Temporal Dynamics Of The Species Composition And Size Fractionated Biomass Of Phytoplankton In Lake Kuriftu Ethiopia

The temporal dynamics of phytoplankton and physic-chemical variables were studied at arncentral station of Lake Kuriftu from December, 2008 to November, 2009. The lake is shallow andrnas a result undergone complete mixing throughout the study period. Secchi depth ranged fromrn0.21 m to 0.6 m. The Lake’s transparency (ZSD) was always less than 0.6 m with smaller valuesrncoincident with high phytoplankton biomass even though there is weak correlation b/nrnphytoplankton biomass and ZSD. The chemistry of the lake was basically similar to the dilute EastrnAfrican lake waters, with maximum pH and total alkalinity values of 9.09 and 4.1 meq/lrnrespectively. Nitrate concentration (μg L-1) ranged from a minimum of 9.33 in November, 2009rnto 65.45 in April, 2009. Soluble reactive phosphate- phosphorus (SRP, in μg L-1) varied from arnminimum value of 14.65 in January, 2009 and May, 2009 to a maximum of 402.57 in August,rn2009. Dissolved silica (Molybdate-reactive silica, SRSi in mg L-1) ranged from 3.051 inrnDecember, 2008 to 97.92 in September, 2009. The phytoplankton community which wasrndominated by Cyanobacteria is 2nd in diversity to chlorophycea which covers 25 species of the 50rnspecies. Cylinderospermopsis Africana was the most dominant species throughout the studyrnperiod.The diversity index was narrow ranging from 1.24 -6.42 bits/individual. Among the threernsize-group of phytoplankton, the nanoplankton (2-20 μm) was the most important contributorrnto the total phytoplankton biomass with its biomass and percentage contributions to the totalrnphytoplankton biomass ranging from 15.44 μg L-1and 51.93 % to 19.74μg L-1 and 73.84 %rnrespectively. The Chl a biomass and percentage contribution to total phytoplankton biomass ofrnthe Netplankton and Picoplankton varied from 1.94 to 6.73 and 6.53 % to 13.87% and 4.68 torn14.63 and 17.5% to 41.54 % respectively. The factors responsible for the observed spatiotemporalrnvariations in the physical,chemical and biological features of the lake are discussed.rnKey words: Lake Kuriftu, Size fractionated Biomass, Phytoplankton, Temporal Variation.

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