N15_uptake Methodology
Raymond Sambrotto

1. Urea - measured by a spectrophotometric procedure based on the
   diacetylmonoxime reaction.
         Reference:
                    Whitledge, T.E., S.C. Malloy, et al. (1981)
                    Automated nutrient analysis in seawater. Department
                    of Energy and Environment, Brookhaven National
                    Laboratory, Upton, NY.

2. Dissolved free Amino Acids -  measured by a fluorometric procedure
   after the o-phthalaldehyde reaction.
         Reference:
                    Parsons, T.R., Maita, Y., Lalli C.M. (1984)
                    A manual of chemical and biological methods for
                    seawater analysis. Pergammon Press, Oxford

3. N-15 uptake -

     A. Uptake rates for nitrate and urea were measured by incubation
   experiments to follow the incorporation of trace additions of N-15
   labeled substrate into particulate material collected by Whatman GF/F
   filters.
         Reference:
                    Sambrotto,R.N., J.H. Martin, et al. (1993) Nitrate
                    utilization in surface waters of the Iceland Basin
                    during spring and summer of 1989. Deep-Sea Research II.
                    Vol. 40, No.1/2, 441-457.

      B. Methodology for ammonium rates was similar to that for nitrate and
   urea, except that the isotope dilution during the incubation was accounted
   for in computing the rates.
         Reference:
                    Glibert, P. M., F. Lipschultz, et al. (1982).
                    Isotope dilution models of uptake and remineralization
                    of ammonium by phytoplankton. Limnology and Oceanography 
                    27: 639-650.

4. Particulate organic carbon and nitrogen - the concentration of
   either particulate organic carbon or nitrogen as measured by collection on
   Whatman GF/F filters, and acid treated to remove carbonates as per the JGOFS
   protocol, followed by analysis on a Europa CHN analyzer.