the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
First phytoplankton community assessment of the Kong Håkon VII Hav, Southern Ocean during austral autumn
- 1Norwegian Polar Institute, Fram Centre, Tromsø, Norway
- 2Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
- 3Institute of Oceanology, Polish Academy of Sciences, Sopot, Poland
- 4Department of Biological Sciences, University of Bergen, Bergen, Norway
- 5Southern Ocean Carbon and Climate Observatory (SOCCO), Council for Scientific and Industrial Research (CSIR), Cape Town, South Africa
- 6Department of Earth Sciences, Stellenbosch University, Stellenbosch, South Africa
- 1Norwegian Polar Institute, Fram Centre, Tromsø, Norway
- 2Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
- 3Institute of Oceanology, Polish Academy of Sciences, Sopot, Poland
- 4Department of Biological Sciences, University of Bergen, Bergen, Norway
- 5Southern Ocean Carbon and Climate Observatory (SOCCO), Council for Scientific and Industrial Research (CSIR), Cape Town, South Africa
- 6Department of Earth Sciences, Stellenbosch University, Stellenbosch, South Africa
Abstract. We studied phyto- and protozooplankton community composition based on light microscopy, flow cytometry and photosynthetic pigment data in the Atlantic sector of the Southern Ocean during March 2019 (early austral autumn). Sampling was focused on the area east of the prime meridian in the Kong Håkon VII Hav, including Astrid Ridge, Maud Rise and a south-north transect at 6° E. Phytoplankton community composition throughout the studied area was characterized by oceanic diatoms typical of the iron-deplete High-Nutrient Low-Chlorophyll (HNLC) Southern Ocean. Topography and wind-driven iron supply likely sustained blooms dominated by the centric diatom Chaetoceros dichaeta at Maud Rise and at a station north of the 6° E transect. For the remainder of the 6° E transect diatom composition was similar to the previously mentioned bloom stations but flagellates dominated in abundance suggesting a post-bloom situation and likely top-down control by krill on the bloom-forming diatoms. Among flagellates, species with haptophyte-type pigments were the dominating group. At Astrid Ridge, overall abundances were lower and pennate were more numerous than centric diatoms, but the community composition was nevertheless typical for HNLC areas. The observations described here show that C. dichaeta can form blooms beyond the background biomass level and fuels both carbon export and upper trophic levels also within HNLC areas. This study is the first thorough assessment of phytoplankton communities in this region and can be compared to other seasons in future studies.
Hanna Maria Kauko et al.
Status: open (until 29 May 2022)
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RC1: 'Comment on bg-2022-45', Anonymous Referee #1, 20 Mar 2022
reply
This manuscript describes the phytoplankton assemblage observed in a relatively under-studied region of the Southern Ocean, based on a variety of analytical tools that together highlight the contributions made via different methods, strengths and weaknesses, and interprets these data within their broader oceanographic context. The authors do an excellent job displaying their data in ways that make sense and do an equally strong job describing patterns in the results. The results section is very detailed, and at first it was a bit slow going to get to the heart of the interpretation. However, I found myself going back to the results and the figures as I was reading the discussion. The strength of this paper lies in the discussion, where the authors key in on the most interesting parts of their complex data set. I do not recommend changes to this manuscript, though I have some suggestions for potential future work, based on their data, and a request for publication of the microscope imagery to assist other researchers, though that may be beyond the scope of this manuscript, and more appropriate for another venue.
Section 4.1. While light microscopy may be the most time-consuming, this tool is critical as a complement to the HPLC/CHEMTAX work, and flow cytometry. The benefit of multiple tools is shown in section 4.1, where coccoliths are suggested by the CHEMTAX, though not observed under the scope, nor are they likely to be present in high numbers so far south, as noted by the authors. In this case, the authors identify the source as “Haptophyte-6-like” – an appropriate decision and one that suggests there is more to learn in this area. I think this is an important lesson for all, that the biochemical work is best combined with old-fashioned microscopy.
Section 4.2. I appreciate the difficulties in interpreting depth-related differences, whether these are related to distinct living assemblages, settling assemblages sinking through the water column, either from directly above or from upstream. Consideration of all is important, and this snapshot study simply doesn’t provide all the answers – which would instead require repeated temporal sampling, either via CTD casts or via sediment trap studies, or both. This kind of work has been done in the Ross Sea, as well as other areas of the Southern Ocean, and might provide a template for future work in this less well-studied region.
Section 4.3. Chaetoceros dichaeta – excellent summary of the oceanographic character of the region and the role of seeding and grazing, in guiding the diatom community – two factors that are often left out of discussion – combined with iron fertilization.
Section 4.4. Astrid Ridge pennates – long history of studies in Antarctica/Southern Ocean, besides those few referenced, that describe the relationship between sea ice algae and marginal ice zone blooms, and the dominance of pennate diatoms within sea ice. Papers by David Garrison, Kurt Buck, Ryszard Ligowski, Sarah McGrath Grossi and Neil Sullivan - for example - might be referenced here.
(line 564, coastal instead of costal)
Section 4.5 Flagellate-dominated post-bloom community – I like the “complete” phytoplankton assemblage study as presented. I think this kind of approach, looking at more than just the diatoms, is going to be increasingly important as environmental change, dominated by warming, but accompanied by factors such as changes in stratification of the upper ocean, nutrient availability, and sea ice extent and duration, becomes more and more critical in driving change in the phytoplankton community at the group level.
General comments:
I realize this may be a big “ask” for this paper, but I think images from the inverted microscopy would be very helpful for other researchers who would like to do similar research, with most phytoplankton researchers familiar with diatom identifications, but less so with the other algal groups. This may be something for a future publication - I think it would be a great contribution.
Any consideration of future sediment coring to address longer-term changes in this sector? It would be a great addition to our background understanding how oceanographic and climatic changes have influenced the ecosystem over time.
Figure 1: contour interval for map?
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AC1: 'Reply on RC1', Hanna Kauko, 18 May 2022
reply
This manuscript describes the phytoplankton assemblage observed in a relatively under-studied region of the Southern Ocean, based on a variety of analytical tools that together highlight the contributions made via different methods, strengths and weaknesses, and interprets these data within their broader oceanographic context. The authors do an excellent job displaying their data in ways that make sense and do an equally strong job describing patterns in the results. The results section is very detailed, and at first it was a bit slow going to get to the heart of the interpretation. However, I found myself going back to the results and the figures as I was reading the discussion. The strength of this paper lies in the discussion, where the authors key in on the most interesting parts of their complex data set. I do not recommend changes to this manuscript, though I have some suggestions for potential future work, based on their data, and a request for publication of the microscope imagery to assist other researchers, though that may be beyond the scope of this manuscript, and more appropriate for another venue.
Thank you very much for the positive feedback and for the appreciation of our work!
Section 4.1. While light microscopy may be the most time-consuming, this tool is critical as a complement to the HPLC/CHEMTAX work, and flow cytometry. The benefit of multiple tools is shown in section 4.1, where coccoliths are suggested by the CHEMTAX, though not observed under the scope, nor are they likely to be present in high numbers so far south, as noted by the authors. In this case, the authors identify the source as “Haptophyte-6-like” – an appropriate decision and one that suggests there is more to learn in this area. I think this is an important lesson for all, that the biochemical work is best combined with old-fashioned microscopy.
Thank you for the thoughts – we agree that multiple methods are needed to give a holistic picture of the communities, as mentioned shortly in the conclusions, and that “old-fashioned” microscopy is still a very valid approach.
Section 4.2. I appreciate the difficulties in interpreting depth-related differences, whether these are related to distinct living assemblages, settling assemblages sinking through the water column, either from directly above or from upstream. Consideration of all is important, and this snapshot study simply doesn’t provide all the answers – which would instead require repeated temporal sampling, either via CTD casts or via sediment trap studies, or both. This kind of work has been done in the Ross Sea, as well as other areas of the Southern Ocean, and might provide a template for future work in this less well-studied region.
Thank you for the advice. We have now also pointed out in the manuscript in the section 4.2 (currently on lines 477-478) that different sampling schemes are needed to properly resolve these patterns.
Section 4.3. Chaetoceros dichaeta – excellent summary of the oceanographic character of the region and the role of seeding and grazing, in guiding the diatom community – two factors that are often left out of discussion – combined with iron fertilization.
Section 4.4. Astrid Ridge pennates – long history of studies in Antarctica/Southern Ocean, besides those few referenced, that describe the relationship between sea ice algae and marginal ice zone blooms, and the dominance of pennate diatoms within sea ice. Papers by David Garrison, Kurt Buck, Ryszard Ligowski, Sarah McGrath Grossi and Neil Sullivan - for example - might be referenced here.
Thank you for these suggestions to strengthen the literature review. We have added articles from the mentioned authors to the manuscript section 4.4 which discusses the relationship between sea ice and pelagic algal communities, as well as to the section 4.1 on general phytoplankton community structure of the region.
(line 564, coastal instead of costal)
Thank you for pointing this out, the error is now corrected.
Section 4.5 Flagellate-dominated post-bloom community – I like the “complete” phytoplankton assemblage study as presented. I think this kind of approach, looking at more than just the diatoms, is going to be increasingly important as environmental change, dominated by warming, but accompanied by factors such as changes in stratification of the upper ocean, nutrient availability, and sea ice extent and duration, becomes more and more critical in driving change in the phytoplankton community at the group level.
Thank you for the comment and acknowledgement of the work – we agree on the importance of studying the different phytoplankton groups.
General comments:
I realize this may be a big “ask” for this paper, but I think images from the inverted microscopy would be very helpful for other researchers who would like to do similar research, with most phytoplankton researchers familiar with diatom identifications, but less so with the other algal groups. This may be something for a future publication - I think it would be a great contribution.
Thank you for the suggestion. Unfortunately, we consider this beyond the scope of the manuscript, also because images were not taken systematically, but we will keep it in mind for future publications. We have included an image of the cyanobacteria present in the samples in the supplementary figures in the appendix.
Any consideration of future sediment coring to address longer-term changes in this sector? It would be a great addition to our background understanding how oceanographic and climatic changes have influenced the ecosystem over time.
Very few paleo records exist from the Kong Håkon VII Hav (e.g. Forsberg et al., 2003, doi: 10.1016/s0031-0182(03)00402-4), but colleagues at the Norwegian Polar Institute are indeed planning new sediment coring in future proposals and projects.
Figure 1: contour interval for map?
This information has now been added to the figure caption.
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AC1: 'Reply on RC1', Hanna Kauko, 18 May 2022
reply
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RC2: 'Comment on bg-2022-45', Anonymous Referee #2, 24 May 2022
reply
The Biogeosciences submission 'First phytoplankton community assessment of the Kong Håkon VII Hav, Southern Ocean during austral autumn' by Kauko et al. shows a lot of potential. The authors have collected a nice dataset about the taxonomy and distribution of the phytoplankton in three distinct regions of the Kong Håkon VII Hav – Southern Ocean. However, the data analysis is very basic and could be greatly improved with the introduction of statistical approaches that allow linking the structure of phytoplankton communities and the environment. The presentation and discussion of results is a simple description (very subjective) of the patterns found in the study region. For example, the separation of sub regions is a great idea, but it comes across very subjective and not quantitatively based at all. How are these phytoplankton-dominated regions determined? I want to see a statistical determination of subregions. In fact, why don't you use a multi-parameter analysis and use the ancillary data, nutrients, mixed layer depth, temperature, salinity, Chl a, phytoplankton assemblage and properly determine these phytoplankton niches? These also need to be clearly mapped out – I want to exactly see these subregions and the conditions that the phytoplankton exist in.
I would encourage the authors to revise the manuscript and submit it again. At its present state, however, I do not feel I can recommend its publication.
Specific comments
- line 143 ‘two types of diatoms…” The HPLC method used cannot separate chlorophyll c1 from c2, so this separation into two types of diatoms becomes difficult and arbitrary. Furthermore, chlorophyll c2 is a poor taxonomic biomarker as it is present in most marine phytoplanktonic groups (red lineage).
- The chemotaxonomic characterization of dinoflagellates-2 and haptophytes-6 is also complicated by the biomarkers (ratios) used – very similar. This separation is only effectively possible if some specific biomarker pigments from each group are used (e.g., gyroxanthin diester; 4-keto-Hex-fuco; Chl c2-MGDG [14/14] and Chl c2-MGDG [18/14] – see Wright & Jeffrey 2006 [https://link.springer.com/chapter/10.1007/698_2_003] or Mendes et al. 2018 [https://doi.org/10.1016/j.dsr2.2017.12.003].
- line 435 ‘Cryptophytes, … also contain similar pigments to haptophytes’ This is not true, right? ... and your table 1 makes that clear.
- lines 437-439 ‘The discrepancies might be partly explained with the relatively small volume filtered (typically 1 L) for HPLC samples…’ This is also a flawed argument, because for microscopy the volume used is much smaller than for HPLC. In fact, the higher volume used for pigment samples (HPLC) is an advantage of chemotaxonomic methods.
- line 195 ‘Two of the sampling locations had an active diatom bloom…’ For me, characterizing a bloom situation with chlorophyll-a values below 1 mg.m-3 is quite strange. I understand that it is a predominantly oligotrophic region, but this delimitation of what is (or is not) a bloom will have to be better defined/discussed.
- The results section will have to be restructured and, essentially, reduced. There is redundant information that does not add content to the discussion of the data presented. I speak, for example, of the graphs with the pigmentary ratios (Figs. 8 and 9) and NMDS analyses (Fig. 5). For this, authors first need to define very well the focus they want to give to the work, as it cannot simply be an unbridled compilation of hard-to-connect data.
Hanna Maria Kauko et al.
Hanna Maria Kauko et al.
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