Methodology:

      1. Ammonium - nanomolar ammonium concentrations, as measured by a
spectrophotometric procedure based on the Berthelot reaction followed
by an extraction step to concentrate the indophenol.
         References: 
		  a) Brzezinski, M.A. (1987) Colorimetric
		     Determination of nanomolar concentrations of
		     ammonium in seawater using solvent extraction.
		     Marine Chemistry, 20, 277-288.
		  b) Selmer, J.S. and F. Sorensson (1986) New
		     Procedure for extraction of ammonium from natural
		     waters for 15N isotopic ratio determinations.
		     Applied Environmental Microbiology, 52, 577-579.

      2. Urea - urea concentration as 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.

      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 modified isotope dilution model was used
to calculate uptake.
         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 carbon and nitrogen - the concentration of either
particulate carbon or nitrogen as measured by collection on Whatman
GF/F filters, followed by Dumas combustion, cryogenic separation and
manometric analysis.