Isotope dilution models of uptake and remineralization of ammonium by marine plankton’

The utility of 15N isotope dilution models for the calculation of uptake and remineralization of NH4+ by marine phytoplankton was examined in light of model limitations when applied to field data either when ambient NH4+ levels border on our limit of detection or when there is no statistically significant difference between ambient NH,+ 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. WC propose modifications to the existing models of NH,+ 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 15N uptake methodology and that regeneration of NH4-’ over relatively brief periods can supply the daily nitrogen requirements of the phytoplankton when there are no losses from the system. Marine phytoplankton living in nitrogen-impoverished waters may be able to exploit a patchy nutrient distribution, since phytoplankton in both laboratory cultures and field assemblages can utilize NH4+ rapidly when it is delivered to the medium in pulse fashion. For example, N-deprived phytoplankton cells grown in NH4+-limited continuous cultures can assimilate this nutrient at rates much faster than their growth rate when exposed to uptake-saturating NH,+ concentrations for 5-min periods (McCarthy and Goldman 1979). Field assemblages can also utilize NH4+ rapidly and efficiently during the initial minutes of 2-h incubations following even trace additions of substrates (Glibert and Goldman 1981). A corollary to the hypothesis of phytoplankton exploitation of nutrient microenvironments is that enough nitrogen to meet the phytoplankton demand must ’ This work was supported by NSF grant <ICE 7726401 and OCE 80-22990 J. J. McCarthy, ’ Present address: Woods Hole Oceanographic Institution, Woods Holc, Massachusetts 02543. be made available by zooplankton excretion and bacterial remineralization. 15N isotope dilution methodology has been used to measure rates of these processes in a freshwater lake (Alexander 1970), southern California waters (Harrison 1978), and Kaneohe Bay, Hawaii (Caperon et al. 1979). A similar methodology has been applied to NH,+ fluxes in the sediment (Blackburn 1979). The analytical methodology in these studies of remineralization varies widely, as do the models used in the calculation of results. Alexander (1970; see also Dugdale 1965) and Harrison (1978; pers. comm.) assumed constant dilution with time to estimate remineralization rates, while Caperon et al. (1979) and Blackburn (1979) used linear differential equations to estimate uptake and remineralization rates of NH4+. We will show that previous isotope models of uptake and remineralization h ave limitations when applied to field data, either when ambient NH,+ levels border on our limit of detection (~0.03 pg-atom * liter-‘) or when there is no statistically significant difference between