| In marine systems, dissolved organic nitrogen (DON) can affect the nitrogen budget, primary production, and potentially the production of nitrous oxide. Water samples were collected in October 1998, January 1999 and May 2000 from Stations ALOHA (22°45'N, 158°W) and Station 2 (16°N, 150°W) located in the oligotrophic North Pacific Subtropical Gyre (NPSG). The concentration and isotope value of isolated DON (>100 Da) were measured simultaneously. DON concentrations generally range between 4--7 muM in surface waters, decreasing with depth to approximately 2 muM. As DON decreases in concentration it becomes enriched in 15N. This inverse relationship is consistent with microbial assimilation and degradation of DON. There is also seasonal variation in the delta 15N values of DON. Samples collected in summer are depleted in 15N relative to those collected in winter, suggesting a shift in nitrogen source. During the summer months stratification of the water column favors Trichodesmium spp., a cyanobacteria that fixes 15N depleted dinitrogen (0‰). DON produced under these conditions can be injected into deeper waters and undergo ammonification, resulting in a large substrate pool for nitrification. Nitrous oxide is a by-product of nitrification and may be vented to the atmosphere via diffusion or deep mixing. Our results support relatively short time scales for DON cycling. Evidence of DON involvement in both primary production and the nitrous oxide pump demonstrates the key role DON can play in the NPSG. |
