Hydrobiologia 288: 1-12, 1994.e 1994 Kluwer Academic Publishers. Printed in Belgium. 1

The Ethiopian Rift Valley lakes: chemical characteristics of asalinity-alkalinity series

Elizabeth Kebede 1,2, Zinabu G.-Mariam 3 & Ingemar AhlgrenIInstitute of Limnology, Uppsala University, Norbyviigen 20, 752 36, Uppsala, Sweden; 2 Biology

Department, Addis Ababa University, P.O. Box 3434, Addis Ababa, Ethiopia; 3 A wassa College ofAgriculture, P.O. Box 5, A wassa, Ethiopia

Key words: alkalinity, Ethiopian lakes, nutrients, Rift Valley, salinity

Abstract

The study on 10 lakes within the Ethiopian Rift Valley during March-May 1991 covered a range ofconductivity (K 25 ) between 286 and 49100 /S cm '. HCO3 - CO32- and Na+ were the dominant ionsin all the lakes. Concentrations of K', Cl- and SO4- increased with increasing salinity and alkalinity,whereas Ca2+ and Mg2 + decreased. Comparison of these data with previous records showed that aten-fold dilution of total ionic concentration occurred over 30 years in Lake Metahara and aboutthree-fold increase occurred over 65 years in Lake Abijata. Concentrations of soluble silica were gen-erally high (12-222 mg SiO21 - 1) and increased with increasing salinity, except for Lake Chamo whichshowed SiO 2 depletion (to < 1 mg SiO21 - 1) over the past three decades.

The relationship between ionic concentration and phosphorus was irregular although high phospho-rus concentrations generally corresponded with increasing salinity. Fitting data to the Dillon & Rigler(1974) chlorophyll a - total phosphorus relationship suggested that lakes Zwai, Awassa and Chamo arephosphorus-limited, whereas others have surplus phosphorus.

Introduction

Development of commercial fishery has been in-tensified over the last decade in some Rift ValleyLakes in Ethiopia: lakes Zwai, Langeno, Awassa,Abaya and Chamo. However, exploitation ofthese resources is developing with little knowl-edge about the fish production potential and basiclimnology of the lakes. The two neighbouringlakes Abijata and Shalla are of special impor-tance to wildlife in providing feeding and breed-ing grounds for a rich bird fauna of over 360species of resident and migratory birds. The twolakes have been enclosed in a national park since1970. But it has not been possible to promote

conservation due to increasing development ac-tivities in the area during the past ten years, whichhave resulted in progressively lower lake levelsand higher salinity in Lake Abijata, the feedingground for the bird populations. Chemical andbiological changes following the increased utili-zation of these aquatic resources in the countryhas not been given attention.

The chemical composition of lakes found inEthiopia is extremely varied. In an attempt toconsolidate data collected from the 1960's and70's, Wood & Talling (1988) surveyed records on28 Ethiopian lakes and gave an overview of therelationships between ionic composition, nutri-ents and phytoplankton. Although a substantial

2

amount of data is available on the major ioniccomposition of these lakes, there is a lack ofinformation concerning nutrients. Continuousenvironmental changes and recurrent droughts inthe country, resulting from both natural causesand human influence, make it necessary to updatethis data base. Thus, the purpose of this study isto update and compare previous data in order tocomplement our knowledge on the chemical com-position of the major lakes in Ethiopia.

Study area

The lakes in this study all lie within the EthiopianRift Valley, running NNE through the middle ofthe country (Fig. 1). These lakes are similar inaltitude and range from 1200 to 1680 m. How-ever, their physical characteristics and catch-ments are diverse (Table 1). The region experi-ences moist subhumid to semi-arid climate withevapotranspiration exceeding rainfall, thus incur-ring rainfall deficit. The mean annual rainfallvaries from about 1000 mm around lakes Abayaand Chamo to between 600-1200 mm aroundlakes Awassa, Shalla and Zwai, and falls below600 mm as the altitude decreases further north inthe rift (Daniel G., 1977). Annual evapotranspi-ration in the rift area south of, and including LakeMetahara, varies between 1100-1600 mm whichcontributes to an annual water deficit of up to1000 mm around Lake Metahara and between500-750 mm further south in the rift valley(Daniel G., 1977).

The impact of seasonal and interannual varia-tion in rainfall and water input is more pro-nounced in small, shallow lakes and terminallakes within closed drainage basins, a commonfeature within the rift valley. Zwai-Shalla basinincludes lakes Zwai, Langeno, Shalla and Abijatawith lakes Zwai and Langeno discharging intoLake Abijata. There is presently no surface out-let from lakes Abijata, Shalla and Awassa. LakeAwassa drains a large swampy area and has re-mained relatively dilute in spite of its closeddrainage. It is believed that seepage accounts forsolute losses in some of the closed basins. Lakes

Abaya and Chamo are connected via a small riverand are part of a much larger drainage basin whichincludes lakes Chew Bahr (Stephanie) andTurkana. The lava-dam lake Metahara, and thecrater lake Chitu are closed basins with no obvi-ous surface outflow. A more detailed descriptionof the hydrology of Ethiopian lakes is given inWood & Talling (1988).

Materials and methods

The study was made during March-May 1991,just after the dry season. Water samples werecollected from a single offshore station from dif-ferent depths and as integrated samples (Table 2)with a weighted, plexiglass tube (2 m height).Samples were then transferred into polyethylenebottles and transported to the laboratory in an icebox. Filtration was done through Whatman GF/Cglass fiber filters within 2 to 6 hours after sam-pling. Electrical conductivity was measured in situwith a conductivity meter (YSI Model 33 S-C-Tmeter) and values were corrected to 25 C as-suming a temperature coefficient of 2.3 % per C(after Talling & Talling, 1965). pH was measuredin situ with a digital pH meter (Canlab,Model 607). Temperature and dissolved oxygenprofiles were measured with a YSI Model 57oxygen meter and water transparency with astandard Secchi disc.

Carbonate-bicarbonate alkalinity was deter-mined by titration with HCI to a pH of 4.5(Golterman etal., 1978). Cl and SO42- wereanalysed by the ion-exchange method (Mack-ereth, 1955), and Cl- was determined separatelyby the argentometric method (APHA et al., 1980).Na+ , K +, Ca2+ and Mg2 + were analysed byICP (Induced Coupled Plasma) analysis. NO3was determined by the cadmium reductionmethod (Golterman etal., 1978), NH+ by theindophenol method as modified by Chaney &Marbach (1962), and dissolved SiO 2 (Wetzel &Likens, 1979) and PO3- (Murphy & Riley, 1962)by molybdate complex formation. Samples fortotal phosphorus (TP) analysis were digested withpersulphate as described by Valderrama (1981).

40°E

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Table 1. Morphometric and physical features of Ethiopian Rift Valley lakes.

Lake Altitude Surface area Max. depth Mean depth Catchment area Secchi depth

(m) (km2 ) (m) (m) (km

2) (cm)

Metahara*(Besaka) 1200 3.2 - 41

Koka 1660 200 - - - 28

Zwai 1636 442 7 2.5 7025 35

Langeno 1582 241 48 17 1600 25-35'

Abijata* 1578 176 14 7.6 1630 65

Shalla* 1558 329 266 87 3920 125

Chitu* 1600 0.8 21 - - 43

Awassa* 1680 90 23 10.7 1250 7 0 -8 0 bC

Abaya 1285 1162 13 7.1 17300 43

Chamo* 1233 551 13 - 2210 65

* Lack visible outlet. Tudorancea et al. (1989); b Demeke K. (1985); c Elizabeth K. (1987). Where references are not cited, Secchi

depth values represent single measurements taken on the sampling dates listed in Table 2.

Chl a was determined according to Tailing &Driver (1963) on pigments extracted in warmmethanol; no correction was made for degrada-tion products.

Results and discussion

Ionic composition

The chemical composition of the lakes is arrangedin order of increasing electrical conductivity(Table 2). Electrical conductivity (K 25) rangesfrom 286 to 49100 #S cm-' and is accompa-nied by an increase in alkalinity from 2.6 to573.7 meq 1 - '. Conductivity and salinity ismainly accounted for by Na + and HCO -CO2 -, which give rise to high pH in the mostsaline lakes. Increasing Na + concentration andalkalinity is accompanied by an increase in K+,C1- and SO4- concentrations (Fig. 2). The di-valent cations Ca2 + and Mg2 + however, show aninverse relationship with salinity and alkalinity,since they are removed from solution when car-bonate precipitates under highly alkaline condi-tions. A detailed discussion of the major ions isgiven in Wood & Talling (1988).

Temporal changes have occurred in the ioniccomposition of two lakes. The lava-dam lakeMetahara showed a decrease in total ionic con-

centration by about a factor of 10 over 30 years(Table 3). The decrease in concentration washighest for Na+ , HCO£ and Cl- (10-12 fold)and lowest for K + (6 fold). Low concentrationsof Ca 2+ and Mg 2+ make comparisons difficult,but it appears that decreasing alkalinity accom-panied by a slight decrease in pH (by 0.5 unit)could have been accompanied by a rise in theconcentration of Ca 2 + . Lake Metahara has in-creased in size over the past 20 years, possiblydue to a combination of subterranean seepagefrom the basin and spillage from the nearby riverAwash. The change in ionic concentration ap-pears to have been accompanied by a shift in thephytoplankton community. Arthrospira fusiformis(Voronich.) Kom. & Lund ( = Spirulina platensis,the characteristic blue-green algal species inAfrican soda lakes, was reportedly dominant in1961 in Lake Metahara (Wood & Talling, 1988),but totally absent in 1991 (unpublished data). In-stead, the phytoplankton community was com-posed of a mixture of green and blue-green algae.Although seasonal variation in species composi-tion can not be ruled out, such a succession isunlikely to occur unless there is a major changein the chemical environment.

The terminal lake Abijata showed a consider-able increase in ionic concentration with themaximum recorded since 1926 (Table 4). Fluc-tuations of ionic concentration can be seen from

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crease in Ca2 + and Mg 2 + concentrations (Fig. 3),as is often the case with saline lakes.

A one-year study in 1981 by Kassahun W.(1982) showed seasonal variation in the ionicconcentration of Lake Abijata. Na+ concentra-tion and alkalinity increased by about 1.6 timesby the end of the dry season, but current valuesstill exceed the maximum concentration recordedin 1981. Past fluctuations of lake level were mainlyattributed to seasonal and interannual variationsin rainfall within the drainage basin. Additionally,the loss of water from Lake Abijata has beenenhanced by recent development schemes in thearea such as pumping of water from the lake forsoda ash extraction, diversion of the feeder riversto Lake Zwai and direct use of Lake Zwai waterfor irrigation. Relatively shallow depth and ter-minal position in the drainage area make LakeAbijata more susceptible than the other threelakes in the basin (lakes Zwai, Langeno, Shalla)to the effects of changes in water input. Wood &Tailing (1988) previously pointed out the salinityincrease in the closed basins of lakes Abijata,Awassa, Abaya and Chamo.

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Fig. 2. Concentration of the major ions in the Ethiopian RiftValley lakes in relation to total ionic concentration (expressedas conductivity). Numbers by the horizontal lines representthe lakes arranged in order of increasing conductivity as inTable 2.

the records over the past few decades. Between1926 and 1991 the salinity has increased bymore than 2.6 times and the alkalinity by about4 times. Cl - showed a two-fold increase over the42 meq I - 1 recorded both in 1926 (Omer-Cooper,1930) and 1938 (Loffredo & Maldura, 1941). Thedominant cation Na+ appears to have been con-centrated more than three-fold. The increase insalinity and alkalinity was accompanied by a de-

Nutrients

Si concentration was variable among the lakeswith a general tendency to increase with increas-ing salinity (Table 2). Values exceeding 100 mgdissolved SiO 2 1 - were found in the saline lakes.Concentrations over 10 mg SiO2 1 ' are com-mon in African lakes due to high mobility of Siin most tropical soils and porous volcanic lavas,relatively high importance of ground water inputs,and enhanced dissolution of Si compounds insaline waters of high alkalinity and pH (Talling &Tailing, 1965).

With the exception of Lake Chamo, concen-tration of Si in the surface waters ranged from14.6 mg 1- in the most dilute Lake Koka to189 mg I - SiO2 in Lake Chitu. The concentra-tion in Lake Chamo was the lowest reported in 25years. It has decreased consistently over the yearsfrom 28 mg I l in 1964 (Wood & Talling, 1988)to less than 1 mg I ' SiO2 in 1991 (Fig. 4). Al-

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Table 3. Change in the ionic composition of Lake Metahara.

Year K2 5 Z Cat E An Na+ K' Ca2 + Mg2 + HCO Cl SO - TP pH(ktS cm C) +CO32 (gl I)

(meq I l)

1961* 74170 784 831 774 10.4 <0.15 <0.6 580 154.6 97.5 11000 9.91991 7440 80 71 79 1.7 0.11 0.04 46 12.6 12.1 1850 9.4

* Data from Tailing & Tailing (1965).

Table 4. Fluctuations with time in ionic composition of the surface water of Lake Abijata.

Reference Date Salinity E Cat Na K' Ca2 Mg2 + Alkal. Cl SO4(g I l) (meq I l)

Omer-Cooper Nov. '26 8.13 125 0.500 0.833 80 42(1930)Loffredo & Apr. '38 8.36 133 130 1.9 0.425 0.50 42 1.4Maldura (1941)De Filippis (1940) Apr. '39 150 140 10.3 0.175 0.067 81 40Tailing & Tailing May '61 19.38 285 277 8.5 <0.15 <0.6 210 91 15.0(1965)Wood & Tailing Mar. '64 16.20 228 222 6.5 <0.1 <0.1 166 51 22.5(1988)Von Damm & Jan./Feb. 12.96 199 194 4.9 0.084 0.046 138 54 0.3Edmond (1984) '76Kassahun W. Nov. '80- 21 238-388 231-378 6.9-9.9 <0.01 <0.01-0.01 180-297 82-121 4.0-5.7(1982) Oct. '81

Mar. '91 26 425 416 9.7 0.120 0.023 326 88 24.0

though not as pronounced, a decline was alsofound in some of the other lakes.

The reason for this decline is not clear. Re-moval of silicate from solution can be significantin lakes dominated by diatoms (e.g. lake Shalla)or in others with large diatom populations (e.g.lakes Zwai, Awassa, Chamo, Abaya). In LakeChamo, Belay & Wood (1982) found a relationbetween decreasing SiO2 concentrations andlarger diatom growth, which simultaneously re-moved much of the nitrogen (N) and phosphorus(P) from the water. Algal blooms and fish killshave been more common in this lake since 1978,when they were first reported (Belay & Wood,1982), and the severe depletion of Si is commonlyassociated with increased diatom growth andlower dissolution rates of silicic acid from diatomfrustules. A 10 fold decline in SiO2 concentrationaccompanied by increased algal production has

been recently reported from Lake Victoria (R.E.Hecky-SIL Congress, 1992). A gradual decreaseof Si concentration with increasing diatom pro-duction (due to increased N and P concentra-tions) has been documented from lakes Michigan(Schelske, 1988) and Ontario (Stoermer etal.,1985), and the Baltic Sea (Wulff& Rahm, 1988).Active dissolution of diatom frustules is inhibitedby organic matter accumulation in the sedimentsas was shown by Hecky & Kilham (1973) forsome alkaline lakes in East Africa. This suggeststhat diatoms are acting as a permanent sink in thelakes with appreciable diatom populations (LakeShalla) and progressive phytoplankton produc-tion (lakes Chamo, Zwai). All comparisons weremade on surface values (Table 2) and samplesfrom Lake Shalla (max. depth, 266 m) would haveprobably shown higher SiO2 concentrations atgreater depths.

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Fig. 3. Fluctuations with time in a) alkalinity level and b)concentrations of calcium (Ca2 ) and magnesium (Mg 2 + ) inthe surface waters of Lake Abijata. Vertical line between twovalues represents the range of measured concentrations. Re-gression between time (years) and the ions gives the equations:alkalinity = 58.000 + 3.3128 years (r2 = 0.668); Ca2 = 0.404-0.00655 years (r2 = 0.722); Mg 2 = 0.598 - 0.01008 years

(r2 = 0.530).

Reverse weathering (neoformation of alumino-silicate minerals) has been proposed as a majorprocess in closed basins of Ethiopian rift lakes,removing Si, base cations and HCO3 from so-lution (Von Damm & Edmond, 1984). An ex-ample of this reaction is

AI2 Si2O5(OH) 4 + 5Mg 2 + Si(OH)4 + 10CO3 + 5H2 0-kaolinite

Mg5 AI2 Si3 Olo(OH)8 + 10HCO-.chlorite

The decline of Si concentration in the Ethiopianrift lakes is not accompanied by decreasing levelsof alkalinity and Mg2+ , but Ca2 + showed a de-crease in the dilute lakes Chamo and Abaya.Other biogenic processes influencing alkalinityand Ca2+ levels might also mask the possibleeffect of reverse weathering in the removal of those

solutes. Wood & Tailing (1988) have cast doubton the proposal by Von Damm & Edmond forEthiopian lakes, the reason being that the massbalance calculations were based on rough hydro-logical information. However, the phenomenonhas been identified as one of the gradual, bio-geochemical pathways in saline lake evolution ofBasuto Lake District, Tanzania (Kilham & Cloke,1990).

No apparent relationship was found betweenionic concentration and N (Table 2). NO- -N wasdepleted in all the saline lakes except in the sur-face waters of Lake Metahara (88.7 Mg 1- ). InLake Abijata, NHg -N increased with depth from88 pg 1- ' in the 0-2 m column of water to 123 pgI - in 3-4 m. The increase was more pronouncedin Lake Chitu where NH+ -N was below detec-tion level in the surface waters and 35 jig 1- at8 m. Dissolved 02 showed sharp discontinuitiesbetween 0.5-1.5 m and between 3-4.5 m, withcomplete deoxygenation below 5 m. The smell ofhydrogen sulphide was detected at depths below8 m. Stratification was sharp in Lake Abijata be-tween 0.50-1.25 m with the temperature drop-ping from 25 C to 21.2 C, and dissolved 02

from supersaturation to 4.2 mg 1- ', which fur-ther decreased to 1.4 mg 1 - ' at 4 m. With a maxi-mum depth of less than 14 m, stratification islargely diurnal in Lake Abijata (Baxter et al.,1965), but can be seasonal in the deeper and pro-tected crater Lake Chitu. Thus the reduced formsof N and S (sulphur) can be transported up fromthe anoxic layer during destratification. Samplingwas done immediately after the dry season, whenthe cooling effect of the short rains and inflowscan disrupt the stability of the water column, animportant factor governing stratification patternsin most African lakes (Talling, 1986). It is alsoworthy to note that NH+ input from excreta ofthe large population of waterfowl (mainly flamin-goes) feeding on and around lakes Abijata andChitu can be large.

Very low NO concentration was found in thedilute, shallow lakes Koka and Zwai. However,NH4 -N concentration was still considerable inLake Zwai (36.3 pg 1 -'), which had a compa-rable phytoplankton density with the most saline

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Fig. 4. Concentration of dissolved silica (mg SiO, 1- ) recorded during 1938-1991 in some lakes of the Ethiopian Rift Valley.Vertical line between two values represents the range of measured concentrations.

Lake Chitu. The highest concentration ofNO3, as well as dissolved inorganic nitrogen(DIN), was found in Lake Abaya where a lowphytoplankton density was estimated (chl a,5 g ').

High ionic concentration corresponded withhigh PO4 -P (SRP-soluble reactive phosphorus)

and TP (Fig. 5), with levels of up to and exceed-ing 2 mg 1- ' P0 4 -P in Lake Chitu. The fresh-water lakes (salinity<3.0 g 1 ') had relativelymuch lower P with P04 -P concentrations belowdetection limit in the productive Lake Zwai (chl a,154/ g 1 '). The P0 4 -P to TP proportion byweight was less than 0.4 in the dilute lakes except

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Fig. 5. Concentrations of soluble reactive phosphorus (SRP)

and total phosphorus (TP) in the Ethiopian Rift Valley lakes

in relation to ionic concentration. Numbers by the horizontal

lines represent the lakes as in Table 2.

in Lake Abaya (0.6) where production is limiteddue to high inorganic turbidity. The proportionwas lowest in the most dilute lakes Koka andZwai and highest (ca 0.9) in the saline lakes Shallaand Chitu. In spite of the high algal standing cropin Lake Chitu (chl a, 224 yg 1 - 1), there was stilla large supply of available P.

Lake Abijata had a relatively low TP concen-tration compared to the other saline lakes. How-ever the actual TP is believed to be higher than themeasured concentration, because at the time ofsampling there was high spatial heterogeneity ofphytoplankton and suspended matter. For ex-ample, the northern shoreline of the lake was cov-ered by a thick algal mat extending about 20-30 m offshore. Surface accumulation of planktoncan be noted from chl a values which show anine-fold difference between the surface and0-2 m water column (Table 2). The proportion ofPO4 -P to TP is only 0.2, and is much lower thanin the saline lakes and most of the dilute lakes.Thus, most of the P is bound in either organic orunavailable forms, but there is still surplus SRP.

Very low inorganic N concentration, in con-trast to their high P content, was suggested byTalling & Talling (1965) to be limiting algal pro-duction in some African lakes. Melack et al.(1982) showed that P was more limiting thanNH4 -N in two Kenyan soda lakes, Elementeitaand Sonachi. They based their conclusion onanalyses of N, P and C (carbon) content of the

100

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Fig. 6. The relationship of chlorophyll a to total phosphorus(TP) in the Ethiopian Rift Valley lakes; the line passing throughthe points is the regression line from Dillon & Rigler (1974).Numbers represent the lakes as in Table 2.

lake water, and P enrichment experiments in LakeSonachi. Inorganic N and P were nearly unde-tectable in both lakes, and enrichment with P inLake Sonachi enhanced algal abundance. The sa-linity and TP concentrations reported from LakeElementeita are comparable to values found inLake Abijata. Available P, however, is muchhigher in Lake Abijata which suggests that P isnot limiting. The net phytoplankton of Lake Abi-jata was dominated by Arthrospirafusiformis anda filamentous blue-green algal species with a highdensity of heterocysts, indicating that they are Nlimited (although N fixation was not measured).

It appears that there was no simple relationshipbetween chl a and TP (Fig. 6), and DIN (Table 2)among the series of lakes. Interpretation would bedifficult without information on total nitrogen(TN) concentration as employed by Kalff(1983).The ratio of dissolved inorganic N and P(DIN:PO4) is lowest (<0.1) in the saline lakesChitu, Shalla and Metahara, and highest (> 3) inthe dilute lakes Zwai, Langeno and Awassa(Table 5). The ratio DIN:TP gives the same trendas DIN:PO4, with the exception of Lake Zwaiwith undetectable PO4-P concentration and highDIN:PO4. From these ratios, N limitation seemsmore likely to occur in the saline lakes withdecreasing importance in the freshwater lakes.

The freshwater lakes Zwai, Awassa andChamo fit the plot of the relationship betweenchl a and TP derived by Dillon & Rigler (1974)

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11

Table 5. The ratios by weight of dissolved inorganic nitrogen,

to dissolved inorganic and total phosphorus (DIN:PO4,DIN:TP) in Ethiopian Rift Valley lakes.

Lake DIN:PO4 DIN:TP

Koka bd:bd bd:224Zwai 40.2:bd 0.2Awassa 3.3 1.1Abaya 1.3 0.8Chamo 1.2 0.2

Langeno 4.7 1.3Metahara 0.1 0.05Shalla 0.005 0.005Abijata 1.0 0.2Chitu bd:1985 bd:2300

bd = concentration below detection.

for temperate lakes and used as an index of Plimitation. Another indication of P limitation isgenerally supported in lakes with high chl a con-tent relative to TP (Fig. 7). The chl a:TP ratiosuggests surplus P in all the lakes where algalproduction is possibly limited by low light condi-tions (Secchi depths, Table 1); this was also notedby Kalff( 1983) for some Kenyan lakes. High chl acontent relative to TP and a good fit to the Dillon& Rigler relationship were found in the twoKenyan freshwater lakes Naivasha and Oloiden(Kalff, 1983), and suggest that they are P limited.

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0

-

A stable colloidal suspension of silt impartshigh turbidity in lakes Langeno and Abaya re-ducing the production zone (Zeu). Wood et al.(1978) have shown that light attenuation due tonon-algal turbidity was high in the four lakes theyinvestigated: lakes Langeno, Shalla, Zwai andAbijata. In the last two lakes, windy conditionsand mixing commonly result in resuspension ofsediments from their shallow depths. High algalbiomass is maintained within the narrow euphoticzone of lakes Zwai (< 1 m) and Chitu (< 1.5 m)and contributes to light attenuation. A deep mix-ing zone in contrast to a relatively shalloweuphotic zone makes Lake Shalla an opticallydeep and light-limited lake. Light can also be alimiting factor in the reservoir-lake Koka(Zeu - 0.6 m) with high silt load transported bythe river Awash, and Lake Metahara (Zeu - 1 m)with an opaque, brown coloured water probablyfrom inorganic sources. Low light climate in sevenout of the ten lakes studied is highly likely toimpart light limitation and mask nutrient limita-tion. Experimental testing of natural populationsof phytoplankton with lake water under differentnutrient and light conditions would help to dis-tinguish between the limiting factors.

Acknowledgements

We are very grateful to Demeke Admassu for hisinvaluable help in the field and suggestions, andC. Hillman for providing us with an outboardengine and a boat. We express our thanks to allcolleagues and friends in Ethiopia who made itpossible for us to travel in the country at a timewith the most severe shortage of petrol. We thankR. Figueroa and J. Petterson for analyses of thecations, and A. Olin for ideas and discussions onion analyses. We are very grateful to J. F. Talling,G. Ahlgren and E. Willen for their valuable

u.u* . 0

IoI

0 St (d v | m"" "~~~~~.0= ~~4 N I In .0

Fig. 7. Chlorophyll a content per unit phosphorus in tlEthiopian Rift Valley lakes. The lakes are arranged in ordof increasing conductivity.

= comments anu improvements on tile IlIanuscnlpt.

,) The study was conducted within the EthiopianFlora Project, a SAREC-sponsored project

le between Addis Ababa University and Uppsalaer University.

- o -to___ - -_ __ 1 o h _- ------

12

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