Time-course of inorganic nitrogen uptake and incorporation by natural populations of freshwater phytoplankton

SUMMARY. 1. Time-course measurements of NH₄⁺ and NO₃^?uptake were made on the natural phytoplankton populations in a eutrophic lake at a time when these nutrients were at their lowest annual concentration.
2. Both NH₄⁺ and NO₃^? uptake was increased at least five-fold during the first 5 min of incubation following near saturating pulses of these nutrients.
3. Elevated uptake was also observed following low level (～2μg N 1^?1) pulses of NH₄⁺ and NO₃^?, but substrate depletion during the first hour of incubation may have been partially responsible for this apparent enhancement.
4. Incorporation of ^I5N into TCA-insoluble material (protein) following the saturating NH₄⁺ pulse was increased less than total cellular ¹⁵N uptake, whereas no elevation of ¹⁵N incorporation into protein was observed following a saturating NO₃^?pulse.
5. The percentage of ^I5N incorporated into protein, with respect to total cellular uptake, was ～32% and ～12% for NH₄⁺ and NO₃^?, respectively, following 5 h of incubation.