Increased ocean carbon export in the Sargasso Sea linked to climate variability is countered by its enhanced mesopelagic attenuation
- 1Bermuda Institute of Ocean Sciences, 17 Biological Lane, St. George's, GE01, Bermuda
- 2Virginia Institute of Marine Science, The College of William and Mary, Chesapeake Bay Hall S206, P.O. Box 1346, Gloucester Pt., VA 23062-1346, USA
- 3Ocean Physics Laboratory and Department of Geography, Ocean Physics Laboratory, EH 1629, University of California, Santa Barbara, CA 93106-3060, USA
- 4Department of Ecology, Evolution and Marine Biology, Marine Biotechnology Building, Room 3147, University of California, Santa Barbara, CA 93106-3060, USA
- 5Institute for Computational Earth System Science, Mail Code 3060, University of California, Santa Barbara, CA 93106-3060, USA
Abstract. 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 >50% increase in euphotic zone integrated autotrophic biomass (estimated from chlorophyll TChl-α), 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. The increased mesopelagic POC attenuation appears mediated by changes in plankton community composition and metabolic activity in both the euphotic and mesopelagic zones. These changes are counter to extant hypotheses regarding inter-relationships between phytoplankton community composition, productivity and carbon export, and have significant impacts on how the Sargasso Sea ecosystem, at least, is modeled. Moreover, these time-series observations suggest that processes in the euphotic zone and mesopelagic are tightly coupled and should be considered together in future research.