# A chlorophyll halo over Maud Rise in the Southern Ocean

Bertrand Ducrocq, Nadine Steiger, Marcel Du Plessis, Jean-Baptiste Sallée, Sébastien Moreau, Sebastiaan Swart

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Bertrand Ducrocq, Nadine Steiger, Marcel Du Plessis, Jean-Baptiste Sallée, Sébastien Moreau, et al. A chlorophyll halo over Maud Rise in the Southern Ocean. Nature Communications, 2025, 16, pp.11302. 10.1038/s41467-025-66458-5. hal-05500456

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# A chlorophyll halo over maud rise in the southern ocean

Received: 2 December 2024

Accepted: 7 November 2025

Published online: 02 December 2025

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Bertrand Ducrocq Ⓔ<fnref n="1" />✉, Nadine Steiger Ⓔ<fnref n="1" />, Marcel du Plessis Ⓔ², Jean-Baptiste Sallée Ⓔ<fnref n="1" />, Sébastien Moreau Ⓔ³ & Sebastiaan Swart²,⁴

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.

Southern Ocean primary production is known to be limited by a lack of iron and light<fnref n="1" />,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 sea floor topography³,¹¹.

Maud Rise is a 3300 m tall seamount isolated in the 5000m-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

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<footnote n="1">¹LOCEAN, Sorbonne Université/CNRS/IRD/MNHN, Paris, France. ²Department of Marine Sciences, University of Gothenburg, Gothenburg, Sweden. ³Norwegian Polar Institute, Tromsø, Norway. ⁴Department of Oceanography, University of Cape Town, Rondebosch, South Africa.</footnote>

<footnote>e-mail: bertrand.ducrocq@ac-orleans-tours.fr</footnote>
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 halo<sup>23,34</sup>. 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 2017<sup>17,18</sup>. However, the link between the specific ocean dynamics associated with Maud Rise and the timing and intensity of the phyto-plankton bloom developing over it remains 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 phyto-plankton bloom in the context of the wider Southern Ocean to highlight its importance.

## Results and Discussion

### 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

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**Fig. 1 | Observations of the chlorophyll halo during the 2021-2022 austral summer.** a Sea Ice Concentration (SIC) on December 1, 2021. The red box delimits the study area over Maud Rise as in b. b Satellite Chlorophyll-a (Chl-a) concentration averaged over the cruise period (6-16 January 2022). Isobaths are in black and depicted every 500 m. The thick blue contour represents the 3500 m isobath. The marks are the locations of the glider (pink dots), CTD (yellow dots), and BGC-Argo floats (red and blue triangles) profiles. c Depth-profiles of the Chl-a concentration

from two BGC-Argo floats - float 6904235 in red and 6903129 in blue - that sampled across the Maud Rise flanks and inside the Taylor column, respectively. The average Chl-a concentration is plotted as a solid line while the filled area of the same color represents 1 standard deviation. Zonal section of the CTD measurements of Chl-a concentration d and conservative temperature e with isopycnals drawn as black lines every 0.1 kg·m⁻³ between 1125.9 and 1126.6 kg·m⁻³.
<figure><img src="image_7.png" /><figcaption>**Fig. 2 | Characterization of the halo structure from satellite observations.**
Seasonal cycle of a Chl-α concentration, b Sea Surface Temperature (SST) and c fraction of sea ice cover over the area of the Maud Rise (SW-10E, 62S-68S). For each of those three variables, each year from 1998 to 2022 is plotted as a solid black line except the cruise year 2021–2022 represented as a solid blue line. The solid red line is the average over the period 1998-2022 and the red filled area represents the</figcaption></figure>

variation by one standard deviation. **d** December climatology (1998–2023) of the Chl-α concentration. **e** Climatology of sea ice concentration over the period mid- November to mid-December between 1998 and 2022. **f** December climatology (1998–2022) for SST. **g** January climatology (1998–2023) for sea surface height (SSH). The thick blue line in **d–g** represents the 3500 m isobath.
<figure><img src="image_8.png" /><figcaption>**Fig. 3 | Correlation maps of satellite variables.** Correlation maps between a SST and Chl-a concentration and b between SST and SSH. Isobaths, plotted as black solid contours, are drawn every 500 m. The 3500 m isobath is drawn in blue.</figcaption></figure>

Hatched areas show significant correlations ($p$ < 0.05). See Figure S13 for more information on the slope of the regression.

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
<figure><img src="image_9.png" /><figcaption>**Fig. 4** | High resolution views of the halo by an underwater glider. (a) Maximum temperature observed between 0-200 m during the glider survey. (b) Glider-observed integrated Chl-a concentration between 0-200 m. For both (a) and (b), isobaths are depicted every 200 m and the 3500 m isobath is shown in blue. Blue lines in the glider sections of (c) Chl-a, (d) temperature, and (e) salinity correspond</figcaption></figure>

to checkpoints marked by the symbols in (a) and (b). Isopycnals are depicted every 0.1 kg·m⁻³ between 1023.0 and 1030.0 kg·m⁻³. The thick black line in (c), (d), (e) is the mixed layer depth (MLD) based on the 0.03 kg m⁻³ density threshold criteria of De Boyer Montégut et al., 2004⁵⁰.
approximately 34.69 PSS-78 and temperature above 1 °C (Figure S7). These water masses are warmer, saltier, higher in $\delta^3$He 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-$\alpha$⁴ 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-$\alpha$ 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-$\alpha$ 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-$\alpha$ 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-$\alpha$ 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 pump⁴,⁴⁴.

## Concluding remarks and perspectives

We have shown that Maud Rise triggers annual phytoplankton blooms on the flanks of the seamount. The Taylor column above Maud Rise creates upward transport of warm water from the deep WDW layer towards the mixed layer. We propose that the WDW layer is enriched with iron that reaches the surface layer via frontal instabilities developing around the seamount, which fuels the phytoplankton bloom during the austral spring and summer. Other fingerprints of this oceanic upwelling includes a halo-shaped minimum in sea ice concentration in spring (mid November to mid December), consistent with subsurface heat impeding sea ice formation in this area, and enhancing melt rate. We also observe a halo-shaped maximum temperature around the flank of the Rise in December, which might be a consequence of an earlier sea ice retreat or a direct consequence of upwelled warm water. Our study suggest that the halo-shaped structures of sea ice concentration, temperature and chlorophyll are not independent but rather connected by the ocean dynamics stemming from the Taylor column of the seamount. As such, phytoplankton blooms over Maud Rise constitute an example of how marine ecosystems are shaped by mesoscale ocean dynamics and ocean circulation-topography interaction. More investigations are needed concerning the iron supply, the role of grazers on bloom dynamics, as well as the impact of atmospheric forcing on the strength of these blooms. Although trace metal nutrient measurements are complex to

make, further trace metal measurements in open-ocean polynya regions, especially in the presence of shallow topography, would help elucidate the nutrient supply mechanisms between the deep ocean and surface layers. Understanding such dynamics and those elucidated in this study would help disentangle the bio-physical processes occurring in these complex polynya systems, with broader implications for Southern Ocean-wide primary production and carbon cycling.

## Method

### 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 4 km × 4 km resolutions, the “Global Ocean Gridded L 4 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-$\alpha$, SSH). The satellite-derived Chl-$\alpha$ 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-$\alpha$ 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-$\alpha$ concentration derived from satellite imagery matches the measurements in situ (Fig. 1 b and 1d).

The correlation map (Fig. 3) was computed first by calculating the yearly times series of Chl-$\alpha$ 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-$\alpha$ - SST correlation was computed by multiplying the Pearson correlation with the ratio of the Chl-$\alpha$ 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° 27.63 S, 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-$\alpha$ and calibrated in the following way: (i) fluorescence units were converted to Chl-$\alpha$ 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-$\alpha$ 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-$\alpha$ 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-α 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 SBE 9plus CTD Unit and SBE 11plus V5.2 Deck Unit. The initial data processing was done with the SBE Data processing software following the standard filters suggested for the SBE 9plus CTD unit. The conversion of fluorescence to Chl-α (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, conductivity, Photosynthetically Active Radiation, Chl-α 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-α 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 floats 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.webod.awi.de/. Transects were contoured using ODV's DIVA gridding algorithm with a signal-to-noise ratio of 10.

## Data availability

The “Global Ocean Colour Plankton and Reflectances MY L3 daily observations” dataset is available at https://doi.org/10.48670/moi-00282. The “Global Ocean OSTIA Sea Surface Temperature and Sea Ice Reprocessed” dataset can be found at https://doi.org/10.48670/moi-00168. The “Global Ocean Gridded L4 Sea Surface Heights And Derived Variables Reprocessed 1993 Ongo-ing” dataset can be found at https://doi.org/10.48670/moi-00148. The files used to produce Fig. 2,S2 and S3 are available at https://doi.org/10.5281/zenodo.16935712. Seaglider data are available at https://zenodo.org/records/15228200. CTD data are available at https://doi.org/10.17882/95314. The files used to produce Figure S9 are available at https://doi.org/10.5281/zenodo.

16935712. Data of BGC-Argo floats WMO6903129 and WMO6904235 can be found at https://fleetmonitoring.euro-argo.eu/float/6903129 and https://fleetmonitoring.euro-argo.eu/float/6904235. The GLODAPv2.2023 dataset of the The Global Ocean Data Analysis Project (GLODAP) can be found on the main page of the project https://glodap.info/ or can be accessed directly via the ODV-online browser tool https://explore.webod.awi.de/.

## Code availability

All scripts used for generating the plots in this paper are available from the corresponding author upon request.

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## Acknowledgements

This study and the research cruise were funded by the EU's Horizon 2020 research and innovation program under grant agreement N821001 (SO-CHIC). S.S and M.D.d.P. thank those part of the collective effort in piloting the glider during the experiment as well as the captain and crew of the RV SA Agulhas II for the deployment of the glider. S.S. is supported by a Wallenberg Academy Fellowship (WAF 2015.0186) and the Swedish Research Council (VR 2019-04400). M.D.d.P is supported by the European Union's Marie Sklodowska Curie Individual Fellowship under Project ID 101032683. S.M. is supported by the Research Council of Norway (RCN) via the project "I-CRYME: Impact of CRYosphere Melting on Southern Ocean Ecosystems and biogeochemical cycles" (grant number 335512), the Centre of Excellence "iC3: Center for ice, Cryosphere, Carbon and Climate" (grant number 332635) and by the EU for the project "WOBEC: Weddell Sea Observatory and Biodiversity and Ecosystem Change" (grant number 350906). We would like to thank Catherine Schmechtig for calibrating the BGC-Argo floats.

## Author contributions

N.S. and J.-B.S. conceptualized and supervised the study. N.S., M.D.d.P and S.S. performed the curation of the in-situ data. B. D., N.S., M.D.d.P, and S.M. carried the data analysis and figures realization. All authors contributed to interpreting the results, writing and editing of the present paper. J.-B.S. as PI and S.S. as Co-PI of the SO-CHIC program acquired the funding.

## Competing interests

The authors declare no competing interests.
**Additional information**

**Supplementary information** The online version contains supplementary material available at https://doi.org/10.1038/s41467-025-66458-5.

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