Hydrobiological Survey of the Bahir Dar Gulf of Lake Tana, Ethiopia

Fortnightly samples were collected from three points at the southern gulf of Lake Tana in the early wet season months of June and July 2007 to investigate physico-chemical condition, phytoplankton and zooplankton assemblages. Water temperature ranged from 22.2 to 25oC, depth, 0.96 to 4m; transparency 0.4 to 0.9m; conductivity 152 to 232μScm-1; dissolved oxygen 1.66 to 7.7mgl-1; pH 7.35 to 8.48; nitrate 0.92 to 4.18mgl-1 and phosphate 0.1 to 0.61mgl-1. Of these parameters water column depth and pH were significantly different in the three sample stations and dissolved oxygen was positively correlated with depth and pH. Sixty one phytoplankton species classified into five divisions in the following order of percentage composition; Chlorophyta 39%, Bacillariophyta 38%, Cyanophyta 13%, Euglenophyta 8% and Dinophyta 2%. Forty four zooplankton species made up of 16 species of rotifers, 16 species of cladocerans and 12 species of copepods and their developing stages were recorded. There was a clear spatial pattern in the phytoplankton and zooplankton distribution in this study. Like the rest of the lake, the pelagic waters of the Bahir Dar gulf was oligotrophic while the inshore areas had greater plankton species diversity and prone to pollution from allochthonous nutrient input from adjacent wetlands and human encroachment.