Temporal Dynamics of Water Quality and Community Structure and Photosynthetic Production of Phytoplankton in Belbela Reservoir, Ethiopia

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Addis Ababa University


The temporal dynamics of the composition, abundance, biomass and photosynthetic productivity of phytoplankton in relation to physico-chemical water quality and zooplankton were studied from September, 2010 to May, 2011 in Belbela reservoir. Water transparency exhibited temporal variation (0.16 m to 0.26 m), which was primarily determined by abiogenic turbidity. The depth profiles of temperature and dissolve oxygen seem to indicate the absence of deep-seated and persistent thermal stratification, which is consistent with the shallowness and complete exposure of the reservoir to wind action. Aggregate chemical parameters were clearly indicative of the very dilute nature of the reservoir water. All nutrients except soluble reactive phosphate were generally at high levels. All chemical parameters including inorganic nutrients exhibited temporal variations with no obvious association with biological variables. The phytoplankton community of the reservoir was primarily constituted by bluegreen algae, green algae and diatoms, with the overwhelming dominance of blue-greens whose dominance seemed to be favored by the turbid, turbulent and nutrient-rich water column. The major contributors to the dominance of blue-green algae include the potentially toxic taxa Cylinderospermopsis, Microcystis and Planktothrix. The impact of the rotifer-dominated zooplankton community, on the phytoplankton seemed to have been weakened by the abundant large-sized colonial and filamentous bluegreens, which are not manageable and probably toxic. Chlorophyll a biomass of phytoplankton varied temporally (20.38 -68.57 Ig L-1) with its peaks corresponding to those of total abundance of phytoplankton and blue-greens but without any clear causal relationship with inorganic nutrients. The vertically compressed depth profiles of gross photosynthesis exhibited temporal variations in their lightsaturated rates (Amax) and depths of their occurrence. Amax varied from about 226 to 891 mg O2 (≈70.5 to 278 mg C) m-3 h-1, corresponding to the lowest and highest phytoplankton biomass values although lack of good correlation between the two was evident. The fairly high and positive correlation between Amax and the biomass-specific rate at light saturation, Pmax (r=0.6938, r2=0.4813 at p=0.0563) provided an explanation for the observed association of high light-saturated rates with low algal biomass. Pmax ranged from ≈ 5.5 to 24.3 mg O2 (mg Chl a)-1 h-1 with most values below 15, but with its maximum value higher than that considered as representative for many African lakes [about 20 mg O2 (mg Chl a)-1 h-1] and an upper limit for lakes of the temperate regions. The hourly integral rate of gross photosynthesis (ΣA, mg O2 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 primarily due to temporal changes in Amax (r=0.7551, r2 =0.5701 at p= 0. 0.0303). The present results seem to suggest that the reservoir is at the verge of a seemingly irreversible environmental degradation. There is, therefore, a need for the assessment of cyanotoxins and the impact of human activities like irrigation, shore-line modification and removal of plant cover with a view to develop strategies of preventing further degradation of the aquatic ecosystem and loss of its resources.