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[1] A synthesis is provided of dissolved organic nitrogen (DON) and phosphorus (DOP) distributions over the Atlantic Ocean based upon field data from eight recent transects, six meridional between 50°N and 50°S and two zonal at 24°and 36°N. Over the entire tropical and subtropical Atlantic, DON and DOP provide the dominant contributions to total nitrogen and phosphorus pools for surface waters above the thermocline. Elevated DON and DOP concentrations (>5 and >0.2 mmol L À1 , respectively) occur in surface waters on the eastern side of the North Atlantic subtropical gyre and equatorial sides of both the North and South Atlantic subtropical gyres, while particularly low concentrations of DOP (<0.05 mmol L À1 ) occur over the northern flank of the North Atlantic subtropical gyre along 36°N. This distribution is consistent with organic nutrients formed at the gyre margins supporting carbon export as they are redistributed via the gyre circulation. The effect of DON and DOP transport and cycling on export production is examined in an eddy-permitting, coupled physical and nutrient model integrated for 40 years: organic nutrients are produced in the upwelling zones off North Africa and transferred laterally into the gyre interior, facilitated in part by the mesoscale eddy circulation, as well as fluxed northward from the tropics as part of the overturning circulation. Inputs of semilabile DON and DOP to the tropical and subtropical Atlantic Ocean play an important role in sustaining up to typically 40 and 70% of the modeled particulate N and P export, particularly on the eastern and equatorward sides of the subtropical gyres.

[1] The spatial extent of N 2 fixation in the Atlantic Ocean is examined by determining the isotopic composition of N in suspended particulate organic nitrogen (d 15 N PON susp ). The samples were collected from zonal and meridional transects of the Atlantic Ocean during a 3-year period. There is a consistent depleted d 15 N PON susp signal extending over the center of the northern subtropical gyre, which partly coincides with a region where the tracer N* increases westward following the gyre circulation. This nonconservative behavior of N* implies that N 2 fixation is responsible for the depleted d 15 N PON susp . A mixing model suggests that N 2 fixation over parts of the northern gyre provides up to 74% of the N utilized by phytoplankton. However, since the PON susp represents only a small fraction of the total N pool, N 2 fixation probably only plays a minor role in supplying new N to the euphotic zone in the surface waters of the northern subtropical gyre.

There is a longstanding discussion on how the macronutrient requirement of the export production in the North Atlantic subtropical gyre is sustained. In this study we asses the role of dissolved organic nitrogen (DON) and phosphorous (DOP) as sources of new nutrients into the North Atlantic subtropical gyre at 24.5° N. We define, based on measurements of DON, DOP, phytoplankton community structure, stable nitrogen isotopic signals, surface mixed layer depth and ocean color as viewed from space, four regions characterized by different nutrient supply regimes. Within these regions, two distinct loci of N₂ fixation occur associated with different plankton assemblages and separated by a region in which N₂ fixation occurs at levels insufficient to leave its distinctive isotopic fingerprint on the isotopic composition of PON. Here, the phosphorus supply pathways to the mixed plankton assemblage appear to be different. In the wester oligotrophic gyre (70–46° W), the lateral advection of DOP supplies the missing P that, together with, shallow mixed layer, almost permanent stratification and high water temperatures, stimulate diazotrophic growth, which augment TON local accumulation. In the eastern oligotophic gyre (46–30° W), DOP cannot support the P demand as it is exhausted on its way from productive areas. This is inferred from DOP turnover rates, estimated form enzymatic clevage rates, which are shorter (11 ± 8 months) than transit timescales, estimated from a 3-D circulation model (>4 yr). A stronger seasonal cycle in chlorophyll and mixed layer depth, favour some nutrient injections from below. Here additional N sources come from the advected DON which has a turnover-time of 6.7 ± 3 yr, instead fast remineralization and little DOP export are needed to maintain the P requirements. We conclude from these observations that organic nutrient utilisation patterns drive diverse phytoplankton assemblages and oceanic nitrogen fixation gradients