The temporal dynamics of plankton, benthic macroinvertebrates and physico-chemical variables werernstudied at a central station in Lake Hora-Kilole from August, 2007 to May, 2008. The lake remainedrnsmall and shallow and consequently underwent complete mixing throughout the study period. Secchirndepth ranged from 0.15 m to 0.78 m with high values coinciding with lower wind speed. Verticalrnextinction coefficient varied temporally (2.39-10.23 ln units m-1) with high values coinciding withrnrelatively low algal biomass and high abiogenic turbidity associated with wind-induced mixing. SRP andrnTP varied from 0.33 to 3.5 and 2.13 to 6.15 Î¼g L-1 respectively. The concentration of NO3 + NO2-N (in Î¼grnL-1) ranged from 17 to 303 while that of NH4++NH3â€“N varied from 1.7 to 49.09 Î¼g L-1. Molybdaternreactive silica (in mg L-1) ranged from 10.4 to 69.7. The phytoplankton community was composedrnprimarily of green algae, euglenoids, diatoms, dinoflagellates and cyanobacteria with the overwhelmingrndominance of dinoflagellates and Cyanobacteria corresponding to the seasonal peaks of Chl a biomass ofrnphytoplankton. The most dominant species of Dinoflagellates and Cyanobacteria were Peridiniumrngatunense and Anabaena cf. agardhii and cylindrospermopsis curvispora respectively. Zooplanktonrnabundance peaked in February, 2008 coincident with the largest peaks of phytoplankton biomass andrnabundance and with Thermocyclops decipiens, Brachionus sp. and Daphnia barbata as the mostrnimportant taxa. Among the macroinvertebrates, chironomids and Lambriculidae-oligochaete wererndominant during the dry period. Total phytoplankton biomass showed seasonal variations (â‰ˆ36 to 148 Î¼grnL-1) with large peaks in October, 2007(â‰ˆ148 Î¼g L-1) and February, 2008 (â‰ˆ147 Î¼g L-1) and the seasonalrnminimum (â‰ˆ 36 Î¼g L-1) in January, 2008. Among the three size-groups of phytoplankton, the netplanktonrn(> 20 Î¼m fraction) was the most important contributor to the total phytoplankton biomass with itsrnbiomass and percentage contributions to total phytoplankton biomass ranging from 13.21 mg m-3 and 32rn% to 132.06 mg m-3 and 95 %, respectively. The percentage contributions of nanoplankton andrnpicoplankton varied from about 0 to 64% and from 4 to 28 %, respectively. Depth profiles of grossrnphotosynthesis were of typical pattern for phytoplankton without surface photoinhibition during the majorrnrainy period. Light-saturated rate of gross photosynthesis (Amax) varied from 370 to 3843 mg O2 (â‰ˆ111.5â€“rn1199 mg C) m-3 h-1 with the maximum value corresponding to the seasonal maximum of phytoplanktonrnbiomass. Biomass-specific rate of gross photosynthesis at light-saturation varied from about 6 to 33 withrnpositive and moderate correlation (r=0.632, r2=0.40 at p=0.0496) with SRP. Hourly integral rate ofrngross photosynthesis, (Î£A), which was positively and strongly correlated with Amax (r= 0.885, r2=0.7895rnat P = 0.0006) and Zeu (r=0.7849, r2=0.616 at p=0.0072), ranged from 0.21 to 6 g O2 (â‰ˆ 0.065 - 1.87 g C)rnm-2 h-1. The marked temporal variations in phytoplankton parameters are discussed in relation to physicochemicalrnand biological variables.and the general knowledge of some biological characteristics of the speciesrn(Garcia et al., 1989).rnLimnology in the tropics has only recently developed past the stage ofrnexploration, but the need of limnological knowledge is, as pressing at tropicalrnlatitudes as in the temperate (Melack, 1996; Talling and Lemoalle, 1998, Lewis,rn2000). Despite the perturbation by humans, vast number of lakes of varied sizernand shape are found in most tropical regions. They are often inhabited byrndensely populated biota and have been the subject of some of the mostrnscientifically informative studies of tropical aquatic ecosystems (Talling, 2001).rnIn Africa, the first assessment of phytoplankton seasonality stemmed fromrncollections made in 1899 by the Fullenborn expedition to Lake Nyassa, andrnlater to Lake Malawi (Talling, 1986). Extensive efforts were made after 1950,rnespecially in East and Central Africa (Talling, 1986). Despite their tropicalrnlocation, African lakes exhibit considerable seasonality related to alternationsrnof warm, wet, cooler and dry seasons. Although numerous studies have beenrnmade on the species composition and photosynthetic production ofrnphytoplankton in various East African lakes (Talling and Lemoalle, 1998),rnrelatively little has been done on this aspect in Ethiopian lakes.rnEthiopia is rich in both natural water bodies such as rivers and lakes andrnman-made lakes (reservoirs) compared to other east African countries. Ethiopiarnalso possesses many great crater lakes (Prosser et al., 1968), among these arernthe Bishoftu crater lakes, which form an extensive series of volcanic explosionrncraters in the vicinity of the city of Bishoftu (Debrezeit). The Bishoftu craterrnlakes are grouped among the most scientifically known lakes of Ethiopia untilrnrecently (Zinabu Gebre-Mariam, 1994). Volcanic crater lakes include lakesrnlocated in crates formed after eruption and they have small surface area andrnsteep crater wall (Hutchinson, 1957). We have some scientific information on the Bishoftu crater lakes dating as farrnback as the early 1930`s and 1940`s (Omer-Cooper, 1930; Vatova, 1940;rnLoffredo and Maldura, 1941; Cannicci and Almacia, 1947, cited in Prosser etrnal., 1968). In the early 60`s, different aspects of these lakes were studiedrn(Baxter et al.,1965; Baxter and wood, 1968; Prosser et al., 1968; Talling et al.,rn1973; Wood et al., 1976, 1976, 1984; Wood and Talling, 1988). It is theserninvestigations which tempted Zinabu Gebre-Mariam (1994) to describe thernBishoftu crater lakes as being among the most studied lakes of Ethiopia. Therernis, however, scanty scientific information on several aspects of these lakes,rnparticularly on some biological aspects.rnLakes throughout the temperate and tropical latitude have been drasticallyrnaltered as a result of the burden they carry when there is an increase inrnpopulation density, economic growth, and change in land cover (Lewis, 2000).rnIt is also known that climatic change especially rainfall, have a prominent effectrnon the limnology of a lake, resulting in changes in different parameters of arnlake. Human interventions are the main cause of lake deterioration and it hasrnvarious consequences. Human interferences are frequently reflected as changesrnin the trophic status of the lake, volume of the water, and consequentrnecological changes. This is a phenomenon observed throughout the world, withrnsome lakes changing from oligotrophic to eutrophic through mesotrophic or thernreverse could happen. Increase or decease in the biomass of plankton and levelrnof nutrients and alteration in the underwater climate are some of thernsymptoms of eutrophicaton and/or oligotrophication. Thus, the trophic statusrnof a lake can be determined from different physico-chemical and biologicalrnparameters. Euphotic depth, concentration of a limiting nutrient,rnphytoplankton biomass, primary productivity, and phytoplankton abundancernare some of the basic and direct indicators of the trophic status of Lake. Onerncan also look at the abundance and biomass of consumers (Zooplankton andrnfishes) to predict tropic status. These, the same parameters can be used torndetermine long term changes of lakes, and for comparison of two or more lakes (Brook Lemma, 2002). The assessment of these physico-chemical and biologicalrnparameters is, therefore, crucial to evaluate the ecological health of aquaticrnecosystems, optimize their exploitation, manage and conserve their resources.rnMost of the Ethiopian lakes including the Bishoftu crater lakes, which are inrnthe vicinity of a fast-growing town surrounded by agricultural lands, arernsubjected to shoreline modification, waste disposal, and other practicesrnassociated with population growth (Zinabu Gebre-Mariam, 1998; ZinaburnGebre-Maraim, et al., 2002). Furthermore, many water bodies (predominantlyrnrivers and lakes) in Ethiopia have been physically degraded or altered seriouslyrnby human manipulation (e.g. L. Hora-Kilole) and habitats have consequentlyrnbeen lost (e.g. L Haramaya) (Brook Lemma, 2002). Human-induced changes ofrnlakes in Ethiopia are exemplified by the eutrophication of Lake Hayk (ElizabethrnKebede et al., 1992), shrinkage of Lake Alemaya (Brook Lemma, 1994; 2002)rnand drastic changes in the water chemistry and species composition andrnproductivity of phytoplankton in Lake Hora-Kilole (Brook Lemma, 1994; 2002).rnLake Hayk has changed greatly in its phytoplankton biomass and transparencyrn(Elizabeth Kebede et al., 1992). This was possible because of the introduction ofrnplanktivorous fish which freed the phytoplankton from grazersâ€™ control. Thernlake was stocked with tilapia species from a crater lake, probably Lake Hora inrn1978 by the Fisheries Department, Ministry of Agriculture, to provide food, andrnharvest by a newly established fish gillnet. The commercial fishery of the lakernincreased to 200 tones per year (84Kg per hectare) (Elizabeth Kebede et al.,rn1992). Lake Alemaya has been used for irrigation, animal watering andrnhousehold consumption and consequently dramatic changes occurred in itsrnvolume has occurred (Brook Lemma, 1994; 2002). Lake Hora-Kilole was oncerngrouped among the unique saline lakes of Africa, such as Arenguade, Chitu,rnAbijata and Shalla in Ethiopia and Nakuru in Keneya (Prosser et al., 1968;rnTalling et al., 1973; Vareschi, 1982; Wood et al., 1984; Elizabeth Kebede et al., 1986; Wood and Talling 1988; Green 1986; 1993; Tudorancea et al., 1999).rnThis lake was also known for its superabundance of Spirulina spp. assemblagernand avifauna. In 1989, the Ministry of Agriculture (MOA), Addis Ababa, in anrnattempt to use the lake as a reservoir to gravitationally irrigate the southernrnand eastern low-lying plains, diverted River Mojo into Hora-Kilole resulting inrnthe complete transformation of the lake ecology (Brook Lemma, 1994; 2002).rnAfter the diversion of the river, few investigations have tried to show ecologicalrnchanges in the Lake (Brook lemma, 1994; 2002, Zinabu Gebre-Mariam, 1994;rnZinabu Gebre-Mariam et al., 2002). Diversion of R. Mojo into Hora-Kilole hasrncaused substantial changes in the morphometric, biological and physicochemicalrncharacteristics of the lake. So the aim of this study is to complementrnand update the physico-chemical and biological data generated for the lake inrnprevious studies, which may help us show the seasonal and long-term changesrnthat have occurred in the lake.