TEMPORAL AND SPATIAL VARIATIONS IN BIOMASS AND PHOTOSYNTHETIC PRODUCTION OF PHYTOPLANKTON IN LAKE CHAMO, ETHIOPIA.

Abstract

The temporal and spatial variations in the hiomass and photosynthetic production of phytoplankton in relation to some physico-chemical environmental factors were investigated at two (offshore and near-shore) stations in Lake Chamo 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 hiomass was high throughout the study period, its contrihution to the extinction of underwater light is expected to he considerahle. Phytoplankton hiomass measured as chlorophyll a varied 43 to 95 (mean = 77.2) and 47 to 108 (mean=69.4) mg Chi a m'^ at the near-shore and central stations, respectively. The vertical distrihutlon of photosynthetic activity was of a typical pattern for phytoplankton with light-inhihition 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'^ h'^ at the central station andtfrom 543 to 1803 mg 02( »169. 42 mg C) m'^ h'^ at the near-shore station. Lack of correspondence hetween phytoplankton hiomass and Amax was evident. Bipmass-specific rate of photosynthetic production at light-saturation, photosynthetic capacity (<Pmax) ranged from 9 to 34 mg O2 (mg Chi a)''' h'^ at the nearshore station and from 10 to 34 mg O2 (mg Chi a)''' h''' at the central station. An inverse relationship hetween photosynthetic capacity and phytoplankton hiomass was also ohserved. Hourly integral photosynthesis ranged from 0.36 to 1.72 g O2 (" 0.11 to 0.54 g C) m'^ h'^ at the central station and from 0.47 to 1.8 g O2 (^ 0.15 to 0.56 g C) m'^h'^ at the near-shore station. The production rate per unit area seems to have heen influenced hy the light-saturated rate of photosynthesis as the strong and positive correlation hetween the two (r = 0.77) suggests. The temporal variations in phytoplankton hiomass and primary production in relation to environmental factors are discussed with a view to identify those environmental variahies that have regulatory role.