These conditions would have favoured
the development of swamps, which would, therefore, have covered far greater
areas than they do now, and would not have been connected with large open
lakes. I n such an environment it is conceivable that the air-breathing organ..
of these fish developed.
It is not intended to suggest any particular geological age during which the
aerial respiratory organs of African fish were evolved. But we can say with
reasonable certainty that these air-breathing fish have not always existed
in an environment such as they inhabit today, where there is free access to
well-oxygenated waters. Before the formation of the Rift Valleys they
probably lived in ‘closed’ swamps, where their aerial respiratory organs
would have been of survival value.
I am much indebted to Dr. G . S. Carter and to Mr. J. T. Saunden for
assistance during the course of this work.
SUMMARY.
(1) The conclusion gained from work in Central South America (Carter and
Beadle, I, 1930) that swamps under tropical climatic conditions support a
comparatively poor fauna and flora of truly aquatic forms i s confirmed by
study of certain East African swamps.
154
MR. L. C.
BEADLE ON TRE
(2) The scarcity of the aquatic faunii can only be attributed (as in the case
of the South American swamps) to the very low oxygen content of thc water,
which is also a characteristic of the t,ropical Africun swamps investigated.
(3) No satisfactory explanation has been found for the scarcity of the aquatic
flora in swimips subjected to a tropical climate in both Central South America
and in East Africa, and for its abundance in similar swamps in a more temperate
clirntlte such as that of Lake Naivasha in Kenya.
(4) The errors in the determinations of oxygen content by the Winkler
method due to the presence of reducing substances such as sulphides in the
water (forwhich no allowance was made in the work on South American swamps)
are not sufficiently grcat to nffect the mein conclusion that lack of oxygen is
a biologically limiting factor in those waters.
( 5 ) This conclusion is further supported by a study of the conditions oxisting
in swamps l,orderingT,akeNaivasha inKenya,which, though closo to theEquator,
lie at an altitude of 6,200 fcct, and are thus not subjected to truly tropical
climatic conditions. In these swamps the fauna and flora are abundant, and
the water contains a compamtivcly large amount of oxygen.
(6) Thc reason for the low oxygen content of tropical swamp waters a t low
altitudes and for the high oxygen content of those a t high altitudes is to be
found solely in t.hc differencci of temperature conditions consequent upon a
diffcrcncc of ;~ltitude.
(7) The oxygcn content of the water is affected by the temperature conditions
in two ways :1. The higher temperature at the lower dtitude cause.~an increaqe in thc?
rate of biickrial decomposition, and a greater amount of oxygen is
thereby removed from the water than is the case a t the higher altitude
wherc the temperature is lower.
2. Under normal conditions the mid-day tcmperaturc gradient in thc water
of swamps in t,he hotter climate is mlatively steep, and the minimum
tLir tempcirature in not sufficiently low to causc a reversal of the gradient
a t night. The water is conreqwntly not oxygenatd by convwtion
currents. In Hwsnips i n the cooler climate the mid-day temperature
gradient is considerably lcss steep ttnd thc minimum air tt!riqxirrtturc
is low cnough to effect a reversal of the gradient a t night, with the result
that the water becomes oxygenated by convection currents.
(8) Most of the swanips of Ct!IltriLl and East Africa are in open communication
with tho well-oxygenated waters of open lakes nnd rivers. Reasons are given
for the assumption that this was not always so, and that a t any rate before the
formation of the Rift Valleys ' closed ' swamps, such as now cxist in Central
South America, were more gcnentl. Urickw such conditions the aerial reapimtory organs of certain African fish, which are now found in well-oxygenated
waters, would have bccn of wrvivrtl vitlue.
155
BIOBOMICS OR SOME EAST AFRICAB SWAXYS
REFERENOES.
ALSTERBERO,
G . Die Winkler’sche Bestimmungsmethodo fur in Wasser geliisten. elementaron Sauerstoff vowio ihm Anwondung bai Anwesonheit oxydiorbarer Substanzen.
Biochom. Zoitschr. clxx, 1926.
CARTER,G.
S.,& BEADLE,
L.C. Tho Fauna of tho Swamps of the Paraguan Chaco in relation
t.o its Erivironmont.
I. Physico-chomical Katuro of the Environmont. Journ. Linn. SOC.. Zool.
vol. xxxvii, no. 251, 1930.
11. Respiratory Adaptations in the Fishes. Ibid. no. 252, 1931.
111. Rospiratory Adaptations in the Oligochaeta. Ibid. no. 253, 1931.
GUEOORY,J. W. The Rift Valleys and Geology of East Africa. London, 1921.
RKTTNER,
F. Hydrographivchu urid hydrochomischo Beobnchtungen auf Java, Sumatra
und Bali. Archiv fur Hydrobiol., Suppl. Bd. viii, Heft 2, 1931.
‘I’HRESII, J. C., & REALE,J. I?. The Examination of Waturs and Wator Supplies.
London, 1926.
WAYLAND,E.J. Summary of Progross of tho Goological Survoy of Uganda for the Years
1919-1929. Ent.ebbe, Uganda, 1931.
LINN, SOURN.-ZOOLOOY,
VOL. XLXVlII
12