Rapid Ammonium Uptake by Freshwater Phytoplankton¹

A natural assemblage of freshwater phytoplankton was removed from an oligotrophic lake and grown at N:P supply ratios of 5:1, 15:1 and 45:1 (by atoms) in semicontinuous culture. After a minimum of 31 days at an average daily growth rate of 0.5 d −1 , experiments were conducted examining the time‐course of saturated ammonium uptake rates. Cultures grown under the two lowest N:P supply ratios demonstrated greatly elevated initial, specific uptake rates for ammonium and were able to sequester between 7 and 21% of their daily N requirement in less than 5 min. The initial rates declined rapidly but were followed by a subsequent increase and decrease over a 120 min period.