Temporal And Spatial Variations In Biomass And Photosynthetic Production Of Phytoplankton In Lake Chamo Ethiopia

The temporal and spatial variations in the biomass and photosynthetic production ofrnphytoplankton in relation to some physico-chemical environmental factors wererninvestigated at two (offshore and near-shore) stations in Lake Chama from August,rn2003 to May, 2004. Inorganic nutrients varied temporally, with their low and highrnconcentration levels generally corresponding to the dry and rainy periodsrnrespectively. Lake's transparency was always low (ZSD < 40 cm). Becausernphytoplankton biomass was high throughout the study period, its contribution to thernextinction of underwater light is expected to be considerable. Phytoplankton biomassrnmeasured as chlorophyll a varied 43 to 95 (mean = 77.2) and 47 to 108 (mean=69.4)rnmg Chi a m-3 at the near-shore and central stations, respectively. The verticalrndistribution of photosynthetic activity was of a typical pattern for phytoplankton withrnlight-inhibition on all sampling dates. The light-saturated rate of photosynthesis (Amax)rnranged from 721 to 1789 mg O2 (""224.95 to 558.17 mg C) m-3 h-1 at the centralrnstation and from 543 to 1803 mg 02( ""169. 42 mg C) m-3 h-1 at the near-shorernstation. Lack of correspondence between phytoplankton biomass and Amax wasrnevident. Biomass-specific rate of photosynthetic production at light-saturation,rnphotosynthetic capacity (¢max) ranged from 9 to 34 mg O2 (mg Chi ay-1 h-1 at the nearshorernstation and from 10 to 34 mg 02 (mg Chi ay-1 h-1 at the central station. Anrninverse relationship between photosynthetic capacity and phytoplankton biomass wasrnalso observed. Hourly integral photosynthesis ranged from 0.36 to 1.72 g 02 ("" 0.11rnto 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 grnC) m-2 h-1 at the near-shore station. The production rate per unit area seems to havernbeen influenced by the light-saturated rate of photosynthesis as the strong andrnpositive correlation between the two (r = 0.77) suggests. The temporal variations inrnphytoplankton biomass and primary production in relation to environmental factorsrnare discussed with a view to identify those environmental variables that havernregulatory role.rnKey words: Biomass, production, phytoplankton, Lake Chama, Ethiopia,

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