Short-term ammonium and nirate dynamics in three plankton size classes were studied in a mesocosm experiment using tracer and 15N isotope dilution techniques. A single site in the Choptank River, a sub-estuary of the Chesapeake Bay, was chosen for initial whole water collection. The samples were then size-fractionated and incubated. Subsamples were extracted over 36 hours and analyzed for ambient concentrations, uptake rates, and regeneration rates of the inorganic nitrogen sources.
In each size class population, initial utilization and regeneration of ammonium, preferred over nitrate, was noted. Absence of light for photosynthesis during the night in the smaller size classes caused nitrogen cycling shifts to affect the larger populations. Nitrate preferences dominated the upper fractions near the end of the 36-hour period. Diel variation was not related to nitrogen source preferences.