Browsing by Author "Major, Yeshiemebet"

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    Plankton Communitiy Structure and Interactions in a Cyanobacteria-Dominated Tropical Reservoir (Koka, Ethiopia)
    (Addis Ababa University, 2016-06) Major, Yeshiemebet; Kifle, Demeke(PhD)
    Koka is a large multi-purpose reservoir impacted by the ever-increasing human activities that resulted in its enrichment with algal nutrients and contamination with chemicals of industrial origin. With a view to come up with scientific information usable in the protection of aquatic resources and public health, temporal dynamics of the community structure and interactions of the major plankton components (phytoplankton and zooplankton) in relation to selected physico-chemical parameters were investigated at monthly intervals from May, 2013, to April, 2014. The observed exceedingly low ZSD, which may have partly resulted from particles resuspended in the water column by windgenerated turbulence to which shallow water bodies like Koka Reservoir are especially susceptible, indicates the remarkably high turbidity of the reservoir. The levels of inorganic nutrients recorded for Koka, which are quite high in comparison to those of Ethiopian rift valley and highland lakes, are typical of reservoirs, which, unlike natural lakes, have large phosphorus and nitrogen loads. The mean values of total phosphorus (0.27 mg L,I), chlorophyll-a (85.79 J.lg L'I), and transparency (0.15 m) recorded for Koka Reservoir WaJTant its classification as a hypereutrophic water body characterized by high biomass production and elevated concentrations of nutrients. The phytoplankton community in the reservoir was constituted by a total of 89 species with the larger number of species belonging to the Chlorophyceae (34) and Cyanophyceae (28). Cyanobacteria were the most important taxa both in terms of \' II l abundance and biomass, with Microcystis and Cylindrospermopsis species altemately dominating the phytoplankton community and accounting for up to 72.29% and 62.87 % and 21.45. % and 20.23% of the total phytoplankton abundance and biomass as biovolume, respectively. The mean concentrations of chl-a (~g L-1) of the nano- (2-20 ,um, 39) and net- (>20 ,um, 40.4) phytoplankton were much higher than those of the picophytoplankton (10.48) as would be expected considering the level of eutrophication of the reservoir. The composition and abundance of cyanobacteria and their toxins, microcystins (MCs) and cylindrospermopsins (CYN), were investigated using samples collected at monthly intervals from the Amudde side of Koka Reservoir from May, 2013, to April, 2014. Analyses of cyanotoxins in filtered samples by HPLC-DAD and LC-IMSIMS identified and quantified 5 variants ofMCs (MC-RR, MC-YR, MC-dmLR, MC-LA and MC-LR) in all samples, with their total concentrations ranging from 1.86 to 28.3 ~g L -I and from 1.71 to 33 ~g L -I, respectively and greatly surpassing WHO's drinking water guideline value of 1 ~g L-1 • The maximum level of MCs occurred in December, 2013, when the phytoplankton community of the reservoir was constituted almost entirely by Microcystis aeruginosa. The total concentrations of MCs measured in freeze-dried plankton samples by HPLC-DAD and LC-MSIMS varied between 312 and 641 ~g (g dwtrl and 351 and 709 ~g (g dwtrl, respectively. Thus to prevent intoxications continuous monitoring of these reservoirs is strongly recommended. This has to be taken into consideration in the management of the reservoir when it used for various domestic purposes. Studies regarding the prediction of the occurrences of toxic blooms and their toxins must be implemented and strengthened in the future in order to avoid or reduce the potential risks associated with human and animals' exposure to the toxins. Despite the presence and occasional abundance of Cylindrospermopsis spp., cylindrospermopsin was not detected by HPLC-DAD and LC-MSIMS in any of the samples collected tlu'oughout the study period. Analysis of the zooplankton community in the reservoir revealed a total of 52 species, 25 of which are new records for Koka Reservoir. With 40 species, rotifers were the most ii I,' \i \' species-rich group followed by cladocerans (8 species), and copepods (4 species). The rotifer Keratella tropica was the species recorded with the highest abundance among the zooplankton taxa contributing 42.11 % of total rotifers abundance. The cladocerans were dominated by small species, Ceriodaphnia corn uta, Diaphanosoma excisum and Moina m/crura, with rare occurrence of the large cladoceran D. barbata during a few months of the sampling period. Total abundance of copepods was constituted largely by Thermocyclops decipiens, which is the most commonly dominant species in tropical inland waters. Calanoid copepods were represented by a single species; Tropodiaptomus sp. Zooplankton abundance was generally higher during the dry season than the rainy season. Eutrophication of Koka Reservoir has resulted in the dominance of cyanobacteria, which are generally regarded as unsuitable food source for zooplankton. It was, therefore, hypothesized that seston may playa role in the diet of the zooplankton community of this reservoir. The contribution of seston fractions (<20 11m, ::::20