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One-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

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One-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
  This is the author's version of the work. It is posted here bypermission of the AAAS for personal use, not for redistribution. Thedefinitive version was published in SCIENCE, 316 , 1021 (2007)doi10.1126/science.1136256 http://www.sciencemag.org.  Eddy-wind interactions stimulate extraordinary mid-ocean plankton bloomsOne-sentence summary : Mid-ocean eddies, together with wind-forced motions, causeepisodic bursts of nutrient supply to the upper ocean, changes in plankton communitystructure, and export of organic material to the deep sea.Dennis J. McGillicuddy, Jr. 1* , Laurence A. Anderson 1 , Nicholas R. Bates 2 , ThomasBibby 3,4 , Ken O. Buesseler  1 , Craig Carlson 5 , Cabell S. Davis 1 , Courtney Ewart 5 , Paul G.Falkowski 3 , Sarah A. Goldthwait 6,7 , Dennis A. Hansell 8 , William J. Jenkins 1 , RodneyJohnson 2 , Valery K. Kosnyrev 1 , James R. Ledwell 1 , Qian P. Li 8 , David A. Siegel 5 ,Deborah K. Steinberg 6  Manuscript revised and resubmitted to Science   January 29, 2007 1 Woods Hole Oceanographic Institution, Woods Hole, MA 02543-1541, USA. 2 Bermuda Institute of Ocean Sciences, Ferry Reach, GE01, Bermuda. 3 Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ08901-8521, USA. 4 School of Ocean and Earth Science, National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, United Kingdom. 5 University of California, Santa Barbara CA 93106, USA. 6 Virginia Institute of Marine Science, Gloucester Pt., VA 23062-1346, USA. 7 Humboldt State University, Arcata, CA 95521, USA. 8 Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL33149, USA.*To whom correspondence should be addressed. Email: dmcgillicuddy@whoi.edu1  Episodic eddy-driven upwelling may supply a significant fraction of thenutrients required to sustain primary productivity of the subtropical ocean. Newobservations in the northwest Atlantic reveal that, although plankton blooms occurin both cyclones and mode-water eddies, the biological responses differ. Mode-water eddies can generate extraordinary diatom biomass and primary production atdepth, relative to the time-series near Bermuda. These blooms are sustained byeddy-wind interactions, which amplify the eddy-induced upwelling. In contrast,eddy-wind interactions dampen eddy-induced upwelling in cyclones. Carbon exportinferred from oxygen anomalies in eddy cores is 1-3 times annual new productionfor the region. Understanding the controls on primary production in the upper ocean is of fundamental importance for two main reasons. First, primary productivity sets a first-order constraint on the energy available to sustain oceanic ecosystems. Second, fixationand subsequent sinking of organic particles removes carbon from the surface ocean (theso-called “biological pump”), which plays a key role in partitioning of carbon dioxide between the ocean and atmosphere. Geochemical estimates of new production ( 1 ) surpassthe apparent rate of nutrient supply by vertical mixing by a factor of two or more insubtropical oceans ( 2-6  ), which constitute some of the largest biomes on earth. Two possible mechanisms to supply the “missing” nutrient locally include nitrogen fixation bycyanobacteria ( 7-10 ), and intermittent upwelling by mesoscale eddies and submesoscale processes ( 11-21 ).2  There are at least three types of mid-ocean eddies in the northwestern subtropicalAtlantic: cyclones, anticyclones, and mode-water eddies (Fig. 1A). Cyclones dome boththe seasonal and main pycnoclines, whereas regular anticyclones depress both densityinterfaces. Mode-water eddies derive their name from the thick lens of water thatdeepens the main pycnocline while shoaling the seasonal pycnocline. Because thegeostrophic velocities are dominated by depression of the main pycnocline, the directionof rotation in mode-water eddies is the same as regular anticyclones. However,displacement of the seasonal pycnocline is the same as in cyclones: both types of featurestend to upwell nutrients into the euphotic zone during their formation and intensification phases. As these eddies spin down, the density surfaces relax back to their mean positions, and thus decaying cyclones and mode-water eddies will have downwelling intheir interiors. This temporal evolution during the life cycle of an eddy is a key regulator of the biogeochemical response ( 22, 23 ).Eddy features are readily discernible via satellite altimetry (Fig. 1B; fig. S1). Accessto these data in near real-time ( 24 ) facilitates tracking of individual eddies and adaptivesampling in shipboard operations. In 2004 and 2005 we sampled a total of ten differenteddies, five more than once (Table S1). Time-series within target features allowsresolution of temporal dynamics in eddy-driven nutrient supply, phytoplankton physiological response, changes in community structure, and biogeochemical fluxes. Wefocus this discussion on cyclone “C1” and mode-water eddy “A4”; findings in other cyclones and mode-water eddies in this study (Table S1) as well as prior investigations(Table S2) are consistent with that presented herein.3  Cyclone C1 was occupied four times between June and August 2004 ( 25 ). Shipboardacoustic Doppler current profiler (ADCP) data documented the counterclockwise flowassociated with C1’s negative sea level anomaly (SLA), and its altimetric historysuggested intensification in May. Uplift of near-surface isopycnals was associated withshoaling and enhancement of the subsurface chlorophyll maximum. The magnitude of the subsurface chlorophyll maximum in C1 was lower than other cyclones (Fig. 2A), yetstill in the upper quartile of all subsurface maxima observed in the Bermuda AtlanticTime-series Study (BATS) ( 26  ) 1988-2003.Phytoplankton species composition in cyclone C1 resembled mean conditions atBATS (Fig. 2B). On average,  Prochlorococcus spp., Synechococcu s spp., pelagophytes,and prymnesiophytes constitute the largest fractions of total chlorophyll a in the 75-140mdepth interval (deep chlorophyll maximum) at BATS; diatoms, dinoflagellates, and prasinophytes contribute comparatively little to total chlorophyll a . The eddy-induced bloom in C1 increased the relative amount of  Prochlorococcus spp., decreased therelative amount of  Synechococcu s spp., and the rare groups became an even smaller fraction of total chlorophyll a .In subsequent occupations of cyclone C1, conditions at eddy center changed from alocal maximum to a local minimum in chlorophyll a and fluorescence. During this latter  phase, integrated primary production at eddy center was not statistically distinguishablefrom climatological summertime conditions at BATS, nor were bacterial production and biomass (Table S3). However, systematic mesoscale variability was observed inmicrobial parameters, with biomass and production enhancement at the periphery relative4
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