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Free Content Distributions of TOP, TON and TOC in the North Pacific subtropical gyre: Implications for nutrient supply in the surface ocean and remineralization in the upper thermocline

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We report measurements of total organic phosphorus, nitrogen, and carbon (TOP, TON, and TOC) along a meridional transect in the eastern subtropical North Pacific. In the surface waters, TOP and TON comprise 83% and 98% of the total dissolved phosphorus and nitrogen pools, respectively. Between the equatorial region and the subtropical gyre, there is a distinct gradient in surface TOP of about 0.25 µM, but very little gradient in TON. TOC increases by nearly 30 µM across the same region. Meridional advection of TOP along its gradient via Ekman transport can provide 40-80% of the phosphorus necessary to supply present estimates of particulate phosphorus export from the subtropical gyre. Mass balance requires an additional input of nitrogen to the surface waters to fuel particulate nitrogen export. This input is likely supported by nitrogen fixation.

As dissolved organic matter (DOM) is degraded along isopycnals that outcrop within the subtropical gyre, TOP and TOC are consumed more rapidly than TON. TOC and TON contribute 70% and 20% to organic matter remineralization along these isopycnals, respectively, and the C:N ratio for remineralization is 30 ± 10. In contrast, along isopycnals that outcrop to the north of the gyre, both TOC and TON contribute 30% to organic matter remineralization and the C:N ratio is 8 ± 1.

Our results suggest that excess TOC, produced within the surface waters of the subtropical gyre during nitrogen fixation, is highly labile and dominates respiration along shallow isopycnals. Preferential remineralization of TOC relative to TON within the gyre results in a negative preformed nitrate signal in the upper thermocline, and may establish a feedback between upper thermocline DOM remineralization and surface ocean DOM production that maintains nitrogen fixation in the surface waters.

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Document Type: Research Article

Publication date: March 1, 2000

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  • The Journal of Marine Research, one of the oldest journals in American marine science, publishes peer-reviewed research articles covering a broad array of topics in physical, biological and chemical oceanography. Articles that deal with processes, as well as those that report significant observations, are welcome. Biological studies involving coupling between ecological and physical processes are preferred over those that report systematics. The editors strive always to serve authors and readers in the academic oceanographic community by publishing papers vital to the marine research in the long and rich tradition of the Sears Foundation for Marine Research. We welcome you to the Journal of Marine Research.
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