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Seasonal and Interannual Variability

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ClickHereforFullArticl eSeasonal and interannual variability of particulate organiccarbon within the Southern Ocean from satellite oceancolor observationsDavid B. Allison,1Dariusz Stramski,1and B. Greg Mitchell2Received 28 February 2009; revised 4 January 2010; accepted 21 January 2010; published 3 June 2010.[1] We use field data of particulate organic carbon (POC) concentration and spectralremote‐sensing reflectance, Rrs(l), to develop an empirical algorithm for estimatingPOC from ocean color in the Southern Ocean. The algorithm based on the band ratioRrs(443)/Rrs(555) is used in conjunction with Sea‐viewing Wide Field‐of‐View Sensorsatellite data to demonstrate seasonal and interannual variability in POC from 1997 to2007. The surface POC concentrations generally range from 30 to 120 mg m−3.Onawhole basin scale (south of 35°S), the monthly means are mostly 70–80 mg m−3.The seasonal signal is weakest at lower latitudes within the Sub‐Antarctic Zone and mostpronounced at higher latitudes (>55°S). The area‐integrated stock of water column POC inthe upper 100 m shows small interannual variations and no clear evidence for long ‐termtrend during the examined 10 year period. The seasonal maximum of the POC stockoccurs in December and reaches a value of about 0.6 Pg of carbon for the entire basinsouth of 35°S. The seasonal range of area‐normalized POC is between about 5.5 and6.6 g m−2. The region south of 55°S provides a dominant contribution to the accumulationof POC within the Southern Ocean during the productive period of the season. Duringthe austral spring, the area‐normalized POC accumulates in these high‐latitude waters atrates from about 0.2 to 0.7 g m−2month−1. The comparison of these rates with large‐scalesatellite‐based estimates of net primary production indicates that only a small fraction(<10%) of production accumulates as POC.Citation: Allison, D. B., D. Stramski, and B. G. Mitchell (2010), Seasonal and interannual variability of particulate organiccarbon within the Southern Ocean from satellite ocean color observations, J. Geophys. Res., 115, C06002,doi:10.1029/2009JC005347.1. Introduction[2] The Southern Ocean is a unique oceanic domain thatencircles the globe providing a link for exchanges of watermasses and climatically significant quantities between theworld’s major ocean basins and atmosphere. Numerousrecent studies have been motivated by a need to advance anunderstanding of the role of the Southern Ocean in regu-lating atmospheric CO2over time scales relevant to climatechange and how the ecosystem structure and biogeochemi-cal cycles of the Southern Ocean respond to climate change[e.g., Sarmiento and Orr, 1991; Sarmiento et al., 2004;Le Quéré et al., 2007]. The Southern Ocean, in particulara region of the Antarctic Circumpolar Current between 40°and 60°S, has been identified as a contemporary net sink foratmospheric CO2on an annual basis, but the magnitude ofthis sink is not firmly established [e.g., Metzl et al., 1999;Takahashi et al., 2002; McNeil et al., 2007].[3] The drawdown of atmospheric CO2into the ocean isfavored by (1) the increase in the CO2solubility in the coldhigh‐latitude surface waters that sink to form the deep watersof the ocean and (2) biological uptake of CO2via phyto-plankton photosynthesis in the euphotic zone of the ocean,which results in the production of particulate and dissolvedforms of organic carbon that is then partly exported into thedeep sea. The ice‐free Southern Ocean is the largest ofseveral oceanic regions with high‐nutrient, low‐chlorophyll(HNLC) characteristics, where major macronutrients (nitrate,phosphate, silicate) occurring in significant concentrationsin surface waters are under utilized by autotrophic processes[Martin et al., 1990; Mitchell et al., 1991; Banse, 1996].These characteristics indicate that the capacity of the bio-logical pump to export organic carbon out of the euphoticlayer within the majority of the Southern Ocean is less thanits potential maximum. Recent experimental and modelingstudies showed, however, relatively high estimates of effi-ciency of the biological pump exporting particulate organiccarbon (POC) [Honjo et al., 2000; Trull et al., 2001;Schlitzer, 2002]. Potential changes in the efficiency of the1Marine Physical Laboratory, Scripps Institution of Oceanography,University of California, San Diego, La Jolla, California, USA.2Integrative Oceanography Division, Scripps Institution of Oceanography,University of California, San Diego, La Jolla, California, USA.Copyright 2010 by the American Geophysical Union.0148‐0227/10/2009JC005347JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, C06002, doi:10.1029/2009JC005347, 2010C06002 1of18Southern Ocean’s biological pump are of interest becausethey may affect air‐sea CO2fluxes and levels of atmosphericCO2[e.g., Knox and McElroy, 1984; Sarmiento andToggweiler, 1984; Siegenthaler and Wenk, 1984].[4] An assessment of biological controls of the carboncycle, including the efficiency of the biological pump andair‐sea CO2fluxes, requires determinations of various carbonreservoirs as well as the processes responsible for transfor-mations and transport of carbon, such as primary production,remineralization, and export of organic carbon. POC in thesurface ocean, which consists of autotrophic and heterotro-phic plankton and biologically derived detrital particles, isone of the reservoirs of substantial importance. The bio-logical pump that exports organic carbon out of the surfaceocean is effected largely by sinking particles, which pro-vides a mechanism for a long‐term storage of atmosphericCO2in the deep ocean [Volk and Hoffert, 1985; Longhurstand Harrison, 1989]. In addition, the net change of POC inthe surface ocean is a component of net community pro-duction (NCP), which is defined as gross primary productionminus respiration by all the autotrophic and heterotrophicorganisms [e.g., Eppley, 1989]. As the NCP describes the netamount of organic carbon produced, it is equivalent to thenet amount of inorganic carbon biologically consumed insurface waters. Therefore, the NCP integrated within theeuphotic layer over a certain period of time determines therole of biological activities for the inorganic carbon budgetin surface waters and can also provide a useful constraint forestimating export production out of the euphotic layer ifsufficient information about carbon mass balance compo-nents, including the net change


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