Temporal Dynamics Of Water Quality And Community Structure And Photosynthetic Production Of Phytoplankton In Belbela Reservoir Ethiopia

The temporal dynamics of the composition, abundance, biomass and photosynthetic productivity ofrnphytoplankton in relation to physico-chemical water quality and zooplankton were studied fromrnSeptember, 2010 to May, 2011 in Belbela reservoir. Water transparency exhibited temporal variationrn(0.16 m to 0.26 m), which was primarily determined by abiogenic turbidity. The depth profiles ofrntemperature and dissolve oxygen seem to indicate the absence of deep-seated and persistent thermalrnstratification, which is consistent with the shallowness and complete exposure of the reservoir to windrnaction. Aggregate chemical parameters were clearly indicative of the very dilute nature of the reservoirrnwater. All nutrients except soluble reactive phosphate were generally at high levels. All chemicalrnparameters including inorganic nutrients exhibited temporal variations with no obvious association withrnbiological variables. The phytoplankton community of the reservoir was primarily constituted by bluegreenrnalgae, green algae and diatoms, with the overwhelming dominance of blue-greens whoserndominance seemed to be favored by the turbid, turbulent and nutrient-rich water column. The majorrncontributors to the dominance of blue-green algae include the potentially toxic taxa Cylinderospermopsis,rnMicrocystis and Planktothrix. The impact of the rotifer-dominated zooplankton community, on thernphytoplankton seemed to have been weakened by the abundant large-sized colonial and filamentous bluegreens,rnwhich are not manageable and probably toxic. Chlorophyll a biomass of phytoplankton variedrntemporally (20.38 -68.57 Ig L-1) with its peaks corresponding to those of total abundance ofrnphytoplankton and blue-greens but without any clear causal relationship with inorganic nutrients. Thernvertically compressed depth profiles of gross photosynthesis exhibited temporal variations in their lightsaturatedrnrates (Amax) and depths of their occurrence. Amax varied from about 226 to 891 mg O2 (≈70.5 torn278 mg C) m-3 h-1, corresponding to the lowest and highest phytoplankton biomass values although lackrnof good correlation between the two was evident. The fairly high and positive correlation between Amaxrnand the biomass-specific rate at light saturation, Pmax (r=0.6938, r2=0.4813 at p=0.0563) provided anrnexplanation for the observed association of high light-saturated rates with low algal biomass. Pmax rangedrnfrom ≈ 5.5 to 24.3 mg O2 (mg Chl a)-1 h-1 with most values below 15, but with its maximum value higherrnthan that considered as representative for many African lakes [about 20 mg O2 (mg Chl a)-1 h-1] and anrnupper limit for lakes of the temperate regions. The hourly integral rate of gross photosynthesis (ΣA, mgrnO2 m-2 h-1) ranged from 0.112 to 0.510 g O2 (≈ 0.035-0.159 g C) m-2 h-1 with its variations being primarilyrndue to temporal changes in Amax (r=0.7551, r2 =0.5701 at p= 0. 0.0303). The present results seem tornsuggest that the reservoir is at the verge of a seemingly irreversible environmental degradation. There is,rntherefore, a need for the assessment of cyanotoxins and the impact of human activities like irrigation,rnshore-line modification and removal of plant cover with a view to develop strategies of preventing furtherrndegradation of the aquatic ecosystem and loss of its resources.

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