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Determination of ammonium uptake and regeneration rates using the seawater dilution method
Authors:S. Neuer  P. J. S. Franks
Affiliation:(1) College of Oceanic and Atmospheric Sciences, Oregon State University, 97331-5503 Corvallis, Oregon, USA;(2) Present address: Max-Planck-Institute for Marine Microbiology, Fahrenheitstr. 1, W-2800 Bremen 33, Germany;(3) Present address: Scripps Institution of Oceanography, University of California at San Diego, 92093-0218 La Jolla, California, USA
Abstract:We have developed a method for the determination of ammonium uptake and regeneration rates applying the principle of the seawater dilution technique. The method is based on the separation of uptake and regeneration processes in the dilution series. A model is used to estimate ammonium uptake and regeneration rates simultaneously, in addition to phytoplankton growth and grazing rates. The method was applied to dilution experiments conducted during a two-year study of the upwelling region off Oregon, USA. Ammonium uptake and regeneration rates determined with our method ranged from 0.5 to 3 mgrmol l-1d-1 and from 0.2 to 2.9 mgrmol l-1d-1, respectively. These values agree well with those from other studies applying 15N tracer techniques in the same or similar environments. We found a close coupling between ammonium uptake and regeneration, and a strong relationship between ammonium regeneration and grazing rates. In addition, the nutritional status of the phytoplankton community could be assessed by comparing instantaneous ammonium uptake rates with the specific phytoplankton growth rates. Using the dilution technique to determine ammonium uptake and regeneration rates of the plankton community is a promising alternative to the application of tracer techniques conventionally used to determine these rates.
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