Physical transport processes in the fjords of South Georgia

Oetjens, Annika; von Appen, Wilken-Jon; Anselin, Joséphine; Ebner, Berenice; Koch, Florian; Ludwichowski, Kai-Uwe; Mole, Ryan; Zanker, Joanna; Young, Emma; Kasten, Sabine

doi:10.5194/egusphere-2026-1808

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https://doi.org/10.5194/egusphere-2026-1808

Preprints

10 Apr 2026

| 10 Apr 2026

Status: this preprint is open for discussion and under review for Ocean Science (OS).

Physical transport processes in the fjords of South Georgia

Annika Oetjens, Wilken-Jon von Appen, Joséphine Anselin, Berenice Ebner, Florian Koch, Kai-Uwe Ludwichowski, Ryan Mole, Joanna Zanker, Emma Young, and Sabine Kasten

Abstract. The Southern Ocean, typically characterized as a high-nutrient, low-chlorophyll region, exhibits recurring chlorophyll blooms downstream of South Georgia island fuelled by iron enrichment of near-surface water. To better understand the physical processes underpinning iron and nutrient transport from South Georgia fjords to the surrounding shelf, extensive data were collected during research cruise PS133.2 of RV Polarstern in late austral spring 2022. Here, we present analyses of these data to characterize water masses, circulation patterns and nutrient distribution inside Cumberland Bay East and King Haakon Bay including the adjacent shelf. Surface currents are influenced by westerly winds, particularly on the eastern side of the island. Fjord-shelf exchange in Cumberland Bay East exhibit weak export and residence times are on the order of days to months driven by a vertical overturning circulation. The highest temperature and chlorophyll values are measured at the mouth of King Haakon Bay. The results highlight the diverse local environment and the significant influence of bathymetry and orography on fjord systems at South Georgia. This study elucidates the physical transport processes of fjord-shelf exchange that may support transport of iron and nutrients as a source for the downstream chlorophyll blooms in the open ocean. Ongoing investigations will further explore the biological and geochemical aspects, providing a comprehensive understanding of the South Georgia system.

Received: 31 Mar 2026 – Discussion started: 10 Apr 2026

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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Annika Oetjens, Wilken-Jon von Appen, Joséphine Anselin, Berenice Ebner, Florian Koch, Kai-Uwe Ludwichowski, Ryan Mole, Joanna Zanker, Emma Young, and Sabine Kasten

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RC1:
'Comment on egusphere-2026-1808', Anonymous Referee #1, 07 May 2026 reply
Oetjens et al. presents an observational study of circulation around South Georgia Island and the implications for what it means for nutrient transport. While I found the physical results and discussion interesting, the study seems to be confirming a lot of previous findings rather than presenting substantive new ones. My biggest concern, however, is the biogeochemical discussion throughout the paper. The authors do not present sufficient methods to describe how much of the data was analysed and from that data draw several conclusions that are not sufficiently supported. For a paper that is trying to link physical transport processes with trace metals and biological activity, the lack of trace metal data (or phytoplankton physiology data as indication of limitation) is a significant limitation of the study. I go into further detail about my concerns with the biogeochemistry below.

Overall, I do not think this paper is ready to be published in its current form. I would highly recommend the authors work with someone who has a background in iron biogeochemistry to help interpret these results more appropriately.

Major Comments:
The authors need to provide more information on how the fluorescence data was processed. On line 166, the note fluorescence values >15 mg m-3. That is quite high. Was the data corrected for quenching? Are the authors aware of the known issue of higher-than-average ratios of fluorescence : chlorophyll concentrations in the Southern Ocean (Schallenberg et al., 2022)? Since it is not included in the methods, I assume no discrete chlorophyll a samples were taken to calibrate the CTD fluorometer? Furthermore, the authors need to acknowledge uncalibrated fluorescence patterns cannot truly be called chlorophyll a Additionally, chlorophyll a concentrations are a proxy for phytoplankton biomass, not primary production.

Line 195-196: While iron and grazing are difficult to assess. I would suspect the authors should be able to comment on whether light was limiting, particularly with a weather station. Was there a PAR sensor on the CTD or anywhere on the ship? Does the weather station assess cloud cover? Similarly, what was the mixed layer depth? All of this would inform the biogeochemical findings.

Line 203: Fluorescence (especially uncorrected!) and AOU ≠ primary production. These two parameters can provide an idea as to how much phytoplankton biomass is in the region and how much microbial respiration is taking place, but they do not inform the rate of primary production. As far as I can tell, the authors do not have any measurements that would allow them to definitely comment on the magnitude of primary production.

Lines 203-205: I don’t believe you can say the sill is acting as barrier for fjord-shelf exchange based on a high patch of chlorophyll. Without knowing whether the fluorescence (and this is fluorescence, not chlorophyll) was quench corrected, it is hard to say whether that signal is real or the result of photoinhibition in the surface. If the fluorescence signal is real, there is nothing to say it is not a standard subsurface chlorophyll maxima, resulting from the biological influences more than the physical influences.

The paper’s ultimate findings on the physical transport of nutrients and residence time of iron ignores the fact that bioavailable iron is readily assimilated by phytoplankton in the region. This would suggest that a purely physically-based residence time is not appropriate in this type of HNLC region.

Throughout the paper, the authors do not write with proper sentence structure. I would suggest grammatical edits to add commas and differentiate clauses.

Minor Comments:
Line 5: Please say what data you are talking about.

Line 10-11: “This study. . .” is a rather passive sentence. Can you strengthen it as this is an abstract.

Line 39: Capitalize “Island”.

Lines 45-46: After a paragraph that focuses on current trajectories, this statement seems to come out of the blue. I would think the buoyancy flux of iron in this region warrants more discussion and would make an appropriate transition into the next section.

Figure 1b: Can you make “KHB” and “CBE” larger? The labels are a little hard to see.

Line 54: I would mention that the HNLC control area mentioned here also represents the majority of the Southern Ocean which is considered an HNLC.

Lines 57-59: This is a very short paragraph for a very important aspect of this system. Can you expand on this?

Lines 60-62: Citation(s)?

Lines 74-75: Can you explain this? Did Borrione et al. find atmospheric trajectories did not deliver particles to the South Georgia region specifically or was the iron not bioavailable?

Line 76: They can also deliver other necessary trace metals besides Fe, including Cu and Mn.

Line 110: Also cite Eppley and Peterson for the f-ratio.

Line 127: This introduction is overall very long. Is it possible to streamline some of the information to reduce the length of the introduction? I don’t know if all the information is necessary.

Line 134: Capitalize the c in chl and italize the a.

Line 142: NH4+ is ammonium. NH3 is ammonia. Similarly, add the charges to each chemical formula.

Line 155: “Back-scatterers” should not by hyphenated.

Line 164: What data exactly is being recorded by the weather station?

Lines 168-169: You say respectively but have actually reversed the order of fluorescence and temperature values.

Lines 173-174: As you haven’t actually defined T and S, I think there is a better way to say this. I would change this to “The deepest water mass is UCDW with temperatures between 2.0-2.6ºC and salinity between 34.3-34.5 psu(?) which is usually found at depths >200 m in the Southern Ocean.”

Line 177: “. . .previously described by Brandon et al. (1999) and Meredith et al. (2005) . . .”

Line 194: Can you provide the values of “low transmissivity”?

Line 197: The mention of AOU comes out of nowhere. This should be discussed in the methods.

Line 209-210: Nitrite values are not particularly informative. Typically, chemical oceanographers report patterns of nitrite + nitrate together.

Lines 219-220: “In general(,) the relatively high concentrations suggest that the phytoplankton have plenty of macronutrients”. This is unsurprising. Macronutrients are not considered limiting in this region.

Lines 230-231: Again, this claim is not appropriately supported.

Line 233: If the authors are suggesting higher fluorescence due to iron limitation alleviation, how does it make sense that this is observed further away from land? If the iron is terrestrially derived, this is counterintuitive.

Line 236: This mention of a “chlorophyll bloom” came out of nowhere. This is fluorescence and not chlorophyll, and there is not sufficient evidence that this is a bloom.

Line 245-246: Are you suggesting NH4+ concentrations are increasing due to increased bacterial remineralization as well as decreased phytoplankton utilization? If so, make that clear as this is the first time remineralization influences on macronutrient concentrations are being mentioned.

Line 247: This is a stoichiometric average. I would mention that the ratio varies between taxa. Additionally, the role of diatoms in the phytoplankton community composition in this region has not yet been mentioned. Are they a typically large component, as with most of the Southern Ocean? Small? Please elaborate.

Lines 292-328: Way too long of a paragraph.

Line 313: Personal communications with who? Zanker? Please clarify.

Lines 313-316: Iron should not be introduced into this transport discussion as you have not actually shown any iron concentration data. There is no indication of the magnitude, which along with biological uptake (which occurs incredibly rapidly), would impact the transport.

Line 318-320: The residence time of iron is also highly dependent on the biological uptake which is generally fast.

Line 321: Does the Ebner et al. paper actually include iron concentrations? If so, I would highly recommend the authors work together to strengthen this paper’s findings by adding in iron data.

Lines 330-332: Personally, I don’t think this type of background information is necessary in a conclusion.

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Citation: https://doi.org/10.5194/egusphere-2026-1808-RC1
RC2: 'Comment on egusphere-2026-1808', Anonymous Referee #2, 20 May 2026 reply

This manuscript presents hydrographic surveys from South Georgia fjords, with a framing of understanding the implications for nutrient transport. I agree with the other reviewer about the limitations to the biogeochemical aspects of the study presented (e.g. equating fluorescence to chlorophyll to primary production). In a revision the results should also be separated from the discussion given the purpose of the study.
My major concern is that there is little presented concerning how the glaciers are modifying the water masses present and the general circulation observed. There is visually apparent glacial modification occurring in these transects, so at the very least meltwater mixing lines should be included in the T-S plots. Understanding the degree to which the fjord hydrography is influenced by subglacial vs. surface discharge sources is absolutely required to understand the island’s downstream nutrient impacts, as subglacial discharge can drive upwelling of benthic/deep water nutrient sources, while surface discharge would be more likely to dilute surface macronutrient concentrations. The glacier-driven influence on the surveys therefore needs to be a larger focus of the manuscript for it to be publishable.

Detailed comments
Introduction: As written, the introduction is largely focused on the biogeochemistry of the island, with less focus on the hydrographic major questions highlighted in previous studies, out of congruence with the data presented
L12: replace “a better” to “an improved”
L79: replace “Nutrient” with “Macronutrient”
L166: how were the fluorescence measurements calibrated?
L179: where would the FWI lie in relation to the Gade mixing line?
L203: remove “primary production according to”
L231: replace “indicating” with “suggesting”
L250: “is likely running into “ rephrase to “likely experienced”
L300: “in a distance” what does this mean
L306: new paragraph here
L318: is there any glacier-driven entrainment of deeper waters to the surface?
L338: “Not so much in KHB, so its hard to generalize the results.” Rephrase
L338: “previous studies” please include those studies here.
L348: remove “This paper is part of a series and future work examining”

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Citation: https://doi.org/10.5194/egusphere-2026-1808-RC2

Annika Oetjens, Wilken-Jon von Appen, Joséphine Anselin, Berenice Ebner, Florian Koch, Kai-Uwe Ludwichowski, Ryan Mole, Joanna Zanker, Emma Young, and Sabine Kasten

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Short summary

Phytoplankton blooms near South Georgia are linked to iron-rich waters, but the pathways are unclear. During a research voyage, we measured currents, temperature, and nutrients in two fjords and nearby shelf waters. We found that winds, underwater landscape, and local circulation control how water and nutrients move. Slow exchange within fjords and strong mixing at their mouths suggest these areas can release nutrients that help sustain blooms in the open ocean.


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