CHROMATOGRAPHIC MEASUREMENT OF NITROGEN MINERALIZATION RATES IN MARINE COASTAL WATERS WITH N-15

Isotope ratios for ammonium were determined directly on seawater filtrates by high performance liquid chromatography (HPLC) for isotope dilution and enrichment experiments in the Mississippi River plume region of the Gulf of Mexico. The 2 isotopic forms could be differentiated by cation exchange chromatography because the ratio of (NH4+)-N-15:(NH3)-N-15 is Slightly greater than the ratio of (NH4+)-N-14:(NH3)-N-14 in aqueous solutions at pH's near the pK for ammonium (ca pH 9). Relatively small (e.g. 60 ml) water samples were fortified in the field with N-15-ammonium or N-15-amino acids and incubated at simulated in situ temperature and light conditions. At 2 to 13 h intervals, subsamples were filtered (0.2 mum pore size) and frozen for later HPLC analysis in the laboratory. Isotope-dilution experiments conducted on water samples collected from different depths in the plume indicated that maximum ammonium regeneration rates occurred in near-surface waters where phytoplankton and bacterial production rates are relatively high. Amino acid and ammonium concentration changes and N-15-NH4 compositional changes were measured at 4 intervals over 21 h after the addition of an N-15-labeled amino acid mixture (4 muM). Comparison of the amount of N-15 recovered as (NH4)-N-15 to that removed from solution as N-15-labeled amino acids indicated that the potential conversion of 'assimilated' N-15-labeled amino acids to dissolved ammonium ranged from about 50 % in surface water to about 90 % in near-bottom (30 m depth) water. These results demonstrate the usefulness of the HPLC approach for measuring nitrogen regeneration rates or conversion efficiencies in small volumes of marine coastal waters.