Inorganic Nutrient Concentrations and Chlorophyll in the Euphotic Zone of Lake Tanganyika

Abstract

Chlorophyll a and inorganic plant nutrient levels were measured in the euphotic zone at a pelagic station in Lake Tanganyika for 12 months. In October, chlorophyll a had a mean concentration of 5.0 μg/l, the highest value obtained during the study period. A secondary peak (mean 2.5 μg/l) was obtained in May. Upwelling in July/August may have been responsible for the high biomass in October, while secondary upwelling in April/May may have caused the secondary peak in May. Secchi disc transparency ranged between 7.71 and 16.25 m. The low transparency coincided with maximum chlorophyll a in October. An inverse relationship was found between chlorophyll and transparency. High nitrate-nitrogen was observed in July and December, when the values were 67 and 76 μg N/l, respectively. Nitrogen fixation by the heterocystous Anabaena flos-aquae may have been responsible for the high nitrate values in October/December. The mean nitrate-nitrogen concentration was 45.64 μg N/l and an inverse relationship was found between chlorophyll a and nitrate-nitrogen. Very low soluble reactive phosphorus levels (range 0.4 and 4.0 μg P/l) were found throughout the 12 month period. A mean SRP concentration of 1.86 μg P/l was found. A positive correlation was found between chlorophyll a and SRP. N:P molar ratio gave a value of 54.33, showing that in the euphotic zone of the lake, P is limiting primary production. Silica concentrations were high throughout the study period with a mean value of 1.56 mg SiO2/l.