Dynamics of dissolved organic nitrogen in the ocean obtained by using high-sensitive and high-resolution method

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A high-sensitive and high-resolution method for measuring dissolved organic nitrogen (DON) in seawater has been established, which consists of a high-temperature combustion unit and a chemiluminescence detector. The international program for instrument comparison for measurement of DON in seawater confirmed that this system has a high performance to obtain accurate and precious data of DON in seawater. By using this system, the detailed distribution of DON in a variety of sea area and biological processes of DON in incubation experiments were obtained as follows: surface oceans in the western North Pacific along 165 E longitude and along A line of Oyashio region, Arakawa estuary in Tokyo Bay, and microbial decomposition process of DON in surface waters in the western. North Pacific and release process of DON by coral. The system is designed for simultaneous measurement of dissolved organic carbon (DOC) with DON, so that it has been made possible that DON dynamics is directly compared with those of DOC, and characteristics of C:N ratio of dissolved organic matter (DOM) is discussed. Consequently it was found that relatively C-rich DOM was stably accumulating in oligotrophic area and unproductive season, whereas N-rich DOM was temporally accumulating in high latitude area especially in productive season. Amino acids in DOM (Total Dissolved Amino acids: TDAA), as typical nitrogenous organic compounds and most major identifiable component of DON, were also determined for some oceanic samples. The proportions of amino acids' carbon (TDAA-C) to DOC (0.8-1.8%) and amino acids' nitrogen (TDAA-N) to DON (8-18%) were obtained. Both proportions of TDAA-C/DOC and TDAA-N/DON showed a tendency to be high in productive area of high latitude and low in oligotrophic area of low latitude, suggesting DOM in productive area contained more biogenic and fresh components, whereas more altered components mainly composed DOM in oligotrophic area.