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Please use this identifier to cite or link to this item: http://hdl.handle.net/123456789/149

Authors: Tadesse, Ogato
Date Added: 25-Sep-2007
Abstract: The seasonal dynamics of phytoplankton in relation to some physico-chemical and biological parameters were studied at a near-shore and central stations in Lake Bishoftu from August, 2006 to May, 2007. Water transparency (ZSD) ranged from 0.50 to 1,05 and from 0.46 to 1.0 m at the Central and Near Shore stations respectively, with high and low values corresponding to dry and rainy periods respectively. Mean vertical extinction coefficient varied temporally with higher values coincident with high algal biomass and abiogenic turbidity following rainy periods. The lake exhibited both superficial and deep-seated thermal stratifications, with ill-defined thermoclines on some occasions and never underwent complete mixing during the study period. Algal nutrients except nitrate-nitrogen showed temporal variations whose changes were inversely correlated with Chl a.The phytoplankton community of the lake, which consisted of five algal classess was dominated by cyanobacteria with Microcystis spp. as the most abundant and persistent taxa and with their contributions to the total abundance of the phytoplankton assemblage of the lake ranging from 58 to 99%. The zooplankton community was dominated by Calanoids and Rotifers with Lovenula africana and Brachionus spp.as the most numerically important taxa. Phytoplankton biomass measured as Chl a showed seasonal variations with maximum values during the rainy and post rainy months at both stations. Depth profile of photosynthesis showed a region of photoinhibition and condensed photosynthetic zone, a feature of some east African lakes. Light-saturated rates of gross photosynthesis (Amax) at the central station varied from a minimum of 410 to a maximum of 1630 mg O2 m-3 h- and with fairly strong and positive correlations with Chl a (R2=0.38, p=0.08), Soluble Reactive Phosphate, SRP (R2=0.46, p=0.07) and Light utilization efficiency, Φ (R2=0.61, p=0.02). Biomass specific rate at light-saturation exhibited unusually high values (29-91) and was positively and significantly correlated with SRP (R2=0.59, p=0.02), Areal rates varied from 0.89 to 3.88 g O2 m-2 h- 1 in association with biomass and Amax. The possible causes for the observed saptio-temporal variations in physico-chemical and biological features of the lake are discussed and suitable conservation strategies recommended.
Description: A Thesis Presented to the School of Graduate Studies of the Addis Ababa University in Partial Fulfilment of the Requirements for the Degree of Masters of Science in Biology.
URI: http://hdl.handle.net/123456789/149
Appears in:Thesis - Biology

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