The Gulf Nutrient Stream

Pelegrí, Josep Lluís; Vallès‐Casanova, Ignasi; Orúe‐Echevarría, Dorleta

doi:10.1002/9781119428428.ch3

“…Therefore, the nutrients diffused up by the double diffusion, which then transported to the north through the lateral stirring caused by rich mesoscale eddies and submesoscale flows in the Kuroshio Extension regions, can generate the upward eddy flux (Lee and Williams 2000) or induction flux (Williams and Follows 2006) in the region north from the Kuroshio Extension (Nagai et al 2019a). The nutrients transported to the south by eddy, on the other hand, are subducted to the deeper layer and recirculated into the subsurface subtropical gyre (Pelegrí and Csanady 1991;Nagai et al 2015c;Pelegrí et al 2019).…”

Section: Discussionmentioning

confidence: 99%

Elevated turbulent and double-diffusive nutrient flux in the Kuroshio over the Izu Ridge and in the Kuroshio Extension

Nagai

¹

,

Quintana

²

,

Gómez

³

et al. 2021

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While the Kuroshio is known to be a nutrient stream, as these nutrients are in dark subsurface layers, they are not immediately available for photosynthesis unless they are supplied to the sunlit surface layers. Recent microstructure observations have revealed that strong diapycnal mixing caused by the Kuroshio flowing over topographic features and double diffusion in the subsurface layers of the Kuroshio. However, it is still unclear how much nutrient flux can be provided by these microscale mixing processes. In this study, using an autonomous microstructure float and nutrient samplings, nutrient flux caused by the Kuroshio over the Izu Ridge, and that caused by double diffusion in the Kuroshio Extension are quantified. The nitrate diffusive flux is estimated to be $$>1 \,\hbox {mmol} \,\hbox {N}\,\hbox {m}^{-2}\hbox {day}^{-1}$$ > 1 mmol N m - 2 day - 1 over a distance, 20–30 km near the Izu Ridge and $$>0.1 \,\hbox {mmol} \,\hbox {N}\, \hbox {m}^{-2}\hbox {day}^{-1}$$ > 0.1 mmol N m - 2 day - 1 , which persists further downstream direction over 100 km along the Kuroshio, increasing the subsurface chlorophyll-a concentration in the region 200 km downstream. The double-diffusion-induced nitrate flux is estimated to be 1-$$10 \,\hbox {mmol} \,\hbox {N} \,\hbox {m}^{-2}\hbox {day}^{-1}$$ 10 mmol N m - 2 day - 1 in the pycnostad 26–$$26.5\,\hbox {kgm}^{-3}$$ 26.5 kgm - 3 of the Kuroshio Extension, suggesting that whether this double-diffusion-induced nutrient flux in the subsurface layers can ultimately contribute to surface primary production depends on additional eddy up- and northward fluxes.

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“…The Kuroshio region is generally characterized by a warm, oligotrophic conditions, with phytoplankton communities generally dominated by pico-and nanophytoplankton (Endo and Suzuki, 2019) similar to the station ALOHA in the North Pacific subtropical gyre (White et al, 2015). However, along strong northward flows of the Kuroshio current, nutrient inputs to the euphotic zone are enhanced due to boundary exchange and diapycnal mixing (Nagai et al, 2019;Pelegrí et al, 2019), which leads to an increase in phytoplankton biomass and a shift in cell size composition toward larger phytoplankton, depending on season and location (Kobari et al, 2019;Nagai et al, 2019). Furthermore, since the Kuroshio current passes ridges with many small islands and seamounts, surface nutrification caused by a topographic flow disturbance (Island Mass Effect) appears to contribute to enhancing productivity in the region (Hasegawa, 2019;Nagai et al, 2021).…”

Section: Introductionmentioning

confidence: 97%

Geographic Variation of Particle Size Distribution in the Kuroshio Region: Possible Causes in the Upper Water Column

Yamada

¹

,

Fukuda

²

,

Umezawa

³

et al. 2021

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Particle size distribution (PSD) in the ocean is a fundamental property that influences carbon export and food webs; however, PSD variation and its causes in oligotrophic oceans are not entirely clear. Here, we used Laser In-Situ Scattering and Transmissometry to investigate PSD (size range 5.2–119 μm) and related variables at 11 stations in the surface layer (0–20 m) of the Kuroshio region of the western North Pacific, where strong current causes dynamic hydrographic and ecological conditions. PSD slopes (range –3.2 to –4.2), derived from the power law model, were steeper at onshore stations and flatter at oligotrophic stations located offshore and at lower latitudes. Notably, slopes tended to become steeper with increasing chlorophyll a concentration, opposing the generally observed relationship between the two variables, whereas they became flatter with increasing transparent exopolymer particle (TEP) concentration. Possible explanations of the above results are localized occurrence of nanophytoplankton and TEP facilitation of particle aggregation. The results support the hypothesis that PSD slopes are controlled by a multitude of factors, including phytoplankton community dynamics and aggregation processes. To determine whether TEP-induced particle aggregation enhances or suppresses carbon export, we need a better understanding of the nature (porosity, density, and sinking velocity) of aggregates in oligotrophic oceans.

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“…During the spring bloom, these nutrients drive primary productivity in the sunlit surface ocean. The high-nutrient waters of the Gulf Stream have been called the nutrient stream, and this advective pathway is thought to be the dominant source of nutrients supporting phytoplankton productivity in the subpolar North Atlantic ( 4 , 5 ). The Gulf Stream originates in the Florida Straits, and as it moves northward along the western boundary of the subtropical gyre, both the mass and nutrient transport of the current are augmented by recirculating water within the North Atlantic basin.…”

mentioning

confidence: 99%

A diminished North Atlantic nutrient stream during Younger Dryas climate reversal

Lynch-Stieglitz,

Vollmer,

Valley

et al. 2024

Science

0

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The high rate of biological productivity in the North Atlantic is stimulated by the advective supply of nutrients into the region via the Gulf Stream (nutrient stream). It has been proposed that the projected future decline in the Atlantic Meridional Overturning Circulation (AMOC) will cause a reduction in nutrient supply and resulting productivity. In this work, we examine how the nutrient stream changed over the Younger Dryas climate reversal that marked the transition out of the last ice age. Gulf Stream nutrient content decreased, and oxygen content increased at the Florida Straits during this time of weakened AMOC. The decreased nutrient stream was accompanied by a reduction in biological productivity at higher latitudes in the North Atlantic, which supports the link postulated in theoretical and modeling studies.

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The Gulf Nutrient Stream

Cited by 6 publications

References 88 publications

Elevated turbulent and double-diffusive nutrient flux in the Kuroshio over the Izu Ridge and in the Kuroshio Extension

Elevated turbulent and double-diffusive nutrient flux in the Kuroshio over the Izu Ridge and in the Kuroshio Extension

Geographic Variation of Particle Size Distribution in the Kuroshio Region: Possible Causes in the Upper Water Column

A diminished North Atlantic nutrient stream during Younger Dryas climate reversal

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