地理ジャーナルと自然災害
オープンアクセス

概要

The Study of Temporal Dynamics of Phytoplankton Biomass and Species Composition in Relation to Some Physical and Chemical Factors, Lake Kuriftu, Oromia Region, Ethiopia

Zelalem Dessalegn Fayissa and Demeke Kifle

Biomass of phytoplankters in relation to the physical and chemical conditions of the water column in Lake Kuriftu was studied from August 2005 to April 2006. The sampling stations of the present study were selected from contrasting segments of Lake Kuriftu as impacted at shore and non-impacted at the center. Secchi depth was estimated with a standard Secchi disc of 20 cm diameter while pH was measured in situ by a portable digital pH meter and temperature was determined with a digital oxygen meter. Nitrate-N was determined by the Zinc Reduction Procedure while phosphate and silica were measured calorimetrically by the Ascorbic Acid and Molybdosilicate methods. Phytoplankters samples were examined with an inverted microscope and identification to genus or species level was made on the basis of the various taxonomic literatures available on phytoplankters. The concentration of Chlorophyll a was calculated by using absorbance measurements made at 665 and 750 nm to calculate the biomass. 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 phytoplankters 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 phytoplankters community which was consistently dominated by cyanobacteria exhibited low species diversity. Phytoplankters 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 variations of the parameters indicates how one is influenced by the other and influences the others in a given ecosystems.