Koka is a large multi-purpose reservoir impacted by the ever-increasing human activitiesrnthat resulted in its enrichment with algal nutrients and contamination with chemicals ofrnindustrial origin. With a view to come up with scientific information usable in thernprotection of aquatic resources and public health, temporal dynamics of the communityrnstructure and interactions of the major plankton components (phytoplankton andrnzooplankton) in relation to selected physico-chemical parameters were investigated atrnmonthly intervals from May, 2013, to April, 2014. The observed exceedingly low ZSD,rnwhich may have partly resulted from particles resuspended in the water column by windgeneratedrnturbulence to which shallow water bodies like Koka Reservoir are especiallyrnsusceptible, indicates the remarkably high turbidity of the reservoir. The levels ofrninorganic nutrients recorded for Koka, which are quite high in comparison to those ofrnEthiopian rift valley and highland lakes, are typical of reservoirs, which, unlike naturalrnlakes, have large phosphorus and nitrogen loads. The mean values of total phosphorusrn(0.27 mg L,I), chlorophyll-a (85.79 J.lg L'I), and transparency (0.15 m) recorded forrnKoka Reservoir WaJTant its classification as a hypereutrophic water body characterized byrnhigh biomass production and elevated concentrations of nutrients.rnThe phytoplankton community in the reservoir was constituted by a total of 89 speciesrnwith the larger number of species belonging to the Chlorophyceae (34) andrnCyanophyceae (28). Cyanobacteria were the most important taxa both in terms ofrn'rnIIrnlrnabundance and biomass, with Microcystis and Cylindrospermopsis species altematelyrndominating the phytoplankton community and accounting for up to 72.29% and 62.87 %rnand 21.45. % and 20.23% of the total phytoplankton abundance and biomass asrnbiovolume, respectively. The mean concentrations of chl-a (~g L-1) of the nano- (2-20rn,um, 39) and net- (>20 ,um, 40.4) phytoplankton were much higher than those of thernpicophytoplankton (10.48) as would be expected considering the level of eutrophicationrnof the reservoir.rnThe composition and abundance of cyanobacteria and their toxins, microcystins (MCs)rnand cylindrospermopsins (CYN), were investigated using samples collected at monthlyrnintervals from the Amudde side of Koka Reservoir from May, 2013, to April, 2014.rnAnalyses of cyanotoxins in filtered samples by HPLC-DAD and LC-IMSIMS identifiedrnand quantified 5 variants ofMCs (MC-RR, MC-YR, MC-dmLR, MC-LA and MC-LR) inrnall samples, with their total concentrations ranging from 1.86 to 28.3 ~g L -I and fromrn1.71 to 33 ~g L -I, respectively and greatly surpassing WHO's drinking water guidelinernvalue of 1 ~g L-1rnâ€¢ The maximum level of MCs occurred in December, 2013, when thernphytoplankton community of the reservoir was constituted almost entirely by Microcystisrnaeruginosa. The total concentrations of MCs measured in freeze-dried plankton samplesrnby HPLC-DAD and LC-MSIMS varied between 312 and 641 ~g (g dwtrl and 351 andrn709 ~g (g dwtrl, respectively. Thus to prevent intoxications continuous monitoring ofrnthese reservoirs is strongly recommended. This has to be taken into consideration in thernmanagement of the reservoir when it used for various domestic purposes. Studiesrnregarding the prediction of the occurrences of toxic blooms and their toxins must bernimplemented and strengthened in the future in order to avoid or reduce the potential risksrnassociated with human and animals' exposure to the toxins. Despite the presence andrnoccasional abundance of Cylindrospermopsis spp., cylindrospermopsin was not detectedrnby HPLC-DAD and LC-MSIMS in any of the samples collected tlu'oughout the studyrnperiod.rnAnalysis of the zooplankton community in the reservoir revealed a total of 52 species, 25rnof which are new records for Koka Reservoir. With 40 species, rotifers were the mostrniirnI,'rnirn'rnspecies-rich group followed by cladocerans (8 species), and copepods (4 species). Thernrotifer Keratella tropica was the species recorded with the highest abundance among thernzooplankton taxa contributing 42.11 % of total rotifers abundance. The cladocerans wererndominated by small species, Ceriodaphnia corn uta, Diaphanosoma excisum and Moinarnm/crura, with rare occurrence of the large cladoceran D. barbata during a few months ofrnthe sampling period. Total abundance of copepods was constituted largely byrnThermocyclops decipiens, which is the most commonly dominant species in tropicalrninland waters. Calanoid copepods were represented by a single species; Tropodiaptomusrnsp. Zooplankton abundance was generally higher during the dry season than the rainyrnseason.rnEutrophication of Koka Reservoir has resulted in the dominance of cyanobacteria, whichrnare generally regarded as unsuitable food source for zooplankton. It was, therefore,rnhypothesized that seston may playa role in the diet of the zooplankton community of thisrnreservoir. The contribution of seston fractions (