Temporal Dynamics of Biomass and Primary Production of Phytoplankton In Relation to Some Phsico-Chemical Factors In Lake Koriftu, Ethiopia

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Addis Ababa University


Biomass and Photosynthetic productivity of phytoplankton in relation to the physical and chemical conditions of the water column in Lake Kuriftu were studied from August, 2005 to April 2006. All the physical, chemical and biological parameters measured varied spatially and temporally. The Lake’s transparency (ZSD) was always less than 0.6 m with smaller values coincident with periods of rainfall and negatively correlated (r=-0.36) with phytoplankton biomass. The chemistry of the lake was basically similar to the dilute East African lake waters, with maximum pH and total alkalinity values of 8.8 and 3.1 meq/l respectively. The inorganic nutrients, which were of moderately high levels varied temporally as a function of hydrological, hydrographic and biological conditions of the lake. The phytoplankton community which was consistently dominated by cyanobacteria exhibited low species diversity. Phytoplankton biomass measured as chlorophyll a varied from 18.35 to 45.18 mg Chl a m-3 at the near-shore station and from 17.24 to 55.6 mg Chl a m-3 at the central station. The light-saturated rate of photosynthesis (Amax), which was positively and strongly correlated with phytoplankton biomass, ranged from 571 to 1136 mg O2 m-3 h-1. Biomass-specific rate of photosynthetic productivity at light saturation (Pmax) ranged from 18.78 to 33 mg O2 (mg Chl a)-1h-1, while the hourly integral photosynthetic production (ΣA) which was positively and strongly correlated with biomass and Amax, varied between 0.686 and 1.05 g O2 m-2 h-1. The factors responsible for the observed spatio-temporal variations in the physical,chemical and biological features of the lake are discussed.