Dynamic seasonal cycling of inorganic carbon downstream of South Georgia, Southern Ocean

Deep-Sea Research II 59-60 (2012) 25–35

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Dynamic seasonal cycling of inorganic carbon downstream of South Georgia, Southern Ocean Elizabeth M. Jones a,b,n, Dorothee C.E. Bakker a, Hugh J. Venables c, Andrew J. Watson a a

School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands c British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK b

a r t i c l e i n f o

abstract

Available online 3 August 2011

The influence of the island mass effect of South Georgia on the seasonal marine carbon cycle was investigated during austral summer (January–February) 2008. South Georgia (54–551S 36–381W) lies on the North Scotia Ridge, strongly influencing the passage of the Southern Antarctic Circumpolar Current Front to the south. Surface waters upstream of the island, in the central Scotia Sea, were characterised by relative high-nutrient low-chlorophyll (HNLC) conditions from winter (September) 2007 to summer, as indicated by satellite and shipboard observations. The fugacity of carbon dioxide (fCO2) was slightly supersaturated and the HNLC waters represented a summertime CO2 source of 2.6 7 1.5 mmol m  2 day  1. Extensive phytoplankton blooms developed in the Georgia Basin, downstream of South Georgia, in October 2007 and persisted until March 2008. The seasonal depletion in dissolved inorganic carbon (DIC) was 947 1 mmol kg  1 and the DfCO2(sea–air) was –92 7 21 matm in the core of the bloom by early February. These conditions created a strong sink for atmospheric CO2 of – 12.9 7 11.7 mmol m  2 day  1. In contrast, wintertime mixing into DIC-rich sub-surface waters created a strong CO2 source of 22.0 7 14.4 mmol m  2 day  1. These processes drive substantial seasonal changes in DIC of up to  0.7 mmol kg  1 day  1 from winter to summer. Similarly to the blooms of Kerguelen and Crozet, the South Georgia bloom is likely to be fuelled by natural iron fertilisation. A DIC deficit of 2.2 7 0.3 mol m  2 upstream of South Georgia suggested that the relative HNLC waters were more productive than indicated by satellites. The DIC deficit more than doubled downstream of South Georgia (4.6 7 0.8 mol m  2) to create the strongest seasonal carbon uptake in ice-free waters of the Southern Ocean to date. & 2011 Elsevier Ltd. All rights reserved.

Keywords: Carbon cycling Scotia Sea Southern Ocean South Georgia Iron fertilisation

1. Introduction The Southern Ocean is an important regulator of the climate system, particularly in buffering atmospheric carbon dioxide (CO2) concentrations through the action of the physical and biological carbon pumps (Eppley, 1972; Sarmiento and Toggweiler, 1984; Siegenthaler and Wenk, 1984; Watson and Orr, 2003). The ice-free waters of the Southern Ocean (62–501S) are a region of uptake of atmospheric CO2 and represent 4% of the global oceanic CO2 sink (Schlitzer, 2002; Takahashi et al., 2009). Some land-remote surface waters of the Southern Ocean reveal high concentrations of macronutrients due to an inefficient biological pump. Natural and artificial iron fertilisation studies have clearly demonstrated that iron is the key limiting factor to

n Corresponding author at: Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands. E-mail address: [email protected] (E.M. Jones).

0967-0645/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2011.08.001

marine productivity in these high-nutrient low-chlorophyll (HNLC) waters (Boyd et al., 2000, 2007; Smetacek, 2001; Bozec et al., 2004; Coale et al., 2004; de Baar et al., 2005; Blain et al., 2007; Pollard et al., 2007). Following iron enrichment events, phytoplankton blooms are observed where biological consumption of dissolved inorganic carbon (DIC) reduces sea surface fugacity of CO2 (fCO2) and enhances CO2 uptake from the atmosphere (Martin, 1990; de Baar et al., 1995; Watson et al., 2000). The Scotia Sea (53–631S 26–601W) is a relatively small ocean basin in the Atlantic sector of the Southern Ocean (Fig. 1, Venables and Meredith, 2012). The northern, southern and eastern margins are bound by the Scotia Ridge, a submarine arc extending from the Antarctic Peninsula to South America. Land mass protrusions along the Scotia Ridge are the South Orkney Islands and the South Sandwich Islands on the South Scotia Ridge (SSR) and South Georgia on the North Scotia Ridge (NSR). To the west, the Antarctic Circumpolar Current (ACC) enters the Scotia Sea from Drake Passage, where waters typically exhibit high-nutrient lowchlorophyll (HNLC) conditions. The primary hydrographic fronts

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E.M. Jones et al. / Deep-Sea Research II 59-60 (2012) 25–35

Fig. 1. Monthly composite images of daily MODIS-Aqua chlorophyll a (mg m  3) for the Scotia Sea and Georgia Basin region during spring (October–November 2007), summer (December 2007–February 2008) and early autumn (March 2008). Depth contours are at 1000 and 2000 m (GEBCO, 2001). The locations of stations Su6.0, Su6.1, Su8.3 and Su9.0 are indicated, with labels displayed in the January 2008 image.

in this region are, from north to south, the Polar Front (PF), the Southern ACC Front (SACCF) and the Southern Boundary (SB) (Orsi et al., 1995; Fig. 10, Venables and Meredith, 2012). The dominant water mass in the ACC is Circumpolar Deep Water (CDW), which occupies mid-levels of the ACC between the PF and the SB (Reid et al., 1977; Whitworth and Nowlin, 1987; Pollard et al., 2002). The NSR projects South Georgia out from the surrounding 3500–4000 m deep ocean by a shelf of up to 300 m deep and 50– 150 km wide (Meredith et al., 2003b). The SACCF exerts an important influence on oceanographic conditions in the vicinity of South Georgia as its flow is perturbed by the island and the NSR due to the underlying bathymetry (Thorpe et al., 2002; Meredith et al., 2003b, 2005; Smith et al., 2010). Drifter data show that the approach of the SACCF from the southwest is directed along the South Georgia shelf edge to the east, after which the main flow

retroflects in the wake of South Georgia before resuming an easterly course (Trathan et al., 1997; Meredith et al., 2003b). Waters either pass directly into the cyclonic circulation of the Georgia Basin or via an anticyclonic flow over the Northwest Georgia Rise (Brandon et al., 2000; Thorpe et al., 2002; Meredith et al., 2003a, 2003b). The retroflection of the SACCF in the Georgia Basin provides a mechanism for CDW to penetrate and enrich the seasonal mixed layer with DIC. The Scotia Sea is recognised as being important for the ventilation and mixing of water masses in the ACC that contribute to the overturning circulation of the Atlantic ocean (Naveira Garabato et al., 2002, 2004, 2007; Meredith et al., 2008). In contrast to the HNLC conditions of much of the Southern Ocean, enhanced phytoplankton productivity is regularly observed in the vicinity of oceanic islands (Blain et al., 2007;

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Pollard et al., 2007; Korb et al., 2008). Previously, the KErguelen Ocean and Plateau compared Study (KEOPS) and CROZet natural iron bloom and Export experiment (CROZEX) have shown that perturbations to the flow of currents in the wake of the Kerguelen (48.5–49.71S 68.5–70.51E) (Park and Gamberoni, 1995) and Crozet (45.5–47.01S 49.0–53.01E) (Pollard and Read, 2001) plateaux, both in the Indian sector of the Southern Ocean, are due to the ‘island mass effect’ (Doty and Oguri, 1956). A consequence of the island mass effect is that waters nearby the islands become enriched with iron through the interaction between the water masses and ¨ topographic features (de Baar et al., 1995; Loscher et al., 1997; Bucciarelli et al., 2001; Coale et al., 2004; Blain et al., 2007). Iron supply and subsequent alleviation of HNLC conditions in the upper ocean enhances marine productivity (Comiso et al., 1993; Moore et al., 1999; Moore and Abbott, 2000; Blain et al., 2001, 2007; Korb et al., 2004; Pollard et al., 2007; Sokolov and Rintoul, 2007). The phytoplankton blooms associated with the Crozet and Kerguelen plateaux are sustained through natural iron fertilisation (Bucciarelli et al., 2001; Planquette et al., 2007) and create regions of extensive carbon uptake (Bakker et al., 2007; Blain et al., 2007; Jouandet et al., 2008; Pollard et al., 2009). The waters around South Georgia support a rich food web (Atkinson et al., 2001; Murphy, 2007) where vast blooms develop from about 150 km downstream (northwest) in the Georgia Basin, between the main flow of the PF and SACCF, and often last for up to 6 months (Korb and Whitehouse, 2004; Korb et al., 2004; Whitehouse et al., 2008). The Scotia Sea region is considered to be an important sink for atmospheric CO2 (Schlitzer, 2002) yet the occurrence of an island mass effect and biological CO2 uptake from in situ measurements remains unresolved. This study investigates the effect of the South Georgia bloom on the marine carbon cycle with respect to seasonal cycling of DIC and air–sea CO2 fluxes. The data comprise shipboard continuous and discrete measurements of the carbon dioxide system from within the bloom and the HNLC ocean. The action of the biological carbon pump is investigated through the changes in sea surface DIC and air–sea CO2 fluxes from winter 2007 to summer 2008 in the two contrasting oceanic regimes. The results are first compared to equivalent work conducted for the Crozet and Kerguelen plateaux (Bakker et al., 2007; Jouandet et al., 2008) and are then put into context by considering summertime carbon dynamics at numerous locations throughout the Southern Ocean.

2. Methods 2.1. Underway and station sampling Data were collected in January and February 2008 during cruise JR177 on R.R.S. James Clark Ross as part of the Discovery2010 program at the British Antarctic Survey. A transect was made from the South Orkney Islands on 12 January, crossing the SB at 58.001S and the SACCF at 57.551S (Venables and Meredith, 2012), reaching the northern part of the Georgia Basin on 4 February. The transect encompassed waters of relative HNLC and bloom conditions as determined from MODIS-Aqua chlorophyll a images (Fig. 1). Satellite chlorophyll a data were downloaded from http://oceancolor.gsfc.nasa.gov as daily, 4 km, level 3 mapped data. An extensive phytoplankton bloom (chlorophyll a 42.0 mg m  3) was identified in the Georgia Basin downstream of South Georgia, which encompassed stations Su8.3 (52.731S 40.151W) and Su9.0 (52.631S 39.101W) on 2 and 4 February, respectively. In the central Scotia Sea (57.5–56.01S 42.5–41.01W) consistently low levels of chlorophyll a ( o0.4 mg m  3) were observed from October 2007 to March 2008. These relative HNLC

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waters were sampled at stations Su6.0 (57.141S 42.431W) and Su6.1 (56.841S 42.261W) on 22 January. Continuous measurements of temperature and salinity in surface water were made from the ship’s underway seawater supply (bow intake at 6.5 m depth). Vertical profiles of potential temperature and salinity were obtained using a conductivity, temperature, depth (CTD) sensor (Seabird SBE9þ) mounted on a rosette equipped with 24  10 L Niskin bottles (Venables and Meredith, 2012). An offset of  0.37 1C (n ¼350) was detected in the ship’s sea surface temperature relative to the CTD surface (5 m) temperature, which has been corrected for. The summer mixed layer depth (MLD) is defined here as the depth where the potential density exceeds that measured at 10 m by 0.05 kg m  3 (Brainerd and Gregg, 1995). Mixed layer depths were calculated from 2 dbar profiles of the potential temperature and salinity from the CTD deployment at each station. The winter mixed layer, marking the remnant of the Winter Water (WW), is defined here as the depth of the potential temperature minimum (ymin). All salinity values are reported on the practical salinity scale. 2.2. Fugacity of CO2 in surface seawater and marine air Quasi-continuous measurements of fCO2 in surface water and marine air were made using an underway fCO2 system described in Schuster and Watson (2007) (Fig. 2A). Atmospheric samples were taken from an air inlet located forward at 15 m height on the bridge of R.R.S. James Clark Ross. Mixing ratios of CO2 and moisture in the marine air and equilibrator headspace were determined by infra-red detection with a LI-COR 7000. The LI-COR was calibrated using three secondary gas standards with CO2 concentrations of 249.170.2, 356.570.2 and 457.470.2 mmol mol  1 in an artificial air mixture (21% O2, 79% N2). All gases underwent pre- and post-cruise calibration against certified primary standards from NOAA at the University of East Anglia. Samples from the equilibrator headspace and marine air were partially dried by being passed through an electric cool box at about 2 1C, prior to analysis in the LI-COR. The fCO2 was computed from the partly dried mixing ratios and the ship’s barometric pressure and then corrected for seawater vapour pressure (Weiss and Price, 1980). Two platinum resistance thermometers positioned in the upper and lower part of the seawater stream determined the temperature of the seawater in the equilibrator. The average warming of the seawater between the intake and the equilibrator was 0.7 1C (s ¼0.2 1C; n ¼622). Sea surface fCO2 data were corrected to sea surface temperature to account for this warming (Takahashi et al., 1993). The precision of the fCO2 data is estimated at 2 matm (Jones, 2010). Air–sea CO2 fluxes were calculated from the difference in fCO2 between seawater and the marine air, DfCO2(sea–air), and wind speed (measured twice daily at10 m height) from QuikSCAT (http://podaac.jpl.nasa.gov) using the Nightingale et al. (2000) gas transfer parameterisation. 2.3. Carbonate system Discrete samples for DIC (Fig. 2B) and alkalinity were collected from the underway seawater supply and from Niskin bottles on the upcast of the CTD. Seawater (250 or 500 ml) was collected in borosilicate glass bottles and saturated mercuric chloride solution was immediately added (0.02% vol/vol). The samples were stored in a dark location at ambient temperature until analysis in April to May 2008. The DIC concentration was determined by coulometric analysis (Johnson et al., 1987) and alkalinity analyses were carried out by potentiometric titration with hydrochloric acid (Dickson, 1981), using a VINDTA system. Two bottles of certified reference material (CRM) from batches 76 or 81 were analysed in duplicate per CTD cast and per 10 samples from the underway supply

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from Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) (Venables and Meredith, 2012), SST was lowest in September 2007, with partial sea ice cover to about 571S (Jones, 2010; Venables and Meredith, 2012). Thus the timing of the winter maximum was set to 12 September, with respect to the start date of the summer transect (Section 2.1), and Dtseason was calculated as 122 days. Determination of surface DIC concentrations during winter 2007 (DICwinter) is based on the assumption that they are equal to concentrations in the WW, at the depth of the ymin (Jennings et al., 1984), during summer 2008. This technique was used as the location of the ymin was well defined for all stations (Fig. 3), however it could lead to a slight overestimation in DICwinter (Jouandet et al., 2008; Jones et al., 2010). The seasonal depletion in DIC (DDICseasonal) was determined from the difference between the average concentration in the summer mixed layer and DICwinter. The WW DIC and alkalinity were used to infer sea surface fCO2 during September 2007 using the CO2Sys programme (Lewis and Wallace, 1998), with thermodynamic dissociation constants for K1 and K2 by Mehrbach et al. (1973), re-fitted by Dickson and Millero (1987). The accuracy of calculated fCO2 is better than 6 matm (Millero, 1995). Atmospheric CO2 data during winter were obtained from Jubany station (62.231S 58.671W), located on King Georgia Island of the South Shetland Islands (Ciattaglia et al., 1999). Jubany atmospheric CO2 concentrations tended to be slightly higher compared to Scotia Sea shipboard data during January and February 2008 (0.33 mmol mol  1; n¼590), indicating a possible overestimation of fCO2(air) during winter 2007. However, this value is within the accuracy of the shipboard CO2 measurement and hence will not add a further degree of uncertainty to the winter CO2 flux estimates.

3. Results 3.1. Winter–summer HNLC waters upstream of South Georgia

Fig. 2. Maps of (A) continuous DfCO2(sea–air) (matm) and (B) discrete sea surface dissolved inorganic carbon (DIC, mmol kg  1) from surface (5 m) CTD (outlined in black) and underway samples along the cruise track. The location of hydrographic stations Su6.0, Su6.1, Su8.3 and Su9.0 are shown (B). Depth contours are at 1000, 2000 and 3000 m (GEBCO, 2001).

(DOE, 2007). The accuracy of the DIC and alkalinity measurements is determined as 2.4 and 2.6 mmol kg  1, respectively, from the average difference between theoretical and measured CRM values (Jones, 2010). The precision of the DIC and alkalinity measurements is 1.5 and 1.0 mmol kg  1, respectively, based on the average difference between CRM in-bottle analyses (Jones, 2010). Depth-integrated DIC deficits were calculated from vertical DIC profiles relative to the concentration of DIC at 100 m depth (Fig. 3B and D). Vertical integration to 100 m depth was selected as summer mixed layers were shallower and winter mixed layers were deeper than 100 m (Table 1), and also for consistency with other inorganic carbon studies in the Southern Ocean (Bakker et al., 2007 and references cited therein). To determine seasonal biological DIC consumption (DDICseasonal bio), the data were corrected for salinity effects by normalisation to salinity 34, average WW salinity, using the technique to account for endmember DIC values at zero salinity (Friis et al., 2003). 2.4. Winter data The winter maximum was considered as the time of year when sea surface temperature (SST) was lowest. Using weekly data

MODIS-Aqua provided adequate spatial and temporal coverage of the surface chlorophyll a concentrations of the Scotia Sea and Georgia Basin before, during and after austral summer 2008 (Fig. 1). Shipboard and satellite chlorophyll a concentrations were largely in agreement during this time, although MODIS appeared to underestimate concentrations in the central Scotia Sea (Whitehouse et al., 2012). From October 2007 to March 2008, surface water chlorophyll a concentrations upstream of South Georgia were usually less than 0.5 mg m  3. This was confirmed in January 2008 by in situ measurements at stations Su6.0 and Su6.1 where average sea surface chlorophyll a concentrations were 0.4 mg m  3 (depth-integrated chlorophyll a was 22.5 mg m  2) within a community dominated ( 490%) by nano- and picoplankton (Korb et al., 2010, 2012; Whitehouse et al., 2012). Sea surface concentrations of silicate and nitrate decreased northwards across the central Scotia Sea in winter (ymin) and summer and showed notable seasonal depletion in the vicinity of the SACCF, where nitrate:phosphate ratios were  11:1 (Whitehouse et al., 2012). Wintertime sea surface fCO2 was slightly below the atmospheric value with DfCO2(sea–air) of 173 matm and mixed layer DIC was 2191 mmol kg  1 (Figs. 2 and 4; Table 1). This region represented a small oceanic CO2 sink of –0.170.2 mmol m  2 day  1, close to the margin of the seasonal sea ice cover (Venables and Meredith, 2012). By January 2008, DIC had been depleted by 3475 mmol kg  1 to create a DIC deficit in the upper 100 m of 2.270.3 mol m  2. The DfCO2(sea–air) increased to 874 matm, primarily due to warming of over 2 1C (Fig. 5A–C), forming a region of CO2 release of 2.671.5 mmol m  2 day  1 (Table 1).

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Fig. 3. Vertical profiles of upper water column (A, C) potential temperature (y, 1C) and (B, D) dissolved inorganic carbon (DIC, mmol kg  1) for HNLC stations Su6.0 (dashed line) and Su6.1 (solid line) and South Georgia (SG) bloom stations Su8.3 (dashed line) and Su9.0 (solid line). Locations of hydrographic stations shown in Fig. 2B.

Table 1 Seasonal inorganic carbon cycling and air–sea CO2 fluxes for the HNLC and South Georgia (SG) bloom stations. The mean value is presented with the difference (as n¼ 2) in parentheses for the summer mixed layer depth (MLD, m), winter mixed layer depth (WMLD, m), summer mixed layer DIC (DICsummer, mmol kg  1), Winter Water DIC (DICwinter, mmol kg  1), seasonal (summer–winter) DIC depletion (DDICseasonal, mmol kg  1), biological DIC depletion (DDICseasonal bio, mmol kg  1), rate of biological DIC depletion (DDICseasonal bio/Dt mmol kg  1 day  1), DIC deficit in the upper 100 m (mol m  2), difference in fCO2 between the sea and marine air (DfCO2(sea–air), matm), rate of change of DfCO2(sea–air) (D(DfCO2(sea–air))/Dt matm day  1) and the air–sea CO2 flux (mmol m  2 day  1) in summer and winter.

Station number (location) MLD (m) WMLD (m) DICsummer (mmol kg  1) DICwinter (mmol kg  1) DDICseasonal (mmol kg  1) DDICseasonal bio (mmol kg  1) DDICseasonal bio/Dt (mmol kg  1 day  1) DIC deficit (mol m  2) DfCO2(sea–air)summer (matm) DfCO2(sea–air)winter (matm) D(DfCO2(sea–air))/Dt (matm day  1) CO2 fluxsummer (mmol m  2 day  1) CO2 fluxwinter (mmol m  2 day  1)

HNLC

SG bloom

Su6.0 (57.141S 42.431W) Su6.1 (56.841S 42.261W) 56 (2) 101 (0) 2157 (5) 2191 (0)  34 (5)  29 (3)  0.2 2.2 (0.3) 8 (4)  1 (3) 0.1 2.6 (1.5)  0.1 (0.2)a

Su8.3 (52.731S 40.151W) Su9.0 (52.631S 39.101W) 58 (10) 126 (10) 2098 (3) 2192 (3)  94 (1)  86 (1)  0.7 4.6 (0.8)  92 (21) 81 (1)  1.4  12.9 (11.7) 22.0 (14.4)

Fluxes were calculated using in situ QuikSCAT winds (measured twice daily) and the Nightingale et al. (2000) parameterisation. Time from winter to summer (Dtseason) is 122 days. a

Partial sea ice cover to 571S in winter 2007 (Jones, 2010).

3.2. Winter deep mixing and carbon enrichment downstream of South Georgia Compared to proximal stations over the NSR, the influence of the SACCF on characteristics of wintertime (ymin) surface waters could be observed in the Georgia Basin by a localised reduction in average surface water temperature of 0.3 1C, increase in salinity (Fig. 4C–D)

and enrichment of nitrate and silicate to similar concentrations measured in the vicinity of the SACCF (Whitehouse et al., 2012). These flow patterns induce an anticyclonic circulation in the Georgia Basin (Atkinson et al., 2001; Meredith et al., 2003a), enabling DIC-rich CDW to infiltrate sub-surface waters (Brandon et al., 2000). Seasonal maxima in DfCO2(sea–air) of 8171 matm was observed in the deep mixed layers at stations Su8.3 and Su9.0 in

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Fig. 4. The latitudinal distribution of sea surface (A) DfCO2(sea–air) (matm), (B) dissolved inorganic carbon (DIC, mmol kg  1), (C) temperature (SST, 1C), and (D) salinity during winter 2007 (black lines) and summer 2008 (grey lines). The locations of the high-nutrient low-chlorophyll (HNLC) stations and South Georgia (SG) bloom stations are indicated.

the central Georgia Basin during winter 2007 (Table 1). At this time, the Georgia Basin was a strong source of CO2 to the atmosphere of 22.0714.4 mmol m  2 day  1.

3.3. South Georgia bloom summertime carbon uptake Extensive phytoplankton blooms developed in the Georgia Basin, downstream of South Georgia, from October 2007 (Fig. 1). The blooms extended across the basin and NSR to reach maximum intensity by January 2008, before declining in March, with an estimated duration of 120714 days. The uncertainty was determined subjectively, based on reduced chlorophyll a concentrations ( o1.0 mg m  3) for at least 50% of the areal extent of the January 2008 bloom, for part of October 2007 and March 2008. Stations Su8.3 and Su9.0 were located in the core of the diatomdominated (75%) bloom and revealed the highest in situ surface ( 46 mg m  3) and depth-integrated (141 mg m  2) chlorophyll a concentrations, relative to the whole transect (Korb et al., 2010, 2012; Whitehouse et al., 2012). Sea surface concentrations of silicate, nitrate and phosphate reached summertime minima with corresponding seasonal depletion ratios for nitrate:phosphate and silicate:nitrate of 12–13:1 and 1–2:1, respectively (Whitehouse et al., 2012). Upper ocean profiles of potential temperature for stations Su8.3 and Su9.0 displayed an active mixing layer, at about 20 m, overlying a distinct seasonal mixed layer (Fig. 3C). This was due to recent stable meteorological conditions, which was accompanied

Fig. 5. Seasonal changes in average (A, D) DfCO2(sea–air) (matm), (B, E) DfCO2(sea–air) normalised to a sea surface temperature of 2 1C (DfCO2T(sea–air) (matm)) and (C, F) sea surface temperature (SST, 1C) for HNLC stations Su6.0 and Su6.1 and South Georgia (SG) bloom stations Su8.3 and Su9.0. Locations of hydrographic stations shown in Fig. 1. Error bars are 7 1 standard deviation of the mean of all data in the given region for each season.

by the lowest DIC concentrations (Fig. 3D). A second mixed layer at 58710 m depth is taken as the actual seasonal mixed layer for better comparisons with upstream HNLC stations (Table 1). The intense phytoplankton growth created a summer DIC minima of 209873 mmol kg  1 (Fig. 4B) and seasonal DIC depletion of 9471 mmol kg  1. Surface waters were strongly undersaturated with respect to atmospheric CO2 with DfCO2(sea–air) of  92721 matm (Fig. 2A). Thermodynamic increases in seawater fCO2 (up to 61 matm) due to seasonal warming of 3.8 1C were strongly counteracted by persistent biological carbon uptake since October 2007 (Fig. 5). The DIC deficit increased from the upstream waters to 4.670.8 mol m  2 with atmospheric CO2 uptake of 12.9 711.7 mmol m  2 day  1 (Table 1). Inspection of temperature–salinity profiles (not shown) and vertical profiles (Fig. 3) showed that stations Su6.0, Su6.1 and stations Su.8.3, Su9.0 were clearly south of the PF with comparable water mass characteristics, well defined ymin close to 0.1 1C and near-identical winter mixed layer DIC (Table 1). This is due to the influence of the SACCF, which was encountered just south of stations Su6.0 and Su6.1 and in a retroflection of the main flow in the Georgia Basin (Venables and Meredith, 2012). Stations Su6.0 and Su6.1 are therefore selected as relative HNLC reference stations to assess the influence of the South Georgia bloom on the seasonal carbon cycle of the Scotia Sea.

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4. Discussion 4.1. An island mass effect downstream of South Georgia With respect to the biological and physical environments at the time of the investigation, stations Su6.0, Su6.1 and Su8.3, Su9.0 were representative for summertime HNLC and bloom conditions, respectively, and are selected as contrasting regions to investigate the effects of the South Georgia bloom on the seasonal cycling of inorganic carbon in the Scotia Sea (Figs. 1 and 2). The relative HNLC (upstream) waters were a negligible CO2 sink in winter, which evolved into a slight CO2 source of 2.671.5 mmol m  2 day  1 by the summer (Table 1). Despite low sea surface chlorophyll a concentrations ( o0.5 mg m  3) and primary production rates of 138 mg C m  2 day  1 (Korb et al., 2012), summer mixed layer had been reduced to create a deficit of 2.2 70.3 mol DIC m  2 by January 2008. Compared to upstream waters, deep winter mixing (Table 1) within SACCF-influenced waters created a strong wintertime CO2 source of 22.0714.4 mmol m  2 day  1 to the atmosphere, downstream of South Georgia. By mid-summer 2008, the region was transformed into a sizable sink for atmospheric CO2 of 12.9 711.7 mmol m  2 day  1 within the extensive phytoplankton blooms that became seeded about 150 km to the northwest of South Georgia during October 2007 (Fig. 1). High primary production rates of 2795 mg C m  2 day  1 (Korb et al., 2012) resulted in considerable DIC uptake, doubling the seasonal DIC deficit to 4.670.8 mol m  2 compared to the HNLC region. Both sites were located in waters of 3500–4000 m depth and show a general inverse relationship between DIC deficit and distance from shallow topography. From a simple calculation, the HNLC stations were about 300 km downstream from elevated (2000 m depth) topography, assuming eastward flow of the ACC. In contrast the bloom stations were 150–200 km downstream of the NSR and South Georgia, following perturbed flow paths of the SACCF. The data indicate an approximate doubling of the DIC deficit as the distance from shallow topography is halved, complimenting an existing relationship between size of DIC deficit and distance from land described by Bakker et al. (2007). These observations demonstrate the presence of an island mass effect downstream of South Georgia, leading to substantial seasonal cycling of inorganic carbon (Figs. 4A and B) and dynamic shifts in air–sea CO2 exchange (Table 1). 4.2. The South Georgia bloom and biological carbon uptake The island mass effect describes the occurrence of elevated marine productivity 150 km downstream of the island of South Georgia. Summer biological carbon uptake in the bloom had reduced winter mixed layer DIC by 8671 mmol kg  1, representing nearly a threefold increase in the seasonal DIC depletion of the HNLC waters (Table 1). Nutrient utilisation was also highest within the bloom, to yield seasonal DICbio:N and DICbio:P depletion ratios of 6:1 and 89:1, respectively. Nitrate depletion was close to Redfield values (Redfield et al., 1963), inferring efficient summertime nitrate utilisation. Low phosphate ratios indicate that greater phosphate usage and/or preferential remineralisation has taken place, discussed further in Whitehouse et al. (2012). The perturbation in flow of the SACCF and ACC downstream of South Georgia provides an indication of a localised iron source, which could include horizontal advection from the NSR, NWGR or South Georgia, at the surface and at depths below the mixed layer that are shallow enough for vertical mixing to supply the surface (Venables and Meredith, 2012). With the occurrence of an island mass effect, high seasonal nutrient depletion downstream of South Georgia indicates an efficient biological pump in an iron

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replete environment (Korb et al., 2004; Holeton et al., 2005; Whitehouse et al., 2008, 2012). These conditions generated a substantial seasonal DIC deficit of 4.670.8 mol m  2 and created an important summertime sink of atmospheric CO2 of 12.9711.7 mmol m  2 day  1 downstream of South Georgia. The presence of significant proportions of diatoms and microphytoplankton in the bloom suggest that biogenic silica and organic carbon is exported to depth (Korb et al., 2012). Following initiation with increasing light levels by October 2007, the South Georgia bloom reached maximum intensity in January 2008 with high diatom abundance and rates of primary production in February (Venables and Moore, 2010; Korb et al., 2012). At this time, the bloom extended for about 80,000 km2 which, based on a DIC deficit of 4.6 70.8 mol m  2 at stations Su8.3 and Su9.0 in early February, could create a DIC deficit in the order of 4.4 70.8 Tg C across the Georgia Basin (Tg C ¼1012 g C). This total DIC deficit estimate is a snap-shot of summertime biological carbon uptake within the bloom, as defined by satellite chlorophyll a concentrations exceeding 2 mg m  3, and is likely to represent a conservative value as (1) adjacent waters with chlorophyll a concentrations below 2 mg m  3 will also exhibit seasonal DIC deficits (Bakker et al., 2007; Section 3.1) and (2) the observed DIC deficit is expected to increase over time within the bloom waters (Bakker et al., 2007) at least until March 2008. Satellite ocean colour analyses reveal that the bloom is a strong and regular feature, persisting for at least 3 months during austral spring, summer and autumn (Atkinson et al., 2001; Korb et al., 2004, 2008, 2012; Venables and Meredith, 2009; Park et al., 2010). By consideration of the in situ and satellite parameters discussed in this issue, the South Georgia bloom represents an area of considerable carbon uptake and potential sequestration in the Atlantic sector of the Southern Ocean. 4.3. The HNLC and CO2 source waters of the central Scotia Sea Satellite and shipboard chlorophyll a ( o0.5 mg m  3) and shipboard primary production rates (138 mg C m  2 day  1) in January 2008 revealed relatively low phytoplankton activity in the central Scotia Sea from winter to the end of summer (Fig. 1) (Korb et al., 2012). These comparatively ‘unproductive’ waters held a small standing stock of phytoplankton by January 2008 and had a DIC deficit of 2.270.3 mol m  2. Sea surface chlorophyll a in the HNLC (upstream) waters was an order of magnitude lower than that of the bloom (downstream) waters, however the accompanying DIC deficit was only half of that within the bloom. This suggests that the HNLC-type waters of the Southern Ocean are often more productive than indicated by satellite derived chlorophyll a alone, as postulated by Schlitzer (2002) and Bakker et al. (2007). This corresponds with results from Arrigo et al. (2008) for pelagic Southern Ocean waters (south of 501S and depth 41000 m) where daily primary productivity, determined from satellite derived chlorophyll a, sea surface temperature and sea ice concentrations, averages 148 mg C m  2 day  1 in one annual cycle. Low but continuous biological activity in mixed layers 450 m deep may appear largely unproductive by satellites and during short term shipboard surveys. For a given sea surface chlorophyll a concentration, a range of production rates can occur due to the influence of other factors such as sea surface temperature and irradiance that should be taken into account in order for estimates to be more realistic (Arrigo et al., 2008). Therefore, using satellite chlorophyll a alone as an indicator of production and carbon export is likely to underestimate rates of both parameters. Whitehouse et al. (2012) compiled a series of data sources, including satellite imagery, seasonal macronutrient deficits and in situ data to determine that standing stocks of phytoplankton across the Scotia Sea transect varied by about a

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E.M. Jones et al. / Deep-Sea Research II 59-60 (2012) 25–35

Fig. 6. Area (km2), mixed layer depth (MLD, m), seasonal change in dissolved inorganic carbon (DDIC, mmol kg  1), the DIC deficit (mol m  2), DfCO2(sea–air) (matm) and the CO2 flux (mmol m  2 day  1) for (A) bloom and (B) reference HNLC regions at South Georgia (this study) and the Kerguelen (Jouandet et al., 2008) and Crozet (Bakker et al., 2007) plateaux. The DIC deficit is the average summer deficit in DIC for the upper 100 m (South Georgia and Crozet) and relative to the temperature minimum (Kerguelen). A negative CO2 flux corresponds to net uptake of atmospheric CO2.

factor 2, with highest values in the South Georgia bloom and lowest values in the upstream HNLC-type waters. This analysis and that of Park et al. (2010) shows that blooms in the central Scotia Sea are ephemeral and variable but do occur. Such enhanced biological activity could be supported by iron inputs from an upstream source, such as the vicinity of the Antarctic Peninsula and SSR (Hewes et al., 2008; Dulaiova et al., 2009; Ardelan et al., 2010; Zhou et al., 2010). Korb et al. (2012) propose that diatom activity that peaks around October and November rapidly exhausts any iron that is supplied to the central Scotia Sea, causing a floral shift to a dinoflagellate population by the summer. This mechanism supports the observed DIC deficit in January 2008, however the effects of seasonal warming negated the biological effects to drive a release of 2.671.5 mmol m  2 day  1 of CO2 to the atmosphere (Fig. 5A–C).

4.4. A seasonal perspective To determine the changes in DIC and CO2 air–sea fluxes from winter to summer, several assumptions are required: (1) parameters measured in the WW are a suitable proxy for sea surface conditions during the preceding winter, (2) the rate of change of fCO2(sea) is governed by the rate of change of mixed layer DIC and sea surface temperature and (3) seasonal changes in fCO2(sea) and DIC are linear with time from the winter maximum to summer minimum (122 days). The Georgia Basin experienced substantial shifts in DfCO2(sea– air) with a seasonal amplitude of over 150 matm (Fig. 4A). The flow patterns of the SACCF and ACC downstream of South Georgia and the NSR induces the formation of an anticyclonic circulation over the Northwest Georgia Rise (NWGR) (Atkinson et al., 2001;

Meredith et al., 2003a). These hydrographic features allow DICrich CDW to upwell along the frontal boundary (Brandon et al., 2000), providing a mechanism for iron and macronutrients to enter the upper ocean (de Baar et al., 1995; L¨oscher et al., 1997; Blain et al., 2007). The island mass effect describes the occurrence of increased productivity downstream of South Georgia, relative to the landremote HNLC waters (upstream) in the central Scotia Sea, and is a further indication of a localised iron source from the NSR and South Georgia. The circulation patterns would enable an iron supply to the waters downstream in the Georgia Basin throughout the winter. Through increased vertical mixing, DIC-rich sub-surface waters become entrained into the winter mixed layer and contribute to the wintertime release of 22.0714.4 mmol m  2 day  1 CO2 to the atmosphere (Table 1). The absence of a strong biological pump enabled sub-surface waters to become enriched with DIC. The presence of the island mass effect in summer led to increased sea surface chlorophyll a concentrations and biological carbon uptake (Section 4.2). Despite seasonal warming of nearly 4 1C (Fig. 5D–F), biological carbon uptake acted to reduce DfCO2(sea–air) and DIC at swift rates of 1.4 matm day  1 and  0.7 mmol kg  1 day  1, respectively, since the winter maximum. From winter to summer, biological activity in the HNLC-type waters had reduced DIC concentrations by 2973 mmol kg  1 at a seasonally averaged rate of 0.2 mmol kg  1 day  1, almost four times less than in the bloom (Table 1). By January 2008, this small but steady productivity created a DIC deficit of 2.2 mol m  2. Sea surface fCO2 did not reflect this modest phytoplankton activity as surface water warming counteracted the biological effects to force a net increase in DfCO2(sea–air) of 0.1 matm day  1 (Fig. 5A–C). The consequence of these processes was that the HNLC-type waters evolved into an efflux of CO2 of 2.671.5 mmol m  2 day  1, which dominated the seasonal budget of uptake and release of CO2 from winter to summer in this region.

E.M. Jones et al. / Deep-Sea Research II 59-60 (2012) 25–35

33

Table 2 The location, year, month, mixed layer depth (MLD, m), DfCO2(sea–air) (matm) and DIC deficit (mol m  2) for a range of locations in the Southern Ocean during austral summer (December–February). Location

Year

Month

MLD (m)

DfCO2 (matm)

DIC deficit (mol m  2)

Reference

Weddell Sea (601S 01W) Pacific sector (48–621S 651E) Seasonal ice zone (65–681S 30–681E) Casey Bay (661S 481E) Prydz Bay (681S 71–781E) Marginal ice zone (64–651S 1401W) Ross Sea (771S 169–1871E) Scotia Sea (57–601S 521W) Weddell Sea (54–661S 17–231E) Crozet plateau (45–471S 49–531E) Kerguelen plateau (50–521S 70–741E) Scotia Sea (56–581S 37–431W) South Georgia (52–541S 38–421W)

1972 1992 1993 1993 1993 1994/5 1995 2001 2003 2004 2005 2008 2008

Jan Feb Feb Feb Feb Dec-Jan Jan-Feb Dec Jan Dec Jan-Feb Jan Feb

60–110 – 50 60 40 10–20 10–80 30–70 – 22 70 56 58

– – – – –  35  80,  150  8,  13 –  55  71 8  92

1.7e 2.5c 0.8–2.8d 1.7–2.8d 1.3–4.0d 0.3–2.5b 1.2–10.8a 1.0–1.2c 2.0b 3.4a 4.4b 2.2a 4.6a

Jennings et al. (1984) Minas and Minas (1992) Ishii et al. (1998) Ishii et al. (1998) Ishii et al. (1998) Ishii et al. (2002) Bates et al. (1998) Sweeney et al. (2000) Shim et al. (2006) Geibert et al. (2010) Bakker et al. (2007) Jouandet et al. (2008) This study This study

–, No data. Methods for determination of DIC deficit. a

DIC deficit in the upper 100 m. DIC deficit relative to the temperature minimum. From net community production. d DIC deficit in the summer mixed layer. e From nitrate deficits relative to the temperature minimum. b c

4.5. Southern Ocean blooms and CO2 uptake The island mass effect and seasonal biological carbon uptake has also been observed during KEOPS and CROZEX (Blain et al., 2001; Bakker et al., 2007; Jouandet et al., 2008) (Fig. 6). Compared to the Crozet and Kerguelen blooms, the South Georgia bloom depleted up to 40 mmol kg  1 more DIC during the summer, creating the largest DIC deficit of 4.670.8 mol m  2 (Fig. 6A). The contrasting HNLC (upstream) waters at all three localities showed summer depletions in DIC (Fig. 6B), which is attributed to low, but persistent marine productivity (Section 4.3). Despite summer reductions in DIC, sea surface fCO2 was close to the atmospheric value for each of the relatively unproductive sites. The HNLC waters upstream of South Georgia and Kerguelen were slight sources of CO2 of 2.6–2.7 mmol m  2 day  1 (Fig. 6B). As for the other island blooms, the characteristics of the South Georgia bloom (Section 4.2) and strong evidence for natural iron fertilisation suggest that this region could be efficient in sequestering carbon to the deep ocean (Blain et al., 2007; Pollard et al., 2009). The characteristics and dynamics of the island mass effect of South Georgia are most comparable to those observed during KEOPS, with time of sampling in January and February, south of the PF (at similar latitudes), similar estimates of bloom duration (Jouandet et al., 2008) and likely macronutrient and iron supply from deeper waters (Blain et al., 2007; Korb et al., 2012). The superior spatial coverage coupled with the sizeable in situ DIC deficit of the South Georgia bloom yields a total summertime DIC deficit of 4.470.8 Tg C (Section 4.2), almost double the DIC deficit (2.471.0 Tg C) generated in surface waters of the Kerguelen bloom (Jouandet et al., 2008). An important consideration here is that these island comparisons are based on snapshots of the blooms during a single season in a given year. It has been well documented that blooms downstream of South Georgia are an annual event but do exhibit a degree of interannual variability in timing of initiation, duration and areal extent (Korb et al., 2004; Park et al., 2010; Whitehouse et al., 2012) and that summertime biological carbon uptake within such blooms can fluctuate by up to 50% on weekly to monthly timescales (Bakker et al., 2007). In a Southern Ocean perspective, the summertime DIC deficit of the South Georgia bloom is one of the largest reported to date and represents the strongest seasonal biological carbon uptake in ice-free waters of the Southern Ocean (Table 2). Blooms in the Ross Sea created substantial DIC deficits of up to 10.8 mol m  2,

considerable summertime fCO2 undersaturation (Bates et al., 1998) and are the most productive in the Southern Ocean (Arrigo et al., 2008). The DIC deficits of the of South Georgia, Kerguelen and Crozet blooms were notably higher (3.4– 4.6 mol m  2) than those in most regions of sea ice melt, which could be indicative of persistent phytoplankton blooms fuelled by a more regular supply of iron into downstream surface waters (Blain et al., 2007; Pollard et al., 2009). The open ocean of the Scotia and Weddell Seas displayed low summertime DIC depletion (1.0–2.2 mol m  2) and further indicated the relationship between the magnitude of the DIC deficit and distance from shallow topography, as postulated by Bakker et al. (2007).

5. Conclusion The influence of the island mass effect downstream of South Georgia on the cycling of inorganic carbon and air–sea CO2 exchange was investigated from a seasonal perspective, by consideration of the driving physical and biological processes from winter (September) 2007 to summer (January–February) 2008. Retroflection of the main flow of the SACCF in the wake of South Georgia provides a mechanism for DIC-rich CDW to penetrate the winter mixed layer. Wintertime mixing promoted DIC enrichment of the surface waters to create a strong source of 22.0714.4 mmol m  2 day  1 of CO2 to the atmosphere. During the winter–summer transition, extensive diatomdominated blooms developed downstream of South Georgia. By February 2008, biological carbon uptake created a sizable DIC deficit of 4.670.8 mol m  2 and transformed the region into an important sink for atmospheric CO2 of 12.9711.7 mmol m  2 day  1. Despite relative HNLC conditions from winter to summer, surface waters upstream of South Georgia in the central Scotia Sea held a small standing stock of phytoplankton by January 2008 and had a DIC deficit of 2.2 70.3 mol m  2. Seasonal warming led to fCO2 supersaturation at the sea surface and the HNLC waters represented an oceanic CO2 source of 2.67 1.5 mmol m  2 day  1. The presence of the island mass effect and characteristics of the phytoplankton blooms provided evidence for iron enrichment and potential carbon export downstream of South Georgia. This scenario is very similar to the naturally iron fertilised blooms of the Kerguelen and Crozet islands. With the largest areal coverage and duration, the South Georgia bloom was the strongest of the three island blooms with a total summertime DIC deficit of

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4.470.8 Tg C. The substantial DIC depletion of the South Georgia bloom created the strongest seasonal carbon uptake in ice-free waters of the Southern Ocean to date.

Acknowledgments The authors would like to thank the British Antarctic Survey and the Captain, officers, crew and scientists onboard R.R.S. James Clark Ross during cruise JR177. The cruise was a component of the Discovery-2010 program. Geraint Tarling, Angus Atkinson and Sophie Fielding have been continually supportive of this work, both during and after the cruise. MODIS-Aqua data were provided by the SeaWiFS Project, NASA Goddard Space Flight Centre and ORBIMAGE. This work was part of Ph.D. research by EM Jones at the University of East Anglia funded by the Natural Environment Research Council (NERC) under the auspices of the Centre for observation of Air-Sea Interactions and fluXes (CASIX) (NER/F14/ G6/115). Participation in the cruise was supported by the Antarctic Funding Initiative (AFI) through the Collaborative Gearing Scheme (CGS8/28). The authors acknowledge the constructive comments by the editors and two anonymous reviewers.

References Ardelan, M.V., Holm-Hansen, O., Hewes, C.D., Reiss, C.S., Silva, N.S., Dulaiova, H., Steinnes, E., Sakshaug, E., 2010. Natural iron enrichment around the Antarctic Peninsula in the Southern Ocean. Biogeosciences 7, 11–25. Arrigo, K.R., van Dijken, G.L., Bushinsky, S., 2008. Primary production in the Southern Ocean, 1997–2006. Journal of Geophysical Research 113 (C08004). doi:10.1029/2007JC004551. Atkinson, A., Whitehouse, M.J., Priddle, J., Cripps, G.C., Ward, P., Brandon, M.A., 2001. South Georgia, Antarctica: a productive, cold water, pelagic ecosystem. Marine Ecology Progress Series 216, 279–308. Bakker, D.C.E., Nielsdo´ttir, M.C., Morris, P.J., Venables, H.J., Watson, A.J., 2007. The island mass effect and biological carbon uptake for the subantarctic Crozet Archipelago. Deep-Sea Research II 54, 2174–2190. Bates, N.R., Hansell, D.A., Carlson, C.A., 1998. Distributions of CO2 species, estimates of net community production, and air–sea CO2 exchanges in the Ross Sea polynya. Journal of Geophysical Research 103, 2883–2896. Blain, S., Tre´guer, P, Belviso, S., Bucciarelli, E., Denis, M., Desabre, S., Fiala, M., Martin Je´ze´quel, V., Le Fe vre, Mayzaud, P., Marty, J.-C., Razouls, S., 2001. A biogeochemical study of the island mass effect in the context of the iron hypothesis: Kerguelen Islands, Southern Ocean. Deep-Sea Research I 48, 163–187. Blain, S., Que´guiner, B., Armand, L., Belviso, S., Bombled, B., Bopp, L., Bowie, A., Brunet, C., Brussaard, C., Carlotti, F., Christaki, U., Corbie re, A., Durand, I., Ebersbach, F., Fuda, J.-L., Garcia, N., Gerringa, L., Griffiths, B., Guigue, C., Guillerm, C., Jacquet, S., Jeandel, C., Lann, P., Lefe vre, D., Lo Monaco, C., Malits, A., Mosseri, J., Obernosterer, I., Park, Y.-H., Picheral, M., Pondaven, P., Remenyi, T., Sandroni, V., Sarthou, G., Savoye, N., Scouarnec, L., Souhart, M., Thuiller, D., Timmermans, K., Trull, T., Uitz, J., van Beek, P., Veldhuis, M., Vincent, D., Viollier, E., Vong, L., Wagener, T., 2007. Effect of natural iron fertilization on carbon sequestration in the Southern Ocean. Nature 446, 1070–1075. Boyd, P.W., Watson, A.J., Law, C.S., Abraham, E.R., Trull, T., Murdoch, R., Bakker, D.C.E., Bowie, A., Charette, M., Croot, P., Downing, K., Frew, R., Gall, M., Hadfield, M., Hall, J., Harvey, M., La Roche, J.G.J., Liddicoat, M.R.L., Maldonado, M.T., McKay, R.M.L., Nodder, S., Pickmere, S., Pridmore, R., Rintoul, S.R., Safi, K., Waite, A., Zeldis, J., 2000. A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization. Nature 407, 695–702. Boyd, P.W., Jickells, T., Law, C.S., Blain, S., Boyle, E.A., Buesseler, K.O., Coale, K.H., Cullen, J.J., de Baar, H.J.W., Follows, M., Harvey, M., Lancelot, C., Levasseur, M., Owens, N.P.J., Pollard, R.T., Rivkin, R., Sarmiento, J., Schoemann, V., Smetacek, V., Takeda, S., Tsuda, A., Turner, S., Watson, A.J., 2007. Mesoscale iron enrichment experiments 1993–2005: synthesis and future directions. Science 315, 612–617. Bozec, Y., Bakker, D.C.E., Hartmann, C., Thomas, H., Bellerby, R.G.J., Nightingale, P.D., Riebesell, U., Watson, A.J., de Baar, H.J.W., 2004. The CO2 system in a Redfield context during an iron enrichment experiment in the Southern Ocean. Marine Chemistry 95, 89–105. Brainerd, K.E., Gregg, M.C., 1995. Surface mixed and mixing layer depths. Deep-Sea Research I 42, 1521–1543. Brandon, M.A., Murphy, E.J., Trathan, P.N., Bone, D.G., 2000. Physical oceanographic conditions to the northwest of the sub-Antarctic Island of South Georgia. Journal of Geophysical Research 105 (C10), 23983–23996. Bucciarelli, E., Blain, S., Tre´guer, P., 2001. Iron and manganese in the wake of the Kerguelen Islands (Southern Ocean). Marine Chemistry 73, 21–36. Coale, K.H., Johnson, K.S., Chavez, F.P., Buesseler, K.O., Barber, R.T., Brzezinski, M.A., Cochlan, W.P., Millero, F.J., Falkowski, P.G., Bauer, J.E., Wanninkhof, R.H.,

Kudela, X., Altabet, M.A., Hales, B.E., Takahashi, T., Landry, M.R., Bidigare, R.R., Wang, X., Chase, Z., Strutton, P.G., Friedrich, G.E., Gorbunov, M.Y., Lance, V.P., Hilting, A.K., Hiscock, M.R, Demarest, M., Hiscock, W.T., Sullivan, K.F., Tanner, S.J., Gordon, R.M., Hunter, C.N., Elrod, V.A., Fitzwater, S.E., Jones, J.L., Tozzi, S., Koblizek, M., Roberts, A.E., Herndon, J., Brewster, J., Ladizinsky, N., Smith, G., Cooper, D., Timothy, D., Brown, S.L., Selph, K.E., Sheridan, C.C., Twining, B.S., Johnson, Z.I., 2004. Southern ocean iron enrichment experiment: carbon cycling in high and low-Si waters. Science 304, 408–414. Ciattaglia, L., Colombo, T., Masarie, K.A., 1999. Continuous measurements of atmospheric CO2 at Jubany Station, Antarctica. Tellus 51B, 713–721. Comiso, J.C., McClain, C.R., Sullivan, C.W., Ryan, J.P., Leonard, C.L., 1993. Coastal Zone Color Scanner pigment concentrations in the Southern Ocean and relationships to geophysical surface-features. Journal of Geophysical Research 98 (C2), 2419–2451. ¨ de Baar, H.J.W., Jong, J.T.M., Bakker, D.C.E., Loscher, B.M., Veth, C., Bathmann, U., 1995. Importance of iron for plankton blooms and carbon dioxide drawdown in the Southern Ocean. Nature 373, 412–415. de Baar, H.J.W, Boyd, P.W., Coale, K.H., Landry, M.R., Tsuda, A., Assmy, P., Bakker, D.C.E., Bozec, Y., Barber, R.T., Brzezinski, M.A., Buesseler, K.O., Boye, M., Croot, P.L., Gervais, F., Gorbunov, M.Y., Harrison, P.J., Hiscock, W.T., Laan, P., Lancelot, C., Law, C.S., Levasseur, M., Marchetti, A., Millero, F.J., Nishioka, J., Nojiri, Y., van Oijen, T., Riebesell, U., Rijkenberg, M.J.A., Saito, H., Takeda, S., Timmermans, K.R., Veldhuis, M.J.W., Waite, A.M., Wong, C.S., 2005. Synthesis of iron fertilization experiments: from the iron age in the age of enlightenment. Journal of Geophysical Research 110 (C09S16). doi:10.1029/ 2004JC002601. Dickson, A.G., 1981. An exact definition of total alkalinity and a procedure for the estimation of alkalinity and total inorganic carbon from titration data. DeepSea Research 28, 609–623. Dickson, A.G., Millero, F.J., 1987. A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Research 34, 1733–1743. DOE, 2007. Guide to Best Practices for Ocean CO2 measurements, vol. 3. In: Dickson, A.G., Sabine, C.L., Christian, J.R. (Eds.), PICES Special Publication, pp. 1–191. Doty, M.S., Oguri, M., 1956. The island mass effect. Journal du Conseil Permanent International de la Exploration de la Mer 22, 33–37. Dulaiova, H., Ardelan, M.V., Henderson, P.B., Charette, M.A., 2009. Shelf-derived iron inputs drive biological productivity in the southern Drake Passage. Global Biogeochemical Cycles 23 (GB4014). doi:10.1029/2008GB003406. Eppley, R.W., 1972. Temperature and phytoplankton growth in the sea. Fishery Bulletin 70, 1063–1085. ¨ Friis, K., Kortzinger, A., Wallace, D.W.R., 2003. The salinity normalization of marine inorganic carbon chemistry data. Geophysical Research Letters 30 (1085). doi:10.1029/2002GL015898. GEBCO, 2001. General Bathymetric Chart of the Oceans Digital Atlas. British Oceanographic Data Centre, Liverpool, UK. Geibert, W., Assmy, P., Bakker, D.C.E., Hanfland, C., Hoppema, M., Pichevin, L.E., ¨ Schroder, M., Schwarz, J.N., Stimac, I., Usbeck, R., Webb, A., 2010. High productivity in an ice melting hot spot at the eastern boundary of the Weddell Gyre. Global Biogeochemical Cycles 24 (GB3007). doi:10.1029/2009GB003657. Hewes, C.D., Reiss, C.S., Kahru, M., Mitchell, B.G., Holm-Hansen, O., 2008. Control of phytoplankton biomass by dilution and mixed layer depth in the western Weddell-Scotia Confluence. Marine Ecology Progress Series 366, 15–29. Holeton, C.L., Ne´de´lec, F., Sanders, R., Brown, L., Moore, C.M., Stevens, D.P., Heywood, K.J., Statham, P.J., Lucas, C.H., 2005. Physiological state of phytoplankton communities in the southwest Atlantic sector of the Southern Ocean, as measured by fast repetition rate fluorometry. Polar Biology 29, 44–52. Ishii, M., Inoue, H.Y., Matsueda, H., Tanoue, E., 1998. Close coupling between seasonal biological production and dynamics of dissolved inorganic carbon in the Indian Ocean sector and the western Pacific Ocean sector of the Antarctic Ocean. Deep-Sea Research I 45, 1187–1209. Ishii, M., Inoue, H.Y., Matsueda, H., 2002. Net community production in the marginal ice zone and its importance for the variability of the oceanic pCO2 in the Southern Ocean south of Australia. Deep-Sea Research II 49, 1691–1706. Jennings, J.C., Gordon, L.I., Nelson, D.M., 1984. Nutrient depletion indicates high primary productivity in the Weddell Sea. Nature 309, 51–54. Johnson, K.M., Sieburth, J.M., Williams, P.J.L., Brandstrom, L., 1987. Coulometric total carbon dioxide analysis for marine studies—automation and calibration. Marine Chemistry 21, 117–133. Jouandet, M.P., Blain, S., Metzl, N., Brunet, C., Trull, T.W., Obernosterer, I., 2008. A seasonal carbon budget for a naturally iron-fertilised bloom over the Kerguelen Plateau in the Southern Ocean. Deep-Sea Research II 55, 856–867. Jones, E.M., 2010. The Marine Carbon Cycle of the Scotia Sea, Southern Ocean. Ph.D. Thesis. University of East Anglia. Jones, E.M., Bakker, D.C.E., Venables, H.J., Whitehouse, M.J., Korb, R.E., Watson, A.J., 2010. Rapid changes in surface water carbonate chemistry during Antarctic sea ice melt. Tellus 62B, 621–635. Korb, R.E., Whitehouse, M.J., 2004. Contrasting primary production regimes around South Georgia, Southern Ocean: large blooms versus high nutrient, low chlorophyll waters. Deep-Sea Research I 51, 721–738. Korb, R.E., Whitehouse, M.J., Ward, P., 2004. SeaWiFS in the southern ocean: spatial and temporal variability in phytoplankton biomass around South Georgia. Deep-Sea Research II 51, 99–116. Korb, R.E., Whitehouse, M.J., Atkinson, A., Thorpe, S.E., 2008. Magnitude and maintenance of the phytoplankton bloom at South Georgia: a naturally iron replete environment. Marine Ecology Progress Series 368, 75–91.

E.M. Jones et al. / Deep-Sea Research II 59-60 (2012) 25–35

Korb, R.E., Whitehouse, M.J., Gordon, M., Ward, P., Poulton, A.J., 2010. Summer microplankton community structure across the Scotia Sea: implications for biological carbon export. Biogeosciences 7, 343–356. Korb, R.E., Whitehouse, M.J., Ward, P., Gordon, M., Venables, H.J., 2012. Regional and seasonal differences in microplankton biomass, productivity and structure across the Scotia Sea and implications to the export of biogenic carbon. DeepSea Research II 59–60, 67–77. Lewis, E., Wallace, D.W.R., 1998. CO2SYS-Program Developed for the CO2 System Calculations. Carbon Dioxide Information and Analysis Centre. Report ORNL/ CDIAC-105. ¨ Loscher, B.M., de Baar, H.J.W., de Jong, J.T.M., Veth, C., Dehairs, F., 1997. The distribution of Fe in the Antarctic circumpolar current. Deep-Sea Research II 44, 143–187. Martin, J.H., 1990. Glacial–interglacial CO2 change: the iron hypothesis. Paleoceanography 5, 1–13. Mehrbach, C., Culberson, C.H., Hawley, J.E., Pytkowicz, R.M., 1973. Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure. Limnology and Oceanography 18, 897–907. Meredith, M.P., Murphy, E.J., Cunningham, N.J., Wood, A.G., Korb, R.E., Whitehouse, M.J., Thorpe, S.E., Vivier, F., 2003a. An anticyclonic circulation above the Northwest Georgia Rise, Southern Ocean. Geophysical Research Letters 30 (2061). doi:10.1029/2003GL018039. Meredith, M.P., Watkins, J.L., Murphy, E.J., Ward, P., Bone, D.G., Thorpe, S.E., Grant, S.A., Ladkin, R.S., 2003b. Southern ACC Front to the northeast of South Georgia: pathways, characteristics, and fluxes. Journal of Geophysical Research 108 (3162). doi:10.1029/2001JC001227. Meredith, M.P., Brandon, M.A., Murphy, E.J., Trathan, P.N., Thorpe, S.E., Bone, D.G., Chernyshkov, P.P., 2005. Variability in hydrographic conditions to the east and northwest of South Georgia. Journal of Marine Systems 53, 143–167. Meredith, M.P., Naveira Garabato, A.C., Gordon, A.L., Johnson, G.C., 2008. Evolution of the deep and bottom waters of the Scotia Sea, Southern Ocean, during 1995–2005. Journal of Climate 21, 3327–3342. Millero, F.J., 1995. Thermodynamics of the carbon dioxide system in the Oceans. Geochimica et Cosmochimica Acta 59, 661–677. Minas, H.J., Minas, M., 1992. Net community production in high nutrient-low chlorophyll waters of the tropical and Antarctic Oceans—grazing vs iron hypothesis. Oceanologica Acta 15, 145–162. Moore, J.K., Abbott, M.R., Richman, J.G., Smith, W.O., Cowles, T.J., Coale, K.H., Gardner, W.D., Barber, R.T., 1999. SeaWiFS satellite ocean colour data from the Southern Ocean. Geophysical Research Letters 26, 1465–1468. Moore, J.K., Abbott, M.R., 2000. Phytoplankton chlorophyll distributions and primary production in the Southern Ocean. Journal of Geophysical Research 105, 28709–28722. Murphy, E.J., Watkins, J.L., Trathan, P.N., Reid, K., Meredith, M.P., Thorpe, S.E., Johnston, N.M., Clarke, A., Tarling, G.A., Collins, M.A., Forcada, J., Shreeve, R.S., Atkinson, A., Korb, R., Whitehouse, M.J., Ward, P., Rodhouse, P.G., Enderlein, P., Hirst, A.G., Martin, A.R., Hill, S.L., Staniland, I.J., Pond, D.W., Briggs, D.R., Cunningham, N.J., Fleming, A.H., 2007. Philosophical Transactions of the Royal Society B 362, 113–148. Naveira Garabato, A.C., Heywood, K.J., Stevens, D.P., 2002. Modification and pathways of Southern Ocean deep waters in the Scotia Sea. Deep-Sea Research I 49, 681–705. Naveira Garabato, A.C., Polzin, K.L., King, B.A., Heywood, K.J., Visbeck, M., 2004. Widespread intense turbulent mixing in the Southern Ocean. Science 303, 210–213. Naveira Garabato, A.C., Stevens, D.P., Watson, A.J., Roather, W., 2007. Shortcircuiting of the overturning circulation in the Atlantic Circumpolar Current. Nature 447, 194–197. Nightingale, P.D., Malin, G., Law, C.S., Watson, A.J., Liss, P.S., Liddicoat, M.I., Boutin, J., Upstill-Goddard, R.C., 2000. In situ evaluation of air-sea gas exchange parameterizations using novel conservative and volatile tracers. Global Biogeochemical Cycles 14, 373–387. Orsi, A.H., Whitworth, T., Nowlin, W.D., 1995. On the meridional extent and fronts of the Antarctic Circumpolar Current. Deep-Sea Research I 42, 641–673. Park, J., Im-Sang, O., Hyun-Cheol, K., Sinjae, Y., 2010. Variability of SeaWiFs chlorophyll-a in the southwest Atlantic sector of the Southern Ocean: strong topographic effects and weak seasonality. Deep-Sea Research I 57, 604–620. Park, Y.-H., Gamberoni, L., 1995. Large scale circulation and its variability in the south Indian Ocean from TOPEX/POSEIDON altimetry. Journal of Geophysical Research 100 (C12), 24911–24930. Planquette, H., Statham, P.J., Fones, G.R., Charette, M.A., Moore, C.M., Salter, I., Ne´de´lec, F.H., Taylor, S.L., French, M., Baker, A.R., Mahowald, N., Jickells, T.D., 2007. Dissolved iron in the vicinity of the Crozet Islands, Southern Ocean. Deep-Sea Research II 54, 1999–2019. Pollard, R.T., Read, J.F., 2001. Circulation pathways and transports of the Southern ocean in the vicinity of the Southwest Indian Ridge. Journal of Geophysical Research 106 (C2), 2881–2898. Pollard, R.T., Lucas, M.I., Read, J.F., 2002. Physical controls on biogeochemical zonation in the Southern Ocean. Deep-Sea Research II 49, 3289–3305. Pollard, R.T., Sanders, R., Lucas, M.I., Statham, P.J., 2007. The Crozet Natural Iron Bloom and Export Experiment (CROZEX). Deep-Sea Research II 54, 1905–1914. Pollard, R.T., Salter, I., Sanders, R.J., Lucas, M.I., Moore, C.M., Mills, R.A., Statham, P.J., Allen, J.T., Baker, A.R., Bakker, D.C.E., Charette, M.A., Fielding, S., Fones, G.R., French, M., Hickman, A.E., Holland, R.J., Hughes, J.A., Jickells, T.D., Lampitt, R.S.,

35

Morris, E.E., Nedelec, F.H., Nielsdo´ttir, M., Planquette, H., Popova, E.E., Poulton, A.J., Read, J.F., Seeyave, S., Smith, T., Stinchcombe, M., Taylor, S., Thomalla, S., Venables, H.J., Williamson, R., Zubkov, M.V., 2009. Southern Ocean deep-water carbon export enhanced by natural iron fertilization. Nature 457, 577–581. Redfield, A.C., Ketchum, B.H., Richards, F.A., 1963. The influence of organisms on the composition of seawater. In: Hill, M.N (Ed.), The Sea. Vol. 2. The Composition of Seawater. Wiley, New York, pp. 26–77. Reid, J.L., Nowlin, W.D., Patzert, W.C., 1977. On the characteristics and circulation of the southwestern Atlantic Ocean. Journal of Physical Oceanography 7, 62–91. Sarmiento, J.L., Toggweiler, R., 1984. A new model for the role of the oceans in determining atmospheric pCO2. Nature 308, 621–624. Schlitzer, R., 2002. Carbon export fluxes in the Southern Ocean: results from inverse modelling and comparison with satellite based estimates. Deep-Sea Research II 49, 1623–1644. Schuster, U., Watson, A.J., 2007. A variable and decreasing sink for atmospheric CO2 in the North Atlantic. Journal of Geophysical Research 112 (C11006). doi:10.1029/2006JC003941. Shim, J., Kang, Y.C., Kim, D., Choi, S.-H., 2006. Distribution of net community production and surface pCO2 in the Scotia Sea, Antarctica, during austral spring 2001. Marine Chemistry 101, 68–84. Siegenthaler, U., Wenk, T., 1984. Rapid atmospheric CO2 variations and ocean circulation Nature 308, 624–626. Smetacek, V., 2001. EisenEx: international team conducts iron experiment in Southern Ocean. US JGOFS News 11 (1), 11–14. Smith, I.J., Stevens, D.P., Heywood, K.J., Meredith, M.P., 2010. The flow of the Antarctic Circumpolar Current over the North Scotia Ridge. Deep-Sea Research I 57, 14–28. Sokolov, S., Rintoul, S.R., 2007. On the relationship between fronts of the Antarctic Circumpolar Current and surface chlorophyll concentrations in the Southern Ocean. Journal of Geophysical Research 112 (C07030). doi:10.1029/ 2006JC004072. Sweeney, C., Smith, W.O., Hales, B., Bidigare, R.R., Carlson, C.A., Codispoti, L.A., Gordon, L.I., Hansell, D., Millero, F.J., Park, M.-O.K., Takahashi, T., 2000. Nutrient and carbon removal ratios and fluxes in the Ross Sea, Antarctica. Deep-Sea Research II 47, 3395–3421. Takahashi, T., Sutherland, S.C., Wanninkhof, R., Sweeney, C., Feely, R.A., Chipman, D.W., Hales, B., Friederich, G., Chavez, F., Sabine, C.L., Watson, A.J., Bakker, D.C.E., Schuster, U., Metzl, Yoshikawa-Inoue, H., Ishii, M., Midorikawa, ¨ T., Nojiri, Y., Kortzinger, Steinhoff, T., Hoppema, M., Olafsson, J., Arnarson, T.S., Tilbrook, B., Johannessen, T., Olsen, A., Bellerby, R., Wong, C.S., Delille, B., Bates, N.R., de Baar, H.J.W, 2009. Climatological mean and decadal change in surface ocean pCO2, and net sea–air CO2 flux over the global oceans. Deep-Sea Research II 56, 554–577. Takahashi, T., Olafsson, J., Goddard, J.G., Chipman, D.W., Sutherland, S.C., 1993. Seasonal variation of CO2 and nutrients in the high latitude surface oceans—a comparative study. Global Biogeochemical Cycles 7, 843–878. Thorpe, S.E., Heywood, K.J., Brandon, M.A., Stevens, D.P., 2002. Variability of the Southern Antarctic Circumpolar Current Front north of South Georgia. Journal of Marine Systems 37, 87–105. Trathan, P.N., Brandon, M.A., Murphy, E.J., Thorpe, S.E., 1997. Characterisation of the Antarctic Polar Frontal Zone to the north of South Georgia in summer 1984. Journal of Geophysical Research 102, 10483–10497. Venables, H.J., Meredith, M.P., 2009. Theory and observations of Ekman flux in the chlorophyll distribution downstream of South Georgia. Geophysical Research Letters 36 (L23610). doi:10.1029/2009GL041371. Venables, H.J., Moore, C.M., 2010. Phytoplankton and light limitation in the Southern Ocean: learning from high-nutrient, high-chlorophyll areas. Journal of Geophysical Research 115 (C02015). doi:10.1029/2009JC005361. Venables, H.J., Meredith, M.P., 2012. Identifying Southern Ocean fronts in satellite dynamic height from water mass properties. Deep-Sea Research II 59–60, 14–24. Watson, A.J., Bakker, D.C.E., Ridgwell, A.J., Boyd, P.W., Law, C.S., 2000. Effect of iron supply on Southern Ocean CO2 uptake and implications for glacial atmospheric CO2. Nature 407, 730–733. Watson, A.J., Orr, J.C., 2003. Carbon dioxide fluxes in the global ocean. In: Fasham, M.J.R. (Ed.), Ocean Biogeochemistry: The Role of the Ocean Carbon Cycle in Global Change. Springer-Verlag, Berlin. Weiss, R.F., Price, B.A., 1980. Nitrous-oxide solubility in water and seawater. Marine Chemistry 8, 347–359. Whitehouse, M.J., Korb, R.E., Atkinson, A., Thorpe, S.E., Gordon, M., 2008. Formation, transport and decay of an intense phytoplankton bloom within the highnutrient low-chlorophyll belt of the Southern Ocean. Journal of Marine Systems 70, 150–167. Whitehouse, M.J., Korb, R.E., Atkinson, A., Venables, H.J., Pond, D.W., Gordon, M., 2012. Latitudinal gradients in primary production and phytoplankton stocks across the central Scotia Sea. Deep-Sea Research II 59–60, 67–77. Whitworth, T., Nowlin, W.D., 1987. Water masses and currents of the Southern Ocean at the Greenwich Meridian. Journal of Geophysical Research 92, 6462–6476. Zhou, M., Yiwu, Z., Ryan, D.D., Measures, C.I., 2010. Dynamics of the current system in the southern Drake Passage. Deep-Sea Research I 57, 1039–1048.