Contrasting primary production regimes around South Georgia, Southern Ocean: large blooms versus high nutrient, low chlorophyll waters

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Deep-Sea Research I 51 (2004) 721–738

Contrasting primary production regimes around South Georgia, Southern Ocean: large blooms versus high nutrient, low chlorophyll waters Rebecca E. Korb*, Mick Whitehouse British Antarctic Survey, Natural Environmental Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK Received 18 March 2003; received in revised form 21 August 2003; accepted 2 February 2004

Abstract During the austral summer of 2002, a large and persistent phytoplankton bloom was detected with SeaWiFS imagery in the Georgia Basin to the north–west of South Georgia, while waters to the east of the island were relatively unproductive. A British Antarctic Survey research cruise in January 2002 confirmed this west/east difference with production values of up to 2.5 g C m
2 d
1 and chlorophyll a (chl a) values up to 15 mg m
3 at stations to the northwest of the island and 0.17 g C m
2 d
1 and 1.3 mg chl a m
3 to the northeast. These differences were not attributable to light limitation as mixed layer depth never exceeded critical depth. Instead, substantial nutrient depletions to the northwest of the island compared with the northeast suggested a difference in nutrient use between the two regions. The exceedingly high nutrient depletions (to o6.0 and 0.3 mmol m
3 for NO3-N and PO4-P, respectively) measured to the northwest were associated with an anticyclonic eddy situated over the Northwest Georgia Rise. Furthermore, differences in NO3-N:PO4-P depletion ratios suggested a greater ability in the northwest phytoplankton to utilise NO3N, and a greater dependence on NH4-N at the northeast stations. Three distinct station groups were identified around the island based on watermass and size-fractionated chlorophyll. To the east, waters were characterised by a high proportion of microplankton and low NO3-N:PO4-P depletion ratios, to the west, by either a high proportion of microplankton and high NO3-N:PO4-P depletion ratios, or a high proportion of nanoplankton and moderate NO3N:PO4-P depletion ratios. We consider this to be indicative of greater Fe availability, promoting NO3-N use, to the northwest of South Georgia. However, an absence of microplankton over the western shelf regions may be due to size selective grazing by krill. Our field data, in conjunction with SeaWiFS imagery, indicated that the Georgia Basin phytoplankton most likely originated upstream of South Georgia. Subsequent interactions with the Northwest Georgia Rise and South Georgia’s south-western shelf promoted increased growth that converged to the west of the island to form a large bloom in the Georgia Basin. r 2004 Elsevier Ltd. All rights reserved. Keywords: South Georgia; Primary production; Phytoplankton blooms; HNLC

1. Introduction *Corresponding author. Fax: +44-1223-221259. E-mail address: [email protected] (R.E. Korb).

In contrast to the majority of the Southern Ocean, which is generally viewed as a high

0967-0637/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2004.02.006

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nutrient, low chlorophyll system (HNLC), the waters surrounding the island of South Georgia often support immense phytoplankton blooms (Whitehouse et al., 1996a; Atkinson et al., 2001; Korb et al., in press). The region of the Georgia Basin, to the northwest of the island, is particularly productive and on a number of occasions, large blooms with chlorophyll a (chl a) concentrations up to 15–20 mg m
3 and mean values X3 mg m
3 have been found to extend at least 500 km from South Georgia to the Polar Front, lasting for as long as 5 months (Korb et al., in press). In marked contrast, waters to the east of the island often support only low levels of phytoplankton (Whitehouse et al., 1999; HolmHansen et al., in review; Korb et al., in press) and are more typical of HNLC waters with chl a concentrations of o0.3 mg m
3. While we are beginning to appreciate the true extent of interannual and spatial variability in phytoplankton growth and accumulation at South Georgia, the precise mechanisms of bloom formation remain poorly understood. Differences in production between the northeast and northwest are doubtlessly due in part to differences in hydrographic regimes. South Georgia lies in the predominantly eastward flowing Antarctic Circumpolar Current (ACC) between the Polar Front (PF) and the Southern Antarctic Circumpolar Current Front (SACCF) (cf. Gordon et al., 1977; Orsi et al., 1995; Thorpe et al., 2002). To the west of South Georgia, much of the ACC is steered northwards over the North Scotia Ridge by the topography surrounding the Scotia Sea, before resuming its eastward course. The PF is diverted northwards over the Shag Rocks Passage to the west of South Georgia before resuming its eastward course and passing north of the island. The SACCF is also steered northwards, looping around the eastern end of the island before retroflecting eastwards again near the Northwest Georgia Rise (Orsi et al., 1995; Thorpe et al., 2002). The different hydrographic regimes around South Georgia exhibit contrasting nutrient and temperature characteristics with waters to the northeast holding higher nutrient concentrations and lower water temperatures compared with the northwest (Whitehouse et al., 1999). In addition,

other critical factors considered to limit phytoplankton growth in the Southern Ocean, such as Fe availability (e.g. Boyd and Law, 2001) and light limitation due to deep mixing (e.g. Nelson and Smith, 1991), may also vary significantly between the eastern and western waters of South Georgia. During the austral summer of 2002, a large and persistent bloom occurred in the Georgia Basin while surface waters to the east of South Georgia held much lower chl a concentrations. A research cruise in Jan 2002 confirmed these observations and collected related physical, biological and chemical data. In this paper, we compare phytoplankton biomass and production between study sites to the northwest and northeast of South Georgia and examine aspects of the physical and chemical environment for insights into the factors influencing phytoplankton dynamics in this area of the Scotia Sea.

2. Methods 2.1. Sampling Shipboard data were collected from the British Antarctic Survey’s (BAS) research vessel RRS James Clarke Ross during cruise JR70 between 6 January–3 February 2002. Measurements were made along two 160 km long transects: one running to the northwest of the island, through the Georgia basin and termed the Western Long Transect (WLT) and a second to the northeast (the Eastern Long Transect or ELT) (Fig. 1a). In addition, a 130  80 km sampling grid located at the western end of South Georgia was surveyed. Three, 130 km long transects in this grid, running parallel to the northwest coast, were situated in shelf, shelf break and off-shelf waters and are termed the Western Transects 1–3 (WT1, WT2, WT3, respectively). The regions predominant water flows during JR70 are illustrated with drifter buoy data in Fig. 1b (from Meredith et al., 2003). A SeaBird 911+ CTD was deployed at all stations and water samples were collected on the upcast of the CTD with 10 l Niskin bottles on the SeaBird 12-position carousel water sampler.

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Fig. 1. (a) Station positions at South Georgia during cruise JR70 with the major bathymetric features of the region. For illustrative purposes, not all stations are numbered along the Western and Eastern Long Transects, however stations run in ascending order from 1 (near shore) to 17 (oceanic waters). The 500 and 2500 m isobaths are represented by dark and pale grey areas, respectively. (b) Drifter buoy trajectories during January to March 2002. Sites of deployment are marked by an asterisk. The 500, 2500 and 3500 m isobaths are indicated by the white, light grey and grey areas, respectively. This plot is reproduced from Meredith et al. (2003).

2.2. Chlorophyll a Chl a was measured at all hydrographic stations on water samples collected with the CTD between

6.5 m (from the ship’s non-toxic seawater supply), and B100 m. An Aqua Tracka fluorometer (Chelsea Instruments) was attached to the CTD frame and fluorescence was measured every 2 m. The

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Aqua Tracka was calibrated against discrete measurements of chl a concentration taken at each CTD station (generally linear regression yielded an R2 value of X0.80). Water samples were filtered through glass fibre filters (Whatman GF/F) and extracted in 90% acetone (Parsons et al., 1984). Fluorescence of the extract was measured before and after acidification on a TD-700 Turner fluorometer. The instrument was calibrated against commercially prepared chl a standards (Sigma). Sub-samples of water collected for primary production (see primary production methods) were also filtered and analysed for chl a as outlined above; however, the Aqua Tracka was not switched on for the primary production CTD drops. Size-fractionated chl a was measured on water samples from a depth of 20 m, filtered onto 47 mm polycarbonate membrane filters (12, 2 and 0.2 mm) using a cascade, then analysed for chl a after extraction in 90% acetone as above. In the literature it may be observed that a filter of 20 mm is commonly used to capture the largest size fraction of chl a, however, on this cruise, the largest filter available was 12 mm. Therefore for the purposes of this study, pico-, nano and micro-plankton were represented by the 0.2–2, 2–12 and >12 mm size fractions. Community chl a (>0.2 mm) was obtained by summing the chl a concentrations for all fractions. To check the quality of the size-fractionated chlorophyll data, community chl a from the sum of the size fractions was compared to community chl a concentrations at 20 m collected on GF/F filters (generally values were within 95% of each other). 2.3. Primary production rates Primary production was measured at 5 stations in the western survey grid, 6 stations along the ELT and 8 stations along the WLT. Primary production was estimated by a method based on the JGOFS 14C-protocol (JGOFS, 1996) in conjunction with a simulated in situ incubator. The incubator consisted of seven clear polycarbonate tubes wrapped in neutral density screening to provide 100%, 50%, 23%, 10%, 2.5%, 1.25%, and 0.38% of incident irradiance (E0). The tubes

were placed in an open, clear acrylic tank (1 m  1 m) in a shade free area on the ship’s deck and maintained at ambient temperature with running seawater. Water samples were drawn from CTD rosette bottles fired at six optical depths corresponding to E0’s (0.38–50%) employed in the deckboard incubator and from a bottle fired at a depth of 1 m (subsequently referred to as the 100% E0 CTD). However, because of equipment failure, light profiles were not obtained at stations along the WLT. As water could not be collected at optical depths corresponding to the incubator E0’s, the CTD was fired at a depth of 20 m only, within the upper mixed-layer depth (UML) along the WLT. Samples (250 ml) from each optical depth were poured into polycarbonate bottles. To determine size-fractionated primary production, an additional 250 ml sample was taken from the 100% E0 CTD bottle. Additional samples were also taken from the 100% and 0.38% E0 CTD bottles for determination of the initial isotope uptake, subsequently referred to as T0 samples. At WLT stations, 250 ml subsamples of the water collected at 20 m were used to fill 10 polycarbonate bottles; 7 bottles were used for deckboard incubations, 2 bottles provided T0 samples and the final bottle was used to determine size-fractionated primary production. To each 250 ml sample, 0.1 ml of NaH14CO3 (activity 0.1 mCi ml
1) was added. Total activity of the isotope was checked by removing 0.25 ml from each of the T0 bottles into 20 ml scintillation vials containing 0.25 ml ethanolamine. The remaining T0 samples were immediately filtered onto Whatman GF/F filters under low vacuum pressure (o70 mm Hg), and the filters were added to 20 ml scintillation vials containing 1 ml of 0.5 M HCl and left uncapped for 24 h in a fume hood. The remaining 250 ml samples were placed into the incubator tubes simulating the E0 at which the samples were collected, and incubated for 24 h. Size-fractionated samples were incubated at 100% E0 for 24 h. At WLT stations, 7 bottles were placed in the incubator tubes, one at each E0 (0.38–100%), and the size-fractionated sample was incubated at 100%; all samples were incubated for 24 h.

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When incubations were complete, samples were filtered and acidified as for the T0 samples. The size-fractionated samples were filtered through a cascade onto 12, 2 and 0.2 mm polycarbonate membrane filters (Millipore), and each filter was added to a 20 ml scintillation vial containing 1 ml of 0.5 M HCl and left uncapped for 24 h in a fume hood. After 24 h acidification, a 10 ml aliquot of Optiphase Hisafe liquid scintillation cocktail was added to each scintillation vial. The radioactivity in each vial was determined by counting on a Packard (model LSC 2900TR) liquid scintillation counter. Column-integrated production rates were derived by integration to the euphotic depth. The chlorophyll-specific rate of photosynthesis (Pb) was estimated by dividing column-integrated primary production rates by column-integrated chl a. As only a limited number of primary production measurements were performed during the cruise (at 19 out of a total of 55 stations), additional estimates were derived with a depth-integrated model (DIM) from Behrenfeld and Falkowski (1997): PP ¼ Pbopt  f ½E0   DL  Ceu ;

ð1Þ
2


1

where PP is the uptake rate (mg C m d ), Pbopt the ‘‘optimum’’ chlorophyll-specific carbon fixation rate determined from the productivity profile (mg C mg chl a
1 h
1), f[E0] is a water column light-dependence function, DL is daylength (h), and Ceu is euphotic zone chlorophyll (mg chl a m
2). All actual uptake rates were plotted against estimates from the model which explained 84% of the variability and closely followed a 1:1 fit. Therefore, the model was considered to provide a robust estimate of depth-integrated primary production at stations where 14C incubations were not performed. 2.4. Nutrient analysis For nutrient analysis, nominal sampling depths were 6.5 m from the ships non-toxic seawater supply) and 20, 40, 60, 80, 100, 125, 150 and 200 m and a further four depths between 200 m and the bottom of the CTD water bottle cast. Water bottles from all CTD casts were subsampled and

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analysed for nutrients. Samples were filtered through a mixed ester membrane (Whatman WME, pore size 0.45 mm), and the filtrate was analysed colorimetrically for dissolved phosphate (PO4-P), nitrate+nitrite (NO3+NO2-N), silicic acid (Si(OH)4-Si), and ammonium (NH4-N), with a Technicon segmented-flow analyser (Whitehouse, 1997). A measure of nutrient depletion was obtained by subtracting near-surface (0–50 m) values from approximately pycnocline (60–80 m) values. Pycnocline values were similar to prebloom and winter values previously recorded for the South Georgia region (see Whitehouse et al., 1996a, 2000). Significant concentration differences were tested for by ANOVA in the statistical software Minitab 13.

2.5. Physical data Further details of CTD measurements are detailed by Meredith et al. (2003). For this paper, density profiles were assessed to establish the UML depth which was defined as a X0.05 kg m
3 density change within 10 m depth. Incident photosynthetically available radiation (PAR, l=400–700 nm) was measured continuously with a 2-p sensor (PAR LITE, Sci-Tec) mounted in a shade-free area on the ships foremast. Incident irradiance was integrated daily. Profiles of downwelling PAR were obtained from a sensor mounted on the frame of the CTD (Biospherical Instruments Inc., model QCD905L) or from an irradiance sensor attached to an undulating oceanographic recorder (Chelsea Instruments Nn Shuttle Mark II). The scalar attenuation coefficient (K0) of the water column was calculated by linear regression of a log-transformed irradiance vs. depth profile after Kirk (1994). The depth of the euphotic zone (Ze ) was defined as that of the 1% incident light level (Kirk, 1994). Because of technical problems, it was not possible to obtain light profiles at all stations along the WLT and Western Transects. Instead, the relationship between K0 and integrated chl a concentration over 100 m (Chlint) throughout the north-western survey areas was used to estimate euphotic depths (K0=0.0004[Chlint]+0.0992, R2=0.81, n=17).

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Critical depths were estimated after Nelson and Smith (1991). 2.6. SeaWiFs data SeaWiFS-derived estimates of surface chl a concentration were obtained from the Goddard Distributed Active Archive Centre with the standard OC4v4 chlorophyll algorithm (O’Reilly et al., 2001). Level 3, monthly composites (9 km resolution) were processed with SEADAS software (NASA), and chl a values extracted from these images.

3. Results 3.1. Chlorophyll distributions and mixed layer depth Marked differences in UML depth and chl a concentrations were observed between the east and the west transects. Along the ELT, the UML depth ranged from 25 to 90 m (Fig. 2a and Table 1). In comparison to the western transects, Chl a concentrations were low throughout the ELT. The pattern of distribution was generally similar to the depth of the UML (Fig. 2a). Between stations 7 and 11, the UML shoaled and chl a concentration increased over this area (>0.75 mg m
3). There was also an indication of a subsurface chl a maxima (B1.3 mg m
3) between 25 and 47 m in this region. However, at station 11, the bloom extended to the surface, and a sudden shallowing of the UML was evident from 56 m at station 11 to 25 m at station 10. Over the shelf, at stations 1–3, the highest chl a concentrations also occurred at depth, at approximately 20–30 m (>0.9 mg m
3). The UML of the WLT (average 33 m) was approximately 1.7 times shallower than the ELT (Table 1, Fig. 2a and b). Chl a concentration throughout the WLT was also much higher than the ELT with minimum surface values of at least 1 mg m
3 and as high as 6–11 mg m
3 at stations 5–11 over the North West Georgia Rise (NWGR; Fig. 2b). There was no evidence of a subsurface chlorophyll maximum along the WLT. Chl a concentrations were generally highest at the sur-

face or near surface and decreased with depth. High biomass in the upper water column was also observed throughout the Western Grid transects. However, there were clear differences in chl a levels between transects. The highest chl a concentration (up to 15 mg m
3) was found in the off-shelf transect WT3, which had the deepest UML (37 m). Chl a concentrations and the UML depth in transects WT1 and WT2 were similar to each other (Table 1, Fig. 2d and e). 3.2. Size-fractionated chl a There did not appear to be a direct relationship between total water column chl a and sizefractionated chl a from 20 m, for example, microplankton were dominant at stations with both high and low chl a biomass. In the off-shelf waters of the ELT (stations 6–17), where chl a concentrations were low, microplankton accounted for at least 40% or more of the total biomass (Fig. 3a). Although biomass was generally higher throughout the WLT than the ELT, microplankton made up less than 40% of the total phytoplankton biomass at 13 stations; at the remaining 4 stations (5, 6, 9 and 11), microplankton accounted for >80% of the total (Fig. 3b). In the Western Grid transects, size-fractionated chl a varied between transects (Fig. 3c–e) and differed greatly from the WLT. The off-shelf transect, WT3, was dominated almost exclusively by microplankton. Biomass along WT2 was dominated by either microplankton (at stations 11, 12 and 14) or nanoplankton (stations 13, 15, 16 and 17). Transect WT1 was composed largely of microplankton with the exception of the easternmost stations, where nanoplankton accounted for 80% of the total. Picoplankton were rare throughout the transects of the Western Grid, and yet this size-fraction was relatively abundant along the WLT as well as the ELT. 3.3. Water column primary production Eastern and western waters showed marked differences in column-integrated production. Along the ELT, production rates remained consistently low, ranging between 44 and

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Fig. 2. Vertical sections of chlorophyll a concentrations from (a) the Eastern Long Transect, (b) the Western Long Transect, (c–e) the Western Grid, Transect 3, 2 and 1, respectively. Note difference in scale for (a). The dashed black line indicates the depth of the UML.

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Table 1 Mean (71 S.D.) euphotic depth (Ze) and upper mixed layer depth (UML) for the Eastern Long Transect (ELT), Western Long Transect (WLT) and the Western Transects (WT1-3) Region

Ze (m)

UML (m)

ELT

57712 (39–83) 3179 (18–52) 2477 (17–36) 3076 (21–41) 2673 (23–32)

55721 (25–90) 33711 (10–52) 37715 (23–60) 25712 (13–50) 21710 (5–30)

WLT WT3 WT2 WT1

174 mg C m
2 d
1 with an average value of 117 mg C m
2 d
1 (Fig. 4a). Production rates were much more variable along the WLT, ranging from 134 to 1325 mg C m
2 d
1 (average 630 mg C m
2 d
1). The highest production rates were centred over the area of the NWGR (stations 5–11) averaging 968 mg C m
2 d
1. Column-integrated production rates in the Western Grid were generally higher (average 879 mg C m
2 d
1) than the WLT, but were variable between transects (Fig. 4b). WT1 and WT2 exhibited similar average values of 784 and 668 mg C m
2 d
1, respectively. The average production rate of WT3 (1184 mg C m
2 d
1) was 1.5–1.75 times higher than the other Western Grid transects, yet rates varied from as low as 150 up to 2505 mg C m
2 d
1. At the subset of stations where it was measured, size-fractionated productivity of surface waters (Fig. 5) showed a distribution similar to that of the size-fractionated chl a biomass at 20 m (Fig. 3). The microplankton contributed 40–60% of total productivity along the ELT (Fig. 5a). In comparison, all size fractions contributed equally to total production along the WLT with the exceptions of stations 5 and 6 where production (>80%) was dominated by microplankton (Fig. 5b). Chlorophyll-specific rates of photosynthesis (Pb) for the algal communities of the WLT and the ELT were examined and found to be relatively low and variable along both transects. Values along the WLT ranged from 3.5 to 15.0 mg C (mg chl

a)
1 d–1 and from 2.9 to 7.1 mg C (mg chl a)
1 d–1 along the ELT (Table 2). However, average values of Pb from the WLT were significantly greater than those of the ELT (t-test, Po0.05) with means of 9.44 and 4.79 mg C (mg chl a)
1 d–1, respectively. 3.4. Nutrient distribution To assess our nutrient data, we subdivided it according to the different water masses/types present within our study site (cf. Brandon et al., 2000; Meredith et al., in review) and the phytoplankton size fractions they contained (Fig. 3). Three distinct groups were considered: group 1 comprised waters lying between the PF and the SACCF and generally contained a high proportion of microplankton, group 2 was a subset of group 1 and comprised mainly on-shelf waters lying to the north of the island and was dominated by pico and nanoplankton, and group 3 comprised waters lying between the SACCF and the Southern Antarctic Circumpolar Current Boundary (SACCB) dominated by microplankton along the ELT (Fig. 6 and 9a). On- and off-shelf pycnocline values were similar in groups 1 & 2 (B24, 1.8, 29 mmol m
3 NO3, PO4 and Si(OH)4, respectively), and generally higher in group 3 (26, 1.9 and 41 mmol m
3 NO3, PO4 and Si(OH)4, respectively). Near-surface values were markedly different with the lowest values evident in group 1 (16, 1.0 and 10 mmol m
3 NO3, PO4 and Si(OH)4, respectively), higher values in group 2 (20, 1.3 and 20 mmol m
3 NO3, PO4 and Si(OH)4, respectively), and the highest values in group 3 (23, 1.5 and 28 mmol m
3 NO3, PO4 and Si(OH)4, respectively. Conversely, pycnocline NH4 concentrations decreased from group 1 to 2 to 3 (2.9, 2.4 and 1.8 mmol m
3 NH4, respectively), with similar values for near-surface waters in groups 1 and 2 (1.3 mmol m
3) and lower values in group 3 (0.8 mmol m
3). 3.5. Irradiance and euphotic depth Daily irradiance (PAR) during the cruise ranged from 9 to 67 mol photons m
2 d
1 (mean 37; Fig. 7) and day-length was around 16 h. K0 was low

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Fig. 3. Size-fractionated chl a at 20 m depths from (a) the Eastern Long Transect, (b) the Western Long Transect, (c–e) the Western Grid, Transect 3, 2 and 1, respectively. Black bars represent the % contribution to the total biomass of microplankton (>12 mm), light grey bars represent the nanoplankton (2–12 mm) and dark grey bars the picoplankton (0.2–2 mm).

along the ELT (mean 0.08 m
1) and higher along all of the western transects (mean 0.17 m
1). Correspondingly, the average euphotic depth

(57 m) of the ELT was almost twice as deep as the western transects (average Ze of 24–31 m; Table 1).

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Fig. 5. Surface, size-fractionated primary production from the (a) Eastern Long Transect, and (b) Western Long Transect. Black bars represent the % contribution to total production of microplankton (>12 mm), light grey bars the nanoplankton (2– 12 mm) and dark grey bars the picoplankton (0.2–2 mm).

Fig. 4. Column-integrated primary productivity (mg C m
2 d
1) along (a) the Eastern and Western Long Transects (black and white bars, respectively), (b) the Western Grid, Transect 3, 2 and 1 (black grey and white bars, respectively).

4. Discussion Open-ocean phytoplankton blooms in the Southern Ocean have been associated with fronts (e.g. Boyd et al., 1995; Barth et al., 2001), islands (e.g. Blain et al., 2001), and bathymetric features (e.g. Sullivan et al., 1993; Moore et al., 1999). Substantial open-ocean phytoplankton blooms at South Georgia have been documented previously and may be linked by all three of these factors (cf. Whitehouse et al., 1996a, b; Atkinson et al., 2001). Recent SeaWiFS-based studies have indicated the true temporal and spatial scales of the South Georgia blooms that may constitute the largest phytoplankton carbon drawdown in the Atlantic sector of the Southern Ocean (Korb et al., in

press). During the austral summer of 2002, SeaWiFS images revealed a dense and persistent bloom to the northwest of South Georgia situated over deep water (Whitehouse et al., in review). This bloom was of a far greater magnitude than any other open-ocean phytoplankton event in the Atlantic sector of the Southern Ocean throughout the austral summer of 2001–2002. Mean SeaWiFSderived chl a values in a 135  135 km area centred over the middle of our north-western study site (53.14 S, 37.79 W) were 5.84, 5.86 and 1.86 mg m
3, respectively for January, February and March 2002 with concentrations of 12– 18 mg m
3 evident on occasions. In contrast, mean values for a similar sized area centred over our northeastern sampling area (53.48 S, 33.90 W) were 0.55, 0.81 and 0.62 mg chl a m
3 for January, February and March, respectively. Here, we explore some of the key factors that contributed to such differential phytoplankton biomass between the north-eastern and north-western waters of South Georgia, as well as examining the contribution of the islands’ southern shelf to the Georgia Basin bloom. 4.1. The Eastern Survey region Although the ELT was the site of least phytoplankton growth and accumulation during

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Table 2 Column-integrated chl a, column-integrated primary production and chlorophyll-specific rates of photosynthesis (Pb) for the Eastern Long Transect (ELT) and Western Long Transect (WLT). Data are shown only for stations where actual 24-h incubations were performed Region

Station number

Chl a (mg chl m
2)

Primary production (mg C m
2 d
1)

Pb (mg C (mg chl a)
1 d
1)

WLT WLT WLT WLT WLT WLT WLT WLT

1 5 6 7 11 12 16 17

103 146 139 163 105 41 50 18

361 670 1325 957 906 391 751 221

3.5 4.6 9.5 5.9 8.6 9.6 15.0 12.5

ELT ELT ELT ELT ELT ELT

5 6 11 12 16 17

22 40 43 24 22 21

156 135 174 69 108 139

7.1 3.4 4.0 2.9 4.9 6.5

our survey, chl a values, primary production measurements, near-surface nutrient depletion and pycnocline NH4-N accumulation all indicated biological activity had occurred, albeit at a relatively modest rate. A SeaWiFS image for November 2001 (Fig. 8) shows an area of elevated chl a biomass in the central Scotia Sea upstream of the ELT. By December 2001, chl a concentrations in this southerly bloom had decreased but the bloom appeared to have moved downstream into the north-eastern survey area. This moderate bloom was presumably responsible for the nutrient depletions observed during the January/February 2002 cruise. Generally, in the Southern Ocean, regions of elevated production are dominated by large-celled phytoplankton, in particular the diatoms, whereas small nano- and picoplankton dominate the phytoplankton community when chl a biomass is low (Gall et al., 2001; Froneman et al., 2001; Landry et al., 2002). Although near-surface chl a was low throughout the ELT (o0.3 mg m
3), concentrations at depths of 20 m were higher, and this is the depth at which size-fractionated chl a samples were obtained. At offshore stations (5– 17), where nutrient depletions indicated growth prior to our cruise, B50% of the total biomass and production was attributable to microplankton (Fig. 3a).

It is unlikely that the light environment could account for the low water column chl a values in the waters along the ELT; average light attenuation was 0.08 m
1 and UML depths never exceeded critical depths. Indeed, a slight subsurface bloom was evident between stations 6 and 11 (up to 1.3 mg m
3 at B40 m) and between stations 13 and 15 (up to 0.7 mg m
3 at B50 m). HolmHansen et al. (1994) speculated that subsurface chl a maxima may occur in pelagic waters whereby winter atmospheric Fe enrichment of the surface water is followed by early summer phytoplankton stripping of the near-surface Fe stock. By midsummer, subsurface phytoplankton continues to grow on a remnant of Fe sitting around the characteristic Antarctic Surface Water temperature minimum just below the UML. From our water bottle data the temperature minimum between stations 6 and 15 along the ELT was at 105 m with a median temperature of 0 C. This is a depth similar to that documented by HolmHansen et al. (1994), although the temperature is considerably higher and our chl a maximum is shallower. Nevertheless, it is conceivable that the subsurface chl a maximum along our ELT may also have resulted from Fe depletion in surface waters as suggested by Holm-Hansen et al. (1994). We also cannot rule out the possibility that

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Fig. 6. Near surface (0–50 m, open bars) and pycnocline (60–80 m, closed bars) nutrient values for three South Georgia water groups.

photoinhibition may account for this subsurface chl a maximum. However, such effects are generally limited to the upper 10–20 m of the water column in Antarctic waters (Tilzer et al., 1985; Holm-Hansen et al., 1977). 4.2. The Western Survey region

Fig. 7. Variation in ship-monitored daily irradiance (PAR) to the north of South Georgia during 8th January–4th February 2002. The dotted line indicates mean irradiance.

The main source region for the extensive bloom that occupied the Georgia Basin during the austral summer of 2002 appeared to be the NWGR. An area of anticyclonic circulation was evident over

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Fig. 8. Monthly SeaWiFS images of the South Georgia sector of the Southern Ocean during November and December 2001. The white lines indicate the 500 and 2500 m isobaths, respectively. Grey patches indicate cloud cover. The average positions and direction of the Polar Front (PF), the Southern Antarctic Circumpolar Current Front (SACCF) and the Southern Antarctic Circumpolar Current Boundary (SACCB) are shown as red lines. Note the difference between the average position of the SACCF and the actual position of the SACCF illustrated in Fig. 9a. Station positions are indicated as black dots.

the NWGR from 12th November 2001 to 8th January 2002, and Whitehouse et al. (in review) speculated that eddy formation and subsequent interactions between the anticyclonic circulation and the island’s shelf would have provided the means of delivering sedimentary micronutrients such as Fe into the surface waters. During the present study, the WLT crossed the NWGR at stations 5–11. Hydrographic data along the WLT showed that water column stability within the anticyclonic circulation reduced surface mixing, and the retention of water near the surface resulted in elevated temperatures >1 C higher than along the ELT and B1 C higher than in the western grid (Whitehouse et al., in review). At stations over the NWGR, light conditions were favourable for growth as critical depths did not exceed the UML. Microplankton were particularly prevalent (75–90% of total) at the periphery of

the NWGR (stations 5, 6, 9 and 10 along the WLT, Fig. 3), primary production was high (median >0.9 g C m
2 d
1) compared with waters either side or upstream (Fig. 4), and NO3-N: PO4-P depletion ratios (Fig. 9b) indicated that phytoplankton were readily using NO3-N. Furthermore, water downstream from the NWGR in the region of transect WT3 showed similar phytoplankton growth characteristics, i.e. abundant microplankton, high production rates, and extensive NO3-N use. Therefore, it seems likely that low-production water containing a high proportion of microplankton entered the South Georgia system from the east. The microplankton benefited from the improved growing conditions over the NWGR and were exported south-westwards through the northern portion of our Western Grid and then into the Georgia Basin.

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The WLT and WT3 were characterised by mixed-layer depths that exceeded euphotic depths, and could potentially shift the carbon balance to increased respiration as phytoplankton spent more time in the aphotic zone (Tilzer and Dubinsky, 1987). Yet WLT and WT3 waters contained some of the highest concentrations of chl a and production rates of our study. However, it is possible that high light attenuation (an average Kd of 0.1770.05 m
1) due to self-shading in western waters may have resulted in light limitation of algal production to some extent. Whilst we reported a production rate of 2.50 g C m
2 d
1 at one station, a value similar to the highest rate previously measured for this region of South Georgia (2.48 g C m
2 d
1, Gilplin et al., 2002), the average rate for all Western Stations was only 0.86 g C m
2 d
1. 4.3. NO3-N:PO4-P depletion ratios and nutrient stress

Fig. 9. (a) The distribution of three water groupings near South Georgia. Note how the actual position of the SACCF varies from the average position shown in Fig. 8. (b) The relationship between surface (0–50 m) NO3-N:PO4-P depletion and surface NO3-N depletion. (c) The relationship between apparent surface NH4-N use (as a % of total N use) compared with surface NO3-N depletion.

Shipboard (e.g. Martin et al., 1990; de Baar et al., 1990; Scharek et al., 1997) and in situ (summarised in Boyd and Law, 2001) Fe-enrichment experiments have provided convincing evidence that phytoplankton growth throughout much of the Southern Ocean is limited by Fe availability. There are no published dissolved Fe data for the waters around South Georgia. However, it seems likely that the island’s downstream waters in the Georgia Basin and beyond receive an input of Fe via island runoff or from shelf sediments (cf. Atkinson et al., 2001; Whitehouse et al., in review). Assimilation rates (Pb), or integrated primary production normalised to chl a (mg C (mg chl a)
1 d–1), may be an indicator of the specific productivity of phytoplankton communities and their adaptation to the ambient environment. For stations where 24-h 14C incubations were carried out, significant differences (t-test, Po0.05) in assimilation rates were observed between the ELT and the WLT (mean Pb=4.79 and 9.44 mg C (mg chl a)
1 d–1, respectively). While we are uncertain as to what the limiting factor was, and the light environment appeared favourable for growth, here we assess the likely role of Fe

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availability by examining NO3-N:PO4-P deficits measured at all of the cruise’s stations. To utilise NO3-N, phytoplankton must reduce this substrate to NH4-N via the nitrate reductase process, which is dependent upon micronutrients such as Fe. Pycnocline NH4-N concentrations and near-surface NO3-N levels were relatively high throughout much of the present survey. Although potentially there were two nitrogen substrates available for phytoplankton use, the interactions involved with the uptake of these two substrates are complex. The suppression of NO3-N utilisation in the presence of NH4-N concentrations >1 mmol m
3 is now considered to be an over simplification (cf. Dortch, 1990). Armstrong (1999) modelled the effects of Fe, light and NH4N co-limitation on NO3-N utilisation by marine phytoplankton and highlighted the importance of Fe. In the open ocean, remote from Fe sources, Mengesha et al. (1998) demonstrated a rapid decrease in f-ratio ([NO3-N uptake]/[NO3-N uptake+NH4-N uptake]) with a relatively small increase in NH4-N availability. A far more gradual decrease in f-ratio has recently been documented for phytoplankton to the north and downstream of South Georgia that indicated a greater ability to utilise NO3-N, and was likely due to a greater availability of Fe (Whitehouse et al., in review; BAS, unpublished data). The different water masses/types around South Georgia during the present study showed very different depletion characteristics. Assuming that South Georgia phytoplankton N:P uptake complied with Redfield ratio, we considered the NO3N:PO4-P deficit ratios against NO3-N deficit for all of the stations occupied (Fig. 9a, b). The groupings are the same as detailed in the above Results, nutrient distribution section. There were significant differences between groups 1 and 3 while group 2 occupied the mid-region between them. Group 3 had the lowest depletion of NO3-N (0.7– 4.6 mmol m
3), significantly lower (Po0.001) than that for group 1 (3.7–11.6 mmol m
3), that was 2–3 times greater. Group 2 NO3-N depletion varied from 2.1 to 7.0 mmol m
3. Similarly, with the exception of one station, there was a significant difference (Po0.001) between the group 3 NO3N:PO4-P depletion ratio (1.9–8.4:1), compared

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with that for group 1 (8.3–12:1), again with the exception of one outlier. Once more, group 2 occupied a mid-range of 6.3–10.9:1, also with one station lying outside of this range. The outliers (circled in Fig. 9) generally occurred at the interface between water types where deficit calculations were subject to greater error. We did not measure NH4-N uptake rate so we have calculated an index of use by subtracting near-surface values (within the depth range of phytoplankton use), from pycnocline values (within the predominant depth range of microbial remineralisation). Previous uptake measurements have shown broadly similar pycnocline and nearsurface values associated with little phytoplankton growth and a large differential where growth was rapid. Although not ideal, this calculation provides a relative index of NH4-N turnover. These values, expressed as a percentage of ‘‘total N use’’ (NH4-N turnover + NO3-N deficit) are illustrated in Fig. 9c. Given the errors inherent in using this method to assess NH4-N use, we do not propose to use it to make any definitive statements. Although there was variability in NH4-N use within each group, there were highly significant differences in NH4-N use between all three groups (Po0.001). Our data would suggest that reduced nitrogen such as NH4-N contributed a far greater proportion of the total nitrogen used by phytoplankton at stations in group 3 compared with group 1. 4.4. South Georgia’s northern shelf Phytoplankton at shelf stations were different from all other stations sampled during JR70 in having a paucity of microplankton; the southern ends of the WLT and ELT were dominated by picoplankton, and stations further west (WT3) were dominated by nanoplankton. The NO3N:PO4-P depletion ratios for this shelf group indicated a moderate use of both NO3-N and NH4-N, (Fig. 9). During the present cruise the eastern shelf waters had an exceedingly deep UML depth and water column stability was low (Whitehouse et al., in review) which may result in unfavourable growing conditions over the long term (i.e. weeks to months). The island’s shelf is

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deep (generally>500 m), and sedimentary micronutrients such as Fe may not be uniformly transported between the shelf and the frequently well defined UML over much of the northern shelf. In Fe-deplete waters of the Southern Ocean, the algal community is dominated by pico- and nanoplankton (Helbling et al., 1991) and we speculate that low Fe concentrations or deep mixing may be responsible for the scarcity of microplankton at eastern shelf station. Zooplankton biomass is greater at South Georgia than elsewhere in the Southern Ocean (see Atkinson et al., 2001). Krill make up about half of the biomass at the island and typically have a strong association with the shelf regions, occurring in significantly higher abundances between depths of 250–1000 m (Trathan et al., 2003). During JR70, krill were persistently found over the shelf in the area of WT3 and the southern ends of the WLT and the shelf break, WT2 (Ward et al., in review). By feeding more efficiently on large diatoms (Meyer and El-Sayed, 1983; Quetin and Ross, 1985) krill are reported to cause flagellates to dominate (Grane! lli et al., 1993; Kopczynska, 1992). While we have no direct evidence to confirm this, it is possible that during the present study, the absence of microplankton over the western shelf regions may be due to size selective grazing by krill which substantially altered the phytoplankton size composition.

5. Conclusions Whilst many areas of the open Southern Ocean are characterised by HNLC conditions, the area downstream of South Georgia often supports intense and long-lasting blooms. During cruise JR70, a large bloom was evidenced in the Georgia Basin and was characterised by high chl a biomass, high rates of primary production and substantial nutrient depletion. In contrast, water to the northeast of the island was more typical of HNLC conditions with low chl a biomass (much of it in the subsurface) and low rates of production, but with nutrient depletion suggesting a modest rate of phytoplankton growth had occurred at some point. During the present cruise, this production differential could not be attributed to irradiance regimes. Instead, phytoplankton distribution and abundance were more likely the result of differential Fe availability in the different water masses made available by mesoscale features such as the anticyclonic circulation over the NWGR. However, to confirm this hypothesis, a comprehensive study of the distribution of Fe in the waters around South Georgia and the mechanisms by which it is made available to phytoplankton is required.

Acknowledgements 4.5. Blooms from the south-western shelf region SeaWiFS imagery for the two months preceding our field measurements (November and December 2001, Fig. 8) clearly shows there to be a substantial bloom on South Georgia’s south-western shelf. During December, blooms from the southern shelf and from the NWGR appear to have converged to the west of the island in a large bloom that occupied the entire Georgia Basin. Without field data for this same sampling period, we speculate that significant upwelling of micronutrients must also occur to the southwest of the island, possibly due to the occurrence of anticyclonic eddies in the area as a means of delivering sedimentary Fe to the surface waters.

This work is a component of the British Antarctic Survey’s DYNAmics and Management of Ocean Ecosystems (DYNAMOE) programme. We would like to thank all the scientists, officers and crew aboard the RRS James Clark Ross for their help and hard work during cruise JR70. Special thanks to Min Gordon and Dr. Peter Enderlein for their assistance with chl a and nutrient analysis. We are grateful to the SeaWiFS Project and the Goddard Earth Sciences Data and Information Services Center for the production and distribution of the SeaWiFS data, respectively. We would like to thank Osmund HolmHansen and two anonymous referees for comments and suggestions on an earlier version of the manuscript.

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