| Summary: | The Gulf Stream provides a ‘nutrient stream,’ an advective flux of nutrients carried in sub-surface waters, redistributing nutrients from the tropics to the mid latitudes. There is a dramatic downstream strengthening in the full depth, volume and nitrate transport diagnosed from synoptic measurements along three sections: 32 Sv and 300 kmol s?1 at 27°N, increasing to 66.7 Sv and 940 kmol s?1 at 35.5°N, and 149.5 Sv and 2100 kmol s?1 at 36.5°N; the transport estimates have uncertainties reaching 10% of their values. The transport-weighted nitrate concentration carried by the Gulf Stream generally decreases downstream in light layers (? ? < 26.8), but increases in denser layers between 35.5°N and 36.5°N. The fraction of nutrients from regeneration within the upper thermocline slightly decreases downstream between 35.5°N and 36.5°N. Hence, the downstream variations in the nitrate concentrations carried by the Gulf Stream are probably due to lateral exchange along isopycnals, rather than diapycnal transfer or biological consumption and regeneration. The Gulf Stream also transports dissolved organic nitrogen (DON) northward within lighter upper waters, providing up to 26% of the total nitrogen transport. An accompanying model study reveals coherent nitrate and DON streams, including significant lateral exchange, running from the tropics along the western boundary of the subtropical gyre and following the separated jet along the inter-gyre boundary. The combined nitrate and DON transport along light surfaces (? ? < 26.8) remains within the subtropical gyre, while the larger nitrate transport along denser surfaces passes into the subpolar gyre, sustaining high-latitude productivity.
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