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Schlagwörter:
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Zusammenfassung:
The ocean's subtropical gyres account for about half of global ocean biological carbon fixation. Their productivity is limited by the rate of supply of essential nutrients to the sunlit surface water, which is continuously depleted by gravitational sinking of organic particles and large-scale, wind-driven downwelling. To address this conundrum of how productivity is sustained in subtropical gyres, we conduct two complementary studies:
1. We conducted a field campaign in in the North Atlantic subtropical gyre, consisting of transects over and off the mid-Atlantic ridge including measurements of turbulent kinetic energy dissipation and nitrate. Diapycnal mixing provides a nutrient supply within the euphotic zone, but a loss of nutrients within the upper thermocline. Eddy stirring augments, and is comparable to, the diapycnal transfer of nutrients within the summertime upper thermocline, while also acting to replenish nutrients within the deeper parts of the thermocline.
2. We diagnose an eddy-permitting simulation of ocean physics, biogeochemistry and ecology over the North Pacific subtropical gyre. Near the base of the sunlit zone, small-scale diapycnal mixing and adiabatic excursions of nutrient-rich density surfaces deliver nutrients into the euphotic zone, depleting the layers immediately below. These sub-euphotic layers are replenished by a combination of the remineralization of sinking particles and along-isopycnal eddy stirring.
In summary, the nutrient supply to the euphotic zone is achieved via a multi-stage mechanism: a diapycnal transfer of nutrients by small-scale turbulence to the euphotic zone, and an isopycnal stirring of nutrients by mesoscale eddies replenishing nutrients in the upper thermocline.
