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

We report measurements of total organic phosphorus, nitrogen, and carbon (T OP, TON, and TOC) along a meridional transect in the eastern subtropical No rth Pacific. In the surface waters, TOP and TON comprise 83% and 98% of the total dissolved phosphorus and nitrogen pools, respectively. Between the e quatorial region and the subtropical gyre, there is a distinct gradient in surface TOP of about 0.25 mu M, but very little gradient in TON. TOC increa ses by nearly 30 mu M across the same region. Meridional advection of TOP a long 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 nitroge n 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 TO N. TOC and TON contribute 70% and 20% to organic matter remineralization al ong these isopycnals, respectively, and the C:N ratio for remineralization is 30 +/- 10. in contrast, along isopycnals that outcrop to the north of th e 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 dominat es respiration along shallow isopycnals. Preferential remineralization of T OC 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 mai ntains nitrogen fixation in the surface waters.