ABSTRACT

In order to reveal the vertical distribution of dissolved organic nitrogen (DON) in seawater, a critical examination is made of two the analytical methods for the constituents, i.e. the persulfate oxidation method and the high-temperature catalytic oxidation (HTCO) method with platinum (Pt) or manganese (Mn) as the catalyst. The analytical values of total dissolved nitrogen (TDN) from the HTCO method agreed very closely with the values obtained by the persulfate oxidation method for several standard organic nitrogen compounds, humic acid prepared from marine and lake sediments, and natural seawater samples. Both of the catalysts, Mn and Pt used in the HTCO method, provided reproducible data for the TDN analysis. The concentrations of TDN changed vertically by both methods in a manner similar to those of nitrate nitrogen. The concentration of DON in seawater, expressed as the difference of TDN and dissolved inorganic nitrogen (DIN), was in the range of 1∼10 μg at N·1^-1 from the surface to 2000 m depth at several stations in the North Pacific Ocean. In the subsurface water from 0 to 200 meter depth, DON showed an inverse correlation to nitrate with statistical significance. The gradient of NO₃/DON by atom base was ca. -4 for the subsurface samples, indicating that about 25% of NO₃ was apparently regenerated from the DON fraction, but no correlation was found below 200 m depth, suggesting. that DON in the deeper layer was relatively refractory for biological activity.