Nutrient Limitation of Phytoplankton in Five Impoundments on the Manyame River, Zimbabwe
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Nutrient limitation was investigated in the Manyame lakes during 2004-05, namely, Harava Dam, Seke Dam, Lake Chivero, Lake Manyame and Bhiri Dam. Selenestrum capricornutum was used as the test organism in one group of bioassays and on the other the lakes natural phytoplankton population. Nitrogen was indicated to be the primary limiting nutrient in Harava Dam, Seke Dam and Lake Manyame. Phosphorus was found to be the primary limiting nutrient in Bhiri Dam while no nutrient indicated to be limiting the growth of phytoplankton in Lake Chivero instead, light was suggested to be limiting the growth of phytoplankton. Since Robarts and Southall (1977), Harava Dam and Seke Dam showed signs of getting enriched owing to the sewage discharge from expanding urban settlements in Ruwa and surrounding areas. Lake Chivero has remained much the same in the last 30 years and was indicated to be acting as a nutrient trap since the dams downstream of it were not found to as eutrophic as itself. Therefore Lake Manyame, Seke Dam and Harava Dam were concluded to be mesotrophic, Bhiri Dam – oligotrophic and Lake Chivero, eutrophic.