Distribution of Dissolved Organic Nitrogen in the North Pacific Ocean

Abstract

Abstract The concentration and composition of dissolved organic nitrogen (DON) in the North Pacific Ocean were investigated to determine the vertical and horizontal variations of DON. The significance of dissolved organic nitrogen in the geochemical cycle of nitrogen was discussed in relation to the hydrographical conditions, biological production and decomposition. Seawater samples were collected from the surface to 2,000 m depth at 33 stations of the subarctic and subtropical regions of the North Pacific Ocean and the DON concentrations were investigated by high temperature catalytic oxidation method. The DON concentrations in the upper waters (0–200 m) of the subtropical region were slightly higher than those in the subarctic region, whereas the concentrations in the middle (200–1000 m) and deep (> 1000 m) waters were nearly the same. Moreover, the vertical DON profiles were almost similar to each other in all the stations: the average DON concentration gradually decreases with depth from 6.4±1.9 μg-at. N/1 at the surface to 2.9±1.9 μg-at. N/1 at and below 200 m, where the DON concentrations were almost constant with depth. In the upper water, DON existed as labile nitrogen compounds which can easily be decomposed by oceanic bacteria, i. e., low molecular weight organic nitrogen compounds. In the middle and the deep waters, however, DON existed more as biochemically refractive nitrogen compounds, i. e., higher molecular weight organic nitrogen compounds. Despite the fact that dissolved inorganic nitrogen (DIN) shows the large horizontal and vertical variations in the ocean, the spatial variation of DON is small both vertically and horizontally. This suggests that a significant source of DIN is not DON but particulate organic nitrogen (PON), and the DON may not be actively involved in the nitrogen cycle in the North Pacific Ocean.