The mean TChl a profiles of the total fraction (TChl aTOTAL) and the size classes (TChl aPICO, TChl aNANO, TChl aMICRO) are presented in Fig. 2a–e for each hydrological zone, and in Fig. A1 (Appendix A) for all stations. Across all zones, the depth of SCM (ZSCM) of TChl aTOTALoccurred between 60 and 100 m and was usually below the ZSML, except in the KER region where the SCM was located above the ZSML. For all zones, the ZSCM was similar for the total fraction and the size classes, except in the KER region where the ZSCM for TChl aMICRO peaked around 40 m while the ZSCM of TChl aNANO was deeper (between 60 and 80 m). Despite vertical variations in TChl a, the TChl a size structure – i.e. the relative contributions of each size class to TChl aTOTAL – remained unchanged with depth for all zones (not shown).

As for TChl a, the mean NPP profiles of the total fraction (NPP_TOTAL) and the size classes (NPP_PICO, NPP_NANO, NPP_MICRO) are presented in Fig. 2f–j for each zone and displayed in Fig. A2 (Appendix A) for all stations. Here, the subsurface NPP maximums were not as marked as the SCM. Moreover, the subsurface NPP maximums coincided with SCM in the STZ and PFZ. Contrary to TChl a, the NPP size structure – i.e. the relative contributions of each size class to NPP_TOTAL – was heterogeneous with depth (not shown). In the STZ, surface NPP was evenly distributed in each size class, while subsurface NPP maximum was dominated by nanophytoplankton at O2 and O3, and by microphytoplankton at O16 (Fig. A2). In the SAZ, PFZ and AZ, surface NPP was mainly supported by nano- and microphytoplankton, while subsurface NPP maximum was dominated by microphytoplankton. In the KER region, NPP was mainly dominated by microphytoplankton. By normalizing NPP to TChl a, we calculated NPP^TChl a (in mgCmgTChla-1d-1) which can reflect photosynthesis efficiency under given environmental conditions (e.g. light/nutrient availability; Cermeño et al., 2005). NPPTOTALTChla was maximal in the first 50 m at all zones – except at O3 and O6 where it peaked below the SML – and decreased with depth (Fig. 2k–o; Fig. A3). Interestingly when considering the size classes, NPPMICROTChla often peaked at depth across all zones and coincided with minima in NPPNANOTChla and NPPPICOTChla, except in the STZ.

Integrated TChl aTOTAL over the ZEL0.01 % ranged from 25.1 mgm-2 at O2 (STZ) to 65.7 mgm-2 at A3 (KER) (Fig. 3a). Stations in the KER region displayed the highest TChl aTOTAL (55.7 ± 11.6 mgm-2), while the remaining stations exhibited lower TChl aTOTAL (32.3 ± 5.3 mgm-2). Across the study area, nano- and microphytoplankton contributed the most to TChl aTOTAL and represented respectively 40 % ± 11 % and 37 % ± 18 % of TChl aTOTAL (Table S3 in the Supplement). It is noteworthy that the picophytoplankton relative contribution to TChl aTOTAL (23 % ± 16 %) was significantly lower than those of nano- and microphytoplankton (p<0.05). In the STZ and SAZ, integrated TChl aTOTAL was similar and dominated by both pico- and nanophytoplankton, which contributed to 42 % ± 8 % and 39 % ± 9 % respectively. In the offshore PFZ and AZ, integrated TChl aTOTAL was similar and dominated by the biomass of nano- (46 % ± 12 %) and microphytoplankton (40 % ± 14 %). In the KER region, microphytoplankton biomass contributed the most to integrated TChl aTOTAL (58 % ± 1 %). Furthermore, the integrated TChl a size structure also varied within specific hydrographic zones. For instance, in the PFZ, integrated TChl a at O7 was dominated by nanophytoplankton (61 %) while the main contributors at O6 and O9 were nano- (48 %) and micro- (40 %) (Table S3).

The lowest integrated NPP_TOTAL over the ZEL0.01 % were observed at O11 (AZ), O16 and O2 (STZ) (553 ± 14 mgCm-2d-1), while the highest values were recorded at A3 (2120 mgCm-2d-1) and O12 (3605 mgCm-2d-1) in KER (Fig. 3b). Such differences highlighted the greater variability of NPP_TOTAL during SOCARB compared to TChl aTOTAL, with a factor of 6.7 for NPP_TOTAL versus 2.7 for TChl aTOTAL. In particular in the STZ, integrated NPP_TOTAL at O3 was twice higher than at O2 and O16 (Fig. 3b). Across the study area, microphytoplankton was the main contributor to NPP_TOTAL (56 % ± 21 %), followed by nano- (35 % ± 17 %) and picophytoplankton (9 % ± 7 %) (Table S3). The relative contributions of each size class to NPP_TOTAL were significantly different from each other (p<0.05). The NPP size structure remained homogeneous within the PFZ, AZ and KER region, where microphytoplankton contributed the most to integrated NPP_TOTAL (66 % ± 13 %), except at O11 where nano- and microphytoplankton accounted respectively for 50 % and 43 %. The STZ was the sole zone with notable heterogeneity: while nanophytoplankton dominated at the western stations O2 and O3 (mean contribution of 63 %), microphytoplankton was the main contributor to integrated NPP_TOTAL at the eastern station O16 (60 %).

Across the global study area, correlations between the total fraction and each size class were significant for both TChl a and NPP (Table 3). TChl aTOTAL exhibited the strongest correlations with TChl aMICRO (ρ=0.87) and TChl aNANO (ρ=0.80), while NPP_TOTAL was most strongly correlated with NPP_MICRO (ρ=0.86). When comparing the hydrographic zones, both TChl a and NPP correlations revealed a clear spatial variability. In the STZ, the total fraction displayed the highest correlations with pico- and nanophytoplankton, while in the PFZ, AZ and KER region it correlated most strongly with nano- and microphytoplankton. NPP_TOTAL was significantly correlated with TChl a for each size class over the global study area, with the strongest correlations found with TChl aNANO (ρ=0.76) and TChl aMICRO (ρ=0.70). When comparing the hydrographic zones, TChl aPICO had a significant impact on NPP_TOTAL only in the STZ, while TChl aNANO and TChl aMICRO had a significant impact on NPP_TOTAL in each hydrographic zone. The SAZ was the sole exception, as no significative correlations were found between NPP_TOTAL and TChl a in any size class, likely due to the small number of samples.

Over the study area, the main contributors to integrated TChl aTOTAL were microphytoplankton diatoms (20 % ± 18 %) followed by nanophytoplankton haptophytes (19 % ± 7 %) and microphytoplankton haptophytes (14 % ± 11 %; Table S5 in the Supplement). Focusing on the contributions of phytoplankton chemotaxonomic groups biomass to integrated TChl a for each size class (Fig. 4a), haptophytes stood out in the three size classes, constituting the dominant and ubiquitous group among all size classes across the study area (Fig. 4a). Focusing on the contributions of each size class to the biomass of the chemotaxonomic groups averaged over the study area (Fig. 4b), each chemotaxonomic group was distributed among each size class with contrasting relative contributions. Diatoms biomass was mostly associated with the microphytoplankton, accounting for 67 % ± 21 % of total diatom biomass. Haptophytes, cryptophytes and chlorophytes biomass were mostly found in the nanophytoplankton, while dinoflagellates biomass was distributed evenly and almost exclusively in the nano- and microphytoplankton (Fig. 4b). Pelagophytes biomass was mainly found in the picophytoplankton, yet nano- and microphytoplankton contributions to total pelagophytes biomass remained notable. As expected, cyanobacteria (Prochlorococcus and Synechococcus) biomass was mainly distributed in the picophytoplankton, however a small fraction (<3.5 %) was also detected in the nano- and microphytoplankton biomass (Fig. 4a and b).

In the STZ, where integrated TChl aTOTAL was dominated by pico- (43 % ± 10 %) and nanophytoplankton (40 % ± 11 %), TChl aPICO was dominated by cyanobacteria (70 % ± 13 %) whereas TChl aNANO was mainly sustained by haptophytes (43 % ± 4 %) and chlorophytes (20 % ± 9 %) (Fig. 5e and f). In the SAZ, despite similar TChl a size structure compared to STZ, TChl aPICO was mainly driven by haptophytes (32 %) and pelagophytes (30 %) while TChl aNANO was mainly sustained by chlorophytes (43 %) and haptophtyes (30 %). In the offshore PFZ and AZ where integrated TChl aTOTAL was dominated by nano- (46 % ± 12 %) and microphytoplankton (40 % ± 14 %), TChl aNANO was firstly sustained by haptophytes (58 % ± 5 %) followed by diatoms (19 % ± 8 %), whereas TChl aMICRO was firstly driven by diatoms (55 % ± 19 %) followed by haptophytes (29 % ± 10 %) (Fig. 5d and e). In the KER region where TChl aTOTAL was dominated by microphytoplankton (58 % ± 1 %), TChl aMICRO was dominated by diatoms and haptophytes whose relative contributions were respectively 54 % ± 9 % and 35 % ± 2 % (Fig. 5d).

The size structure of phytoplankton chemotaxonomic groups biomass shifted within each size class across the study area. The microphytoplankton community shifted from haptophyte dominance in the STZ (48 % ± 9 % of TChl aMICRO) towards diatom dominance in the PFZ, AZ and KER (54 % ± 15 %) (Fig. 5d). Within the nanophytoplankton, chlorophytes were the secondary contributors in the STZ (20 % ± 9 % of TChl aNANO), but were replaced by diatoms in the PFZ, AZ and KER region (21 % ± 7 %) (Fig. 5e). The picophytoplankton community shifted from cyanobacteria dominance in the STZ (70 % ± 13 % of TChl aPICO) to haptophyte dominance in the PFZ and AZ (50 % ± 15 %) (Fig. 5f). This shift across the study area was also observed for the total fraction (Fig. S4 in the Supplement). Indeed, the SAZ acted as a “boundary zone” within the study area, delineating distinct community structures. North of the SAZ, the community in the STZ appeared relatively diversified, with four groups (cyanobacteria, haptophytes, chlorophytes and pelagophytes) each contributing more that 10 % to TChl aTOTAL. In contrast, south of the SAZ, the community in the PFZ, AZ and KER region appeared relatively less diversified, with only two groups (diatoms and haptophytes) contributing more than 10 % to TChl aTOTAL (Fig. S4b). In the SAZ, the community at O14 was relatively diversified, with four groups (haptophytes, chlorophytes, pelagophytes and diatoms) each contributing more than 10 % to TChl aTOTAL, alongside a marked increase in diatom biomass and a concurrent decline in cyanobacteria.

In all zones within the study area, except in the STZ, NPPMICROTChla peaked at depth and coincided with minima in NPPNANOTChla and NPPPICOTChla (Fig. 2k–o). These maximums could reflect the adaptability of microphytoplankton in low-light environments to take advantage from the nutrients diapycnal diffusion (Tagliabue et al., 2014). At O6, O9 (PFZ) and all stations in the AZ and KER, diatoms were the main contributor to TChl aMICRO at these NPP^TChl a maximums (data not shown). Large diatoms are known to thrive in such environmental niches thanks to their high growth efficiency under low-light conditions (Fisher and Halsey, 2016), their ability to regulate buoyancy (Villareal et al., 1996) and to exploit nutrient pulses through enhanced uptake and storage (Kemp and Villareal, 2013). However, haptophytes were the main contributor to TChl aMICRO at O14 (SAZ) and O7 (PFZ), where NPPMICROTChla peaked. For these stations, several hypotheses could explain our results. First, haptophytes have been shown to produce transparent exopolymer particles and form microphytoplankton size aggregates (Riebesell et al., 1995; Leblanc et al., 2009). Second, some Phaeocystis species such as P. globusa or P. antarctica are haptophytes known to form microphytoplankton size colonies from nano- and picophytoplankton size single cells, in response to environmental factors such as high irradiance and iron repletion (Feng et al., 2010; Bender et al., 2018), grazing (Long et al., 2007) or NOx limitation (Riegman et al., 1992). We rule out the latter hypothesis for explaining colonies formation at O14 and O7, as they featured HN-LSi-LC conditions. Further studies are needed to evaluate the recurrent or exceptional aspect of this outstanding feature and the preceding hypotheses.

Across the study area, microphytoplankton was the main contributor of NPP_TOTAL (56 % ± 12 %) while the main contributors to TChl aTOTAL were nano- (40 % ± 11 %) and micro- (37 % ± 18 %). At the scale of the study aree, our results in TChl a size structure are similar with previous studies conducted during the austral summer, in the South Indian Ocean using phytoplankton functional pigments approaches to the bulk fraction (Mishra et al., 2020), and in the South Atlantic and the Atlantic sector of the Southern Ocean from size-fractionation approaches (Froneman et al., 2001). However, the NPP size structure in our study differed from that of TChl a, while Froneman et al. (2001) reported that NPP displayed similar size structure with TChl a. This concerns especially the microphytoplankton, as its contribution to NPP_TOTAL in our study was superior than of TChl aTOTAL. This result suggested that microphytoplankton could be more efficient in CO2 fixation than the other size classes, which corroborate with previous studies from in situ experiments (Cermeño et al., 2005) and photophysiological models (Uitz et al., 2010). More specifically, the higher microphytoplankton photosynthetic efficiencies might be associated with a higher photochemical efficiency characteristic of certain taxonomic groups such as diatoms (Cermeño et al., 2005). We support this hypothesis as microphytoplankton diatoms formed the main contributor of bulk TChl a biomass in our study, by representing 20 % of TChl aTOTAL (Table S5).

Pigment chemotaxonomy has constituted a valuable tool for estimating the contribution of phytoplankton groups to TChl aTOTAL and analysing phytoplankton communities (Higgins et al., 2011; Kramer et al., 2024). Yet its application remained limited to the bulk fraction. Our study coupling pigment chemotaxonomy with size fractionation brings novel insights to dive deeper into the size structures of the phytoplankton community and of the phytoplankton chemotaxonomic groups. To our knowledge, Rodríguez et al. (2006) and Nunes et al. (2019) are the only two studies that have applied these approaches – with CHEMTAX algorithm and two size classes (<3 and >3 µm) – to investigate phytoplankton communities in the northwestern Iberian basin and in the surface Atlantic Ocean, respectively. Consistent with these studies, our results highlight that each phytoplankton chemotaxonomic group was not strictly associated with one specific size class (Fig. 4). These results underline the limitations of phytoplankton functional type approaches used to estimate phytoplankton size structure from bulk measurements (e.g. Uitz et al., 2006; Hirata et al., 2011). For instance, diatoms and dinoflagellates, which are commonly associated with the microphytoplankton in such approaches, were also distributed in the pico- and nanophytoplankton size classes (Fig. 4). This likely reflects the presence of nanoplanktonic dinoflagellate genera such as Amphidinium, Gymnodinium, Protoperidinium and Prorocentrum which have been reported in the Indian sector of the Southern Ocean (Georges et al., 2014; Hörstmann et al., 2021; Sreerag et al., 2023). Additionally, pico- and nanoplanktonic diatom genera such as Minidiscus and Fragilariopsis, as well as bolidophytes, a eukaryotic picophytoplankton group genetically very close to diatoms and sharing a similar pigments composition (Guillou et al., 1999), have previously been observed in the South Indian and South Atlantic Oceans (Hinz et al., 2012; Leblanc et al., 2018; Nunes et al., 2019; Deteix et al., 2024). Also, the presence of haptophytes in the picophytoplankton could be attributed to some coccolithophore genera such as small Emiliana, and to other genera such as Chrysochromulina and Phaeocystis (Poulton et al., 2007; Hinz et al., 2012; Patil et al., 2017; Hörstmann et al., 2021). Notably, Nunes et al. (2019) have shown that phytoplankton functional types approaches based on bulk measurements predicted a high contribution of nano- and microphytoplankton in the Atlantic sector of the Southern Ocean, while the size fractionation approach indicated the dominance of picophytoplankton.

Furthermore, our findings revealed that Prochlorococcus, Synechococcus and chlorophytes were mainly distributed in the picophytoplankton, but were also detected in the nano- and microphytoplankton (Fig. 4). This result, also reported in previous studies (Rodríguez et al., 2006; Nunes et al., 2019), can be explained by the size fractionation methodology, as the 3 and 20 µm pore sizes may retain a part of these organisms due to aggregation and/or adhesion. In addition, the attribution of pigments like zeaxanthin – associated with Synechococcus in this study – to larger size classes may be influenced by the presence of this pigment in nanophytoplankton (e.g. UCYN-B; UCYN-A in symbiosis within nanophytoplankton haptophytes) and microphytoplankton (e.g. Trichodesmium spp.; diatom diazotroph associations) previously detected in the STZ of the South Indian Ocean (Metzl et al., 2022; Chowdhury et al., 2024) or in some diatoms under high irradiance (Lohr and Wilhelm, 1999). Thus, our result underline the importance of interpreting phytoplankton size structure data in the context of methodological constraints. The presence of picophytoplankton groups in larger size classes and the dominance of haptophytes in the picophytoplankton underscore the need for complementary validation using microscopy, flow cytometry or molecular techniques.

The size structures of integrated biomass and primary production clearly shifted between the oligotrophic subtropical waters and the Southern Ocean waters, consistent with previous studies conducted in the Indian and Atlantic sectors of the Southern Ocean (Froneman et al., 2001; Mishra et al., 2017, 2020). Result from the multivariate analysis showed that potential temperature (theta) and NOx concentration (and DIP due to its strong correlation with NOx, see Sect. 2.6) were the major factors driving the spatial variability between the different zones (Fig. 6). Indeed, temperature and NOx concentration are recognized as key factors to shape phytoplankton biomass and productivity size structures, with picophytoplankton usually prevalent in warm and oligotrophic waters (Marañón, 2009; Hörstmann et al., 2021; Berthelot et al., 2025). In our study, the TChl a and NPP size structures in the PFZ, AZ and KER were mainly sustained by nano- (TChl a: 40 % ± 12 %; NPP: 30 % ± 12 %) and microphytoplankton (TChl a: 47 % ± 14 %; NPP: 66 % ± 13 %), which were consistent with previous studies conducted in the Atlantic and Indian sectors of the Southern Ocean, encompassing both HNLC and HN-LSi-LC low-productivity waters as well as high-productivity waters near the Crozet and Kerguelen Islands (Froneman et al., 2001, 2004; Seeyave et al., 2007; Uitz et al., 2009). By contrast, the TChl a size structure in the SAZ was mainly dominated by pico- (41 %) and nanophytoplankton (36 %), consistent with observations from other sectors of the SAZ in the Atlantic (Froneman et al., 2001) and western Pacific sectors (Boyd et al., 1999; McKay et al., 2005; Gutiérrez-Rodríguez et al., 2020). Similarly, the TChl a size structure in the STZ was mainly driven by the pico- (43 % ± 10 %) and nanophytoplankton (40 % ± 10 %). To our knowledge, there is no size-fractionated data for biomass and NPP in the literature in the STZ of the South Indian Ocean. Nevertheless, our results were similar to those of Froneman et al. (2001) in the South Atlantic STZ, which reported a TChl a size structure driven by pico- (49 % ± 10 %) and nanophytoplankton (39 % ± 6 %), but differed from other studies conducted in the northern and southern subtropical Atlantic, where picophytoplankton accounted for 60 %–75 % of TChl aTOTAL (Marañón et al., 2001; Morán et al., 2004). As temperature and NOx concentrations were similar between our study and the latter, the differences in TChl a size structure between the Atlantic and the Indian basins may be attributed to factors such as regional-scale hydrodynamics and/or atmospheric inputs (Marañón, 2009). Nevertheless, Zhang et al. (2012) reported a latitudinal variability of TChl a size structure in the western Pacific, with picophytoplankton being less dominant in subtropical than in tropical regions. For all that, extensive researches are needed in the South Indian Ocean to better understand the potential factors in shaping the size structures of phytoplankton biomass and productivity, especially in the STZ.

Phytoplankton chemotaxonomic groups biomass also varied in association with changes in TChl a size structure. Cyanobacteria, pelagophytes and chlorophytes mainly sustained picophytoplankton in the STZ – typical for LNLC areas with low mixing – while diatoms, haptophytes and dinoflagellates mostly sustained nano- and microphytoplankton in the PFZ, AZ and KER region – typical in areas where these opportunistic taxa are particularly well suited to take advantage of excess nutrient (Figs. 5 and 6) (Schlüter et al., 2011; Leblanc et al., 2018). Possible species that may account for much of the biomass of these phytoplankton chemotaxonomic groups include: Synechococcus and Prochlorococcus for cyanobacteria; Pelagomonas, Micromonas for pelagophytes; Chloroparvula, Chloropicon for chlorophytes; Chaetoceros, Corethron, Coscinodiscus, Eucampia, Fragilariopsis, Thalassiosira for diatoms; Gephyrocapsa, Chrysochromulina, Phaeocystis for haptophytes; and, Amphidinium, Gymnodinium, Protoperidinium, Prorocentrum for dinoflagellates (Armand et al., 2008; Lasbleiz et al., 2016; Patil et al., 2017; Irion et al., 2020; Hörstmann et al., 2021; Sreerag et al., 2023, 2025; Thyssen et al., 2024). Recent studies have nevertheless underlined that some eukaryotic picophytoplankton groups, such as prasinophytes – belonging to the green algae lineage within chlorophytes – can also benefit from enhanced nutrient conditions (e.g. ammonium; Irion et al., 2020) or deep-mixing and low-light regimes in HNLC open ocean waters (Gutiérrez-Rodríguez et al., 2023). Also, when focusing on the size structure of phytoplankton chemotaxonomic groups biomass, several common features were observed between Nunes et al. (2019) in the South Atlantic Ocean and our study in the South Indian Ocean, for similar latitudes. First, we observed that haptophytes was the main and ubiquitous group within each size class across the study area; a feature also observed by Nunes et al. (2019) across the Atlantic Ocean. Second, Nunes et al. (2019) reported in the subtropical and tropical Atlantic Ocean the dominance of cyanobacteria in the picophytoplankton (70 % of TChl aPICO), and the dominance of haptophytes and dinoflagellates in the >3 µm fraction (63 % of TChl aNANO + MICRO). These results are in good agreement with ours in the STZ as cyanobacteria represented 70 % ± 13 % of TChl aPICO (Fig. 5f) and the sum of haptophytes and dinoflagellates represented 53 % ± 6 % of TChl aNANO and 57 % ± 10 % of TChl aMICRO (Fig. 5d and e). Third, we observed noteworthy contributions of diatoms to TChl aPICO in KER (20 % ± 4 %; Fig. 5f). Nunes et al. (2019) reported similar findings in the Atlantic sector of the Southern Ocean in Patagonian waters (∼40 % of TChl aPICO) and explained this result by the presence of picophytoplankton diatoms such as Minidiscus sp. and bolidophytes (see Sect. 4.1.2). Together, these findings across two ocean basins highlight the utility of combining pigment chemotaxonomy with size fractionation to reveal size-specific shifts in phytoplankton communities from subtropical to polar regions. The methodological consistency and alignment of results between these two studies offers promising avenues to refine global assessments of phytoplankton size structure and composition.

Stations O2 and O3, located in the western STZ at similar longitudes, exhibited strong differences in TChl a stocks and NPP fluxes, despite comparable size structures of TChl a and NPP. The integrated TChl aTOTAL and NPP_TOTAL at O3 were respectively 31 % and 110 % higher than at O2, primarily due to increases in the nano- (TChl a: +32 %; NPP: +98 %) and microphytoplankton (TChl a: +77 %; NPP: +217 %) (Fig. 3). However, results from phytoplankton chemotaxonomic groups biomass did not display any noticeable shifts between O2 and O3 (Fig. 5). These differences were likely driven by higher nutrient contents at O3 (NOx: +115 %; DIP: +67 %; DSi: +49 %; Table S6 in the Supplement). Satellite-derived sea surface height data (GLORYS product) indicated the presence of a cyclonic eddy at O3 characterized by a shallower ZSML (Table 2) and nitracline depth than at O2 (data not shown), leading to nutrient upwelling and enhanced productivity and biomass. These hydrographic and biogeochemical features are consistent with previous observations of cyclonic eddies in the Mozambique Channel and Basin, which are characterized by a shallower ZSML and nitracline, and a deeper euphotic zone (Lamont and Barlow, 2017). Our NPP fluxes are similar to previous studies, reporting a 20 %–100 % increase in integrated NPP_TOTAL in cyclonic eddies compared to non-eddy areas in the Bay of Bengal (Prasanna Kumar et al., 2007; Sarma et al., 2020), in the South Indian Ocean (Dalabehara and Sarma, 2021) and in the subtropical North Pacific Ocean (Landry et al., 2008). Moreover, Sarma et al. (2020) reported no significant differences in the NPP size structure between cyclonic eddy and non-eddy areas, which supports our findings. About TChl a, there is a lack of previous studies focusing on eddies in the Indian Ocean to compare with our dataset. Nevertheless, Vaillancourt et al. (2003) reported a similar 28 % increase of integrated TChl a in cyclonic eddy compared to non-eddy areas in the subtropical North Pacific Ocean. In addition, Beatty et al. (2025) reported, in the latter region, based on amplicon sequencing data, that protistan community composition showed no response to eddy forcing in the water column, which is consistent with our results from phytoplankton chemotaxonomic group biomass.

Stations O6, O7 and O9 follow a west-east transect, from the Crozet Plateau toward the northwest continental margin of Kerguelen Plateau (Fig. 1). The lower integrated TChl aTOTAL observed at O9 compared to O6 and O7 may be attributed to the island mass effect, where the persistent micronutrients supply downstream of Crozet shape phytoplankton biomass that decline with distance, as micronutrients become depleted in the SML (Graham et al., 2015; Robinson et al., 2016). Notwithstanding, the TChl a size structure at O7 differed from O6 and O9, despite similar NPP_TOTAL and NPP size structure. Indeed, at O7, the microphytoplankton contribution to TChl aTOTAL was reduced while that of nano- increased (Fig. 3; Table S3). The decrease in the TChl aMICRO contribution at O7 was due to the decrease of microphytoplankton diatoms biomass by 4.3–6.2 mgm-2 compared to O6 and O9 (Fig. 5a). Conversely, the TChl aNANO contribution increase was caused by the increase of nanophytoplankton haptophytes and chlorophytes biomass by 3.9–7.5 mgm-2 in comparison to O6 and O9 (Fig. 5b). This community shift corroborated with lower integrated DSi content over the ZEL0.01 % at O7, on average 60 % lower than at O6 and O9 (Table S6). This decrease at O7 likely resulted from a low DSi surface water mass intrusion from the SAZ, leading to the growth limitation of microphytoplankton diatoms in favour of a non-silicifying nanophytoplankton community dominated by haptophytes. This transect is known to be influenced by the southern branch of the SAF current (Park et al., 1993), especially around 55–58° E where a signal of higher SST and lower fCO2 has been previously observed (Poisson et al., 1993; Leseurre et al., 2022). Indeed, underway continuous measurements during SOCARB recorded an SST increase and fCO2 decrease between Crozet and O7 around 54–56° E (data not shown).

The HN-LSi-LC station O10, located southwest of the Kerguelen plateau, and the KER stations (A3, E and O12), exhibited similar NPP size structure, with a dominance of the microphytoplankton (75 % ± 10 %), although NPP_TOTAL was approximately 2.5 times higher at the KER stations (Fig. 3b, Table S3). Likewise, the TChl a size structure at O10 and the KER stations was similar, with a dominance of microphytoplankton (59 % ± 2 %) mainly sustained by diatoms, although TChl aTOTAL was approximately 1.6 times higher at the KER stations (Figs. 3a and 5d, Table S3). No major differences were observed in the chemotaxonomic biomass structure – except a slightly higher contribution of pelagophytes in the TChl aMICRO at O10 compared to the KER stations (Fig. 5d). In contrast, the offshore HNLC station O11 displayed distinct size structures relative to the other stations. Both NPP_TOTAL and TChl aTOTALwere dominated by nanophytoplankton (mainly haptophytes and diatoms) with respective contributions of 50 % and 57 %. In addition, both NPP_TOTAL and TChl aTOTAL at O11 were lower than those measured at station O10. Our results are consistent with the study by Uitz et al. (2009), conducted during the austral summer, which reported a dominance of microphytoplankton (mainly diatoms) in the Fe-fertilized waters of the Kerguelen Plateau and an increasing contribution of nanophytoplankton at offshore HNLC stations.

We now compare the southeastern and northeastern blooms in KER, at stations A3 and O12, respectively. The highest integrated TChl a and NPP at A3 and O12 reflected the well-documented natural Fe fertilization. Despite sharing similar TChl a and NPP size structures, integrated NPP displayed variability, as integrated NPP_TOTAL at A3 was 70 % lower compared to O12 (Fig. 3). This difference may not be attributed to the phenology, as satellite-derived surface TChl a (MODIS product) did not highlight major differences neither in the timing, nor in the magnitude of phytoplankton biomass (Fig. S5 in the Supplement). Previous studies raised potential factors in explaining the spatial variability in integrated NPP in iron-fertilized areas, such as Si concentrations, phytoplankton community shifts, grazing pressure or light-mixing regime (e.g. Seeyave et al., 2007). First, A3 and O12 exhibited DSi/NOx ratios in the SML lower than the Brzezinski ratio (Table 2), indicating a potential Si limitation; this indicates that Si availability does not explain the observed difference in NPP_TOTAL. Second, phytoplankton chemotaxonomic groups biomass displayed a noticeable difference in the phytoplankton community structure at A3 compared to O12. The relative contributions of dinoflagellates and cryptophytes to integrated bulk biomass increased by 17 % – to the detriment of diatoms and haptophytes by 18 % – primarily due to increases in the nanophytoplankton (31 %) (Figs. 5 and S4). Third, to investigate the light-mixing regime, we computed the ratio of the diadinoxanthin (DD) and diatoxanthin (DT) concentrations to TChl a ((DD+DT):TChla). Although DD and DT have limited chemotaxonomic values, they have a photoprotective role, with concentrations that respond rapidly to changes in irradiance (Demers et al., 1991). Because most phytoplankton contain these pigments, the (DD+DT):TChla ratio provides useful information on the vertical mixing rates in the water column along with the light regime (Moline, 1998). The (DD+DT):TChlaTOTAL ratio at A3 was homogeneous within the SML (Fig. 7a), indicating that the vertical mixing rate was – or had recently been – faster than the photoprotective response (Moline, 1998). Our result corroborated with Uitz et al. (2009), which also studied A3 in late austral summer 2005 and underlined the lack of relationship between the bloom occurrence and the light-mixing regime previously described by Park et al. (2008). In contrary, the (DD+DT):TChla ratio at O12 decreased with depth within the SML (Fig. 7b), implying that the vertical mixing rate was – or had recently been – slower than the photoprotective response (Moline, 1998). Therefore, the variability in integrated NPP_TOTAL between A3 and O12 could result from contrasting phytoplankton communities and/or light-mixing regimes. Our results highlighted the heterogeneous distribution of phytoplankton communities within the nano- and microphytoplankton size classes in similar productive regimes around the Kerguelen Plateau in late austral summer, which was previously raised only during austral spring by Lasbleiz et al. (2014).