Eddy-wind interactions stimulate extraordinary mid-ocean plankton bloomsOne-sentence summary: Mid-ocean eddies, together with wind-forced motions, cause episodic bursts of nutrient supply to the upper ocean, changes in plankton community structure, and export of organic material to the deep sea. Understanding the controls on primary production in the upper ocean is of fundamental importance for two main reasons. First, primary productivity sets a firstorder constraint on the energy available to sustain oceanic ecosystems. Second, fixation and subsequent sinking of organic particles removes carbon from the surface ocean (the so-called "biological pump"), which plays a key role in partitioning of carbon dioxide between the ocean and atmosphere. Geochemical estimates of new production (1) surpass the apparent rate of nutrient supply by vertical mixing by a factor of two or more in subtropical oceans (2-6), which constitute some of the largest biomes on earth. Two possible mechanisms to supply the "missing" nutrient locally include nitrogen fixation by cyanobacteria (7-10), and intermittent upwelling by mesoscale eddies and submesoscale processes (11-21).
[1] We report iron measurements for water column and aerosol samples collected in the Sargasso Sea during July-August 2003 (summer 2003 and April-May 2004 (spring 2004. Our data reveal a large seasonal change in the dissolved iron (dFe) concentration of surface waters in the Bermuda Atlantic Time-series Study region, from $1-2 nM in summer 2003, when aerosol iron concentrations were high (mean 10 nmol m À3 ), to $0.1-0.2 nM in spring 2004, when aerosol iron concentrations were low (mean 0.64 nmol m À3 ). During summer 2003, we observed an increase of $0.6 nM in surface water dFe concentrations over 13 days, presumably due to eolian iron input; an estimate of total iron deposition over this same period suggests an effective solubility of 3-30% for aerosol iron. Our summer 2003 water column profiles show potentially growth-limiting dFe concentrations (0.02-0.19 nM) coinciding with a deep chlorophyll maximum at 100-150 m depth, where phytoplankton biomass is typically dominated by Prochlorococcus during late summer.
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