Heme b concentrations in the SS varied from 0.6 to 21 pM with an average of 1.9 ± 1.7 pM and were thus higher than those determined in the IB (Figure 5). Particulate organic carbon and PON were not determined for the SS samples. Phytoplankton counts were undertaken at a depth of 20 m at each station [Korb et al., 2010] and heme b concentrations were therefore compared to the calculated phytoplankton carbon biomass (phyC). However, it should be noted that these calculations do not include the biomass from nanoeukaryotes or picoeukaryotes and are thus likely to overestimate actual heme b:phyC ratios. Results of the comparison are reported in Table 4. Heme b to phyC ratios observed at SSC2, SSC4, and SSP3 were similar to heme b:C ratios observed in nutrient replete phytoplankton grown in the laboratory [Honey et al., 2013]. Bioassay experiments carried out close to SSC2 and SSP3 indicated that phytoplankton were not iron limited [Hinz et al., 2012]. The low heme b:phyC ratio observed at stations SSP24, SSP28, and SSR3 to the northwest of South Georgia and in the SS indicates that the phytoplankton community was growing under low-nutrient conditions at these stations. Iron bioassay experiments near station SSP24 indicated that the phytoplankton community was iron limited in this area [Hinz et al., 2012]. The estimates of heme b:phyC determined for stations in the SS are broadly compatible with changes in Fv/Fm recorded in the region (Table 4). Fv/Fm was highest around station SSR3, where the highest heme b:phyC ratio was observed. Furthermore, south of South Georgia at stations SSP24 and SSP28, Fv/Fm was also observed to be lower (<0.3, Table 4), consistent with the depleted heme b:phyC ratios. However, at other stations (SSC2, SSC4, and SSP3) with similar Fv/Fm values, we observed a wide variability in heme b:phyC ratios. This could have been a result of underestimation of the heme b:phyC ratio due to omission of C from nanoeukaryotes and picoeukaryotes, likely to be especially important at SSC2 and SSC4, as these areas were dominated by small phytoplankton species [Korb et al., 2011]. In addition, comparison between Fv/Fm and heme b:phyC in the region is also likely influenced by changes in the community composition [Suggett et al., 2009] as the dominant taxanomic groups varied from cryptophytes at station SSC2, dinoflagellates at SSC4, and diatoms at SSP24, SSP28, SSP3, and SSR3 [Korb et al., 2010, 2011]. It was notable that chl a:phyC ratios at stations SSP24, SSP28, and SSR3 were close to the average observed for all the stations (252, 265, and 590 μmol mol−1, respectively), showing that while heme b was depleted relative to biomass at these stations, chl a was not. Integrated mixed-layer depth values for chl a and heme b calculated for the SS are given in Table 2. Chlorophyll a to heme b ratios ranged between 80 and 1940, with a mean value of 533 (Figure 5). As with the IB, no significant correlation between heme b and chl a was observed (Table 3). Integrated chl a:heme b ratios were below 500 north of the South Orkney Islands and increased going north across the SS to a value of 528 just south of South Georgia. West of South Georgia mixed-layer chl a:heme b ratios decreased to 240. This region is strongly influenced by iron inputs from the South Georgia Ridge and the station sampled was close to an area reported to be iron replete at the time of sampling (Figure 6) [Hinz et al., 2012]. Northwest of South Georgia, however, we observed high chl a:heme b ratios (> 1400), combined with low heme b:phyC ratios. This station (SSR3) was characterized by high chl a (~ 5 μg L−1), complete drawdown of Si (< 0.15 μM) and relatively depleted N concentrations (~ 14 μM). The phytoplankton community at both stations was dominated by diatoms [Korb et al., 2010, 2011]; however, Fv/Fm was lower at SSR3 than at SSP3 (Table 4). Dissolved iron concentrations were not recorded at the station itself; however, comparison of dFe and nutrient data showed that the closest underway dFe samples (situated 15 and 27 km to the southwest of the station; upstream in the ACC) had dFe concentrations of 0.16 ± 0.01 nM and 0.61 ± 0.1 nM, respectively (Figure 6). The phytoplankton community at SSR3 was dominated by centric diatoms over 10 μm in size [Korb et al., 2010] which are thought to have relatively high iron requirements [Ho et al., 2003; Sunda and Huntsman, 1995]. Therefore, it is possible that the phytoplankton at this station have exhausted the nutrient supply to the extent that they have reduced their hemoprotein content.