a discussion on the biology of an equatorial lake: lake george, uganda || geographical, historical...

Geographical, Historical and Physical Aspects of Lake GeorgeAuthor(s): A. B. Viner and I. R. SmithSource: Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 184, No.1076, A Discussion on the Biology of an Equatorial Lake: Lake George, Uganda (Dec. 8, 1973),pp. 235-270Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/76175 .

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Proc. R. Soc. Lond. B. 184, 235-270 (1973) Printed in Great Britain

Geographical, historical and physical aspects of Lake George BY A. B. VINERt AND I. R. SMITHS

t School of Biological Sciences, Untversity of Malaya, Kuala Lumpur, Malaysia

1 The Nature Conservancy, 12 Hope Terrace, Edinburgh

[Plate 383

The equatorial location gives to Lake George (surface area 250 kM2) a climatic regime which makes for an exceptionally unvaried physico-chemical environment within the water mass throughout the year. This is enhanced by peculiarities of the local geomorphometry and the morphometry of the lake.

Incident solar energy, according to 10-day running means, varies during the year only ?13% of the daily mean of 1970 J cm-2. The mean air temperature of 23 ?C is always equable (mean daily temperature = 29.5 + 1.5 0C, mean nightly, 16.5 + 1.5 ?C), and is in equilibrium with the minimum water temperature of the bottom layers of the lake, the surface temperatures of which are usually about 30 'C. The effect of two, approximately equal, dry seasons is offset by the presence of mountainous catchment areas which have a high runoff and thus permit a continuous flow through the lake, which amounts to a mean flushing rate of 2.8 times the mean lake volume per year, so sustaining the supply of primary nutrients. These features promote a continuous productivity, although the allochthonous supplies of nutrients are not thought to be important compared to the total flux of nutrients within the lake.

The wind r6gime, which is strongly influenced by local convectional rather than continental forces, assists convection currents in the water in effecting a nocturnal turbulence which provides an efficient recirculation of nutrients within the shallow water column (2.4 m). This turbulence, alternating with diurnal stratification, against the background of the relative constancy of the climate, means that the lake is dominated by a 24 h physiological cycle rather than the seasonal succession of other latitudes.

Some evidence indicates that the concentric distribution of plankton in the lake is established by wind initiated rotary currents. Wave movements at the sedimentlwater interface are thought to disturb the majority of the sediment surface area possibly down to between 5 and 14 cm in depth, with a mean frequency of an order of magnitude of once every 3 weeks.

Progressive organic enrichment has existed throughout the lake's history (3600 + 90 years), but during the last approximately 700 years an equilibrium appears to have been reached as shown by a retarded rate of change of organic deposition.

I. INTRODUCTION

Given an objective of studying a tropical lake for comparison with those of higher latitudes, the selection of a water body at the equator should provide the medial situation between the variations of conditions which Can occur within the area bordered by the Tropics of Cancer and Capricorn. But this severely limits the choice of lake. Out of East Africa there are no suitable waters, except possibly in Sumatra.

There are only two lakes in the world through which the equator passes, these are Lakes George (long. 300 12'E) and Victoria. There are a few important lakes where the equator passes very near, such as Lakes Edward, Albert and Kioga. All these waters are in, or are part of Uganda, and of these Lake Kioga is almost riverine in its characteristics and as such is hardly comparable with the others. Lakes Victoria, Edward and Albert are among the largest lakes in the world. Their depth

[ 235 ]



236 A. B. Viner and I. R. Smith (Discussion Meeting)

allows them to stratify permanently or for long periods, and their vast surface areas, especially in that they stretch away from the equator, means that many differences of limnology may occur between one part and another, both vertically and horizontally.

In addition to these there is a group of five smaller lakes in Kenya nearer to the area and depth of Lake George and all within latitude of 0? 50' N or S of the equator. Of these, Nakuru, Elmenteita and Hannington are highly saline and thus represent special cases of the range of aquatic environments that sets them apart from a latitudinal comparison. Lakes Baringo and Naivasha could have great value for such a comparison, but again Baringo is slightly saline, and Naivasha presents an anomaly that would complicate a biological study in that it appears to have a closed drainage system and yet remains non-saline, implying a sublacustrine water flow. Lakes any farther away from the equator than these would not reflect the true equatorial conditions. The Sumatran lakes are also exceptional cases in being deep and high volcanic crater lakes.

Lake George has none of these special case complications, and with its compara- tively small surface area and depth allows for any forces arising out of its unusual geographical position to affect the whole water mass in approximately the same manner. It is thus unrivalled as a water body in which the limnological manifesta- tions of the equatorial situation may be studied.

The following account describes those factors imposed by the latitude and also further features peculiar to the locality, which are known to, or which might, affect the lake.

2. GEOGRAPHY AND GEOMORPHOLOGY

Lake George occupies a shallow depression southeast of the Ruwenzori Moun- tains, which rise to over 5000 m. This depression is, in effect, a 'bay' of the main western limb of the East African Rift Valley (figures 1 and 2, plate 38; and figure 3). The fault lines of the Rift Valley curve around the southern end of the mountains, but whereas the faults have formed steep rift walls to the west and south of the mountains, farther to the east, where the lake is, the slopes are much more gentle, and a subsidence rather than rift has formed. The true rift wall appears again to the east of the lake and continues to its south.

By far the majority of inflow water comes from the catchment areas of the Ruwenzori Mountains, but a portion comes from the east of the lake. The single effluent is to the southwest along the Kazinga Channel to Lake Edward which is one of the larger Rift Valley lakes.

The average depth of Lake George is 2.4 m but can vary seasonally + 0. 1 m of this mean (figure 4c). As the basin is fairly flat there is little depth change across the lake. (N.B. The depth used in this account is the mean depth for the 5 years of 1965 to 1969. Other depths quoted by other workers refer to the mean depth during the particular year for which they are presenting data and which require a depth value for their calculation.)



Viner & Smith Proc. R. Soc. Lond. B, volume 184, plate 38

.

. ..

.

.

0 '- !4N ,J,i '.: '..; . ... D g.g wio 0

I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~- -- --- - -

Background: Ruwenzori Mountains. FIGuR?E 2. Geographical features from same position as in figure 1, but looking northeast.

Middle distance: Lake George. Background: Rift wall.

(Facing p. 2 3 6)



Geographical, historical and physical aspects 237

1201)m :% < 1 E-4~~~~~~~4

== =z = / ~~~~~~~~~c~Crater lakes

___ -- -----__ - 7 F; K 1200m~~~~~~~~~k

FIUR 3. MpoLae Gerge an it hitrad Th lae' positio ~~~~~~~i Afic is shw iset.

z=S= C,;Qs /KASES

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~~~~~amukng 120am y

LkmSari 5 0 10 20 30~~~EQUTU

FIG$U:ET 3. Mapof LakeGeorganithierndTe kespiio

ir . B.ic P. LAw ist




The surface area of 250 km2 is bounded by a somewhat diamond-shaped outline with a large extension leading from it to the northwest, Hamakungua Bay, which is extremely shallow (about 0.5 m). Accumulations of floating macrophytes, mainly Pistia stratiotes, frequently cover the majority of the surface of this area and sometimes constrict it from the main water mass. Hamakungua Bay is quite atypical of the lake proper and has not been dealt with in our studies. Three large islands in the western half of the lake complicate the otherwise simple morphometry. The shoreline is not complex in that it is not particularly indented. The friable recent sedimentary deposits around the lake have enabled a sharp step to be wave eroded in many places which has precluded the development of a true zonation of littoral communities, although there are also other reasons (see ? 6, and Lock (I973) concerning macrophytes). But large parts of the shore, particularly in the north, are more gradually sloping where alluvium has been deposited by the inflows, and here the shore is fringed with emergent macrophytes, especially Cyperus papyrus.

months

|J |F |Mq A |M |J |J |A |S | 0 N |D|

(a) * 170O e 150

130 S

200

100

2 L.~~~~~~~~~~George

0C J^F^ AM J~ _1O2.1 8

-~z2.5 ~

12.2 1965--- 1966----- 1967----- 1968-- 1969-&-

FiGuRE 4 (a) to (c). For legend see facing page.




, 10 61

J F |M |A |M| J |J |A |S | 0 N |D)i J |F|

'20

(e) - 15M

A 1/

9 <d1~02

0

- 0

Sebwe R.

"'uNyamugasani R. -5

J JF M A M J J A S O N D|

months

FIGuRE 4. The relations between rainfall, evaporation, runoff, lake level and lake volume. (a) Evaporation: relatively unchanging and not related to seasons. (b) Comparison of mean monthly rainfall for the Ruwenzori Mountains and for the

eastern catchments for the lake, and directly into the lake. The seasons are approximately equal in magnitude and duration. Data from 20 years.

(c) The variations in lake level during the 4 years, 1966 to 1969. (d) Comparison of lake volume, 0, calculated from (c), and volume of direct rainfall

onto the lake, 0, calculated from (b), showing out of phase relation. (e) Runoff rate for four typical Uganda rivers, showing conformity with the rainfall

regime, (b).

To the immediate southwest and south of the lake the terrain is flat and com- posed of deposits laid under a geologically earlier lake. It is covered mainly with savanna grassland vegetation and includes the area of the Queen Elizabeth National Park (figures 1 and 2, plate 38).




The mean water level is at an altitude of 913 m (2997 ft) which when compared to many lakes is of considerable height, but because it is at the bottom of the Rift Valley, it is low for East Africa.

The precise positioning of Lake George on the equator in the middle of the Inter Tropical Convergence Zone (I.T.C.Z.) means that there must be, in principle, two pairs of equal seasons in relation to the Sun passing directly overhead as it travels through its solstices and equinoxes twice during the year. In practice, on the equator in East Africa, the second rainy season is slightly less than the first, but the reasons for this have not been established.

The Sun is directly overhead at the equator on 21 March and 23 September (the equinoxes), having a six-monthly interval. It is farthest away from this position (at the solstices, i.e. directly overhead at the Tropics of Cancer and Capricorn) on 22 June (Cancer) and 22 December (Capricorn). But the real effect of the Sun's movement is only experienced up to 6 weeks after its being overhead. The I.T.C.Z. low-pressure belt, caused by the Sun heating to the maximum the region over which it passes, follows the Sun through its course, but is followed in turn by a high-pressure zone bearing with it rain. These rainy seasons for Uganda, including Lake George, are April to May and October to November (Jameson & McCallum 1970).

The actual rainfall may well not be as clearly defined as this description might imply. In practice, as at Lake George, the wet seasons merge imperceptibly into the dry, and for any one year an impression of complete irregularity of precipitation might be obtained. The pattern becomes clear when records from many years (20) are plotted, as in figure 4b.

Although local geography will modify the basic pattern, in general, the annual rainfall becomes increasingly aggregated into a single rainy season as one moves north or south away from the equator, so that at about latitude 40 S the rains occur during November to April and at about latitude 4? N they occur during May to October. At latitudes greater than this a single peak of rainfall becomes more pronounced, and the seasonal contrast increases, which has obvious limnological and other ecological repercussions.

Surrounded as it is by vast areas of the African continent, and especially by the desert regions to the north, Uganda receives less rainfall than African equatorial coastal areas, and far less than other equatorial regions, for example, northern S. America and SE Asia which are influenced more by moist oceanic winds. Rain- fall also increases with altitude, and as the Rift Valley is lower than the other parts of Uganda, Lake George is in one of the driest parts of the country. It gets an annual mean of 82 cm year-1. But this is equivalent to one-tenth of the run off from the catchment as most of the influent water comes from the Ruwenzori Mountains and reference to figure 4b will show that the Mountains have far more rain - 200 cm year-l- even on their drier Uganda side. Consequently the lake is flushed through much more than it would otherwise be at this latitude and altitude, both in the wet and the dry seasons, because of the proximity of the mountains.



Geographical, historical and physical aspects 241 In addition to this, contributions of melt water from the glaciers and temporary snow would be enhanced in the warmer dry seasons thus reducing the seasonal range of fluctuation by increasing the minimum dry season flow.

3. HISTORICAL ASPECTS

There are two historical considerations of importance to the present study. First, the manner of the geomorphological formation, which indicates from where the fish fauna originated and thus shows what species were available to establish the assemblage that one may see today. Secondly, the degree to which the organic productivity has changed with time, as recorded in the sediments.

The evidence for the origin of Lake George (Doornkamp & Temple I966) seems to show that a river of one sort or another has flowed through the present area of the lake since early Tertiary times if not before this. It was a flow from the east, from what is now Kenya. The contribution from the Ruwenzori Mountains is uncertain, but probably increased during the course of the Mountains' formation throughout the Tertiary, but particularly from the mid-Pleistocene

During mid-Pleistocene there was a large rift lake covering what is now the Lakes George and Edward area, but subsequent drying up in late-Pleistocene (starting about 100000 years ago) isolated the piece of water which became Lake George, but which still has a river passing through it to Lake Edward in the original position of the river that fed the larger water-body.

The sediments of Lake (George are sharply divided, with a layer of soft, richly organic mud up to about 2.5 m thick, depending upon the locality, overlying the deposits of an earlier lake. It is not known for certain whether during the shrinking of the earlier lake there was a period when the area of present Lake George was completely dry or whether there was only a sharp change in limnological conditions. Evidence does seem to suggest that there was an extensive period of drying up. Bishop (1970) says, on the basis of de Heinzelin's (1957) study involving a magmatically related area to the north of Lake Edward, that volcanic activity around Lake George ceased about 8000 years ago. Our 14C dating of the bottom of the Lake George muds shows that the deposition of these began 3600 + 90 years ago. These muds are laid upon a grey clay of varying thickness, again depending upon the locality, which is mineralogically similar to the volcanic ashes associated with the crater lakes on the west and south side of the lake (figure 3). The discontinuity is extremely abrupt, except that there is a narrow band of very coarse alluvial silica sand mixed with the upper 1 to 2 cm of clay. Assuming that the clay was deposited during volcanic activity in the area, and immediately after by inwash, then there must have been a period of about 4000 years when the Lake George area was dry, or at least nor lacustrine. It is reasonable to say that, as far as our modern study is concerned, Lake George began at the time of the beginning of the mud deposition 3600 years ago.

Analysis of the mud shows that there has been an increase through time of the




amount of organic material, as a proportion of total dry weight, deposited in the sediment. There was apparently a steady increase of deposition of organic material from 1 g m-2 a-1 at 240 cm to a maximum rate of 32 g m-2 a-1 at about 25 cm. But in the top 70 cm, equivalent to about 700 years, the rate has been between 28 and 32 g m-2 a-1. The increase to 70 cm level does not in itself necessarily indicate increasing fertility but it is the most reasonable explanation since there is some supporting evidence from analyses of the various fractions of phosphate and of the fossil diatoms that limnological conditions must have changed with time towards a more eutrophic state (Viner & Haworth, in preparation).

The reduction in rate of change of deposition in the upper 70 cm is especially interesting as it would tend to indicate that deposition of material and recycling has reached a steady state. This could mean that the decomposition rate is increasing at the same rate as an increase of deposition of material, or that erosion of the sediments (see below) is likewise increasing with increasing deposition, or, far more probable, that the deposition rate is controlled by a steady state of primary production. There is evidence that this latter explanation might well be true (Ganf & Viner, this volume).

The isolation of Lake George apparently occurred after the drainage of western Uganda had been separated by tectonic movements from the Lake Victoria drainage farther eastwards. The invading fish fauna for the early Lake George must therefore have either come from the riverine assemblage of the inflows or from Lake Edward. The rapid organic enrichment of the lake would have severely limited a riverine invasion, and the lake now exhibits characteristics, among equa- tic environments, which are the extreme opposite of riverine conditions in that it has a very high standing crop of plankton and there are large diurnal physico- chemical changes in oxygen, pH and temperature stratification. If riverine fauna did enter the early lake the subsequent changes there would have placed such species at a disadvantage. The fish must have come from the shallower parts of Lake Edward as they would have been preadapted to the developing smaller lake. As the ichthyofauna of Lake Edward was already impoverished due to other factors of a tectonic or volcanic nature (Worthington I937; Greenwood I959) history has allowed only a limited number of species to occupy Lake George.

It is clear that in a lake where such a large portion of the total biomass is phytoplankton (Burgis et al., this volume) that phytoplanktophagous species will be selected for from the pool of potential invaders, and not surprisingly the fish assemblage is dominated by two species of such feeding habits; Tilapia nilotica and Haplochromris nigripinnis, both from Lake Edward. But the lake lacks some of the well-known piscivores of the East African lakes and many of the carnivores are restricted in both species and biomass. It is possible that, even if carnivores such as Hydrocynus or Late8 had persisted in Lake Edward (Greenwood 1959) they would not have been able to tolerate the eutrophic conditions in Lake George. But Lates exists in shallow Lake Kioga and in the often very rich blue-green algae dominated (as in Lake George) bays of Lake Rudolf.




The relevance of this historical aspect for our present biological study is to show that, in deciding what might delineate the faunal assemblage, even to the existence or not of a particular trophic relationship, it is mandatory to consider historical contingencies as well as modern environmental characteristics of the lake, especially with respect to the principle of the reduction of species in eutrophic conditions.

4. THuE WATER BALANCE

(a) Kazinga Channel hydraulics The simplest means of estimating how much water passes through a given

water mass is to measure the flow and dimensions of the effluent. For Lake George this is the Kazinga Channel. It is about 1.5 km wide and 3 m deep, and has a cross-sectional area of approximately 4.5 x103 M2. The average velocity in the Channel is the flow rate divided by the sectional area, i.e. 53.5/4.5 x 103 m s-1 or about 1.2 cm s-1. Wind-induced currents can have a velocity an order of magnitude greater than that required to discharge the outflow from. Lake George, and the surface slope associated with this outflow is negligible. Consequently the direction of flow has been shown to reverse very readily with changes in wind stress (Viner 1970a). The Kazinga Channel is thus a lake rather than a river.

Because of its overall negligible surface slope, conditions in the Channel are likely to be affected by seiches in both Lakes Edward and George. In support of this, casual observations do suggest rapid transitory changes in slope in the Channel surface.

The complexity of the linked three-lake system indicate that attempts to use evidence of water movement in the Channel in the analysis of the water budget were likely to be futile and were not attempted. As a result of this, assessments of budget have been based upon rainfall in the catchment areas, corrected for evaporation.

(b) Rainfall If the rainfall data are compared with the changes in lake level (figure 4d) it

is seen that the fluctuations are out of phase by 1.5 to 2.5 months. One expects some delay in effect of rainfall with highly vegetated catchment areas such as on the Ruwenzoris, but the out of phase period in this case is far larger than the delay of 1 to 4 weeks usually caused by vegetation retarding flow. Also, figures 4b and e show that runoff does indeed have a close relation with the rainfall cycle involving very little delay.

The reasons for the lake level/rainfall relation must be sought in the difference in height between Lake George and Lake Edward along the Kazinga Channel, which is less than 1 m, and important alterations in flow rate could occur because of temporary differences in relative heights of the two lakes. These are almost certain to exist during the year as the lakes are both very different in size and in

i6 Vol. I84. B.



244 A. B. Viner and I. R. Smith (Discussion Meetimg)

the origins of their influents. As has been pointed out above, the balance of flow between the two lakes is obviously extremely delicate.

(c) Evaporation

The evaporation rate from the lake surface has been calculated by the Penman formula (Penman 1948) to be a mnean of 5 mm day--, and there seems to be little variation throughout the year (figure 4a). The tendency is for the diurnal variation to be rather greater and to follow the diurnal changes in air temperature.

The significance of the rainfall and evaporation for the lake may be illustrated by comparing the annual evaporation and direct precipitation figures listed in table 1 with the mean volume of the lake, which is 600 x 106 M3. It is seen that the direct rainfall throughout the year amounts to about one third (0.34) of the mean volume of the lake, but that the evaporation is more than twice this rainfall and is about three-quarters (0.76) of the lake volume. If all influent and direct rainfall could be imagined suddenly to cease the lake would disappear by evaporation within 460 to 500 days, depending on the initial depth. This could be extendecl to between 820 and 900 days if the direct rainfall is added.

During the dry seasons the outflow from the lake can decrease to about 0.8 x 106 m3 day-l, but the evaporation exceeds this as 1.25 x 106 m3 day-'-. Consequently there is a net drying up of the lake for parts of the year. A fall in 0.2 m in lake level during 3 months between wet seasons is equivalent to 50 x 106 m3, but the equivalent evaporation for this period would be 152 x 106 m3 which could therefore easily account for the fall in lake level. During the wet seasons the amount of water coming from the mountains is in fact very great and cancels out the loss by evaporation by many times.

(d) Runoff

The method of calculation is indirect because the inflow records are either absent or incomplete. To make the analysis possible supplementary information has been used from rivers which do not actually flow into Lake George but which happen to have complete gauging data. An extrapolation has been made from such rivers to those of the Lake George drainage by comparing the areas of catchment and the rainall for all relevant rivers against the runoff distribution of these rivers for which it is known. That is, the annual discharge has been expressed as depth over the catchment area. This has been considered a legitimate approach as catchment area and rainfall information is good and the geological and vegeta- tional characteristics of the catchments are similar.

The details of this method of calculation, together with tablature of the results, are given in appendix 1. The final conclusions are presented below.

Figure 4 e shows the seasonal cycle of runoff. The River Sebwe is the only one of the four rivers illustrated which actually flows into the lake. The remaining three from other catchment areas are included to show the general conformity of pattern over a very wide area. The Nyamugasani does have a very similar type of




catchment to the Lake George influents, although it flows to Lake Edward. The Muzizi, somewhat to the northeast of Lake George, is of the Lake Albert catchment, and the Ruizi, to the southeast of Lake George flows to Lake Victoria. The two latter rivers pass over rather flat, swampy terrain, whereas the others are of mountain origins. It appears that the Ruwenzori streams have the least pronounced seasonal changes. The effect of the mountains in this regard has been mentioned

TABLE 1. ANNUAL WATER BALANCE OF LAKE GEORGE

(see also figure 3 and appendix I for locations of catchment areas.) 106 m3 inflow proportions % of total

inflow 1948 1. Ruwenzori Mountains direct rainfall on lake 205 (Rukoki/Kamilukwezi, Sebwe, hence total to lake 2153 Mubuku, Ruimi, Upper evaporation from lake 456 Mpanga plus areas 3 and 4) 57

hence discharge down Channel 1697 2. Eastern Plateau (Lower Mpanga plus areas I and 2) 38

in ?2. The slightly greater time lag between the occurrence of maximum and minimum rainfall and the times of high and low flow for the eastern rivers is probably due to the retention effect of the swamps, but there may be some error in this observation as the graphs are based upon monthly readings, and more fre- quent data might well have shown different peaks not displaced from those of the rainfall.

Table 1 is a summary of the water balance, assuming that there is no seepage loss from the lake, and taking into consideration the evaporation rate and direct precipitation. The mean retention time, i.e. the ratio of lake volume to annual inflow, is 4.3 months. The outflow volume is equivalent to a flow rate of 53.5M3 s-1 (4.6x 106 m3day-).

(e) The significance of the water budget In this account accent is placed upon the calculation of the water balance

because of its importance in assessing both the lake's nutrient budget (Ganf & Viner 1973, this volume) and the amount of planktonic material flushed out through the channel effluent. Considering the great amount of biomass incorporated within the plankton (cf. Burgis et al., this volume) it is essential to have some value for the emigration from this part of the biota so as to estimate production rates.

Although the mean replacement rate is 2.8 times the lake volume per year the actual rate at which this is carried out at any one moment may vary by an order of magnitude during the year.

At times of minimal rainfall the flushing rate could, in theory, decrease 9.6 m3 s-1 and increase during maximal rainfall to 97 in3 S-1. In terms of lake volume this difference could be made more extreme because of runoff being out of phase with lake volume such that, on average, maximum runoff affects the lake at its

i6-2



246 A. B. Viner and I. RI. Smith (Discussion Meeting)

minimum volume and minimum runoff affects the lake at about mean volume (figure 4d). A day of minimum rainfall could replace 0.83 x 106 m3 of water, or 1/720 of the lake, whereas a day of maximum rainfall could replace 8.4 x 106 m3,

or 1/68 of the lake. Such large changes in the degree of dilution of the lake water by river water

should be reflected in concomitant changes in the concentration of suspended plankton. It can be shown (Burgis et al., this volume; Ganf & Viner, this volume) that such changes do not actually occur. Either the sudden growth of the plankton, in response to increasing nutrient supplies, manages to replace any increased emigration in times of high flow, or alternatively, although there may be considerable changes of water coming into the lake, these are not transferred as similar changes in the effluent flow. Like the phytoplankton, the zooplankton, with a much longer life cycle than that of the algae under optimal growth conditions, do not show major fluctuations, thus arguing for the latter alternative. Also, increased runoff appears to be very readily translated into increased depth of lake according to figures 4d and e implying increased ponding back of water rather than increased washout.

Probably the most important factor in the control of this passive removal of planktonic material is the small slope between Lakes George and Edward, which is so slight that flow may be reversed as described above. This acts as a mechanism to damp out sharp fluctuations in flow rate which thus maintains Lake George as a lake rather than the expanded river which the large quantity of water passing into the basin during the year would otherwise produce. It is therefore a demonstra- tion of the remarkable balancing out of climatic and hydrological factors which enables a stability to be maintained such that the lake level varies only ? 0.1 m or + 4 % of the mean.

Nutrient supplies via the influents will vary with the fluctuations in water flow. In particular, high peaks of nutrients of very short duration are likely to appear in the stream waters as a result of flushing of the soils of the catchment during heavy but brief tropical storms. As there are many more storms in the mountainous catchment areas than around the lake itself it is impossible to gauge when such peaks might be observable.

In lieu of continuous recording, it has been assumed that the 30 sets of analyses of inflow chemistry which were made were distributed randomly with respect to the nutrient concentration changes.

The nutrients with which we have been most concerned are phosphate and the forms of nitrogen. The range of concentrations found for NH+ nitrogen was 3.0 to 135.0 ,g 1-l with a mean of 24.4 Vg I-1. For N03 nitrogen it was 0.0 to 2390 ,ug 1-1 with a mean of 533 ,g 11. And for PG4 phosphorus 2.5 to 215 ,g 1I- with a mean of 78.8 ,ug 1-1. As can be seen, the concentrations vary by two orders of magnitude, but, as far as may be determined by the analysis, such variations can occur at any time irrespective of season, so that the mean concentrations may be used for any time of the year.




When it is considered that the flow of water can also vary seasonally by an order of magnitude (see above) then clearly the total amounts of nutrients transported will vary in an approximately similar manner. In many lakes this would be sufficient to cause great changes of biomass amongst the primary producers, but the fact that this does not occur in Lake George is indicative of the subordinate role of the allochthonous nutrient supplies for the nutrient dynamics of the lake. This is so even when the contribution of the solutes in the rainfall are added into the budget. During 1968 and 1969, 19 sets of chemical analyses of rainfall were made. The ranges for the nutrients considered here were: NH4 nitrogen, 0.128 to 4.1 mg 1-h; NO3 nitrogen, trace to 0.89 Mg 1'-; P04 phosphorus, 0.004 to 1.7 mg 1-'. Atmospheric dust affected the higher values. When the means of these ranges are multiplied up for the mean rainfall during the year, the total inorganic nitrogen amounts to only a quarter of that from the influents, although the phosphate is two-thirds of that from the influents. The relative importance of the various origins of nutrient supplies within the total flux is discussed further by G-anf & Viner in this volume.

It is possible to calculate hlow much material is washed out of the lake. This is best done as an annual mean, thereby allowing the more ephemeral oscillations to cancel each other out. The mean masses of important planktonic constituents expressed in metric tonnes for the whole lake are: dry weight, 26000; carbon, 9000; nitrogen, 1200; phosphorus, 80; chlorophyll a, 107. If the water volume is replaced 2.8 times during the year then multiplying these values by 2.8 gives: dry weight, 73000; carbon, 25000; nitrogen, 3400; phosphorus, 220; chlorophyll a, 300. These are minimum estimates of the amount of constituents that must be fixed by the plankton (95 % algae, cf. Burgis et al., this volume) during the year, as they do not include values for permanent sedimentation nor for grazing.

In terms of the mean that must be fixed per day during the year, using a daily replacement rate of 1/130 of the lake volume calculated from the mean flow rate quoted above, the planktonic constituents, which on average must be replaced daily, are (in tonnes): dry weight, 200; carbon, 70; nitrogen, 9.2; phosphorus, 0.6; chlorophyll a, 0.8.

This carbon value has particular interest as it has direct relevance to photosynthetic experiments. It is equivalent to 0.28 g m-2 day-' net carbon fixed. Or, using a simple stoicheiometric conversion for equivalent oxygen produced, 0.74 g 02 m-2 day-'.

5. THE WIND REGIME With such a shallow lake the effect of wind is especially important for the

distribution of nutrients and plankton. Much of the raw data for the following analysis has been obtained from the

East African Meteorological Department, and is for the Kasese airstrip station 14 km north of the lake. The terrain is quite flat between here and the lake and the records are considered suitable for our purposes inasmuch as they demonstrate




the general pattern of wind regime for the whole area concerned. Suitable directional records were not obtainable at the Royal Society laboratory, but some data are included here from Mweya (see figure 5d).

A position on the equator should receive prevailing winds in accordance with the Trade Winds shift, i.e. northeasterly from 1 November to 30 April, and south- easterly from 1 May to 31 October (actually, tending towards southerly in Uganda). Figure 5 a clearly shows that for the Lake George areathisis notthe case. The analysis

---May-Oct --Nov-Apr 09h -12h --15h

(a) N NE (b) N NE

NW NW j SV < ~~ ~ ~ ~~---E *~ * = - - - ---

SW , \ SW\N

SE t SE

f58 Sm S-1

(c) N NE (d) N

W < -F. NW\ >. E

Wi \W I SE scales in 5% intervals S

FIGURE 5. Wind kites showing the distribution of wind direction with time and with wind strength. (a), (b) and (c) Kasese airstrip data for 3 years 1966-8, (d) Mweya data for 1967 (see figure 3 for localities).

(a) Wind directions during two periods when Trade Wind influence could create contrasting directions but apparently do not.

(b) Wind at three periods during the day. No significant change in direction is shown. (c) Wind at two velocities above which they are likely to have some hydrodynamic

effect (see ?8d). (d) Wind directions at Mweya showing similarity to (a) but with an added southwest

component.




of 2121 wind direction records for the 3 years 1966-8 have been used to compare the two periods, and the wind kites constructed show an almost identical shape, with neither showing a prevailing direction in relation to the Trade Winds.

mXean 1966-1969 2

' 3 i

1

ss~~~~~~~~ , .196

0~~~~~~~~

_) 's,#R 1968

NE Trades J SE Trades tNE Trades

LIiF 1 M 1 M J J A OJ IJ %. _ 1.0 ,.

lJ~~~10 J. L6

months

FIGURE 6. Seasonal changes in wind strength. The periods during which the Trade Winds could effect the wind regime are included to show that there is no obvious relation between them and the local conditions.

Instead, the dominant wind is from the east. Kasese is in a position somewhat more protected by the mountains than is Lake George, so that it might be suLspected that the wind direction at the lake itself would have a greater proportion of wind from the west. The data from Mweya, however, indicate that this is not likely to be so since even at this less protected position (in relation to the mountains) the dominant wind is easterly, although in this case there is an added component from the southwest, due to the exposure to the winds across Lake Edward (figure 5d).

In spite of this lack of seasonality of direction the monthly means of wind velocity (5043 values for 4 years 1966-9) do show some annual pattern with two peaks corresponding approxinmately with the wet seasons (means of 20 years) (figure 6, compare with figure 4b). The relation is rather loose, however, as when taken individually these years do not always exhibit much of this pattern.

The mean wind speeds (figure 7b) shows that the strongest winds occur during mid-afternoon (15 h) decreasing to a minimum during the night. A modified picture emerges, however, if the relative frequencies of wind speeds are analysed. Then it can be seen (figure 8a) that there is a greater likelihood of stronger breezes blowing around 18 h. In other words, the winds change their character during the afternoons, on average, from being sustained light breezes to being shorter gusts later.




This is clearly a convectional wind effect unrelated to any continental climatic factor. But there is no significant change in dominant wind direction associated with this diurnal fluctuation (figure 5b). And nor is there a change in dominant direction at the stronger wind intensities (figure 5c).

The conclusions to be drawn from these data are that the Lake George/Kasese area is almost entirely protected from the continental wind characteristics by the mountains on the one side and the Rift wall on the other, the former being the more important as very little wind comes from that quarter. (The winds that do

(a) (b)

5 - +

al, ,/ ,/ , 1, 3 -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~I

06 09 12 15 18 21 06 09 12 15 18 21 solar hours

FIGuRE 7. Diurnal changes in wind strengths. (a) Mean velocities from 4 years (O 1966; A, 1967; V, 1968; El, 1969) plotted independently to show degree of differences between the years. (b) Means of data of (a).

20C IIIIIV[II IIllHI 171 j11111

(a) 7

I+

10 0/ / A -I

0 ---~~~~~~~~~~~~~~--

10 30 50 70 90 95 99 99.8 99.99 30 50 70 90 95 99 99.8 99.99 percentage of time when wind speed was less than as indicated

FIGuRE 8. Probability of occurrence of wind strengths. Data for 1966 to 1970. (a) Percentage of time winds of less velocity than as indicated at @, 09 h; A, 12 h; V, 15 h; E, 18 h; KO, 21 h. (b) Mean of data in (a).




come from due west are probably deflexions from the mountains of the other winds, and also results of the complex convectional downdrafts along the mountain valleys caused by extreme temperature difference at top and bottom of the mountains.)

The directional pattern must then be controlled by local geomorphological peculiarities, and this is imposed upon the wind speeds created by the local convectional regime. The wet season increases in wind speeds must therefore be due to an enhancement of the convection factors, e.g. the increasing range of temperature variations brought about by storms. Such a pattern is inherently unpredictable, and it is not surprising that the years for which data are supplied show such variability. If there are any seasonal reflexions in the biota they are evidently not going to be repeated in the same manner from year to year.

Because of the protected situation, wind velocities are, in general, very low, even when corrected for the increase appropriate to the greater exposure of the lake proper (cf. ? 8). This results in a distinctive hydrodynamic sequence in the lake, as is discussed (? 8) below.

6. THE SOLAR REGIME

In spite of the Sun being directly overhead at midday twice during the year and never at angle greater than 23 0 away from this, the 10-day means of solar energy at Lake George have an annual variation between 1720 and 2210 J cm-2 day-1, with a mean about 1970. This is an annual variation of + 13 % of the mean. But individual days may show differences as far apart as 860 and 2760 J cm-2 day-l (derived from Ganf I969). For comparative purposes this variation is quite un- important; the various observation stations throughout Uganda, even though all are so near the equator, show greater differences between their annual means than does each station for its 10-day means. Small peculiarities of locality such as cloudiness exert an influence. Even an observation station about 1O km away from Lake George, but in superficially the same terrain, shows an annual mean difference of 84 J cm-2 day-l, and the Kasese mean is about 245 J cm-2 day-l lower, due mainly to the sunlight being shielded by the Ruwenzori Mountains and the clouds associated with them.

As a contrasting example the solar radiation in the United Kingdom at a latitude of 51 'N varies during the year by almost 90 % of the mean of about 1250 J cM-2 day-'l. This is largely because of changes in day length which, in the United King- dom, has a range of 8 h 4 min to 16 h 23 min; at the equator, day length is 12h 7min. +1 min.

Compared to other latitudes, then, the amount of incident solar energy at the equator might be considered as a constant environmental feature.

The distribution of this energy in the lake is highly complex, however. It is very sharply attenuated, a great deal being lost by scatter and absorption by the great amount of suspended material and by the pigmentation of the water. The penetration of photosynthetically active light does not provide a net gain of photo-




synthesis below about 0.7 m. Owing to the dominating influence of the plankton upon the penetration of light even small changes in the population of plankton can have significant affect upon the light penetration for photosynthesis. This is discussed further by Ganf & Viner (this volume). In a similar manner the day-to-day changes in incident radiation, even though the 10-day means are so similar, can have considerable repercussions on experimentally measured photosynthetic activity and algal growth, since what may be observed at one time is much dependent upon the phytoplankton's immediate past physiological history (cf. Viner 1973). Thus, although the radiation is constant enough to provide for a continuously productive season, this is not necessarily so where the 24 h cycles of physiological activity within the lake are concerned.

Details of the lake's internal light climate are more properly discussed else- where (Ganf 1969, and in preparation) in conjunction with photosynthetic studies. However, although of no precise physiological value, Secchi disk readings are a useful rough indication of light penetration for interlake comparison. Such readings in Lake George were between 26 and 46 cm.

This very small light penetration is probably the most important reason for the general absence of submerged aquatic macrophytes in the lake. Macrophytes do occur, however, in river mouths and lagoons within the northern fringe of the lake. The water here, although in other respects very similar to that of the main lake, is lacking algae and is extremely clear (see Lock 1973 for full discussion).

7. THE TEMPERATURE REGIME AND THE THERMAL STRATIFICATION

OF THE WATER

Because the solar radiation does not have a seasonal decrease comparable with that of higher latitudes the air temperatures are uniformly high. The daily mean temperature is 29.5 0C, and the mean minimum during the night is 16.5 'C. The annual mean is 23 'C. An indication of the constancy of this is illustrated by the fact that neither the means of the maximum nor minimum vary in their 10-day means more than about + 1.5 'C. Maxima on isolated days however can go as high as 35 0C.

Consequently, the temperature of the water remains high. The bottom layers are about equal to the mean air temperature, 23 to 25 'C, and thermally are the most constant part of the water column. But the water surface heats up progressively during the day with the passage of the Sun (but with a time lag) so that the maximum is at mid-afternoon (ca. 15 h) rather than at midday. Turbulence during this time is not sufficient to transfer this heat gain to the whole water column so that extreme stratification results. The surface has been known to reach 35 0C but is usually nearer 30 'C at the peak. Thus the vertical temperature gradient can be up to about 10 'C within 2.4 m. Physiologically this might be thought a very considerable change of environment, at least for micro- and meio-organisms. It is equivalent to the amount of change that could be expected in a temperate




water during the march of the seasons, but in Lake George the temperature is never low enough to inhibit metabolism. There is a better likelihood of the vertical change of temperature having greater significance in the more subtle relations of photosynthesis and respiration among the algae.

Between 17.00 and 19.00 h there is a cooling of the surface, and turbulence is created by the resulting convection currents, assisted by any breezes which might occur. Mixing of the lake then takes place, probably to the whole depth of the water column for most days, but sustained recording of this has not been done, and the completeness and frequency of the mixing has been assumed from other

temperature/0C

24 27 30 33 0

09 11 ~13 15 16.30

2

FIGURE 9. Typical changes in water temperature profiles during the day. The profiles represent one half cycle of the diurnal stratification and destratification. The numbers against the profiles indicate solar hours.

evidence, including the fact that the lower part of the water column remains at the mean temperature of the air, and so must be in thermal equilibrium with it. This also means that the net gain of heat during the day is, on average, completely lost during the night. The heat flux for the whole lake therefore must be very great. The typical sequence of stratification is illustrated in figure 9. Using the data in this figure it is possible to calculate a heat budget for the lake. The changes in temperature during the day for a number of depths along the water column have been integrated and the mean change in temperature calculated. This was found to be 2.6 'C. With a value of 2.5 x 1012 cm for the lake surface and 6 x 1014 cm3 of total lake volume which attains this mean temperature shift, the energy intake must be 2.6 kJ cm-2 day-l. Hutchinson (1957) has compiled a list of heat budgets for various lakes, but unfortunately direct comparison is not possible with these because the lengths of the warming season(s) are not included so that the mean flux per day cannot be calculated. But some comparison is possible if the daily heat loss for Lake George is summed to make a hypothetical figure for the whole year. This




is 950 kJ cm-2. Hutchinson, quoting Str0m (1944), says that the equatorial Sumatran lakes should have an annual budget of about 1260 kJ cm-2, which is not very dissimilar to Lake George (but it is difficult to see how Strom's value of 6280 kJ cm2 for Lake Tanganyika is possible). In fact, the 2.6 kJ cm2 day-l for Lake George is lost again as part of the diurnal cycle of events in the lake.

8. LAKE CURRENTS AND THE CIRCULATION OF WATER

(a) General principles and methods of measurement

The wind-induced circulation in any lake must satisfy at least two conditions: First, the requirement of continuity must be met in that the flow into and out of any part of the lake must be equal. Secondly, there must be equilibrium in the horizontal forces acting on the water. Such a force balance is usually achieved by the wind stress on the water surface being opposed by hydrostatic forces due to differences in water level, or by being dissipated in overcoming fluid friction. A third influence on the circulation pattern in larger lakes, namely the Earth's rotation, does not affect Lake George since it lies on the equator. The current velocity depth profile at any point in a lake is usually the net result of several current-producing mechanisms such as the drift current due to the actual drag of the wind setting the water in motion and slope or radial currents due to the distortion of the water surface.

The characteristic wind regime at Lake George, which is predominantly light winds and short duration storms, results in two distinctive forms of water motion. During light winds, the equilibrium conditions stated above are met and a steady- state circulation pattern results, which is the first form of motion. The short duration storms end as suddenly as they develop so that immediately after their occurrence the horizontal force equilibrium is destroyed resulting in the second form of motion, unstable oscillations of the water mass. The equilibrium is also destroyed if there is a sudden change in wind direction.

For both types of circulation the current speeds and directions were measured using a small aluminium drogue (figure 10). The usual procedure was to anchor the boat and, when the boat was steady, to drop an anchored buoy overboard so that any drag on the boat's anchor would be detected immediately. The drogue was then released with a fine line attached, the other end being wound on a large fishing reel. The distance travelled in a given time was indicated by the amount of line payed out once it was considered that the motion of the drogue was free from the influence of the boat. Care was taken to ensure that the line was always slack during the run so that the drogue movement was unimpeded. Direction was recorded with a compass.

(b) Currents during light winds

A number of observations of current speed and direction at different depths were made during the day as the stratification built up, mostly at a standard




station (figure 12). Some typical results are shown in figure 11. The obvious effect of stratification is an increase in the surface velocity and a decrease of depth of the current. The streaming motion at the surface, which can easily be seen during calm conditions, only extends for the top 20 to 25 cm. Temperature and

polyethylene bottle

length of line variable

e O~~~~~1cm

FIGURE 10. Details of drogue used for current measurement.

velocity/cm s-1 0 So 10 5 10- 15 ~ 5 10

T~~~~~~ 1

~~~~~~~velocity

temperature

20 9h20 12h40 16h45

20 25 20 25 30 20 25 30 5 May 1972

5 10 15 5 10

10 - ~ ~ ~ ~ -

14h3 170h15 I I / ~~~~~~~~~~~8 May 1972

20 25 30 20 25 30 temperature/0C

FIGuRE 11. Velocity and temperature profiles at various times of day when light winds are blowing.




density gradients inhibit the turbulent transfer of momentum downwards from the water surface and, if the energy supplied by the wind is unchanged, this must result in an increased velocity near the surface.

g '<i~~~standard station

FIGURE 12. Circulation in Lake George when light winds are blowing. The arrows indicate the measured direction of drogue travel. The angle, a, indicates the range of wind directions for observations at the standard station.

All observations at the standard station were made with winds from a generally northern direction and the water flow towards the southeast (figure 12). Drogue runs along the western shore also indicated flow towards the Kazinga Channel. Further observations were therefore made along a transect through the standard station towards the northeast shore (figure 12) in order to determine the general form of the circulation pattern satisfying the continuity requirement. The results indicate that the main circulation feature during steady, light, northerly winds is an anti-clockwise rotation. At times this appears contrary to purely visual observation of the surface as the drift component moves in the direction of the wind even though the flow due to the distortion of the water surface is in the opposite direction. Thus the movement of coloured river water from northwest to southeast along the northeast shore is only in the upper centimetres. There is limited evidence that the water rotation is clockwise when the wind is from a southerly direction.

(c) Lake motion after storm Direct evidence for the occurrence of oscillations or seiches in Lake George can

be found in the records from the continuous water level recorder near the Royal Society laboratories (figures 3 and 13). Examination of the charts for 10 days, chosen




at random, indicate an average of 15.2 peaks per day which is equivalent to a period of 1.7 h. The period, T, of a primary seiche in a rectangular basin is given by

T = 21/V(gD), where I is the length of basin, g, the acceleration due to gravity, and D is the water depth. Taking I as 15 km for Lake George, the completed period is 1.6 h which is very near to that observed from the recorder charts.

24h

5cm[

FIGURE 13. Typical record from water level recorder.

In the narrow channel between the laboratories and the islands (cf. figure 3), only north-south oscillations are likely in the open lake (Hutchinson I957). WVhere the length and breadth are roughly equal, transverse seiche, i.e. at right angles to the wind direction, occur as well as longitudinal ones. In circular basins, oscillations are even more complex. Nodal lines, where there is no vertical movement of the water surface, can occur on both diameters and circles, and the resultant wave form may rotate round the basin.

Such complex oscillations are best investigated by examining the records from a series of continuous water-level recorders. This was not possible and the only observations were of drogue movements immediately after storms or changes in wind direction. Typical results are shown in figure 14. Complex oscillations are clearly discernible but no obvious pattern can be detected.

These complex seiche movements suggest a possible explanation for the concentric phytoplankton distribution observed in the lake (Burgis et al. I973, this volume). If a circular bowl, containing, for example, cigarette ash, is given a sudden jerk, then, after the oscillations have died down, the ash is seen to be concentrated in the centre of the bowl. This 'slopping basin' effect is considered a more likely cause for the horizontal distribution in the lake than the steady-state rotational motion since the energy imparted to the lake during storms is much greater and, in addition, algae tend to sink into the lower, more or less stagnant, water during stratification and thus out of the zone of transport by the steady state motion.

(d) Sediment disturbance due to wave action Many of the characteristics of a lake depend on the inter-action between its

sediment and water. There are clear indications that the sediments in Lake George are physically disturbed. This has immediate implications for the flow of nutrients and oxygen consumption within the system and for the stability of the benthic organisms' environment.




Cores show that viable algae can occur more than 20 cm below the mud surface, and it has been shown that the chlorophyll a concentration in the water increases from the bottom upwards during a storm. A surface water sample taken during a storm had a concentration of 1.05 g 1-1 of mud solids, while at the sediment

I J /~~~~ o 2.0

wind / / direction , 1 /

(a) 0.5,2. 0.5,1.5

1.0,1.5,2.00 0.5

15h45 16h30 17 hOO

1.75

1.75 0,0.75

15h45 0,0.75 ! 16h20

(b) 0 0

0.75 0.75

17h10 / 18h20

calm

/ 1.75 1.75

FIGURE 14. Current directions when lake is oscillating: (a) movement after a sudden drop in wind speed; (b) movement after a sudden change in wind direction. The numbers beside the arrows indicate the depth of drogue in metres. The other numbers indicate time of day.

surface mud solids are about 30 g 1 1. The implied depth of erosion sufficient to have caused this mud concentration gradient in the water depends on the assumed shape of the gradient but it was likely that the mud was disturbed to between 5 and 10 cm.

The physical mechanisms concerned with the disturbance of sediment by wave action can be considered in two stages: the critical velocity for the disturbance of the liquid mud ((i) below); the wave characteristics capable of developing this




critical velocity at the sediment water interface, and the wind speeds and fetches capable of generating such waves ((ii) below).

(i) Instability of lake muds The erosion of the sediment concerns the stability of the interface between a

viscous, non-turbulent fluid and a flowing water. Such a situation is very similar to Mortimer's (196I) discussion of motion in thermoclines, i.e. waves are formed at the interface between the fluids of different density, and, as the relative velocity of the two layers increases, these waves become unstable and ultimately collapse, resulting in water from both layers being mixed together. An appropriate criterion for instability with a non-turbulent lower layer is discussed by Ippen & Harleman (1952). In this, waves of length, A, begin to break when

where VJ is the critical relative velocity of the layers, g, the acceleration due to gravity, Ap, the density difference between the layers, and p, the density of water. At the onset of instability the wavelength A = 7rd where d is the depth of the lower layer.

A mud profile from central Lake George indicated that in our case d = 25 cm. The density of the upper liquid mud layer is more or less constant, and a single but characteristic sample showed that Ap/p = 0.01 approximately. By inserting these values into the equation above it was found that the critical relative velocity, that is the threshold velocity required to move a water particle at the surface of the sediments, is about 14 cm s-1.

The critical velocity refers to water movement in one direction only and is not strictly applicable to wave induced instability. Nevertheless, it can be used to give a general indication of the wave characteristics causing erosion. The progressive transformation of the motion of a water particle beneath the waves is shown in figures 15 and 16. The theoretical mean velocity at the sediment surface is, in shallow water, related to the wave characteristics by the following equation:

UD= + 2H/{T sinh(27r/AD)}, where H is the wave height, T, the wave period, A, the wavelength, and D, the water depth.

Also, Umax 2 7TD where Umax is the maximum instantaneous velocity.

In addition, this theoretical mean velocity at the sediment surface during one half cycle is not the true relative velocity of the two layers. Figure 17, taken from Ippen & Harleman (I 952), show that the velocity of the two layers adjust so as to avoid an abrupt discontinuity when it is the lower, denser layer that is in motion. Figure 18 shows how this adjustment is likely to take place when it is the overlying water that is initially in motion. It appears that the actual relative velocity at the onset of instability must be about twice the theoretical.

I7 Vol. I84. B.



260 A. B. Viner and I R. Smith (Discussion Meeting)

This approximate analysis suggests that the sediment will become unstable when UD, in the above equation, exceeds 25 to 30 cm s-1. The extent of erosion may not increase very rapidly when the relative velocity exceeds the critical so that the frequency of storms may be more important than their intensity provided that the threshold value has been exceeded.

HI

D 2.5m

d-z25cm viscous fluid mnid

FiGuRE 15. Water particle motion beneath waves in shallow water showing the progressive transformation from a circular orbit at the water surface to linear oscillation at the sediment surface. The orbital paths are not to scale.

T

40

time

FiGuREF 16. Water particle motion at sediment surface during one complete period.

It is difficult to provide an estimate for the actual depth of mud which might be suspended in the water during such wave disturbances, but it is probable that the semi-empirical observations mentioned earlier in which the sediment was thought to have been mixed down to 10 cm represent a far from unusual occurrence. Such adepth is equivalent to about Wd.




(ii) Wave characteristics for Lake George Relations between wave characteristics, wind speed and fetch, i.e. the distance

over water along which the wind is blowing for lakes in which the water depth has no effect on the waves, can be expressed by the following equations (U.S. Army I962): gH/W2 =0.0026 [gEl W2]047, gT/W 0.46 [gF/W2]0228

/ O ~ profile in absence of overlying water

velocity

FIGauRE 17. Velocity-depth profile at the sediment/water interface when a density current is flowing under initially still water. (From Ippen & Harleman 1952.)

U'D

mean velocity of viscous layer

velocity

FIGURE 18. Assumed velocity-depth proffle at the sediment/water interface when water is flowing over an initially still, viscous fluid.

I7-2




where H is the significant wave height, W, the wind speed, g, the acceleration due to gravity, F, the fetch, and T, the wave period.

The wavelength, A, can be calculated from the following general relation for deep water waves: A = 1.56T2.

Fetch is not the greatest straight line distance that the wind travels over water since this can easily lead to unrealistic values because of the presence of islands and irregular shorelines. The exact definition of fetch and other details of the limnological application of these relations are more accessible in Smith & Sinclair (I972)-

TABLE 2. PERCENTAGE AREA OF LAKE BED WITH FETCH

BETWEEN INDICATED LIMITS

fetch/km % area < 2.5 26

2.5-5.0 23 5.0-7.5 17 7.5-10 17 >10 17

For Lake George, fetches were calculated for an easterly prevailing wind (see ? 5) at each intersection point of a 2 km x 2 km grid superimposed on the lake, and the values obtained used to estimate the percentage areas of the lake bed with fetches between given limits (see table 2). The lake length is about the same as its breadth so that the percentage areas for a wind from any other direction is not likely to be greatly different from those in the table.

The height and period of waves depends not only on the fetch but also on the length of time during which the wind is blowing. Initially waves grow larger as the wind continues to blow, but this time dependence does not continue indefinitely. The m'inimum storm duration, td, i.e. the shortest time for which wave characteristics at a fixed point cease to grow when a steady wind is blowing, is given by the following equation (U.S. Army I962)

td= KF/T.

Where K is a constant dependent on the units: K = 29.8 if td is in minutes; the fetch, F, is in kilometres and the wave period, T, is in seconds.

For the fetches occurring in Lake George, the minimum storm duration may be several hours and thus larger than actual duration of storms at the lake. The wave characteristics generated during storms lasting less than the minimum duration are related to the steady wave characteristics by the following approximate equations (I. R. Smith, unpublished, cf. appendix 2 for summary)

1it 8.95 t/rd Tt 4.35t/td H 1 + 7.95 t/td) H 1 +3.35t/td'

where Ht is the wave height generated during a storm of duration, t, and H, the wave height generated when storm duration td.




The difference between the two equations reflects the increased steepness of the waves developed during short duration storms.

The three previous sets of questions have been combined to predict the wave characteristics required to compute the theoretical water velocities at the sediment/ water interface (table 3). The equations have been adjusted to a standard storm duration of an hour. The assumption has been made that the lake depth has no influence on the wave height and period, and under such conditions the previous equation for UD is only approximately correct. The theoretical limit for the validity of this is that the wavelength must not be greater than twice the water depth but no serious error is introduced until A/D > 4. For the fetches occurring in Lake George this limit is not passed until the wind speed reaches 19 m s-' (force 8 on the Beaufort Scale). Table 3 shows that, at such wind speeds, the water velocities are considerably greater than the critical (25 to 30 cm s-1) so that errors in the estimation of wave characteristics are not likely to alter the general conclusions. These are that 19 m s-L and probably 15 m s-' (force 7) winds disturb by far the greater part of the lake bed. A third of the bed is affected by a 12.5 m s- (force 6) wind, and a (force 5) wind has no effect.

TABLE 3. THEORETICAL WATER VELOCITIES AT THE SEDIMENT/WATER INTERFACE (cm s-1)

Beaufort wind scale

5 (- 9.5 ms-1) 6 (12.5 ms-1) 7 (15 m s-) 8 (i 9 m s1) fetch/km UD UD UD UD

2.5 9.8 16.8 26.1 36.9 5.0 12.9 22.3 34.8 50.4 7.5 14.1 24.5 39.1 57.2

10.0 14.3 25.6 37.5 61.9

Winds at Kasese airstrip (figure 6) will underestimate the wind speed over water. Norrman (I964) indicates that the wind speed recorded at a lake shore must be multiplied by a factor of approximately 1.3 to give the over water wind speed at fetches greater than 5 km. Increasing the factor to 1.5 to allow for the under-exposure of the airfield site and so increasing the values given in figure 6b, it can be shown that wind speeds greater than 12.5 m s l over water occur at 0.2 % of the time or 17 h per year. This suggests that significant areas of the lake bed are disturbed once every three weeks on average if the mean storm duration is 1 h.

It is important to stress the approximate nature of all calculations relating to the disturbance of the sediment by wave action and that any conclusions reached can only give a rough assessment of the extent of erosion.




9. CONCLUSIONS

It is well known that the equatorial situation has a reduced seasonal climatic change, and that the temperature and light conditions will make for a continuious productive season. But for the purposes of the biological study of Lake George it is necessary to delineate as clearly as possible what degree of change does exist and whether this is important to the biology of the lake.

When one examines closely the climatic data it is apparent that, although Lake George might not suffer any extremes of climate, in fact what dictates the precise nature of the environment are the local peculiarities of geomorphology which shelter the lake from the continental features of rain and wind, and instead impart an enhanced flush of water from the high mountains (particularly important during true dry seasons) and a gentle wind regime created by local convections.

These characteristics would not be of such great significance if it were not for the lake having such a high surface area: depth ratio, as this means that the effect of surface stress by wind is maximized and can influence the whole volume of water, so that responses in the hydrology and thus the biota might be expected even from subtle climatic changes.

In this manner Lake George exaggerates the features which might be expected from only its equatorial position, which in itself creates a fairly unchanging environment, but which might be exemplified limnologically by other lakes near the equator both in East Africa and in Sumatra. The lake might therefore be thought of as being at the end of a continuous sequence of lake types which have a progressively lessening degree of seasonal envrironmental fluctuation.

Instead of an annual seasonal change the lake is characterized by the mrxajor physical and chemical changes occurring during a 24 h cycle. This dominance of the diel rhythm is well known for terrestrial tropical ecology but does not seem to have been investigated in detail for aquatic habitats, except for the survey of various tropical lakes by Baxter et al. (1965) which provides a useful comparison with Lake George for some physico-chemical features, and the photosynthesis investigations of Talling (X957).

Such a temperature and circulation cycle will also occur in the uppermost zone of deep tropical lakes (see, for example, Worthington 1930, Talling 1957, Viner I97o0b), but the shallowness of Lake George allows this feature to affect the whole of its water mass. Consequently, the lake might also be thought of as being near the end of a sequence of lakes with progressively increasing periods of climatically induced physical disturbance of the water column.

Emphasis has been placed here upon the analysis of the hydrodynamics within the lake. Under particular but very common circumstances wind-induced movements of water at the surface, and internal seiche type movements below these, can be rotary in direction. Also, the central parts of the lake are the most exposed to the wave action that is capable of affecting the nature of the sedimenits and the organisms in them. It is not surprising therefore that the total hydrodynamic



Geographical, historical and physical aspects 265 pattern imposed upon the biota is likely to be a concentric one. The direct implications of this pattern uponl the distribution of organisms is amplified by other con- tributors to this volume.

APPENDIX I. METHOD OF CALCULATION OF THE RUNOFF FROM

THE LAKE GEORGE CATCHMENT AREA

The water-balance calculation has been based upon published raw data of the Uganda Water Development Department (U.W.D.D. I969), in addition to unpublished data from the same authority.

Kauipere Mpanga * g

e rea 3

t uimi 111 201 /s f vX >, t}7 1 1 area 2 5

Mubuku - E f\ A Y44area 4 Dd l< {measured annual runofft

t \ ; lSOOm ; ; g J M5d~~~~~~6estimated annual runoffJ \ffivllok-D KO -1? a8f 27 gauging station |amulkclf 5ebwe , angaV

<~~~~~K 12502

?~~~~~~~~~~~~~~~~0 10 0 }

areas.~ ~ ~~ara



266 A. B. Viner and I. R. Smi'th (Discussion Meeting)

cl cq 06 ii

ci Ci C;

C) ul cq to t- C> o

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06 C; 6 C;

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00

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Two classes of records are available: (i) complete records, where the quoted monthly flow is the average of several flow estimates for every day of the month. Only one station with such records is within the Lake George catchment. (ii) Incomplete records where the quoted flow is a single determination at some time during the month. The gauging records account for about 58 % of the Lake George catchment. The remaining ungauged areas, for which the method outlined in ?4(d) and below has been used, have been divided into areas above and below

TABLE 5. RUNOFF DATA FROM STATIONS WITH INCOMPLETE RECORDS

Rukoki/ catchment Kamulikwezi Mubuku Ruimi Mpanga Mpanga Chambura

gauging station A+B D E F G H (cf. figure 19)

catchment area/km2 183 256 266 401 4670 660 period of record 1962-8 1962-8 1962-8 1962-8 1966-8 1962-8 average flow (i) Q 6.0 12.3 6.6 5.3 14.0 10.0

m- - (ii) Q33 4.1 12.5 6.0 4.2 11.5 9.5 equivalent annual

runoff/mm (i) during period of 714 1540 711 330 - 454

record, using mean of Q and Q33

(ii) adjusted - 78 (1954-68)

TABLE 6. CATCHMENT AREAS AND ANNUAL RUNOFF

(cf. table 1)

catchment area annual runoff inflow to lake kn2 mm 106 m3

entire catchment (adjusted 179 for differences in areas of comnponent catchments and including lake area)

surface area of lake 250 gauged areas Ruwensori

Rukoki/Kamulikwezi 183 714 130.7 Sebwe 83 788 777 64.5 Mubuku 256 1540 394.2 Ruimi 266 711 189.1

gauged area, Mpanga 4670 85 397.0 total gauged inflow 1175.5

ungauged area below 1200 m 1994 50 99.7 above 1200 m 1 833 400 333.2

2 7331 200 146.6 3 471 2253 250 117.7 4 216 350 75.6

total ungauged flow 772.8 total area = 9955 total inflow to lake 1948.3



268 A. B. Viner and 1. R. Smith (Discussion Meeting)

1200 m altitude, and that above 1200 m further divided into four regions (table 6, figure 19).

Table 4 shows the results of the analysis for stations with complete records. For stations with incomplete records it has been assumed that the monthly flow determinations form a random sample so that the mean of all the monthly estimates is approximately the average flow (Q in table 5). This assumption could be wrong, particularly because hydrologists usually try to measure as wide a range of flows as possible. This would bias the estimate of the average by giving too much weight to the infrequent high flows. Probability distributions of daily flows have the same general form irrespective of the size and climate of the catchment area, the average flow being exceeded two-thirds of the time. This has been used to provide a second estimate of the average flow (Q33 in table 5) and to calculate the equivalent annual runoff.

There are only 2 years of records from the lower Mpanga gauging station (G): 1966-8. The full records for the table 4 stations showed that the runoff in this

period was, on average, only 88 % of that for the period 1954-68. The Mpanga runoff, therefore, has been increased in proportion.

The annual runoff figures are shown on table 6 and marked on the map (figure 19). On the basis of these estimates, the ungauged inflow accounts for 40% of the total so that a 25 % error in the runoff estimate for the ungauged areas would result in a 10 % error in the total runoff volume.

APPENDIX 2. WAVE CHARACTERISTICS FOR STORMS

OF LIMITED DURATION

The duration of storms at Lake George is often less than the minimum time for steady-state conditions to be established, i.e. for waves at a fixed point to cease growing when a steady wind is blowing. It is necessary, therefore, to develop additional prediction equations for more characteristics when the storm duration is less than this minimum time, td.

Figure 134 in Sverdrup, Johnson & Fleming (1942), presents some experimental evidence on the growth of wave height and period for large ocean waves. The equation for the minimum storm duration, td, already referred to, namely, td = 29.8 F/T, is found to be accurate for large ocean waves as well as lake waves. It has, therefore, been assumed that the curves of figure 134 represent the general form of the relation for all values of td.

Table 7 (below), derived from figure 134, shows how wave height increases with time. If HtIH is plotted against t/td, the curve has the general equation y = ax/( 1+ bx) (fig. 20). The constants a and b are easily obtained by plotting ylx against y since this is a straight line (y/x =a -yb). The leads to the following prediction equation for wave height for storms of duration less than td,

Ht 8.95t/td H =1 + 7 .95t/td




An exactly similar procedure can be carried out to give a prediction equation for wave period, namely

Tt 4.35t/td T 1+3.35tItd

The difference between the two equations reflects the increased steepness of 'young' waves. Although the experimental evidence on which these equations are based is rather limited, it is believed that they adequately represent wave characteristics for storms of limited duration.

TABLE 7. GROWTH IN WAVE HEIGHT WITH TIME AT A FIXED POINT

height at time, t time, t/h t/td Ht/m HtIH

5 0.1 2.75 0.49 10 0.2 3.92 0.70 15 0.3 4.48 0.78 20 0.4 4.86 0.87 30 0.6 5.19 0.92 40 0.8 5.46 0.97 50 1.0 5.61 1.00

H is the wave height for steady state conditions.

_ y=ax/(l+bx)

i 0.5 1.0

t/td

FIGURE 20. The general form of the relation showing the increase in wave height with time.

The authors would like to thank Mr. W. G. Owen of the Uganda Water Develop- meent Department, and the East African Meteorological Department for the use of their river-flow and wind records.




REFER1ENCES (Viner & Smith)

Baxter, R. M., Prosser, M. V., Talling, J. F. & Wood, ZR. B. I965 Stratification in tropical African lakes at moderate altitudes (1,500 to 2,000 m). Limnol. Oceanogr. 10, 510-520.

Bishop, W. W. I970 Pleistocene stratigraphy in Uganda. Geol. Surv. Uganda Mem. 10. de Heinzelin, J. I957 Les fouilles d'Ishango. Inst. des Parcs. Nat. du Congo Belge, Explor.

du Parc Nat. Albert. Mission J. de Heinzlin (1950) Fasc. 2, 1-128. Doornkamp, J. C. & Temple, P. H. i966 Surface drainage and tectonic instability in part

of Southern Uganda. Geogr. J. 132, 238-252. Ganf, G. G. I969 Physiological and ecological aspects of the phytoplankton of Lake George,

Uganda, Ph.D. thesis. Lancaster University. Greenwood, P. H. I959 Quaternary fish fossils. Inst. des Parc Nat. du Congo Belge, Explor.

du Parc Nat. Albert. Mission J. du Heinzlin (1950). Fasc. 4, 1-80. Hutchinson, G. E. I957 A treatise on limnology, vol. 1. New York: John WViley. Ippen, A. T. & Harleman, D. R. F. I952 Steady state characteristics of subsurface flow. In

Gravity waves. Circ. U.S. Bur. Stand. 521, no. 12, 79-93. Jameson, J. D. & McCallum, D. I970 Climate. In Agriculture in Uganda (2nd Edn.) (ed.

J. D. Jameson). London: Oxford University Press. Lock, J. M. I973 The aquatic vegetation of Lake George, Uganda. Vegetatio (inl the Press). Mortimer, C. H. I96I Motion in thermoclines. Verh. int. Verein. theor. angew. Limnol. 14,

79-83. Norrman, J. D. I964 Lake Vattern. Investigations on shore and bottom morphology.

Geogr. Annaler 46, 1-238. Penman, H. L. I948 Natural evaporation from open water, bare soil, and grass. Proc.

R. Soc. Lond. A 193, 120-145. Smith, I. R. & Sinclair, I. J. I972 Deep water waves in lakes. Freshwat. Biol. 2, 387-399. Sverdrup, H. U., Johnson, M. W. & Fleming, R. H. 1942 The oceans. Englewood Cliffs,

N.J.: Prentice Hall. Talling, J. F. I957 Diurnal changes of stratification and photosynthesis in some tropical

African waters. Proc. R. Soc. Lond. B 147, 57-83. U.S. Army i962 Waves in inland reservoirs. Tech. Memo. no. 132. Beach Erosion Board,

U.S. Corps of Engineers. U.W.D.D. I969 Summary of hydrological records 1962-1968. Uganda Water Development

Department. Entebbe, Uganda. Viner, A. B. 197oa Ecological chemistry of a tropical African lake. Ph.D. thesis. University

of London. Viner, A. B. I97ob Hydrobiology of Lake Volta, Ghana. I Stratification and circulation of

water. Hydrobiologia 35, 210-228. Viner, A. B. I973 Responses of a mixed phytoplankton population to nutrient enrichments

of ammonia and phosphate, and some associated ecological implications. Proc. R. Soc. Lond. B 183, 351-370.

Worthington, E. B. I930 Observations on the temperature, hydrogen-ion concentration and other physical conditions in the Victoria and Albert Nyanzas. Int. Rev. Hydrobiol. 24, 328-357.

Worthington, E. B. I937 On the evolution of fish in the Great Lakes of Africa. Int. Rev. Hydrobiol. 35, 304-317.



a discussion on the biology of an equatorial lake: lake george, uganda || geographical, historical...

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