Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 7 1 2010 10.5194/bg-7-57-2010 http://www.biogeosciences.net/7/57/2010/ http://www.biogeosciences.net/7/57/2010/bg-7-57-2010.html http://www.biogeosciences.net/7/57/2010/bg-7-57-2010.pdf 57 70 2010-01-05 Increased ocean carbon export in the Sargasso Sea linked to climate variability is countered by its enhanced mesopelagic attenuation M. W. Lomas michael.lomas@bios.edu D. K. Steinberg T. Dickey C. A. Carlson N. B. Nelson R. H. Condon N. R. Bates Bermuda Institute of Ocean Sciences, 17 Biological Lane, St. George's, GE01, Bermuda Virginia Institute of Marine Science, The College of William and Mary, Chesapeake Bay Hall S206, P.O. Box 1346, Gloucester Pt., VA 23062-1346, USA Ocean Physics Laboratory and Department of Geography, Ocean Physics Laboratory, EH 1629, University of California, Santa Barbara, CA 93106-3060, USA Department of Ecology, Evolution and Marine Biology, Marine Biotechnology Building, Room 3147, University of California, Santa Barbara, CA 93106-3060, USA Institute for Computational Earth System Science, Mail Code 3060, University of California, Santa Barbara, CA 93106-3060, USA Photosynthetic CO₂ uptake by oceanic phytoplankton and subsequent export of particulate organic carbon (POC) to the ocean interior comprises a globally significant biological carbon pump, controlled in part by the composition of the planktonic community. The strength and efficiency of this pump depends upon the balance of particle production in the euphotic zone and remineralization of those particles in the mesopelagic (defined here as depths between 150 and 300 m), but how these processes respond to climate-driven changes in the physical environment is not completely understood. In the Sargasso Sea, from ~1996–2007, we have observed a decade-long &gt;50% increase in euphotic zone integrated autotrophic biomass (estimated from chlorophyll TChl-<i>&alpha;</i>), prokaryotic phytoplankton, primary production and shallow (150 m) POC export coinciding with a shift in the mean phase of the winter North Atlantic Oscillation (NAO) from consistently positive to neutral but variable. During this same period mesopelagic POC flux attenuation has doubled such that carbon sequestration below 300 m, the maximum winter/spring ventilation depth, has not changed. 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