A chlorophyll halo over Maud Rise in the Southern Ocean

Domain: earth_science
Published: 2025-12-02
Editorial outcome: major_revision

Abstract
Phytoplankton blooms above the seamount Maud Rise in the Antarctic Ocean have been reported but their emerging mechanisms and their importance for the wider Southern Ocean are not well known. We use satellite data spanning over the last two decades and in-situ data collected from a ship, an underwater glider and Biogeochemical-Argo profiling floats to understand the processes involved in the formation of Maud Rise phytoplankton blooms. We find that the seamount generates upwelling of warm deep water that transports heat, and likely dissolved iron, to the surface via diapycnal mixing. This creates a recurring annular structure of chlorophyll concentration (or chlorophyll halo) in correspondence with the previously observed warm water and sea ice halo over Maud Rise. The in-situ observations reveal integrated chlorophyll-a concentrations of up to 100 mg·m−2, which suggests exceptionally high phytoplankton biomass within the Southern Ocean, thus making the seamount a regional phytoplankton hotspot.

Introduction
Southern Ocean primary production is known to be limited by a lack of iron and light1,2. On the contrary, the Southern Ocean is comparatively rich in macronutrients (nitrate, phosphate, and silicate), and so the processes that enhance iron supply to the surface layer have the potential to increase primary production. Annual phytoplankton growth in the subpolar Southern Ocean starts typically in spring when sea ice retreats, thus leading to an increase in sunlight exposure of the ocean surface3. At this time of the year, iron is typically available in the photic zone, having been resupplied by processes such as deep winter mixing4, bacterial remineralization5, and sea ice melt6. Phytoplankton quickly takes up iron hence their accumulation is typically observed in the vicinity of iron sources7–10. For example, enhanced primary production is often found around topographic features, where ocean currents such as the Antarctic Circumpolar Current, interact with the seafloor topography3,11.

Maud Rise is a 3300 m tall seamount isolated in the 5000 m-deep abyssal plain of the eastern Weddell Sea. It is located within the westward flowing southern limb of the Weddell Gyre, near 65S, 2E (Figure S1). The scientific interest of oceanographers for Maud Rise stems from the occasional occurrence of an open ocean polynya, first documented as the large-scale Weddell Polynya in 1974–197612 and more recently the smaller Maud Rise Polynya13. These rare polynyas are associated with deep ocean convection, which mixes warm and nutrient rich subsurface waters to the surface, thereby melting sea ice and likely enhancing primary production12,14–18. These openings within the sea ice have typically been explained by the upwelling or entrainment of deeper warm waters, such as those associated with the Maud Rise ocean current-topography interaction19,20, Ekman effects13,21–26, as well as atmospheric drivers through anomalous storm events27 and positive air-sea heat flux28,29.

According to the Taylor-Proudman theorem, the water column above a seamount is stiffened by Earth’s rotation which effectively traps the water masses within the column and blocks any incoming flow as if the seamount extended up to the surface24,30,31. This concept known as Taylor column is a simple theoretical model, but describes well the Maud Rise ocean dynamics. The hydrography of the stagnant water masses overlying the seamount can significantly differ from the incoming flow from the surrounding Weddell Sea15. The Maud Rise Taylor column is associated with upwelling of warm and saline Warm Deep Water (WDW)32,33 along the flanks of the Rise, which has been observed to leave an imprint on the sea ice cover, with a pattern of reduced sea ice concentration that follows the shape of the warm water halo23,34. The dynamics around Maud Rise and the associated polynya opening can also leave an imprint on the primary production, as described for the strong phytoplankton bloom that occurred over the Rise in 201717,18. However, the link between the specific ocean dynamics associated with Maud Rise and the timing and intensity of the phytoplankton bloom developing over it remain poorly understood, while in-situ observations showing these linkages have been absent.

At the beginning of austral summer 2021-2022, a polynya occurred over Maud Rise before merging with the retreating sea ice edge (Fig. 1a). Following this polynya opening, we performed an extensive field campaign on board the icebreaker R/V S.A. Agulhas II between 6 to 16 January 2022 to observe the dynamics occurring in the Maud Rise area (Fig. 1b). During the measurement period, satellite images of chlorophyll-a (Chl-a) showed the occurrence of a phytoplankton bloom over Maud Rise with a particularly high Chl-a concentration up to 1.6 mg·m−3 on the flanks (Fig. 1b).

In this paper, we describe this phytoplankton bloom occurring over Maud Rise and discuss its spatial variability and frequency using 25 years of satellite observations of Chl-a, SST and sea ice cover. We also use a unique high-resolution glider survey of the Maud Rise, observations from Biogeochemical (BGC)-Argo floats and from the ship-based survey to gain a better understanding of the drivers of the halo-shaped phytolankton bloom. We then set the Maud Rise phytoplankton bloom in the context of the wider Southern Ocean to highlight its importance.

Results
A recurring halo-shaped phytoplankton bloom over Maud Rise
We use more than two decades of satellite data to assess the significance of the chlorophyll halo observed in January 2022 and to study the phytoplankton bloom phenology in context with sea ice concentration and sea surface temperature. In the long-term mean, Chl-a concentration averaged over the Maud rise area (5W-10E, 62S-68S) starts to increase in late-November and reaches a maximum of 0.85 mg·m−3 in January (Fig. 2a). The year-by-year evolution of the Chl-a concentration indicates that the bloom over Maud Rise is a recurring feature and typical of the Southern Ocean Sea Ice Zone bloom phenology3,35. The onset of the phytoplankton bloom coincides with the retreat of the sea ice cover (Fig. 2a, c), followed by an increase in sea surface temperature (Fig. 2b). The Chl-a concentration reaches a maximum in the second week of January, when the sea ice has completely melted. The bloom reduces in intensity throughout late-January and February (Fig. 2a) and reaches a post-blooming phase in March, possibly due to increases in the grazing pressure or nutrient limitation3,36.

The December-climatology of Chl-a concentration from 1998 to 2023 shows an annular phytoplankton bloom centered over the seamount (Fig. 2d). The Chl-a concentration reaches its highest magnitudes on the flanks, especially on the western and eastern flanks (≈0.85 mg·m3). The time evolution and the spatial distribution of Chl-a (Fig. 2a, d) thus suggest that a Chl-a halo over Maud Rise is a recurring feature that occurs almost annually during December (Figure S2). The average satellite observations of sea ice concentration and sea surface temperature show a low sea ice concentration halo shape as reported in Lindsay et al.23 and a sea surface temperature halo37 (Fig. 2e, f). The Taylor column overlying the seamount is observable by remote sensing of the sea surface height (Fig. 2g).

Focusing on the 2021–2022 austral summer, when we have high-resolution in-situ observations, the sea surface temperature and the fraction of area covered by sea ice are representative of the longer term conditions but the Chl-a concentration is higher than in other years (exceeding the standard deviation of the long-term mean in January) and peaks later by about a week (blue line in Fig. 2a–c). Overall, the observed Chl-a halo during 2021–2022 was a rather intense late bloom, with a maximum concentration of 1.48 mg·m−3 in January.

The seasonal cycle of the phytoplankton bloom shows strong inter-annual variability but moderate spatial variability, with recurring high biomass along the flanks of Maud Rise. The warm water pattern often matches the phytoplankton bloom pattern (Figures S2 and S3) which is supported by a positive correlation between the yearly time series of SST and Chl-a on the southeastern flank of Maud Rise (Pearson correlation coefficient r in the range of 0.4–0.8 with p-values under 0.05 - Fig. 3a). The correlation is significantly lower on the Northwestern flank (r between 0. and 0.2) probably due to eddy formation on this side of the seamount30,37. Therefore, we suggest that the inter-annual variability of the chlorophyll halo can be partly explained by the inter-annual variability of the warm water halo38. In addition, we suggest that anomalies in easterly winds may be responsible for enhanced upwelling in this part of the Southern Ocean and, therefore, lead to higher phytoplankton biomass as shown by Moreau et al.10 in the region directly Southeast of Maud Rise.

A significant correlation between the SST and SSH yearly time-series was also observed, with values reaching up to r ≈ 0.6 on the Northeastern flank directly facing the incoming westward flow (Fig. 3b). These two correlations, between SST and SSH between and SST and the Chl-a concentration, hint at the pivotal role of ocean-topography interaction in the formation of chlorophyll halos as discussed in the next section.

Linking phytoplankton biomass to ocean circulation dynamics
We used high-resolution underwater glider observations with ship-CTD and BGC-Argo float data to investigate the structure of the phytoplankton bloom together with the physical characteristics of the water column. The glider undertook seven crossings of the Maud Rise flank and through the Maud Rise halo. We analysed the first four glider transects (white triangle, circle, square and diamond in Fig. 4a, b) when the phytoplankton bloom was ongoing (late November to early January). Both glider and CTD data (Figs. 4c, d, e, 1e and S4) indicate that the water column can, in a simplified view, be classified as a three-layer system structured from top to bottom. The first is an upper layer of relatively homogeneous temperature and salinity (≈50 meters) within which the mixed layer is contained. The upper layer is bounded below by the cold Winter Water (WW) layer39, where the temperature remains below -0.7°C. The third layer contains the warmer, saltier Warm Deep Water (WDW). The glider Chl-a and temperature sections (Figs. 4c and 4d) indicate that all the elevated Chl-a is contained within the upper layer. There is a decline of Chl-a concentration with depth starting between 40 and 50m, consistent with the depth of the upper boundary.

The glider flank crossings show significant spatial variability, with higher Chl-a concentration (>1.5 mg·m−3) consistently found where the glider approaches the steepest topography of the Maud Rise flank, which is located around the 3500 m isobath. This isobath is used here as a reference depth throughout this study. The BGC-Argo floats further support the spatial distribution of Chl-a observed by the glider and seen in the satellite views, with float 6903129 in the Taylor column registering a substantially lower mean Chl-a concentration than float 6904235, which was bound to the Maud Rise flank (maximum recorded Chl-a values between 0 and 50m for each float are 0.6 mg·m−3 and 1.0 mg·m−3, respectively; Fig. 1c). The ship-based CTD sections support a similar finding as shown in Fig. 1d and S4.

These sharp spatial gradients of Chl-a concentration are closely mirrored by changes in deeper ocean temperature and salinity (Fig. 4d, e), suggesting that the distribution of the surface layer Chl-a is closely tied to the deeper ocean fronts and circulation dynamics. In each dataset (ship CTDs, gliders and floats), the water mass distribution and its vertical extent changes abruptly over the Maud Rise flank, where the seafloor is approximately 3500 m deep. In particular, there is a sharp transition between the thick WW (up to 120 m thick) layer over the Maud Rise Taylor column, associated with cooler deep waters (below 0.4°C), and the thin WW layer (20-40 m; Figure S5) bounded by warmer deep waters (≈1.2°C) outside of the 3500 m isobath.

Such spatial patterns in WW thickness, as well as elevated warmer waters just below the WW at the flanks of Maud Rise suggest that ocean current-topography interactions lead to enhanced water mass interactions between the surface and deep water layers in these regions, leading to a response in phytoplankton production. Recent observations and modelling efforts have shown that the spatial pattern in upper ocean properties of the Maud Rise region are linked to the local bathymetry, with the flanks acting as a transition zone between the deeper waters of the Taylor column and the WDW of the surrounding ocean20,38. Steeply sloping isopycnals seen over the flank (Fig. 4d, e) may give rise to vertical transports, such as the upwelling of warm WDW through the WW and into the base of the mixed layer, as suggested through interleaving water masses observed by Mohrmann et al.20. Additionally, the sharper vertical temperature and salinity gradients at the base of the WW at the flanks would promote double-diffusive convection, such as salt fingering40, thereby exchanging deeper water masses vertically to the upper ocean. These amplified oceanic vertical transports over the Maud Rise flank have the potential to supply iron from the iron-rich deep ocean to the notably iron-deficient upper sunlit layers9,10, where it is made available to phytoplankton photosynthesis, there by leading to the higher values of Chl-a concentration that we observe as a more productive halo surrounding the Maud Rise.

We verified this iron supply hypothesis with the GLODAP database and its δ3He measurements throughout the Southern Ocean. 3-helium (3He) is released to the deep ocean by hydrothermal vents along with dissolved iron. There it can be transported thousands of kilometers before being upwelled closer to the surface to support primary production9,41. Therefore, measurements of δ3He can be used as a proxy for estimating the presence of hydrothermal iron. A transect of δ3He along the 0° meridian from 60°S to 70°S (Figure S6) gives a cross-section view of Maud Rise (approximately 65°S-66°S). We noticed two δ3He anomalies above the flanks of the seamount (68°S-66°S and 65°S-64°S) in the range 200-900 meters depth (Figure S7). By looking at two additional transects of temperature and salinity, we find that the water masses, where the two δ3He anomalies are, have a salinity of approximately 34.69 PSS-78 and temperature above 1°C (Figure S7). These water masses are warmer, saltier, higher in δ3He and supposedly higher in dissolved iron concentration of hydrothermal origin than other water masses in the area (20°W-20°E, 50°S-70°S) hence supporting the hypothesis of iron supply from the deep ocean (measurements of these water masses are within the black rectangle in Figure S8). It is also probable that some part of the iron suspended in the upper sunlit layers originates from sea ice melting6.

Relevance of the Maud Rise bloom for the Southern Ocean
Given the strong potential impact of the ocean dynamics over Maud Rise on the Chl-a concentration, the question arises how strong the Maud Rise bloom is compared to the rest of the Southern Ocean. Based on the in situ observations, the depth-integrated Chl-a concentration over the upper 200 meters reaches values up to 98.2 and 89.4 mg·m−2 for Float 6904235 and CTD casts over the flanks of Maud Rise, respectively. Using all BGC-Argo floats42 available for the period 2012-2020 in the Southern Ocean, we only find 39 profiles out of 8560 profiles in total (covering all areas of the Southern Ocean and all seasons), where the integrated Chl-a over the upper 200 meters is larger than 100 mg·m−2 (Figure S9). These 39 profiles do not include the 2 floats which were deployed over Maud Rise during the summer 2021-2022 to avoid any sampling bias.

Interestingly, the distribution of the locations with these exceptionally high Chl-a biomasses in the BGC-Argo floats database are (1) in the notoriously productive Scotia Arc, (2) the sea ice zone of the Ross and Weddell Seas (including Maud Rise), and (3) a few locations near sub-Antarctic Islands in the Indian sector of the Southern Ocean where sedimentary iron is typically supplied by the inter-action of the Antarctic Circumpolar Current with shallow bathymetry7.

The flanks of Maud Rise themselves account for 11 of these 39 profiles with exceptionally high Chl-a biomass (ranging between 106.7 and 160.5 mg·m−2 on the flanks of Maud Rise, compared to an average 132.6 mg·m−2 for these 39 profiles). This adds evidence for the role of dynamic processes on the Maud Rise flanks leading to exceptionally high phytoplankton biomass accumulation in the Southern Ocean. This also suggests that Maud Rise may well be a critically important area for the Southern Ocean ecosystem and biological carbon pump43,44.

Methods
Satellite observations
The satellite observations used in this study are based on the “Global Ocean Colour Plankton and Reflectances MY L3 daily observations” dataset with daily and 4km × 4km resolutions, the “Global Ocean Gridded L4 Sea Surface Heights And Derived Variables Reprocessed 1993 Ongoing” dataset with daily and 0.125° × 0.125° resolutions, and the “Global Ocean OSTIA Sea Surface Temperature and Sea Ice Reprocessed” dataset with daily and 0.05° × 0.05° resolutions. The three datasets extend from 1998 to 2022 (SIC, SST) or 2023 (Chl-a, SSH). The satellite-derived Chl-a concentration is evaluated by observations in the visible domain of the ocean surface implying that cloud cover and sea ice hinder the retrieval of upper ocean Chl-a concentrations45. The sea surface temperature and sea ice concentration estimates used in this study rely on the sea surface reflectance in the near infrared domain which is little affected by clouds but is subject to sea ice reflectance interferences and possible sensor calibration issues.

Based on the satellite-derived sea ice concentration, we define the fraction of the Maud Rise area covered by sea ice as the number of pixels with sea ice concentration above 15% in the domain (5°W-10°E, 62°S-68°S) divided by the total number of pixels in the same domain. Chl-a concentration derived from satellite imagery matches the measurements in situ (Fig. 1b and 1d).

The correlation map (Fig. 3) was computed first by calculating the yearly times series of Chl-a concentration and sea surface temperature (Figures S2 and S3) then computing the Pearson correlation coefficient map for these two time series. The slope of the regression of the Chl-a-SST correlation was computed by multiplying the Pearson correlation with the ratio of the Chl-a and SST standard deviations. Similarly, The slope of the regression of the SSH-SST correlation was computed by multiplying the Pearson correlation with the ratio of the SSH and SST standard deviations.

Seaglider observations
A buoyancy-driven Seaglider (SG640) was deployed during the SO-CHIC cruise on January 10, 2022 on the flank of Maud Rise (63°27.63S, 02° 26.59E). Such gliders sample the physical and biogeochemical state of the upper ocean at high temporal and spatial resolution, with each V-shaped dive occurring between the surface and 1000 meters depth. The spatial resolution between profiles was 3.1 ± 1.5 km, while the mean temporal resolution was 3 hours. SG640 was equipped with sensors to measure salinity and temperature (SeaBird CT Sail), chlorophyll fluorescence at 695 nm and optical backscattering at 470 and 700 nm (WETLabs ECO Puck), and dissolved oxygen (Aandera optode 4831). SG640’s last dive occurred on April 7, 2022. SG640 fluorescence was converted to Chl-a and calibrated in the following way: (i) fluorescence units were converted to Chl-a units (mg·m−3) using the manufacturer calibration scale factor, (ii) the ship-board CTD was calibrated against in-situ bottle samples (Figure S10), and (iii) SG640 manufacturer-corrected Chl-a was corrected to the calibrated ship-board CTD by finding the linear regression at the cast taken directly after the Seaglider deployment (Figures S11 and S12). The glider calibrated Chl-a was subsequently corrected for non-photochemical quenching according to the methods in Xing et al.46.

CTD observations
We use ship-based CTD and Chl-a measurements that were taken at 58 stations over the Maud Rise. The stations were organised in 4 sections, two over the northern flank (Section N and Section NE) of which Section N aligns with the first glider section, and two sections that cross the Maud Rise from center to east and center to west, respectively (here shown as combined Section EW). The distance between the CTD casts is about 9 km and the depth alternatively 1000 m and full depth; for the center-west section, the distance is about 18 km and they cover the full depth. The rosette was equipped with 24 Niskin bottles for water samples, and the instrumentation was a SBE9 plus CTD Unit and SBE11 plus V5.2 Deck Unit. The initial data processing was done with the SBE Data processing software following the standard filters suggested for the SBE9 plus CTD unit. The conversion of fluorescence to Chl-a (mg·m−3) was done using the bottle samples and a quenching correction as described in the previous paragraph. Isopycnals were calculated based on the Thermodynamic Equation of Seawater-2010 via the gsw-python library that calculates in-situ density of seawater from absolute salinity and in-situ temperature.

BGC-Argo floats observations
We use data from Biogeochemical-Argo (BGC-Argo) floats WMO6903129, deployed over the flank of Maud Rise at 64°32.860’S, 03°12.533’ E and WMO6904235, deployed on top of Maud Rise at 64°31.0’S, 1°30.0’E. The float data are available at the Coriolis data platform. BCG-Argo floats provide insight into the water column physical state and submerge down to 1000 meters. They have no means of propulsion i.e. they drift according to currents, providing a quasi-Lagrangian view of the ocean. They are equipped with sensors for pressure, temperature, dissolved oxygen, conduc- tivity, Photosynthetically Active Radiation, Chl-a fluorescence, irradiance at 380, 412, 490 nm and backscattering at 700 nm. WMO6904235 data spans 29 descent-ascent cycles from 2021-12-11 to 2022-06-03 while WMO6903129 data spans 27 cycles from 2022-01-08 to 2022-05-22. We use the profiles of the ascent and corrected slope, non-photochemical quenching and the deep offset of Chl-a based on the backscattering data at 490 nm according to Xing et al.47. The processing is an adaptation of what was done in Schmechtig et al.48. The data are linearly interpolated onto a vertical grid of 1 db.

To produce Figure S949, we used float observations collected and made freely available by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) Project funded by the National Science Foundation, Division of Polar Programs (NSF PLR-1425989 and OPP-1936222 and 2332379), supplemented by NASA, and by the International Argo Program and the NOAA programs that contribute to it (https://argo.ucsd.edu, https://www.ocean-ops.org). The Argo Program is part of the Global Ocean Observing System.

GLODAP database
The figures S6, S7 and S8 were created with the ODV-online browser tool https://explore.webodv.awi.de/. Transects were contoured using ODV’s DIVA gridding algorithm with a signal-to-noise ratio of 10.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review Report: A Chlorophyll Halo over Maud Rise in the Southern Ocean\nThis study investigates the phenomenon of a halo-shaped phytoplankton bloom over Maud Rise, a seamount in the\nSouthern Ocean. Although the concept/mechanism shown in this study is already known, the study used extensive\nobservations from satellite data spanning 25 years, in-situ ship-based measurements, Seaglider observations, and\nBiogeochemical-Argo floats to provide concrete evidence on the physical and biogeochemical drivers that contribute to this\nrecurring bloom. The authors highlight the Taylor cap mechanism, which promotes upwelling of warm, iron-rich deep water,\nstimulating phytoplankton growth.\nThe study provides an insight into physical-biogeochemical coupling in the Southern Ocean, particularly the interactions\nbetween ocean-topography and phytoplankton blooms.\nThe paper is organized, systematically progressing from background, data collection, findings, discussion, and conclusions.\nFigures and maps are good, effectively illustrating chlorophyll concentration variations, sea ice patterns, and temperature\nprofiles.\n1. Areas for improvement and suggested revisions\n1.1 Clarification on the mechanisms of iron Supply\nThe study infers iron upwelling as a driver of phytoplankton blooms but lacks direct iron measurements or flux estimations.\nInclude references to existing iron measurements in the region or hypothesize about potential iron sources (e.g., sediment\nresuspension, sea ice/glacial melt).\n1.2 Quantification of phytoplankton productivity\nThe study focuses on Chlorophyll-a concentration but does not estimate net primary productivity (NPP) or carbon fixation\nrates. Provide a comparative analysis of productivity rates using remote sensing estimates or existing productivity models for\nthe Southern Ocean.\n1.3 Discussion on interannual variability\nThe study states that the chlorophyll halo is a recurring feature, but there is limited discussion on interannual variability and\npotential climate drivers. Incorporate a more detailed comparison of bloom intensity across different years and discuss the\ninfluence of ENSO, Southern Annular Mode, or changing wind patterns on bloom strength.\n1.4 Addressing uncertainties & data limitations\nThere is no dedicated section discussing uncertainties in remote sensing data, in-situ sampling constraints, and potential\nmethodological biases. Acknowledge the limitations of satellite-derived Chl-a estimates, cloud cover interferences,\ncontamination of signals from sea ice reflectance and possible sensor calibration issues.\n2. Future research directions\nThe study concludes with significant findings but lacks recommendations for further research. Suggest future work such as\nnumerical modeling to simulate phytoplankton bloom dynamics, iron tracer experiments to confirm nutrient flux pathways,\nand zooplankton grazing studies to determine bloom termination mechanisms.\n3. Recommendation\nThe paper contributes well to ocean biophysical research and is suitable for publication with major revisions. Key revisions\nshould focus on adding biogeochemical context (iron supply), interannual variability, and addressing methodological\nlimitations. This paper presents an advancement in understanding regional marine productivity in the Southern Ocean. The\nuse of multi-source data and the focus on mesoscale physical-biological interactions are explained well. The revisions\nsuggested above will further enhance the scientific depth and impact of the manuscript. The authors are encouraged to\naddress these points before final submission.", "reviewer_2": "This paper provides a comprehensive description of the Maud Rise Bloom. Both the annual change in chlorophyll, sea\nsurface temperature and sea ice cover from satellite data, and also provide in situ observations (from CTD, glider, and float\nprofiles) of the bloom that forms a halo around the north west side of Maud Rise each year. The authors provide\ncomprehensive support for their conclusion that the magnitude and shape of the bloom are driven by increased upwelling of\ndeep water that is most likely rich in iron. This upwelling is in turn a result of the circulation driven by the Taylor column that\nsits above Maud Rise. The authors place the Maud Rise bloom in the context of the magnitude and location of other\nSouthern Ocean blooms, ranking it among the some of highest biomass recorded by Argo float profiles throughout the\nSouthern Ocean.\nI find this work mostly well-supported and interesting. Recurring blooms in the Southern Ocean driven by processes that\nhave not yet been well-characterized are important for our understanding of how changes in temperature and salinity may\nimpact primary production. The Maud Rise bloom appears to be one of these recurring blooms.\nHowever, I have one major issue with the manuscript and that is the lack of discussion of the interannual variability in the\nMaud Rise bloom. The authors briefly mention that it may be related to the warm water halo (line 88) and discuss the\nprocesses that cause the warm water halo (lines 132-146). Given the satellite data included in the paper, it should be very\npossible to pursue this idea. Does the magnitude of the bloom track the intensity and extent of the warm water halo?\nGiven the arguments that the authors make about the causes of the bloom (the upwelling of warm Weddell Deep Water) it is very\nodd that they do not pursue this line of inquiry. The strength of the upwelling should have a direct impact on the measured\nsst. The authors state that the impact of this paper, and how it is different from previous similar studies of the Taylor column\nand the Maud Rise bloom, is that they are characterizing “the link between the specific ocean dynamics around Maud Rise\nand the timing and intensity of the phytoplankton bloom developing over it.” (Lines 51-53). But the authors do not currently\nmeet this threshold. This needs to be addressed.\nMedium-sized issues:\nThe authors rely heavily on the presence of a Taylor column/cap above Maud Rise. The authors need to define this term.\nThe authors use the terms Taylor cap and Taylor column interchangeably, do they mean the exact same thing? A sentence\nabout the general characteristics of a Taylor column and its impacts on circulation.\nThe authors also need to characterize the spatial extent and position of the Taylor column, beyond the temperature and\nsalinity sections. Ideally the approximate boundaries of the column would be calculated overlayed as a contour on any of the\nnumerous map figures included in the paper. Given how strongly the circulation above Maud Rise differs from the circulation\naround Maud Rise, this should be visible in surface current data from satellite altimetry. Alternatively, if the average\nposition/extent of the column has been characterized in another paper, the position of the column from that paper could be\nused. The Taylor column is such a fundamental part of this work, and it needs to be visualized.\nThe oceanographic setting of the Maud Rise bloom is difficult to follow. Specifically, the pattern of sea ice retreat, its position\nrelative to the Weddell gyre, and its location relative to the rest of the Weddell Sea.\nThe authors mention the Maud Rise polynya several times, and then transition to talking about the bloom. They discuss the\npolynya as a winter phenomenon, but when the winter polynya opens, it also impacts sea ice retreat (sea ice retreats\nsignificantly earlier) and therefor primary production. Perhaps a climatology of sea ice concentration of a slightly larger\nregion for each of the months during sea ice retreat in the Supplemental. It was hard to visualize how sea ice retreats\nthrough this area.\nThe authors mention the topography of the Weddell region, but only ever show very close-cropped view of Maud Rise. They\nalso mention that Maud Rise is positioned in the westward flowing limb of the Weddell Gyre, but there is no figure showing\nthe Weddell Gyre. Authors need a figure showing the regional bathymetry and average position of the relevant part of the\nWeddell Gyre. The fact that Maud Rise falls in the path of the gyre seems important to the presence of the Taylor column.\nOnly people who work specifically in the Weddell region will have an image of this in their minds.\nMore details on the oceanographic setting of Maud Rise and the associated bloom, related to the above comments, need to\nbe added to the text as well. Lines 45-53 region.\nFigure 3 is very important to the arguments being made in the paper, and extremely difficult to make sense of. There has got\nto be an easier way to label figure 3. Use different symbols and then put those symbols above the lines on the sections.\nWrite above the sections when the float is above the rise and when it isn’t. Just something to make this more digestible. It is\ntoo hard to digest this figure and even when I thought I had the orientation of everything correct, I wasn’t sure. Considering\nthe importance of this figure to the paper it needs clearer annotation. Figure C1 is much clearer and easier to get oriented to.\nThe authors need to provide the data for the integrated chlorophyll values that they calculated from Argo float profiles. This is\nthe data on which the importance of the Maud Rise bloom is based.\nSmall issues:\nIn the caption for Figure 1 the Authors mention isopycnals drawn on the sections, but I cannot find the details of how density\nwas calculated. This needs to be a sentence in the methods.\nFor figure 2e, the caption mentions sea ice contours, but all I see are bathymetry contours."}, "author_response": "We thank the reviewer for their feedback to improve our manuscript, which we have revised accordingly. We appreciate the time and\nenergy dedicated to evaluating this manuscript, and we recognise how the constructive and critical feedback has helped us improve\nthe quality and clarity of the work presented. Below are the original comments from the reviewer in black and our corresponding\nresponses in green. Text from the manuscript is shown in green and italic.\nThis study investigates the phenomenon of a halo-shaped phytoplankton bloom over Maud Rise, a seamount in the Southern\nOcean. Although the concept/mechanism shown in this study is already known, the study used extensive observations from satellite\ndata spanning 25 years, in-situ ship-based measurements, Seaglider observations, and Biogeochemical-Argo floats to provide\nconcrete evidence on the physical and biogeochemical drivers that contribute to this recurring bloom. The authors highlight the\nTaylor cap mechanism, which promotes upwelling of warm, iron-rich deep water, stimulating phytoplankton growth.\nThe study provides an insight into physical-biogeochemical coupling in the Southern Ocean, particularly the interactions between\nocean-topography and phytoplankton blooms.\nThe paper is organized, systematically progressing from background, data collection, findings, discussion, and conclusions. Figures\nand maps are good, effectively illustrating chlorophyll concentration variations, sea ice patterns, and temperature profiles.\nWe thank the reviewer for these positive comments.\n1. Areas for improvement and suggested revisions 1.1 Clarification on the mechanisms of iron Supply\nThe study infers iron upwelling as a driver of phytoplankton blooms but lacks direct iron measurements or flux estimations. Include\nreferences to existing iron measurements in the region or hypothesize about potential iron sources (e.g., sediment resuspension,\nsea ice/glacial melt).\nOur analysis hypothesized that the observed bloom over Maud Rise is associated with iron upwelling supply from deeper waters.\nSuch a mechanism assumes that a deep iron source provides the necessary iron that is needed to allow for such high biomass\nbuild-up. As discussed in Moreau et al. (2023) or in Ardyna et al. (2019), hydrothermal iron can be transported thousands of\nkilometers from its origin and be a key driver of surface primary production. In the sector of Maud Rise, meridional section of Helium\nisotopic ratio (δ3He), which is a tracer of primordial helium originating from the Earth’s mantle, commonly used to detect plumes\ndownstream of hydrothermal vents, shows elevated ratios directly below the surface layer (see Figure 3b of Moreau et al., 2023).\nThis clearly indicates that water directly below the surface layer in our region of interest does have influence of hydrothermal vents\nthat are known to be loaded in iron. We have clarified these aspects in the new version of the manuscript. Also, based on the\nrecommendation of the reviewer regarding the potential role of sea ice, we added the following sentence stating that some of the\niron supply could come from the sea ice melt (lines 156-157):\n“ It is also probable that some part of the iron suspended in the upper sunlit layers originates from sea ice melting [6].”\nHowever, we do not think that sediment resuspension could play a role in this deep-sea environment, the shallowest parts of Maud\nRise being deeper than 1,000 m deep.\nReferences:\nArdyna, M., Lacour, L., Sergi, S. et al. Hydrothermal vents trigger massive phytoplankton blooms in the Southern Ocean.\nNat Commun 10, 2451 (2019). https://doi.org/10.1038/s41467-019-09973-6\nMoreau, S., Hattermann, T., de Steur, L. et al. Wind-driven upwelling of iron sustains dense blooms and food webs in the eastern\nWeddell Gyre. Nat Commun 14, 1303 (2023). https://doi.org/10.1038/s41467-023-36992-1\nD. Lannuzel, M. Vancoppenolle, P. van der Merwe, J. de Jong, K.M. Meiners, M. Grotti, J. Nishioka, V. Schoemann; Iron in sea ice:\nReview and new insights. Elementa: Science of the Anthropocene 1 January 2016; 4 000130.\nhttps://doi.org/10.12952/journal.elementa.000130\n1.2 Quantification of phytoplankton productivity\nThe study focuses on Chlorophyll-a concentration but does not estimate net primary productivity (NPP) or carbon fixation rates.\nProvide a comparative analysis of productivity rates using remote sensing estimates or existing productivity models for the Southern\nOcean.\nThe reviewer is right and the study of Net primary productivity (NPP) could bring interesting results to the present study, such as\nusing the remote-sensing based VGPM or CbPM models of Berenfeld et al. (1997, 2005). However, there are also inherent\nuncertainties associated with these models and the carbon- and chlorophyll-based models typically result in opposing trends as\nshown by Pinkerton et al. (2021). In addition, we believe that presenting NPP data will not improve, in this context, our\nunderstanding of the formation mechanism of the annular phytoplankton blooms. Given the amount of data we already present in\nthe article, for e.g., with more than 2 decades of information about satellite-derived ocean colour, we do not wish to increase overly\nthe size of the manuscript by adding the study of remotely-sensed ocean NPP, also to remain within the standard size of articles\npublished in Nature Communication. We believe that the study of the phytoplankton biomass (inferred through satellites but also in\nsitu with CTD casts, floats and gliders) is relevant to make a strong link with ocean dynamics as we present in this study. We leave it\nto the editor and the reviewer to decide whether they wish to include such an analysis of the satellite-derived NPP to the article.\nReferences :\nBehrenfeld et al. ; A consumer’s guide to phytoplankton primary productivity models ; 22 December 2003 ;\nhttps://doi.org/10.4319/lo.1997.42.7.1479\nBehrenfeld et al. ; Carbon-based ocean productivity and phytoplankton physiology from space ; 25 January 2005 ;\nhttps://doi.org/10.1029/2004GB002299\nPinkerton et al ; Evidence for the impact of climate change on primary producers in the southern ocean ; 17 May 2021 ;\nhttps://doi.org/10.3389/fevo.2021.592027\n1.3 Discussion on interannual variability\nThe study states that the chlorophyll halo is a recurring feature, but there is limited discussion on interannual variability and potential\nclimate drivers. Incorporate a more detailed comparison of bloom intensity across different years and discuss the influence of\nENSO, Southern Annular Mode, or changing wind patterns on bloom strength.\nThe interannual variability of the phytoplankton bloom over the area of Maud Rise is shown in figure 2a and appendix figure A1.\nFigure 2a suggests that there is a strong interannual variability in phytoplankton biomass while appendix figure A1 suggests a\nmoderate spatial variability of the summer phytoplankton bloom, although with recurring high biomass along the flanks of Maud\nRise.\nTo respond to the reviewer’s comment, in the new version of the manuscript, we added figure 3 and appendix figures A2 showing\nthe interannual variability in the warm water halo around Maud Rise (figure A2) and the significant correlation between the warm\nwater halo and the annular phytoplankton bloom (figure 3). In addition, we added a paragraph discussing this aforementioned link to\nthe end of the section “A recurring halo-shaped phytoplankton bloom over Maud Rise” (lines 101-110):\n“The strength of the phytoplankton bloom show strong interannual variability but moderate spatial variability, with recurring high\nbiomass along the flanks of Maud Rise (Figures 2a and A1). The warm water pattern generally matches the phytoplankton bloom\npattern (Figures A1 and A2). The correlation map (Figure 3) between the monthly time series of SST and Chl-a concentration\nindicates a significantly positive correlation on the flanks of the Maud Rise (nearly r = 0.7 on the eastern side directly facing the\nincoming flow of the Weddell gyre). Therefore, we suggest that the interannual variability of the chlorophyll halo can, to a great\nextent, be explained by the interannual variability of the warm water halo [35]. In addition, we suggest that anomalies in easterly\nwinds may be responsible for enhanced upwelling in this part of the Southern Ocean and, therefore, lead to higher phytoplankton\nbiomass as shown by [10] in the region directly Southeast of Maud Rise.”\nWe also do not believe that ENSO or SAM play a significant role on phytoplankton dynamics around Maud Rise, as there are years\nwhen strong positive and negative ENSO or SAM anomalies were detected but with no discernable anomalies in the intensity of the\nannular phytoplankton blooms. Although phytoplankton dynamics have been shown to be related to the ENSO or SAM index in\nwestern parts of Antarctica like the Antarctic Peninsula (e.g., Vernet et al., 2008; Moreau et al., 2015), which the reviewer is perhaps\nreferring to, such links do not seem to exist in eastern parts of Antarctica as suggested by Moreau et al. (2023) who carried these\nanalyses in the region directly Southeast of Maud Rise.\nAs the reviewer suggested, however, changing wind patterns may indeed have an impact on phytoplankton blooms as also\nsuggested by Moreau et al. 2023 who investigated the drivers of a recurring phytoplankton bloom Southeast of Maud Rise. The\nauthors found that this anomalously productive bloom was linked to negative anomalies in easterly winds which, in the Southern\nOcean, indicate enhanced upwelling. A similar but unpublished relationship was observed between phytoplankton biomass and\neasterly winds anomalies at Maud Rise (L. de Steur, pers. comm.) Therefore, following the reviewer’s comment, we added the\nfollowing sentences to the new version of the manuscript (lines 103-110) :\n“The correlation map (Figure 3) between the monthly time series of SST and Chl-a concentration indicates a significantly positive\ncorrelation on the flanks of the Maud Rise (nearly r=0.7 on the eastern side directly facing the incoming flow of the Weddell gyre).\nTherefore, we suggest that the interannual variability of the chlorophyll halo can, to a great extent, be explained by the interannual\nvariability of the warm water halo [35]. In addition, we suggest that anomalies in easterly winds may be responsible for enhanced\nupwelling in this part of the Southern Ocean and, therefore, lead to higher phytoplankton biomass as shown by [10] in the region\ndirectly Southeast of Maud Rise.”\nReferences :\nVernet et al. Primary production within the sea-ice zone west of the Antarctic Peninsula : I-sea ice, summer mixed layer, and\nirradiance ; September 2008 ; https://doi.org/10.1016/j.dsr2.2008.05.021\nMoreau et al. Climate change enhances primary production in the western Antarctic Peninsula ; 28 January 2015 ;\nhttps://doi.org/10.1111/gcb.12878\n1.4 Addressing uncertainties & data limitations\nThere is no dedicated section discussing uncertainties in remote sensing data, in-situ sampling constraints, and potential\nmethodological biases. Acknowledge the limitations of satellite-derived Chl-a estimates, cloud cover interferences, contamination of\nsignals from sea ice reflectance and possible sensor calibration issues.\nWe have now added the following to more fully describe the satellite based observations methods that are found in Methods (lines\n207-211).\nThe satellite-derived Chl-a concentration is evaluated by observations in the visible domain of the ocean surface implying that cloud\ncover and sea ice hinder the retrieval of upper ocean Chl-a concentrations [41]. The sea surface temperature and sea ice\nconcentration estimates used in this study rely on the sea surface reflectance in the near infrared domain which is little affected by\nclouds but is subject to sea ice reflectance interferences and possible sensor calibration issues.\nWe were not sure what further details the reviewer is requesting for the in situ observations. The in situ glider and ship-based\nobservations are described in detail in the Methods section. Aspects such as measurement quality control, calibrations against other\nmeasurements and CTDs, corrections (including correction for non-photochemical quenching) are provided.\n2. Future research directions\nThe study concludes with significant findings but lacks recommendations for further research. Suggest future work such as\nnumerical modeling to simulate phytoplankton bloom dynamics, iron tracer experiments to confirm nutrient flux pathways, and\nzooplankton grazing studies to determine bloom termination mechanisms.\nThe following sentences were added at the end of conclusion serving the purpose of suggesting further research (lines 191-199) :\nAs such, phytoplankton blooms over Maud Rise constitute a good case study of how marine ecosystems are shaped by mesoscale\nocean dynamics. More investigations are needed concerning the iron supply, the role of grazers on bloom dynamics as well as the\nimpact of the local state of the atmosphere on the strength of the blooms. Although trace metal nutrient measurements are complex\nto make, further trace metal measurements in open-ocean polynya regions, especially in the presence of shallow topography, would\nhelp elucidate the nutrient supply mechanisms between the deep ocean and surface layers. Understanding such dynamics\nexplained in the present study would benefit from a computer simulations-based investigation including biogeochemical\ncomponents. This could help disentangle the bio-physical processes occurring in these complex polynya systems.\n3. Recommendation\nThe paper contributes well to ocean biophysical research and is suitable for publication with major revisions. Key revisions\nshould focus on adding biogeochemical context (iron supply), interannual variability, and addressing methodological limitations. This paper\npresents an advancement in understanding regional marine productivity in the Southern Ocean. The use of multi-source data and\nthe focus on mesoscale physical-biological interactions are explained well. The revisions suggested above will further enhance the\nscientific depth and impact of the manuscript. The authors are encouraged to address these points before final submission.\nWe thank the reviewer for their constructive comments that we believe improved the quality of the manuscript.\nReviewer #2 (Remarks to the Author):\nThis paper provides a comprehensive description of the Maud Rise Bloom. Both the annual change in chlorophyll, sea surface\ntemperature and sea ice cover from satellite data, and also provide in situ observations (from CTD, glider, and float profiles) of the\nbloom that forms a halo around the north west side of Maud Rise each year. The authors provide comprehensive support for their\nconclusion that the magnitude and shape of the bloom are driven by increased upwelling of deep water that is most likely rich in iron.\nThis upwelling is in turn a result of the circulation driven by the Taylor column that sits above Maud Rise. The authors place the\nMaud Rise bloom in the context of the magnitude and location of other Southern Ocean blooms, ranking it among the some of\nhighest biomass recorded by Argo float profiles throughout the Southern Ocean.\nI find this work mostly well-supported and interesting. Recurring blooms in the Southern Ocean driven by processes that have not\nyet been well-characterized are important for our understanding of how changes in temperature and salinity may impact primary\nproduction. The Maud Rise bloom appears to be one of these recurring blooms.\nWe appreciate the time and energy dedicated to evaluating this manuscript, and we recognise how the constructive and critical\nfeedback has helped us improve the quality and clarity of the work presented.\nHowever, I have one major issue with the manuscript and that is the lack of discussion of the interannual variability in the Maud Rise\nbloom. The authors briefly mention that it may be related to the warm water halo (line 88) and discuss the processes that cause the\nwarm water halo (lines 132-146). Given the satellite data included in the paper, it should be very possible to pursue this idea. Does\nthe magnitude of the bloom track the intensity and extent of the warm water halo? Given the arguments that the authors make about\nthe causes of the bloom (the upwelling of warm Weddell Deep Water) it is very odd that they do not pursue this line of inquiry. The\nstrength of the upwelling should have a direct impact on the measured SST. The authors state that the impact of this paper, and how\nit is different from previous similar studies of the Taylor column and the Maud Rise bloom, is that they are characterizing “the link\nbetween the specific ocean dynamics around Maud Rise and the timing and intensity of the phytoplankton bloom developing over it.”\n(Lines 51-53). But the authors do not currently meet this threshold. This needs to be addressed.\nThank you for bringing this to our attention. By being in the details of the analysis we may have missed this crucial part of the study,\nand your comment has considerably helped us to bring this SST-chl relationship more prominently into the study. As you will see\nwith the figure 3 below, there is indeed a remarkably strong relationship between SST and chl and therefore between the ocean\ndynamics and the phytoplankton bloom over Maud Rise. We now bring these updated findings into the manuscript. Thank you again\nfor this insightful point.\nIn more details, we added the two figures below to the main manuscript and to the section A of the appendix as well as the following\nparagraph at the end of the section 3.1 “A recurring halo-shaped phytoplankton bloom over Maud Rise” (lines 101 - 110) :\nThe strength of the phytoplankton bloom show strong interannual variability but moderate spatial variability, with recurring high\nbiomass along the flanks of Maud Rise (Figures 2a and A1). The warm water pattern generally matches the phytoplankton bloom\npattern (Figures A1 and A2). The correlation map (Figure 3) between the monthly time series of SST and Chl-a concentration\nindicates a significantly positive correlation on the flanks of the Maud Rise (nearly r = 0.7 on the eastern side directly facing the\nincoming flow of the Weddell gyre). Therefore, we suggest that the interannual variability of the chlorophyll halo can, to a great\nextent, be explained by the interannual variability of the warm water halo [35]. In addition, we suggest that anomalies in easterly\nwinds may be responsible for enhanced upwelling in this part of the Southern Ocean and, therefore, lead to higher phytoplankton\nbiomass as shown by [10] in the region directly Southeast of Maud Rise.\nFigure A2 : Annual climatologies of the warm water halo. Spatial maps of SST concentration obtained by averaging satellite\ndata over December for each year. Isobaths, plotted as black solid contours, are drawn every 500 meters. The 3500 meter isobath is\ndrawn in white.\nFigure 3 : Correlation map between the SST and the Chl-a. The two time series used to compute the correlation map are the\nmonthly time series used to plot the Figures A1 and A2. Isobaths, plotted as black solid contours, are drawn every 500 meters. The\n3500 meter isobath is drawn in white.\nMedium-sized issues:\nThe authors rely heavily on the presence of a Taylor column/cap above Maud Rise. The authors need to define this term. The\nauthors use the terms Taylor cap and Taylor column interchangeably, do they mean the exact same thing? A sentence about the\ngeneral characteristics of a Taylor column and its impacts on circulation.\nThe following paragraph was added to the introduction (lines 45-49) :\nAccording to the Taylor-Proudman theorem, the water column above a seamount is stiffened by Earth’s rotation which effectively\ntraps the water masses within the column and blocks any incoming flow as if the seamount extended up to the surface [25, 31,32].\nThis concept known as Taylor column is a simple theoretical model, but describes well the Maud Rise ocean dynamics. The\nhydrography of the stagnant water masses overlying the seamount can significantly differ from the incoming flow [16].\nThe term “Taylor cap” has been replaced by “Taylor column” throughout the manuscript. We thank the review for bringing this detail\nup to our attention.\nThe authors also need to characterize the spatial extent and position of the Taylor column, beyond the temperature and salinity\nsections. Ideally the approximate boundaries of the column would be calculated overlayed as a contour on any of the numerous map\nfigures included in the paper. Given how strongly the circulation above Maud Rise differs from the circulation around Maud Rise, this\nshould be visible in surface current data from satellite altimetry. Alternatively, if the average position/extent of the column has been\ncharacterized in another paper, the position of the column from that paper could be used. The Taylor column is such a fundamental\npart of this work, and it needs to be visualized.\nThank you for raising this point. As the reviewer suggests, the Taylor column should be visible in surface current data, and indeed, a\ndoming is visible in the climatology of the sea surface height (SSH), which we added to figure 2g in the new version of the\nmanuscript (see below). However, even though the theory of the Taylor column is based on the rotating shallow water hypothesis, at\nMaud Rise, the Taylor column and its SSH signal are attenuated by the stratification (isopycnals doming) (Chapman et Haidgovel,\n1991). Previous simulations-based work in the literature have shown that, under a strong flow, the Taylor column can be swept off\nfrom the seamount (Chapman et Haidgovel, 1991). The warm water halo is well hugged by the 3500 meter isobath and we decide to\nuse it as an approximate characterization of the Taylor column's extent. For clarity, the 3500 meter isobath has been added to\nmultiple maps across the manuscript (Figure 1b, 2d.g, 3, 4a-b, A1, A2, A3, and C1a-b).\nReference :\nChapman et Haidgovel, 1991, Formation of Taylor caps over a tall isolated seamount in a stratified ocean.\nhttps://doi.org/10.1080/03091929208228084\nFigure 2 : Characterization of the halo structure through climatologies of the satellite observations. Seasonal cycle of (a)\nChl-a concentration, (b) Sea Surface Temperature (SST) and (c) fraction of sea ice cover over the area of the Maud Rise (5W-10E,\n62S-68S). For each of those three variables, each year from 1998 to 2023 is plotted as a solid black line except the cruise year\n2021-2022 represented as a solid blue line. The solid red line is the average over the period 1998-2023 and the red filled area\nrepresents the variation by one standard deviation. (d) January climatology of the satellite Chl-a concentration between 1998 to\n2023. (e) Climatology of sea ice concentration over the period mid-November to mid-December between 1998 and 2022. (f)\nDecember climatology for SST averaged between 1998 to 2022. (g) January climatology for sea surface height (SSH) averaged\nbetween 1998 to 2023. The thick blue/white line in (d), (e), (f), (g) is the 3500 meter isobath.\nThe oceanographic setting of the Maud Rise bloom is difficult to follow. Specifically, the pattern of sea ice retreat, its position relative\nto the Weddell gyre, and its location relative to the rest of the Weddell Sea. The authors mention the Maud Rise polynya several\ntimes, and then transition to talking about the bloom. They discuss the polynya as a winter phenomenon, but when the winter\npolynya opens, it also impacts sea ice retreat (sea ice retreats significantly earlier) and therefore primary production. Perhaps a\nclimatology of sea ice concentration of a slightly larger region for each of the months during sea ice retreat in the Supplemental. It\nwas hard to visualize how sea ice retreats through this area.\nWe thank the reviewer for bringing this up to our attention. Based on the reviewer’s comment, a climatology of sea ice concentration\nand the major oceanographic currents of a slightly larger region encompassing the Weddell Gyre were plotted and added to the\nappendix (Figure A3). We hope that this provides the reader with a broader context of the connections between the Maud Rise, sea\nice concentration and the phytoplankton blooms described in this study.\nFigure A3 Sea Ice Concentration Climatology of the Austral Summer : spatial maps of sea ice concentration obtained by\naveraging satellite data over 4 different periods from 1998 to 2022. The 3800 meter isobath is drawn as a solid black line. ACC,\nWDW and CDW stand for Antarctic Circumpolar Current, Weddell Deep Water and Cold Deep Water respectively.\nThe authors mention the topography of the Weddell region, but only ever show a very close-cropped view of Maud Rise. They also\nmention that Maud Rise is positioned in the westward flowing limb of the Weddell Gyre, but there is no figure showing the Weddell\nGyre. Authors need a figure showing the regional bathymetry and average position of the relevant part of the Weddell Gyre. The fact\nthat Maud Rise falls in the path of the gyre seems important to the presence of the Taylor column. Only people who work specifically\nin the Weddell region will have an image of this in their minds.\nMore details on the oceanographic setting of Maud Rise and the associated bloom, related to the above comments, need to be\nadded to the text as well. Lines 45-53 region.\nIn response to the reviewer’s comment, and as described in our response to the previous comment, arrows representing the major\noceanographic currents in the Weddell Gyre were added to the new figure A3 showing the development of the Maud Rise Polynya,\nthus giving a better insight of the general circulation in the Weddell Gyre and around Maud Rise.\nFigure 3 is very important to the arguments being made in the paper, and extremely difficult to make sense of. There has got to be\nan easier way to label figure 3. Use different symbols and then put those symbols above the lines on the sections. Write above the\nsections when the float is above the rise and when it isn’t. Just something to make this more digestible. It is too hard to digest this\nfigure and even when I thought I had the orientation of everything correct, I wasn’t sure. Considering the importance of this figure to\nthe paper it needs clearer annotation. Figure C1 is much clearer and easier to get oriented to.\nWe thank the reviewer for this suggestion that improved the readability of this Figure. In response, we added different symbols\nwhere the seaglider was deployed and above the corresponding lines, to better link the sections with the maps (see the new version\nof the figure 3).\nFigure 3 High resolution probing of the halo by seaglider during the austral summer 2021-2022 : (a) Maximum temperature\nmeasured between 0 and 200 meters by the glider survey. (b) Glider observed integrated Chl-a concentration between 0 and 200\nmeters. For both (a) and (b), isobaths are drawn every 200 meters and the 3500 meter isobath is shown in blue. Blue lines in the\nglider sections of (c) Chl-a, (d) temperature, and (e) salinity correspond to checkpoints marked by different blue-bounded white\nsymbols in a) and b). Isopycnals are drawn every 0.1 kg/m^3 between 1023.0 and 1030.0 kg/m^3. The white star and hexagon in (a)\nand (b) marks where the time series shown in (c), (d) and (e) begin and end. The thick black line in (c), (d),(e) is the mixed layer\ndepth based on the definition used by Xing et al, 2012.\nThe authors need to provide the data for the integrated chlorophyll values that they calculated from Argo float profiles. This is the\ndata on which the importance of the Maud Rise bloom is based.\nThe link to the data used for assessing the importance of the Maud Rise bloom was mistakenly put in the Acknowledgement section.\nThe Data availability section now reads (lines 202-217) :\nThe satellite observations used in this study are based on the \"Global Ocean Colour Plankton and Reflectances MY L3 daily\nobservations\" dataset with daily and 4 km × 4 km resolutions, the \"Global Ocean Gridded L4 Sea Surface Heights And Derived\nVariables Reprocessed 1993 Ongoing\" dataset with daily and 0.125°× 0.125° resolutions, and the \"Global Ocean OSTIA Sea\nSurface Temperature and Sea Ice Reprocessed\" dataset with daily and 0.05° × 0.05° resolutions. The three datasets extend from\n1998 to 2022 (SIC, SST) or 2023 (CHL-a, SSH). The satellite-derived Chl-a concentration is evaluated by observations in the visible\ndomain of the ocean surface implying that cloud cover and sea ice hinder the retrieval of upper ocean Chl-a concentrations [42]. The\nsea surface temperature and sea ice concentration estimates used in this study rely on the sea surface reflectance in the near\ninfrared domain which is little affected by clouds but is subject to sea ice reflectance interferences and possible sensor calibration\nissues. Based on the satellite-derived sea ice concentration, we define the fraction of the Maud Rise area covered by sea ice as the\nnumber of pixels with sea ice concentration above 15% in the domain (5°W-10°E, 62°S-68°S) divided by the total number of pixels in\nthe same domain. Chl-a concentration derived from satellite imagery matches the measurements in situ (Figures 1 b and 1 d). The\ncorrelation map (Figure 3) was computed first by calculating the yearly time series of Chl-a concentration and sea surface\ntemperature (Figures A1 and A2) then computing the Pearson correlation coefficient map for these two time series.\nSmall issues:\nIn the caption for Figure 1 the Authors mention isopycnals drawn on the sections, but I cannot find the details of how density was\ncalculated. This needs to be a sentence in the methods.\nThe following sentence was added at the end of the subsection “In-situ Observations” of the “Methods” section (lines 242-244) :\nIsopycnals were calculated based on the Thermodynamic Equation of Seawater - 2010 via the gsw-python library that calculates\nin-situ density of seawater from absolute salinity and in-situ temperature.\nFor figure 2e, the caption mentions sea ice contours, but all I see are bathymetry contours.\nWe thank the reviewer for noticing this mistake. We decided not to draw sea ice contours as we thought it does not significantly help\nto understand the figure 2e. Therefore, we corrected the caption to no longer mention sea ice contours. We also decided to remove\nthe sea surface temperature contours for the same reasons in figure 2f."}, {"round": 2, "reviewer_comments": {"reviewer_2": "Review Round 2\nThis paper describes the phytoplankton bloom that forms around Maud Rise in the eastern\nWeddell Sea. They link phytoplankton abundance to upwelling of warm/salty deep water layer\ndue to the mesoscale dynamics on the edges of the Taylor column. They show the thinning of\nthe winter water layer and the intrusion of the warm/salty deep water with glider, argo float,\nand ctd data. They have also now linked changes in chlorophyll a concentration with changes in\nSST indicative of the upwelling of the warm deep water.\nI very much want to see this paper published. I think the findings are very interesting. However,\nI still think there are issues that need to be addressed. I’ve detailed them in three sections\nbelow.\nStats reporting\nThe analysis of the relationship between chl and sst in Figure 3 does a good job of addressing\nmy comments about interannual variability from the last round of reviews. However, there\nneeds to be more information given on the relationship identified beyond just the correlation\ncoefficient. I think it would be appropriate to report the p-value and the slope of the\nrelationship. I don’t expect that the p-value would tell a very different story than the correlation\ncoefficient already presented, but it should be included. The slope gives us additional\ninformation on the magnitude of the relationship not just the strength of it. How much does the\nchl change with increasing or decreasing SST? I would recommend adding these to the\nsupplemental and referencing them briefly in the text.\nI also feel like you are leaving something on the table by not taking your analysis further by now\nlooking at the relationship between SSH and SST, or chl and SSH. It would be a very powerful\nconnection if there is in fact a significant correlation between either of these variables and SSH.\nThen you tie the strength of the upwelling (and the bloom) to the strength of the taylor column.\nI’m not going to insist on it, but it would be an excellent addition to your narrative. And\nseemingly with all of the data you have put together and the code you have presumably already\ndeveloped, it should be a relatively easy analysis.\nData availability\nYou need to make the data that you used in your analyses available publicly. In my last review I\nasked for the publication of the Argo data that you used to make the maps in Figure A4, and you\nresponded with a paragraph of text about satellite data. Clearly this was a mistake, however the\ndata still have not been published.\nI mocked up a version of the same figure as A4 from some float profiles that I have for the\nPacific sector of the SO that shows all of the profiles with integrated chl (down to 200 meters)\nabove 100 mg m-2. There are a lot more than what you show in your figure A4. Out of 1227\nprofiles I have 146 with integrated chl above 100 mg m-2 (11%) versus the 39 out of 8560 that\nyou found (0.4%).\nIt’s not that I don’t believe your results, it’s that I don’t think you provide enough information\nfor me to get the same answer. You hang the importance of the Maud Rise bloom on the\nmagnitude relative to other blooms in the Southern Ocean, at a minimum you need to publish\nthese data, and you should also add more detail on the chl fluorescence corrections used. If the\nversion of the Argo chl data that you used is a publicly available product, then link to that. But\nthe link to the Argo website and the original (presumably uncorrected) float data results in the\nanswer that I show in the figure below. I need to understand why it’s different and how to get\nthe answer that you get, and then you need to provide the data.\nI didn’t ask for you to publish the satellite data in my last review, because you were only using\nthem for descriptive purposes, but now you are running statistical analyses and you need to put\nthe data that you used in a public repository. This includes the data for the images in figures A1\nand A2 (the December averages for chl and sst for each year) as well as the data used to\ncalculate the sea ice and SSH climatologies in Figure 2. Linking to the website that provides the\ndaily satellite images is not enough. This would require someone to download daily images for\nthe relevant time period for each year (~775 images for each of 4 different satellite products)\nand then create averages for each year, just to evaluate your findings. You do state that code to\nmake the figures is available upon request. This is not the current standard for publication. The\ndata need to be publicly available. You should probably include the glider data too.\nDeep iron concentrations\nIn reading through the comments from Reviewer 1, my attention was drawn to the source of\nthe iron being delivered from the deep water, and I went down a bit of a rabbit hole and found\nseveral issues. As reviewer 1 identified, you do not really resolve the issue of whether the water\nmass being upwelled does in fact have enough Fe to generate the observed bloom. And the\nchanges that you made in response to their comment, do not really address this issue.\nThis needs to be addressed but I think it is a simple fix. In your response to reviewer 1 you\nmention hydrothermal Fe and the elevated 3He found in the eastern Weddell, but you never\nmention findings on this in the paper and only reference papers that say this without ever\nmentioning the word hydrothermal. I have read the Moreau paper that you reference\npreviously, but in looking back at it, one of the transects that measures 3He, practically runs\nover Maud Rise. You can directly address the origin of the Fe by plotting a section of the 3He\nfrom this transect and then do a water mass analysis that shows that the elevated 3He is in fact\nassociated with the WDW water mass. This was the method used to link phytoplankton blooms\nnear the SWIR to hydrothermal vents upstream in the Ardyna paper that you also reference,\nand would satisfy me that we could reasonably expect elevated Fe concentrations in the WDW\nwater mass.\nTo satisfy my own curiosity and to not be that reviewer that sends you down a horribly\nunproductive blind alley, I took a look at the 3He data for the transect in question (shown\nbelow; bottom left shows the map with the 3He transect shown in the top right outlined in red,\ntop left shows a map of the maximum 3He value for each profile in color, and bottom right is a\nTS plot with 3He shown in color). You can actually see in the section plot where the transect\npasses over the shoulder of Maud Rise at 65S and there is clearly an elevated 3He anomaly\nthere. This should be a pretty straightforward connection to make simply by referencing the T/S\nrange of your warm/salty water mass and mapping it onto the T/S space for the transect.\nLastly, the paper that you reference when you first mention Weddell Deep Water (Lines 50-52),\ndoes not say anything about Weddell Deep Water, it is only about the warm water halo around\nMaud Rise. I think it would be wise to reference a paper that actually discusses this water mass\nand the properties associated with it. I found a few of these (in a search to satisfy my own\ncuriosity) and also found that this water mass is usually referred to as Warm Deep Water,\ninstead of Weddell Deep Water. I leave it up to you to decide on your naming conventions, but\nwhatever paper you reference for a detailed description of this water mass should use the same\nname. I think including a reference that provides more details about this water mass is\nnecessary as its properties are important to your conclusions, and many people will be looking\nfor more information."}, "author_response": "We thank the reviewer for the positive and constructive comments about our paper. We\nrespond below to all these new comments. We believe that they have significantly improved the\nquality of the manuscript and we hope that both the editor and the reviewer will find this new\nversion of the paper acceptable for publication in Nature Communication.\nStats reporting\nThe analysis of the relationship between chl and sst in Figure 3 does a good job of addressing\nmy comments about interannual variability from the last round of reviews. However, there\nneeds to be more information given on the relationship identified beyond just the correlation\ncoefficient. I think it would be appropriate to report the p-value and the slope of the\nrelationship. I don’t expect that the p-value would tell a very different story than the correlation\ncoefficient already presented, but it should be included. The slope gives us additional\ninformation on the magnitude of the relationship not just the strength of it. How much does the\nchl change with increasing or decreasing SST? I would recommend adding these to the\nsupplemental and referencing them briefly in the text.\nWe thank the reviewer for suggesting that we include the p-value and slope. Statistically\nsignificant (p-value < 0.05) regions were hatched on the correlation maps. The slopes of the\nSSH-SST and Chl-a - SST correlation maps were added as a single figure in the appendix (Figure\nA4).\nI also feel like you are leaving something on the table by not taking your analysis further by now\nlooking at the relationship between SSH and SST, or chl and SSH. It would be a very powerful\nconnection if there is in fact a significant correlation between either of these variables and SSH.\nThen you tie the strength of the upwelling (and the bloom) to the strength of the taylor column.\nI’m not going to insist on it, but it would be an excellent addition to your narrative. And\nseemingly with all of the data you have put together and the code you have presumably already\ndeveloped, it should be a relatively easy analysis.\nAlthough no interesting correlation was found between SSH and Chl-a concentration, a positive\ncorrelation between SSH and SST was found as explained in a new paragraph (lines 112-115)\nadded at the end of the section titled “A recurring halo-shaped phytoplankton bloom over\nMaud Rise”:\nA significant correlation between the SST and SSH yearly timeseries was also observed, with\nvalues reaching up to r ≈ 0.6 on the Northeastern flank that directly faces the incoming\nwestward flow (Figure 3b). These two correlations, between SST and SSH between and SST and\nthe Chl-a concentration, point towards the pivotal role of ocean-topography interaction in the\nformation of chlorophyll halos as discussed in the next section.\nThe correlation map was added in the main text as Figure 3b).\nData availability\nYou need to make the data that you used in your analyses available publicly. In my last review I\nasked for the publication of the Argo data that you used to make the maps in Figure A4, and you\nresponded with a paragraph of text about satellite data. Clearly this was a mistake, however the\ndata still have not been published.\nWe apologize for this oversight in our previous response to the reviewer’s comment. As\ndescribed below, we now provide all the BGC-Argo float data used in this paper.\nI mocked up a version of the same figure as A4 from some float profiles that I have for the\nPacific sector of the SO that shows all of the profiles with integrated chl (down to 200 meters)\nabove 100 mg m-2. There are a lot more than what you show in your figure A4. Out of 1227\nprofiles I have 146 with integrated chl above 100 mg m-2 (11%) versus the 39 out of 8560 that\nyou found (0.4%).\nIt’s not that I don’t believe your results, it’s that I don’t think you provide enough information\nfor me to get the same answer. You hang the importance of the Maud Rise bloom on the\nmagnitude relative to other blooms in the Southern Ocean, at a minimum you need to publish\nthese data, and you should also add more detail on the chl fluorescence corrections used. If the\nversion of the Argo chl data that you used is a publicly available product, then link to that. But\nthe link to the Argo website and the original (presumably uncorrected) float data results in the\nanswer that I show in the figure below. I need to understand why it’s different and how to get\nthe answer that you get, and then you need to provide the data.\nWe apologize for not describing this more clearly in the previous versions of the article. We\nused BGC-Argo floats data from the SOCCOM program to produce these results. For each float,\nboth an uncorrected Chlorophyll-a variable (named “Chl_a”) and a corrected Chlorophyll-a\nvariable (named “Chl_a_corr”) are provided. We have consistently used the corrected\nChlorophyll-a variable (“Chl_a_corr”) which is consistently smaller than the uncorrected\nChlorophyll-a variable (“Chl_a”). We wonder if the reviewer used the uncorrected Chlorophyll-a\nvariable to produce the results mentioned above.\nIn detail, the \"Chl_a_corr\" variable provided with SOCCOM BGC-Argo floats contains corrected\nchlorophyll-a values, where several post-processing steps have been applied to improve\naccuracy, including dark correction to minimize sensor baseline drift, NPQ (non-photochemical\nquenching) correction to account for the underestimation of fluorescence in sunlit surface\nwaters, and quality control steps in accordance with BGC-Argo standard protocols.\nHowever, to make sure that there are no further doubts on the type of data we are using, and as\nthe reviewer suggested, in the new version of the manuscript, we now provide a link to the\nBGC-Argo floats data used as well as the code used to produce the integrated Chl-a biomass\npresented in the manuscript (Figure B1). We hope that the reviewer will find this approach\nacceptable.\nLine 301 : The code and files used to produce Figure B1 are available at the following location:\nhttps://doi.org/10.5281/zenodo.16935712.\nI didn’t ask for you to publish the satellite data in my last review, because you were only using\nthem for descriptive purposes, but now you are running statistical analyses and you need to put\nthe data that you used in a public repository. This includes the data for the images in figures A1\nand A2 (the December averages for chl and sst for each year) as well as the data used to\ncalculate the sea ice and SSH climatologies in Figure 2. Linking to the website that provides the\ndaily satellite images is not enough. This would require someone to download daily images for\nthe relevant time period for each year (~775 images for each of 4 different satellite products)\nand then create averages for each year, just to evaluate your findings. You do state that code to\nmake the figures is available upon request. This is not the current standard for publication. The\ndata need to be publicly available. You should probably include the glider data too.\nThe data used for plotting figures 2e, 2g, A1 and A2 is now publicly available at\nhttps://doi.org/10.5281/zenodo.16935712. This is mentioned in the text, lines 248-249.\nWe added the following link for the Seaglider data (lines 264-265):\nSeaglider data are available at https://zenodo.org/records/15228200.\nReferencing the float numbers and website for the float data that you used seems fine as those\nare curated in a way that makes them very easy to access.\nDeep iron concentrations\nIn reading through the comments from Reviewer 1, my attention was drawn to the source of\nthe iron being delivered from the deep water, and I went down a bit of a rabbit hole and found\nseveral issues. As reviewer 1 identified, you do not really resolve the issue of whether the water\nmass being upwelled does in fact have enough Fe to generate the observed bloom. And the\nchanges that you made in response to their comment, do not really address this issue.\nThis needs to be addressed but I think it is a simple fix. In your response to reviewer 1 you\nmention hydrothermal Fe and the elevated 3He found in the eastern Weddell, but you never\nmention findings on this in the paper and only reference papers that say this without ever\nmentioning the word hydrothermal. I have read the Moreau paper that you reference\npreviously, but in looking back at it, one of the transects that measures 3He, practically runs\nover Maud Rise. You can directly address the origin of the Fe by plotting a section of the 3He\nfrom this transect and then do a water mass analysis that shows that the elevated 3He is in fact\nassociated with the WDW water mass. This was the method used to link phytoplankton blooms\nnear the SWIR to hydrothermal vents upstream in the Ardyna paper that you also reference,\nand would satisfy me that we could reasonably expect elevated Fe concentrations in the WDW\nwater mass.\nTo satisfy my own curiosity and to not be that reviewer that sends you down a horribly\nunproductive blind alley, I took a look at the 3He data for the transect in question (shown\nbelow; bottom left shows the map with the 3He transect shown in the top right outlined in red,\ntop left shows a map of the maximum 3He value for each profile in color, and bottom right is a\nTS plot with 3He shown in color). You can actually see in the section plot where the transect\npasses over the shoulder of Maud Rise at 65S and there is clearly an elevated 3He anomaly\nthere. This should be a pretty straightforward connection to make simply by referencing the T/S\nrange of your warm/salty water mass and mapping it onto the T/S space for the transect.\nThanks for the careful analysis to help us improve our argument that iron is upwelled from\nWDW. Following the reviewer's suggestion we added the following paragraph to the new\nversion of the manuscript, lines 163-176, as well as 3 figures in the appendix, Figure C1,C2 and\nC3 :\nWe verified this iron supply hypothesis by using the GLOPDATA database and its δ3He\nmeasurements throughout the Southern Ocean. 3-helium (3He) is released to the deep ocean by\nhydrothermal vents along with dissolved iron. There it can be transported thousands of\nkilometers before being upwelled closer to the surface to support primary production (Riesing et\nal., 2015; Ardyna et al., 2019). Therefore, measurements of δ3He can be used as a proxy for\nestimating the presence of hydrothermal iron. A transect of δ3He along the 0° meridian from\n60°S to 70°S (figure C1) gives a cross-section view of Maud Rise (approximately 65°S-66°S). We\nnoticed two δ3He anomalies above 9 percent at the flanks of the seamount (68°S-66°S and\n65°S-64°S) in the range of 200-900 m (figure C2). By looking at two additional transects of\ntemperature and salinity, we find that the water masses, where the two δ3He anomalies are,\nhave a salinity in the range 34.50-34.75 PSS-78 and temperature above 1°C (figure C2). These\nwater masses are warmer, saltier, higher in δ3He and expectedly higher in dissolved iron\nconcentration of hydrothermal origin than other water masses in the area\n(20°W-20°E,50°S-70°S), hence supporting the hypothesis of iron supply from the deep ocean\n(measurements of these water masses are within the black rectangle in Figure C3).\nLastly, the paper that you reference when you first mention Weddell Deep Water (Lines 50-52),\ndoes not say anything about Weddell Deep Water, it is only about the warm water halo around\nMaud Rise. I think it would be wise to reference a paper that actually discusses this water mass\nand the properties associated with it. I found a few of these (in a search to satisfy my own\ncuriosity) and also found that this water mass is usually referred to as Warm Deep Water,\ninstead of Weddell Deep Water. I leave it up to you to decide on your naming conventions, but\nwhatever paper you reference for a detailed description of this water mass should use the same\nname. I think including a reference that provides more details about this water mass is\nnecessary as its properties are important to your conclusions, and many people will be looking\nfor more information.\nWe thank the reviewer for pointing out this inconsistency. Weddell Deep Water has been\nreplaced by Warm Deep Water throughout the text. We thank the reviewer for providing us\nwith references about characteristics of the Warm Deep Water. We included these references in\nour text line 50."}, {"round": 3, "reviewer_comments": {"reviewer_2": "Review Round 3\nThis paper describes the phytoplankton bloom that forms around Maud Rise in the eastern\nWeddell Sea. They link phytoplankton abundance to upwelling of warm/salty deep water layer\ndue to the mesoscale dynamics on the edges of the Taylor column. They show the thinning of\nthe winter water layer and the intrusion of the warm/salty deep water with glider, argo float, and\nctd data. In addition to the previous addition showing the connection between changes in\nchlorophyll a concentration and changes in SST they have now added a figure showing the\nconnection between changes in SST with changes in SSH.\nThe authors have addressed all of my previous comments to my satisfaction. I have one more\ngeneral comment, and a few specific comments shown below. I believe these are all very easily\nhandled with very minor changes to the manuscript.\nGeneral comment:\nIn the section where you talk about the relevance of the Maud Rise Bloom and use the Argo\nfloat data to highlight how productive it is compared to other blooms in the Southern Ocean, you\nhighlight that Maud Rise is responsible for 11 of the 39 profiles you find in the Argo record with\nintegrated Chl over 100 mg m-3. Unless I am misunderstanding something this feels like an\noverstatement. Not that it isn’t true, but what I understand from your paper is that these profiles\nare from the float that you deployed into these productive waters, while the other high chl\nprofiles were encountered by chance. If I have misunderstood this, please make it clearer in\nyour paper that this was not the float shown in Figure 1b that was specifically deployed into the\nbloom. If I have this correct, then it is inappropriate to play this up as you have introduced a\nsampling bias into the integrated chl data set by deploying a float into a known area of high chl\nand then comparing it to the frequency of other regions of high chl in the rest of the dataset.\nThis needs to be made clear in your text. Either by clarifying that this data is from a different\nfloat or by acknowledging the sampling bias and addressing it directly in the text.\nSpecific comments:\nFigure 1 caption: You say that (d) and (e) show a longitudinal transect when it is clearly\nlatitudinal, judging from the x-axis which shows it going from east to west. If you’re looking to\nsay this in a single word zonal or latitudinal work.\nFigure 4 caption: Add the definition that you use for MLD back to the caption. You had it there in\na previous version, please put it back.\nFigure 4: You don’t include the symbols above the sections that correspond to the symbols in\nthe glider track maps. You had these in the last version. Please add them back.\nLine 163: do you mean GLODAP instead of GLOBDATA?\nLine 168 and Line 171: you left the “He” out of 𝛿3He\nFigure A4: Adjust the colorscale in Figure A4a to fit the range of the data which appear to go from -1 to 1 rather than -5 to 5.\nFigure A4: Add hatching to indicate area where p-value was less than 0.05 on both A4 figures\nCasey Schine"}, "author_response": "We thank the reviewer for the positive and constructive comments about our paper. We respond\nbelow to all these new comments. We hope that both the editor and the reviewer will find this\nnew version of the paper acceptable for publication in Nature Communication.\nGeneral comment:\nIn the section where you talk about the relevance of the Maud Rise Bloom and use the Argo\nfloat data to highlight how productive it is compared to other blooms in the Southern Ocean, you\nhighlight that Maud Rise is responsible for 11 of the 39 profiles you find in the Argo record with\nintegrated Chl over 100 mg m-3. Unless I am misunderstanding something this feels like an\noverstatement. Not that it isn’t true, but what I understand from your paper is that these profiles\nare from the float that you deployed into these productive waters, while the other high chl\nprofiles were encountered by chance. If I have misunderstood this, please make it clearer in\nyour paper that this was not the float shown in Figure 1b that was specifically deployed into the\nbloom. If I have this correct, then it is inappropriate to play this up as you have introduced a\nsampling bias into the integrated chl data set by deploying a float into a known area of high chl\nand then comparing it to the frequency of other regions of high chl in the rest of the dataset.\nThis needs to be made clear in your text. Either by clarifying that this data is from a different\nfloat or by acknowledging the sampling bias and addressing it directly in the text.\nWe thank the reviewer for pointing out this lack of information. The 39 profiles do not include the\nfloats deployed over Maud Rise. We clarified this in the text lines 182-183 :\nThese 39 profiles do not include the 2 floats which were deployed over Maud Rise during the\nsummer 2021-2022 to avoid any sampling bias.\nSpecific comments:\nFigure 1 caption: You say that (d) and (e) show a longitudinal transect when it is clearly\nlatitudinal, judging from the x-axis which shows it going from east to west. If you’re looking to\nsay this in a single word zonal or latitudinal work.\nWe apologize for this mistake. We actually meant zonal. This has been corrected.\nFigure 4 caption: Add the definition that you use for MLD back to the caption. You had it there in\na previous version, please put it back.\nThe MLD definition that we used was placed in the Methods section. We put it back at the end\nof the caption of Figure 4 as in the previous version.\nFigure 4: You don’t include the symbols above the sections that correspond to the symbols in\nthe glider track maps. You had these in the last version. Please add them back.\nWe thank the reviewer for pointing out this problem. We changed the pixel resolution of the\nfigure and during the process, the symbols went astray. This has been corrected as shown\nbelow :\nLine 163: do you mean GLODAP instead of GLOBDATA?\nWe thank the reviewer for bringing this typo to our attention. We meant GLODAP. This has been\ncorrected\nLine 168 and Line 171: you left the “He” out of 𝛿3He\nWe thank the reviewer for notifying us about this typo. It has been corrected.\nFigure A4: Adjust the colorscale in Figure A4a to fit the range of the data which appear to go\nfrom -1 to 1 rather than -5 to 5.\nFigure A4: Add hatching to indicate area where p-value was less than 0.05 on both A4 figures\nWe thank the reviewer for pointing out this inconsistency. We scaled back the colorscale of\nFigure A4a) to [-1:1] and we hatched the areas where the p-value is less than 0.05 on both\nFigures A4 a) and b)."}]

Decision letter