Temporal and Spatial Variations in Biomass and Photosynthetic Production of Phytoplankton In Lake Chamo, Ethiopia

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Date

2004-06

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Addis Ababa University

Abstract

The temporal and spatial variations in the biomass and photosynthetic production of phytoplankton in relation to some physico-chemical environmental factors were investigated at two (offshore and near-shore) stations in Lake Chama from August, 2003 to May, 2004. Inorganic nutrients varied temporally, with their low and high concentration levels generally corresponding to the dry and rainy periods respectively. Lake's transparency was always low (ZSD < 40 cm). Because phytoplankton biomass was high throughout the study period, its contribution to the extinction of underwater light is expected to be considerable. Phytoplankton biomass measured as chlorophyll a varied 43 to 95 (mean = 77.2) and 47 to 108 (mean=69.4) mg Chi a m-3 at the near-shore and central stations, respectively. The vertical distribution of photosynthetic activity was of a typical pattern for phytoplankton with light-inhibition on all sampling dates. The light-saturated rate of photosynthesis (Amax) ranged from 721 to 1789 mg O2 (""224.95 to 558.17 mg C) m-3 h-1 at the central station and from 543 to 1803 mg 02( ""169. 42 mg C) m-3 h-1 at the near-shore station. Lack of correspondence between phytoplankton biomass and Amax was evident. Biomass-specific rate of photosynthetic production at light-saturation, photosynthetic capacity (¢max) ranged from 9 to 34 mg O2 (mg Chi ay-1 h-1 at the nearshore station and from 10 to 34 mg 02 (mg Chi ay-1 h-1 at the central station. An inverse relationship between photosynthetic capacity and phytoplankton biomass was also observed. Hourly integral photosynthesis ranged from 0.36 to 1.72 g 02 ("" 0.11 to 0.54 g C) m-2 h-1 at the central station and from 0.47 to 1.8 g 02 ("" 0.15 to 0.56 g C) m-2 h-1 at the near-shore station. The production rate per unit area seems to have been influenced by the light-saturated rate of photosynthesis as the strong and positive correlation between the two (r = 0.77) suggests. The temporal variations in phytoplankton biomass and primary production in relation to environmental factors are discussed with a view to identify those environmental variables that have regulatory role. Key words: Biomass, production, phytoplankton, Lake Chama, Ethiopia,

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Biomass, production, phytoplankton, Lake Chama, Ethiopia,

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