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

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Abstract
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.