The stoichiometry of particulate nutrients in Lake Tanganyika - implications for nutrient limitation of phytoplankton

We studied the potential nutrient limitation of phytoplankton by means of s eston nutrient stoichiometry and nutrient enrichment bioassays in the epili mnion of Lake Tanganyika. In most cases, the particulate carbon to phosphor us (C:P) ratio was high and indicated moderate P deficiency, while the resp ective C:N ratio mainly suggested moderate N deficiency. The N:P ratios of seston indicated rather balanced N and P supply. In three two-day enrichmen t bioassays in April-May 1995, a combined addition of P, N and organic carb on (glucose) always increased primary production in comparison to untreated controls. Primary production also slightly increased after the addition of phosphate-P, while the additions of single ammonium-N and glucose had no e ffect. Although the measured turnover time of P was short and our few nutri ent enrichment experiments suggested that P may be the most limiting single nutrient, the particulate nutrient ratios and the strong stimulation of pr imary production after the combined addition of P and N mostly suggest that in the upper epilimnion of Lake Tanganyika plankton experience a restricte d, but approximately balanced nutrient supply.