Isotope dilution models of uptake and remineralization of ammonium by marine plankton1

Abstract

The utility of 15 N isotope dilution models for the calculation of uptake and remineralization of NH 4 + by marine phytoplankton was examined in light of model limitations when applied to field data either when ambient NH 4 + levels border on our limit of detection or when there is no statistically significant difference between ambient NH 4 + at the beginning and end of an incubation. Through specific examples of field and laboratory data we show that the limitations are a function both of analytical error inherent in the methodology and of changes in rates of uptake and remineralization over the course of a given experiment. We propose modifications to the existing models of NH 4 + uptake and remineralization which overcome some of these limitations. The results show that uptake rates have been traditionally underestimated by a factor of ≈2 in routine 15 N uptake methodology and that regeneration of NH 4 + over relatively brief periods can supply the daily nitrogen requirements of the phytoplankton when there are no losses from the system.