Lake Bosomtwe is a closed, stratified, natural freshwater body located in the Ashanti Region of Ghana. Recently residents along the Lake shore complain of foul smells emanating from the Lake and also there have been occasional fish deaths there. This led to the study of the composition of phytoplankton community dominating the lake. Sampling was done during the rainy and dry seasons. Samples were collected from twelve towns along the lakeshore. Phytoplankton samples were collected using clean 100-ml bottles and fixed with Lugol’s solution. In the laboratory, phytoplankton was identified using a Carl Zeiss inverted microscope. The results obtained during the dry season (August) showed the dominance of blue-green algae in all the twelve stations, comprising more than 90% of the biomass as was also found in the rainy season. This implied that the presence of the blue-green algae in Lake Bosomtwe was not seasonal but all year round, and is also nutrient-dependent as shown by the chemical data collected during the studies. However, the green algae population reduced from 6.2 to 3.7%, in August while the diatoms increased from 0.9 to 3.5%. Notable among the blue-green algae were Cylindrospermopsis, Planktothrix and Spirulina for both the dry and rainy seasons. A positive correlation was obtained between temperature, phytoplankton and dissolved oxygen. This was expected as the blue-green algae which form over 90% of the phytoplankton counts are favoured by elevated water temperatures, a phenomenon which is notable of tropical Lakes. A mean temperature of 27°C was recorded in Lake Bosomtwe. High mean temperature (27°C) and high nutrient of the lake, coupled with its stagnant nature had led to its eutrophic state and the proliferation of blue-green algae. Some blue-green algae contain toxins that are toxic to both man and animals. These could be the cause of the occasional fish deaths and foul smells emanating from the Lake.
Key words: Lake Bosomtwe, fish deaths, phytoplankton, sustainability, blue-green algae, eutrophic state, Ghana.
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