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kimnologica 32, 169-179 (2002) © Urban & Fischer Verlag http://www.urbanfischer.de/journals/limno LIMNOLOGICA

The effects of wet and dry seasons on concentrations of solutes and phytoplankton biomass in seven Ethiopian rift-valley lakes

Gebre-Mariam Zinabu*

Debub University, Faculty of Natural Sciences, Awassa, Ethiopia

Received January 21, 2001. Revised December 5, 2001 .Accepted December 12, 2001

Abstract

The chemical characteristics and phytoplankton biomass (measured as chlorophyll-a concentrations) of seven lakes and one reservoir in the Ethiopian rift-valley were studied during the wet and dry seasons between 1990 and 2000. Mean concentrations of three major plant nutrients (nitrate-nitrogen, soluble reactive phosphorus, and silicate) increased during the wet seasons in four of the seven lakes, presumably as a result of mixing events and input from runoff. The changes in the major nutrient concentrations in the rest of the lakes were variable, but concentrations were usually higher during the dry seasons, most likely as a response to temporal variation in the phytoplankton biomass, pH measurements of the lakes did not show marked differences between the wet and dry seasons. Salinity (measured as conductivity) and total ions seemed to increase during the wet seasons in some of the lakes, possibly as a result of inflows that might carry high concentrations of solutes due to the heavy rains. Chlorophyll-a concentrations were higher during the dry seasons in most lakes except in three relatively more productive lakes. The results suggest that there could be light limitation in some of the Ethiopian rift-valley lakes, and events associated with the wet and dry seasons could bring about contrasting changes in nutrient levels and phytoplankton biomass in lakes, depending on the physical characteristics of the lakes.

Key words: Ethiopian rift-valley lakes - seasons - salinity - nutrients - chlorophyll-a - light limitation

Introduction

Factors that regulate seasonal changes in nutrient dynamics and phytoplankton biomass in tropical aquatic systems, where seasonal variation in temperature is low, are not the same as those in the temperate systems. In tropical systems the alternation of wet (rainy) and dry seasons may dictate seasonal variation in net water input and water loss, which may lead to changes in solute concentrations (TALLING & LEMOALLE 1998). Moreover, rains may affect the phytoplankton biomass through

their effects on other limnological variables like mixing, light and temperature regimes, etc. (ZINABU • TAYLOR 1989).

Limnological studies on the Ethiopian rift-valley lakes have frequently been done by short-term visitors who did not make repeated visits to the lakes or did not maintain short interval samplings of the lakes (WOOD & TALLING 1988) and thus not many seasonal studies have been made. The seasonal studies that were carried out in the 1980's focused on relatively few lakes (WODAJO & BELAY 1984; KEBEDE ~; BELAY 1994; TILAHUN 1988;

*Corresponding author: Dr. Gebre-Mariam Zinabu, Visiting Scientist, Uppsala University, Department of Limnology, Norbyv~gen 20, SE - 752 36 Uppsala, Sweden; Phone: +46(0)18-471-5720, Fax: +46(0)18-53 11 34, e-mail: [email protected]

0075-9511/02/32/02-169 $15.00/0 Limnologica (2002) 32, 169-179

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170 G.-M. Zinabu

ZINABU & TAYLOR 1989; KIFLE ~: BELAY 1990), and the time span of the studies did not go much over 12 months, making information still incomplete. Because of various local fluctuations in climate from year to year, analyses of seasonal studies should extend several years (WETzEL 2001). Therefore, repeated seasonal studies concurrently carried out in many lakes over an extended period of time would be more informative than short-term studies in one or two lakes. Since seasonality in Ethiopia is dominated by the alternation of wet and dry seasons (Wood & TALLINO 1988), systematic studies of the effects of wet and dry seasons on Ethiopian lakes based on long-term sampling during the wet and dry seasons are needed, and such studies will contribute to our knowl- edge of seasonality in tropical lakes. The objective of this study was to examine how the wet and dry seasons influence the chemistry and chlorophyll-a concentrations (phytoplankton biomass) of Ethiopian rift-valley lakes.

Description of the Study Area

A map and morphometric features of the Ethiopian rift- valley lakes included in this study are given in Fig. 1 and Table 1, respectively. The lakes lie within the Ethiopian rift-valley running NNE through the middle of the country at altitudes between 1200 and 1680 m. These lakes exhibit a wide range of salinity, and bicarbonate/carbonate are the major anions and sodium is the dominant cation - a common feature of the East African lakes (TALLING & TALLING 1965; VON DAMM & ED- MOND 1984; WOOD & TALLINO 1988). TELFORD (1998) speculates that a possible explanation for the high concentrations of bicarbonate/carbonate in these lakes is the inflow of sodium bicarbonate rich groundwater de- rived from the dissolution of silicate minerals and glass by carbonic acid, formed from the solution of carbon dioxide. The proportions of maj or cations in these lakes are Na > K > Ca > Mg unlike most of the lakes and

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Limnologica (2002) 32, 169-179

Wet and dry seasons in Ethiopian lakes 171

Table 1. Altitude and morphometric features of the Ethiopian rift-valley lakes included in this study. The table is reproduced from KEBEDE et al. (1994) and WOOD • TALLING (1988). Lakes that lack visible outlets are indicated by asterisks (*).

Lake Altitude Surface area Max. depth Mean depth Volume Catchment area (m) (km 2) (m) (m) (km 3) (km 2)

Koka 1660 200 . . . . Zwai 1636 442 7 2.5 1.6 7025 Abijata* 1578 176 14.2 7.6 1.1 1630 Langano 1582 241 48 17 5.3 1600 Shalla* 1558 329 266 87 36.7 3920 Awassa* 1680 90 22 11 1.34 1250 Abaya 1285 1162 13.1 7.1 8.2 17300 Chamo* 1233 551 13 - - 2210

rivers in the temperate region where the relationships are Ca > Mg > Na > K (TUDORANCEA et al. 1999; WET- ZEL 2001). Closed drainage basin, a major feature among several of these lakes (see Table 1), plays an important role in ionic composition and salinity, especially in the shallow basins (I~BEDE et al. 1994). Rivers and streams arising in the highlands feed most of these lakes, and the most dilute waters (waters of very low ionic content hence electrical conductivity) may be found in these rivers and streams (HARRISON & HYNES 1988). Since the water in the rivers is flowing, there is much less time available for accumulation of dissolved substances than there is in the lakes, and the rivers are generally less saline than the lakes in the same vicinity (TALLING ~; LEMOALLE 1998). However, the addition of saline thermal waters that obtain the solutes from the rocks through which they pass, may sometimes lead to higher concentrations of solutes in the rivers (TELFORD 1998).

The climate of the rift-valley region is dry sub-humid with distinct wet and dry seasons, and lower altitudes in the region experience reduced precipitation and elevated evaporation (MAKIN et al. 1975; GAMACHU 1977; GASSE et al. 1980). These lakes are, therefore, situated in a region of high rainfall deficit with evapotranspiration exceeding precipitation, and the balance between rainfall and evaporation influences the hydrologic regime of the lakes (TUDORANCEA et al. 1999 and references cited therein). Evapotranspiration values exceeding 1600 mm are found in most of the rift-valley (MAKIN et al. 1975), and they could be as high as 3000 mm in the southern part of the rift-valley (GASSE et al. 1980). Due to the fact that the evapotranspiration exceeds the rainfall in the Ethiopian rift-valley area, the drainage basins in this region may suffer seasonal or annual water deficit. The ef- fect of such water deficit is that many of the lakes have no outflow and all are concentrated relative to their inflows. In a nutshell, direct precipitation, water inflow, evapotranspiration, and water deficit are the main pro- cesses in the hydrological budget of the lakes in the

Ethiopian rift-valley (TELFORD 1998). More detailed de- scriptions and hydrology of the lakes are given in WOOD & TALLING (1988).

Materials and Methods

Surface water samples were collected from seven Ethiopian rift-valley lakes - Zwai, Abijata, Shalla, Langano, Awassa, Abaya, Chamo and the Koka Reser- voir - during the wet and dry seasons between 1990 and 2000. Samples were collected at least twice a year from Lakes Abijata, Awassa, Abaya and Chamo, and at least twice every other year from the rest of the lakes making sure that samples were taken in different seasons of the year. Analyses of major ion concentrations were carried out on samples collected from all lakes at least twice a year every alternate year during 1991 to 2000, except for samples from Lakes Awassa and Shalla whose ionic concentrations were measured less regularly. Surface water samples from all the lakes were taken at offshore stations (usually about the center of the lakes), using acid- washed 2-L polyethylene bottles. Whenever samples were collected from inflows (rivers), a sample for each inflow was taken from the deepest location close to the site where the inflow enters the lake. Samples were transported to the laboratory in dark boxes containing lake water that maintained the temperature of the samples close to that in situ. Samples were filtered through Whatman GF/C glass-fiber filters within 2 to 8 hr of sampling and stored in polyethylene bottles at tempera- tures close to 4 °C for nutrient analyses. Nutrient analyses of filtered samples were done within 48 to 72 hr of sample collection.

Conductivity and pH were usually measured in the field with the exception of samples for Lakes Abaya and Chamo, for which measurements were made in the laboratory within eight to ten hours of sample collection. A YSI model 33 S-C-T conductivity meter and a 3311 probe were used to measure conductivity, and all values

Limnologica (2002) 32, 169-179

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172 G.-M. Zinabu

were corrected to 25 °C using a temperature coefficient of 2.3% per °C (TALLING & TALLING 1965). A portable digital pH meter model CG 818 (Schott Ger~ite, Ger- many) was used to measure pH.

Alkalinity (bicarbonate + carbonate) was determined by titrating to pH 4.5 with standard acid within less than 8-10 h of sample collection. Ammonia- nitrogen (NH 3 + NH4 +- N) was determined by the phenol-hypochlorite method after CHANEY & MAR- BACH (1962). Nitrate-nitrogen plus nitrite-nitrogen ( N O 3- + N O 2- - N) w a s measured as nitrite by the cad- mium reduction method (GOLTERMAN et al. 1978). Solu- ble reactive phosphorus (SRP) was determined by the modified method of MURPHY & RILEY (1962) as outlined by MACKERETH et al. (1978). Silica (SiO2) was determined as its molybdate complex as outlined by WET- ZEL & LIKENS (1979).

Calcium, magnesium, potassium, and sodium were determined by the atomic absorption spectrophotometric method. Chloride was determined by the mercuric nitrate-nitrogen titrimetric method, and sulfate was determined by the barium sulfate turbidimetric or gravimetric method depending on the concentrations in the samples (APHA, AWWA & WPCA 1986).

Chlorophyll-a concentrations of phytoplankton was determined spectrophotometrically after filtering 200-250 ml of water samples through Whatman GF/C glass-fiber filters and extracting the filters in cold 90% methanol overnight in the dark. The abbreviated formula of TALLING & DRIVER (1963) was used to calculate the concentration of chlorophyll-a. No correction was made for degradation products.

Rainfall data were obtained from three nearest meteorological stations to each of the lakes. For Lakes Abaya and Chamo data were obtained from the state farms of southern Omo (Arbaminch), for Lake Awassa from Awassa Institute of Agricultural Research, and for the rest of the lakes from Adamitulu Research Center. A "wet" (rainy) month and "dry" month was distinguished according to GAMACHU (1977) and references cited therein. A "rainfall coefficient" for each month was cal- culated for each of the three lake regions. Rainfall coefficient is the ratio between the monthly rainfall and one- twelfth of the annual rainfall (the latter is called the "rainfall module"). A month is designated "rainy" when the monthly rainfall coefficient is greater than or equal to 0.6 (60% of the rainfall module).

average amount of rain in most of the dry seasons for all lakes was always less than 40% of that in the wet seasons, there were a few dry season months when rainfall was as high as 59% of the wet season's rainfall. Howev- er, typically the dry seasons had only 10 to 15 % of the rainfall in the wet seasons. Rainfall data obtained during this study period show that rainfall patterns were not the same from year to year and a month designated as wet in one year could be a dry month in another year. For ex- ample, according to the data from Adamitulu Research Center, July and August were the only months that could be classified as "wet" 100% of the time, and December was the only month that was classified as "dry" all the times during the study period. November and January, which are considered to be dry, were wet months 10% of the time.

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Rainfall regime In each region the dry seasons always had less than 40% of the rainfall in the wet seasons (Fig. 2). Although the

Fig. 2. Rainfall pattern in the Ethiopian rift-valley lakes region during the wet and dry seasons of 1989-2000. Note that data were obtained from meteorological stations in Arbaminch for Lakes Abaya and Chamo, from Awassa for Lake Awassa, and from Adamitulu for the rest of the lakes.

Limnologica (2002) 32, 169-179

Major nutrients, pH and chlorophyll-a

Mean concentrations of almost all nutrients increased during the wet seasons in Lakes Abijata, Awassa, Zwai, and Shalla, except that concentrations of ammonia-nitrogen in Abijata and silicate in Shalla decreased. In Lake Shalla ammonia-nitrogen and nitrate-nitrogen were not detected during the dry seasons (Table 2). On the other hand, in Lake Chamo mean concentrations of all nutrients except silicate were higher during the dry seasons. In both Lakes Shalla and Chamo silicate con-


centrations were relatively lower during the seasons when nitrate and phosphate were higher. The changes in Lakes Abaya and Langano were variable. In these two lakes, concentrations of soluble reactive phosphorus (SRP) and ammonia-nitrogen were higher during the dry seasons. Seasonal differences in pH were not conspicu- ous, and slightly higher values were associated with wet seasons in five out of the seven lakes. Chlorophyll-a concentrations were higher during the dry seasons in all the lakes except in Lakes Abijata, Awassa, and Zwai (Fig. 3).

Table 2. Mean concentrations and ranges (in parentheses) of major plant nutrients and pH in seven Ethiopian rift-valley lakes during the dry and wet seasons of 1990-2000. Data are not available for the reservoir Lake Koka. All concentrations are given in pg 1-1, except SiO2 which is given in mg I ~, (N = Number of samples; N.D. = Not Detected).

Lake Season N SiO2 SRP NO3-+NO2-- N NH3+NH4+- N pH

Abaya Dry 11 18.0 187.9 47 32.3 8.96 (9-27) (139-284) (19-81.9) (0-70) (8.7-9.3)

Wet 10 22.8 I20.8 128.5 23.6 8,67 (17-29) (77.2- 147) (5-180) (7.5-50) (7.85-9.0)

Abijata

Awassa

Chamo

Langano

Shalla

Zwai

Dry 11 65.9 180.5 1.8 78 9.91 (36-106) (24-576) (0 - 5.2) (0-234) (9.70-10.05)

Wet 12 81.4 1009.4 9.5 36 9,93 (53-122) (115-2790) (2.8-16.2) (0-95) (9.50-10.3)

Dry 12 36.1 14.2 7.3 29.5 8.79 (19-61) (0-59) (0.75-13.9) (0-75) (8.31-9.00)

Wet 14 40.1 18.8 20.6 179.2 8.87 (32-65) (1.5-35) (0-58.3) (3-1328) (8,7-9.1)

Dry 11 1.3 39.2 54.4 21.4 9.22 (0-3.3) (11- 82) (5-165) (6.2-35) (9.05-9.6)

Wet 11 1.7 14.8 27.3 20.9 9.12 (0.35-4) (6.4 - 25) (5-60) (3-47) (8.60-9.5)

Dry 7 24.5 22.1 31.6 11.4 8.91 (17-32) (5.40-45) (13.5-67) (6-16.8) (8.50-9.1)

Wet 6 30.7 16.4 64.8 8.5 9.13 (28-34) (11 - 20) (5.3 - 132) (0-15.5) (8.95-9.50)

Dry 6 53.5 276 N.D. N.D, 9.67 (1,2-103) (82-618) (9.40-9.80)

Wet 6 45.6 951.7 1.7 9.3 9.76 (1.74-63) (52-2178) (0-9.65) (0-28) (9.4-10.2)

Dry 6 19.0 9.8 10.6 48.5 8.55 (13.4-31) (3-18) (3-23) (32-65) (8.13-8.80)

Wet 7 22.9 59.1 13.6 297 8.59 (14.7-37.5) (2.5-220) (3.9-29.9) (9-585) (7.30-9.30)

Limnologica (2002) 32, 169-179

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Fig. 3. Chlorophyll-a concentrations of phytoplankton (mg m -3) in the Ethiopian rift-valley lakes during the wet and dry seasons of 1990-2000. Data shown are mean, standard deviation, and sample size (at the base of column).

Major ions and conductivity

Concentrations of total cations and total anions were higher during the dry seasons than wet seasons in all of the Ethiopian rift-valley lakes except in Lake Abijata and Koka Reservoir, and measurements of conductivity followed a similar trend in most lakes. Nevertheless, there were disparities between ionic concentration and conductivity measurements in Lakes Zwai and Shalla (Table 3). Although the changes in concentrations of in- dividual ions were variable, Ca 2+ concentrations were consistently higher during the dry seasons in all the lakes except in Lake Chamo, whereas concentrations of K ÷ and SO42- were notably higher during the wet seasons in all the lakes except in Lakes Langano and Koka, respectively.

Discussion

Rainfall regime

The large differences in the amount of rainfall were use- ful in classifying a month as "dry" or "wet". Such varia-

tions in rainfall could have direct effects on lakes as they have been shown to be responsible for within-year variations of water level and associated change in salinity in a number of lakes, and alteration in phytoplankton and other communities can occur (TALLING & LEMOALLE 1998). Therefore, an ideal situation was cre- ated in this study to look at the effects of the rains (wet seasons) on the ecology of the Ethiopian rift-valley lakes. On the other hand, classification of the months into dry and wet seasons based on the conventional seasonal classification did not always agree with the rainfall data obtained during this study. For most of the Ethiopian rift-valley it is assumed that March to Octo- ber is wet season and November to February is dry season (GAMACHU 1977). This trend is slightly different for the southern most lakes (Lakes Abaya and Chamo), where seasons are shifted. Given the present unpre- dictable rainfall pattern in Ethiopia, it is no longer possible to take for granted that a certain month is always wet or dry based on general classification. It is, therefore, essential for ecological studies to obtain actual rainfall data of each sampling month as was done in this study.

Limnologica (2002) 32, 169-179

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Major nutrients, pH and chlorophyll-a The higher levels of nutrients during the wet seasons in most lakes are presumably the result of mixing events that redistribute nutrients to the surface water, and inputs from runoff. Especially that silicate and nitrate-nitrogen concentrations were consistently higher during the wet seasons could be due to input from the drainage basin. Higher nutrient concentrations during rainy season could also reflect that phytoplankton is light-limited due to mixing and cloud cover, and less capable of assimilat- ing nutrients. Morphological differences among the lakes and/or high nutrient load in their inflows may have contributed to the seasonal differences. Biogenic and abiogenic transformations in the lakes, and nutrients from atmospheric precipitation, are also very likely to bring about some of the changes (TALHNG & LEMOALLE 1998).

It may be important to mention that the silicate concentration is notably low in Lake Chamo where concentrations of less than 1 mg 1-1 were recorded. This indi- cates the possibility of silicate limitation (at least period- ically) in phytoplankton growth in this shallow productive lake where diatoms are one of the dominant phytoplankton components (KEBEDE et al. 1994; KEBEDE & WlI.LEN 1998). That nitrate- and ammonia-nitrogen were not detected in Lake Shalla is an indication that phytoplankton in this lake could be nitrogen limited, and this is consistent with earlier findings of KEBEDE et al. (1994). On the other hand, the fact that silicate concentrations in Lakes Chamo and Shalla were relatively lower during the dry seasons when concentrations of nitrate and phosphate were higher could be due to the fact that loading of nitrate and phosphate depletes silicate (WETZEL 2001).

While a generalization concerning why some lakes increased in chlorophyll-a during the rainy season and some decreased requires further study, I propose that the low chlorophyll-a concentrations in dry and wet seasons reflect nutrient and light limitation, respectively. Lakes that responded to rainy season with higher chlorophyll-a (Zwai, Abijata, and Awassa) are presumably those that are nutrient-limited during the dry season. According to KEBEDE et al. (1994) the chlorophyll-a and total phosphorus relationships in Lakes Zwai, Awassa and Chamo indicated that these lakes were phosphorus limited while others had surplus phosphorus. It is worth noting that these three lakes, and Lake Abijata, are among the most productive of the Ethiopian rift-valley lakes. Similar results were obtained in year-round seasonal studies made in these three lakes (WODAJO 1982; KEBEDE 1987; TILAHUN 1988; ZINABtJ & TAYLOR 1989). Mixing of the water bodies (that results from cooling and winds associated with the rains), which releases nutrients into the euphotic zone, could be among the factors that regulate chlorophyll-a levels (ZINABU & TAYLOR 1989).

The reasons why chlorophyll-a levels in the rest of the Ethiopian rift-valley lakes were higher during the dry seasons are unclear, but I suggest that these lakes have a greater tendency to light limitation. Lakes Abaya and Langano and Koka Reservoir are turbid (discolored) from the high silt colloid they contain, and are thus less productive than the other Ethiopian rift-valley lakes. Lake Shalla, the deepest lake in North Africa (max. depth -- 266 m), is an unproductive lake with high phosphate concentrations (mean SRP concentrations ranging from 276 to 952 ~g 1-1). These lakes could be more light- limited during rainy seasons presumably due to their un- favorably low euphotic depth to mixed depth relation- ship. However, Lake Chamo is neither a deep lake nor is its water particularly turbid (BELAY & WOOD 1982). Therefore, the changes in the chlorophyll-a concentrations of this lake may be dictated by other factors. In some of these lakes it is possible that there could be a lag between the time of the highest nutrient concentrations and the increase in chlorophyll-a as response to the rise in nutrient concentrations.

Major ions and conductivity Conductivity measurements of the lakes were expected to be lower during the wet seasons due to dilution from precipitation and less evaporation during the mostly cooler days. However, except for the southernmost lakes (Abaya and Chamo), the mean conductivity values were higher during the wet seasons in all the Ethiopian rift-valley lakes. On the other hand, the ranges of the conductivity values during the wet and dry seasons show that in some of these lakes (e.g., Awassa and Langano) the highest conductivity values were measured during the dry seasons. Since conductivity measurements were taken much more frequently than ionic concentrations, intermittent rains during the dry seasons could have contributed to the low conductivity values of the dry seasons in some of the lakes. It is also possible that conductivity is lower early in the dry season than early in the wet season, which means conductivity lags behind rainfall, so that the high conductivity associated with dry season develops slowly, and may even persist into the start of the rainy season. More- over, except in Lakes Abijata and Koka, the higher conductivity measurements during the wet seasons were not related to the total ionic concentrations. Such irregular relationships between conductivity measurements and total ionic concentrations were also observed in studies made in Lakes Awassa (KIFLE & BELAY 1990), Zwai (TILAHUN 1988), andAbijata and Langano (WODAJO 1982). Sources of such irregularities, besides human error, could be differences that might arise from the fact that different ions could contribute differently to conductivity and the contribution per unit concentration may change with salinity (WOOD & TALLING 1988). The possibility that organic

Limnologica (2002) 32, 169-179

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compounds could be contributing to conductivity is something that needs further consideration. All these is- sues are beyond the scope of this paper.

It is interesting to note that concentrations of certain ions were consistently higher during either the wet or dry seasons. For instance, concentrations of K ÷ and SO42- were always higher during the wet seasons, and those of Ca 2÷ were always higher during the dry seasons. Given the fact that SO42- is biologically active and has a high redox potential, the scenario depicted here could be due to biological activity and/or change in redox potential as a result of mixing of the lakes during the wet seasons.

The fact that rains tend to cause lowering of solute concentration in natural waters, especially those of short retention time (TALLING & LEMOALLE 1998), ex-


plains why concentrations of total ions were lower during the wet seasons in most of the lakes. In cases where the opposite had occurred, as in Lake Abijata and Koka Reservoir, the inflows must have contributed to the increased solute concentrations during the wet seasons. Total ion concentrations were determined in samples taken from the inflows of some of the lakes at the end of the dry season (January/February) and during the onset of the heavy rains (July/August) of the year 2000. The concentrations of total ions measured during the beginning of the heavy rains were much higher than those at the end of the dry season in the River Horaccel- lo, one of the inflows of Lake Abijata. The difference in cation concentrations between the two sampling peri- ods was nearly five fold (Fig. 4A), and the conductivity

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1 Fig. 4. Concentrations (meq k ) of total cations (A) and measurements (mS cm ~) of conductivity (B) in some inflows of the Ethiopian rift-valley lakes at the end of the dry season (January/February) and at the onset of the heavy rains (July/August) of the year 2000. The lakes into which the rivers flow are shown in parentheses.

Limnologica (2002) 32, 169-179

178 G.-M. Zinabu

measurement showed similar results (Fig. 4B). Since the River Horaccello originates from Lake Langano, a less saline lake, the high concentration of the solutes in this river is likely to be due to the new water, from the heavy rains, that displaced solutes from accumulations in soils in the drainage system. It is possible that concentrations of solutes in this inflow could start out high as accumulated salts are dissolved from soils, and de- cline during the rainy season. Such a follow-up study has not been made and conclusions are difficult at this point.

The foregoing discussion portrays a very significant phenomenon given the fact that the salinity of Lake Abi- j ata has increased in the last three decades (KEBEDE et al. 1994; ZINABU 1998). Lake Abijata is a shallow, saline terminal lake whose volume varies considerably with climate. Water salinity in this lake is assumed to have increased due to high evaporation, diversion of water from inflow rivers and other factors related to human activity (ZINABU 1998). While these and a number of other factors have been assumed to have contributed to the increase in the salinity of Lake Abijata (Z~NABU & ELIAS 1989; ZINABU 1998), the possible contribution of the inflows to changing salinity of the lake has never been considered. The fact that the catchment through which River Horaccello flows is completely deforested and heavily grazed could be one of the factors for the high solute concentration of the river. The Koka Reservoir also had much higher values of total ions and conductivity during the wet seasons, presumably from a similar ef- fect of its inflows. Although a number of other inflows of the Ethiopian rift-valley lakes flow through deforested and overgrazed catchments (ZINABU & ELIAS 1989; ZINABU 1998), a similar event to that which occurred in the River Horaccello was not observed in other rivers, presumably due to the different soil types in the drainage basins.

Considering that analytical errors in ion determination were more likely than errors in the conductivity measurements, one can conclude that the salinity of some of the Ethiopian rift-valley lakes (at least those of Lake Abijata and Koka Reservoir) increased during the wet seasons, possibly as a result of inflows that may carry high concentrations of solutes due to the rains. One other possible explanation for higher salinity during the wet season could be exchange with sediments within the water body especially during frequent mixing (which mostly occurs during the rainy seasons) in the shallow lakes. Although higher mean conductivity measurements were observed in Lakes Awassa and Langano during the wet seasons, the ranges show that there were some higher values recorded during the dry seasons. Therefore, it is not possible to rule out that conductivity was higher during the dry seasons in these lakes.

Summary and Conclusions

Although the seasonal changes in this study could have been confounded by inter-annual changes, the study has clearly shown that wet and dry seasons affect concentrations of nutrients and phytoplankton biomass in the Ethiopian rift-valley lakes, and probably not in the way that one would expect, wet seasons often leading to higher values and dry seasons to lower values. It is possible that events associated with the wet and dry seasons bring about contrasting changes in nutrient levels and phytoplankton biomass depending on the physical characters of the lakes. The possibility of light limitation in some of the Ethiopian rift-valley lakes can be implied from this study. Vertical re-distribution, associated with de-stratification, and changing phytoplankton uptake are likely to affect concentrations of nutrients and major ions to some extent. The variable area of catchment that each lake drains could be one reason for the unpre- dictable changes observed among lakes, especially with respect to ionic composition. It is very likely that the ionic compositions of these lakes are strongly affected by new water input, which depends on hydrology and retention time of the lakes. The relatively shallow depth and terminal position make some of these lakes, like Lake Abijata, very susceptible to the effects of changes in water/solute input. The fact that solute concentrations of at least one inflow of one of the lakes (Lake Abijata) were much higher during the wet seasons than the dry seasons, probably due to increased solute concentrations from accumulations of salts in soils, reinforces the view that catchment management is an important component of lake restoration (ZINABU 1998).

It remains to be seen whether or not these changes in the concentrations of solutes, levels of nutrients and phytoplankton biomass associated with the dry and wet seasons dictate the rates of primary production and dynamics of other organisms including fish. Future studies should focus on the importance of spatial variability and of deforested catchments, as well as the possible effects of thermal stratification on the seasonality in these tropical lakes. The contributions from atmospheric precipitation and thermal springs to changes in the concentrations of solutes also deserve attention.

Acknowledgements

I thank Drs. J.F. Talling (Freshwater Biological Association, U.K.), W.D. Taylor (University of Waterloo, Canada) and C.A. Chapman (University of Florida, USA) for critically reading an earlier draft of the manuscript and for their valuable com- ments. I also thank Dr. Ingemar Ahlgren (Uppsala University, Sweden) and the two anonymous referees for their construc- tive suggestions. Zerihun Desta, Ali Seid, Kedir Woleye, Debebe Moges and Tafese Kefyalew offered technical assis-

Limnologica (2002) 32, 169-179

tance during the study period. Elias Dadebo and Tafere Gebre- Egziabher helped in bringing samples from Lakes Chamo and Abaya. Chemical analyses of ions were done at the water laboratory of the Ethiopian geological surveys. Rainfall data were obtained from the meteorological stations of Adamitullu and Awassa Institute of Agricultural Research as well as from the State Farms of Southern Omo. This study was partially sup- ported by research grants from the Ethiopian Science and Technology Commission (ESTC) and the International Foun- dation for Science (IFS). Grant for writing this paper was pro- vided by the Fulbright Senior Scholarship Program through CIES, USA. The Department of Zoology, University of Flori- da, USA, through Dr. L.J. Chapman kindly made computer and other office facilities available to the author.

References

APHA, AWWA & WPCF (1986): Standard methods for the determination of water and wastewater, 16 a~ ed. American Public Health Association, New York.

BELAY, A. & WOOD, R. B. (1982): Limnological aspects of an algal bloom in Lake Chamo in Gamo Goffa Administrative Region of Ethiopia in 1978. SINET: An Ethiopian Journal of Science 5: 1-19.

CHANEY, A.L. & MARBACH, E.R (1962): Modified reagents for the determination of urea and ammonia. Clinical Chemistry 8: 130-132.

GANACHU, D. (1977): Aspects of climate and water budget in Ethiopia. Addis Ababa University Press, Addis Ababa.

GASSE, F., ROGNON, P. & STREET, EA. (1980): Quaternary his- tory of the Afar and Ethiopian rift-valley lakes. In: M.A.J. WILLIAMS & H. FAURE (eds.), The Sahara and the Nile, pp. 361-400. Rotterdam.

GOLTERMAN, H.L., CLYMO, R.S. & OHNSTAD, M.A.M. (1978): Methods for the physical and chemical analysis of freshwa- tel', 2nd ed., Oxford.

HARRISON, A.D. & HYNES, H.B.N. (1988): Benthic fauna of Ethiopian mountain streams and rivers. Arch. Hydrobiol./ Suppl. 81: 1-36.

KEBEDE, E. (1987): Seasonal studies of phytoplankton composition and biomass in Lake Awassa. M.Sc. Thesis, Addis Ababa University, Addis Ababa.

KEBEDE, E. & BELAY, A. (1994): Species composition and phytoplankton biomass in a tropical African lake (Lake Awassa, Ethiopia). Hydrobiologia 288:13-32.

KEBEDE, E., ZINABU, G.M. & AHLGREN, I. (1994): The Ethiopi- an rift-valley lakes: chemical characters along a salinity-alkalinity gradient. Hydrobiologia 288: 1-12.

Fd~BEDE, E. & WILLEN, E. (1998): Phytoplankton in a salinity- alkalinity series of lakes in the Ethiopian rift-valley. Algo- logical Studies 89: 61-69.

KIFLE, D. & BELAY, A. (1990): Seasonal variation in phytoplankton primary production in relation to light and nutrients in Lake Awassa, Ethiopia. Hydrobiologia (The Hague) 196: 217-227.

MACKERETH, F.J.H., HERON, J. & TALLING, J.F. (1978): Water analysis: some revised methods for limnologists. Scientific Publications of the Freshwater Biological Association, U.K., No. 36.


MAKIN, N.J., KINGHAM, T.J., WADAMS, A.E., BIRCHAL, C.J. & TEFERRA, T. (1975): Development prospects in the southern rift-valley, Ethiopia. Land resources study 21, Land Resources Division, Ministry of Overseas Development, Surrey, England.

MURPHY, J. & RILEY, J.R (1962): A modified single solution method for the determination of phosphate in natural water. Analytica Chimica Acta 27: 31-36.

TALLING, J.E & DRIVER, D. (1963): Some problems in the esti- mation of chlorophyll-a in phytoplankton. - Proceedings, Conference of Primary Productivity Measurement, Marine and Freshwater, pp. 142-146. US Atomic Energy Comm., TID-7633.

TALLING, J.E & LEMOALLE, J. (1998): Ecological dynamics of tropical inland waters. Cambridge.

TALLING, J.E & TALLING, I.B. (1965): The chemical composition of African lake waters. Int. Rev. ges. Hydrobiol. 50: 421-463.

TELFORD, R.J. (1998): Diatom stratigraphies of Lakes Awassa and Tilo, Ethiopia: Holocene records of groundwater variability and climate change. Ph.D. Thesis, University of Wales.

TILAHUN, a . (1988): A seasonal study on phytoplankton primary production in relation to light and nutrients in Lake Zwai, Ethiopia. M.Sc. Thesis, Addis Ababa University, Addis Ababa.

TUDORANCEA, C., ZINABU, G.M. & ELIAS D. (1999): Limnology in Ethiopia. In: R.G. WETZEL & B. GOPAL (eds.), Limnology in developing countries, Vol. 2, pp. 63-118. New Delhi.

VOW DAMN, K.L. & EDMOND, J.M. (1984): Reverse weather- ing in the closed basin of the Ethiopian rift. American Jour- nal of Science 284: 835-862.

WETZEL, R.G. (2001): Limnology: Lake and river ecosystems, 3 rd ed. San Diego.

WETZEL, R.G. & LIKENS, G.E. (1979): Limnological analysis. Philadelphia.

WODAJO, K. (1982):~Comparative limnology of Lake Abij ata and Lake Langano in relation to primary and secondary production. M.Sc. Thesis, Addis Ababa University, Addis Ababa.

- & BELAY, A. (1984): Species composition and seasonal abundance of zooplankton in two Ethiopian rift-valley lakes - Lakes Abijata and Langano. Hydrobiologia 113: 129-136.

WOOD, R.B. & TALLING, J.F. (1988): Chemical and algal relationships in a salinity series of Ethiopian inland waters. Hydrobiologia 158: 29-67.

ZINABU, a. M. (1998): Science in Africa - Emerging water management problems. In: J. SCHONEBOOM (ed.), Human interactions and water quality in the Horn of Africa, pp. 47 - 61. A publication of the symposium at the 1998 American Association for the Advancement of Science (AAAS), Annual Meeting, Philadelphia.

ZINABU, a . M. & ELIAS, D. (1989): Water resources and fish- eries management in the Ethiopian rift-valley lakes. SINET: An Ethiopian Journal of Science 12: 95-109.

ZINABU, G. M. & TAYLOR, W.D. (1989): Seasonal and spatial variation in abundance, biomass and activity of het- erotrophic bacterioplankton in relation to some biotic and abiotic variables in an Ethiopian rift-valley lake (Awassa). Freshwater Biology 22: 355-368.

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