Seasonal variation of phytoplankton biomass in Lake Tana (Ethiopia)

Abstract

A record of seasonal dynamics of major phytoplanktonic communities estimated by biovolume in a tropical high altitude lake studied from June 2003 to July 2004 is presented. Four seasons were clearly distinguishable; Main-rainy season (MRS), Post-rainy season (PORS), Dry season (DS) and Pre-rainy season (PRS) with different phytoplankton assemblages. Total phytoplankton biovolume ranged from 708.5mm3ml-1during the main-rainy and dry seasons to 302751.5mm3ml-1 during the post rainy season. The major divisions were Bacillariophyta, Chlorophyta and Cyanophyta. Blue greens were clearly the most dominant taxonomic group during the post-rainy season while diatoms were dominant during the other three seasons. However, the predominance of diatoms generally occurred during periods of strong vertical mixing and that of blue green algae under relatively less mixed water column (post-rainy season). Generally, phytoplankton seasonality was influenced more by hydrological (runoffs associated nutrient availability) and hydrographic (internal mixing) changes. In addition, the fact that the lake is shallow and lacks marked thermal stratification has enabled it to have its own pattern of seasonality compared with other large African lakes.