The seasonal dynamics of phytoplankton in relation to some physico-chemicalrnand biological parameters were studied at a near-shore and central stations inrnLake Bishoftu from August, 2006 to May, 2007. Water transparency (ZSD) rangedrnfrom 0.50 to 1,05 and from 0.46 to 1.0 m at the Central and Near Shore stationsrnrespectively, with high and low values corresponding to dry and rainy periodsrnrespectively. Mean vertical extinction coefficient varied temporally with higherrnvalues coincident with high algal biomass and abiogenic turbidity following rainyrnperiods. The lake exhibited both superficial and deep-seated thermalrnstratifications, with ill-defined thermoclines on some occasions and neverrnunderwent complete mixing during the study period. Algal nutrients exceptrnnitrate-nitrogen showed temporal variations whose changes were inverselyrncorrelated with Chl a.The phytoplankton community of the lake, which consistedrnof five algal classess was dominated by cyanobacteria with Microcystis spp. asrnthe most abundant and persistent taxa and with their contributions to the totalrnabundance of the phytoplankton assemblage of the lake ranging from 58 to 99%.rnThe zooplankton community was dominated by Calanoids and Rotifers withrnLovenula africana and Brachionus spp.as the most numerically important taxa.rnPhytoplankton biomass measured as Chl a showed seasonal variations withrnmaximum values during the rainy and post rainy months at both stations. Depthrnprofile of photosynthesis showed a region of photoinhibition and condensedrnphotosynthetic zone, a feature of some east African lakes. Light-saturated ratesrnof gross photosynthesis (Amax) at the central station varied from a minimum ofrn410 to a maximum of 1630 mg O2 m-3 h- and with fairly strong and positiverncorrelations with Chl a (R2=0.38, p=0.08), Soluble Reactive Phosphate, SRPrn(R2=0.46, p=0.07) and Light utilization efficiency, Î¦ (R2=0.61, p=0.02). Biomassrnspecific rate at light-saturation exhibited unusually high values (29-91) and wasrnpositively and significantly correlated with SRP (R2=0.59, p=0.02), Areal ratesrnvaried from 0.89 to 3.88 g O2 m-2 h- 1 in association with biomass and Amax. Thernpossible causes for the observed saptio-temporal variations in physico-chemicalrnand biological features of the lake are discussed and suitable conservationrnstrategies recommended.phytoplankton was based on short-term observations, which were not systematic.rnWood and Talling (1988) have compiled the sporadic information on therndominant or most common species of phytoplankton reported in variousrnlimnological papers. The systematic and detailed taxonomic studies conductedrnon phytoplankton to date are those of Tsegaye Mihrete-Ab (1988), ElizabethrnKebede and Amha Belay (1994), Elizabeth Kebede (1996) and Elizabeth Kebedernand WillÃ©n (1998). Also several studies (Talling et al., 1973; Amha Belay andrnWood, 1984; Girma Tilahun, 1988; Brook Lemma, 1994; Taylor and ZinaburnGebre-Mariam 1989; Demeke Kifle and Amha Belay, 1990 and Eyasu Shumbulo,rn2004; Girma Tilahun, 2006) have investigated the dynamics of phytoplankton inrnrelation to some limnological aspects in Ethiopian lakes.rnAlthough the Bishoftu crater lakes have been subjects of many limnologicalrninvestigations, some dating as far back to the early 1930's (Prosser et al., 1968),rninformation on the temporal dynamics of the community structure andrnphotosynthetic production of phytoplankton in relation to physico-chemical factorsrnin Lake Bishoftu is non-existent. The early short-term comparative studies madernon the Bishoftu lakes focused mainly on the assessment of general trends inrnphysico-chemical limnological features. Even the few published reports on thernphytoplankton biomass (Zinabu Gebre-Mariam, 1994; Wood and Talling,1988)rnand photosynthetic production (Talling et al., 1973) in the Bishoftu crater lakesrnwere based on short-term observations although it was indicated thatrnphytoplankton of tropical waters exhibit temporal changes in their speciesrncomposition, biomass and primary production (Talling,1986).rnIn Lake Bishoftu, some-short term studies have been reported on bathymetry,rnstratification, some chemical and morphometric aspects by expeditionaryrnlimnologists such as Baxter et al. (1965), Baxter and Wood (1965), and Woodrnand Talling (1988). These short-term studies were based on occasional samplingrnfor the purpose of comparison among crater and tropical Rift Valley lakeComparative study with another soda lake, Lake Aranguade, on zooplanktonrnstructure and dynamics has been conducted by Afeworki Ghebrai (1992).rnTherefore, in comparison with other Debre-Zeit and Rift Valley lakes, someonerncan confidently talk of that Lake Bishoftu is not studied in phytoplankton aspectrnover extended periods.rnZinabu Gebre-Mariam (1994), Brook Lemma (2002) and Zinabu Gebre-Mariamrnet al. (2002) have noted that the Ethiopian Rift Valley and crater lakes have beenrnundergoing changes in their limnological features during the last two decades orrnso because of increased human interventions. The present study lake is nornexception. Lake Bishoftu is surrounded by a fast growing town, Debre Zeit, andrnits shores are used for washing clothes, watering livestock and recreation andrnthe like. Shoreline modifications made on almost all sides of the lake for variousrnpurposes (e.g. for the construction of resort hotels and residences in therncatchments) introduce enormous amounts of particulate wastes, which formrnsuspensions in the water column thereby reducing light penetration at least in thernnear-shore regions of the lake. The lake has also been serving as a majorrndumping site for domestic wastes (solid and liquid wastes) originating fromrnhouseholds, hotels and small industrial operations, including a privately-ownedrntannery. These activities could lead to changes in the physico-chemicalrnlimnological features of the lake resulting in the disruption of the lake ecosystem,rnwith consequent changes in the species composition and photosyntheticrnproduction of phytoplankton, which determine the pelagic food web structure andrnfish production in the lake. It is also possible that degradation of this aquaticrnecosystem may go on unnoticed for a long time and eventually result inrnunexpected disastrous effects. The assessment of qualitative and/or quantitativernchanges taking place in this lake, therefore, necessitates the generation of basicrninformation on the physical, chemical and biological aspects of this water bodyrnover an extended period of time.Thus, the purpose of this research project was to look into the temporal variationrnin the species composition, seasonal abundance and photosynthetic productionrnof phytoplankton in relation to some physico-chemical variables in Lake Bishoftu