Geophysical and geochemical controls on the megafaunal community of a high Arctic cold seep

Sen, Arunima; Åström, Emmelie K. L.; Hong, Wei-Li; Portnov, Alexey; Waage, Malin; Serov, Pavel; Carroll, Michael L.; Carroll, JoLynn

doi:https://doi.org/10.5194/bg-15-4533-2018

Articles | Volume 15, issue 14

https://doi.org/10.5194/bg-15-4533-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-15-4533-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 15, issue 14

Research article

|

25 Jul 2018

Research article |

| 25 Jul 2018

Geophysical and geochemical controls on the megafaunal community of a high Arctic cold seep

Arunima Sen, Emmelie K. L. Åström, Wei-Li Hong, Alexey Portnov, Malin Waage, Pavel Serov, Michael L. Carroll, and JoLynn Carroll

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Final revised paper (published on 25 Jul 2018)
Preprint (discussion started on 11 Jan 2018)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Interactive comment on “Geophysical and geochemical controls on the megafaunal community of a high Arctic cold seep” by A. Sen et al.', Paul Dando, 09 Mar 2018
- AC1: 'Response to first reviewer (Paul Dando)', Arunima Sen, 24 Mar 2018
RC2: 'additional comment', Paul Dando, 20 Mar 2018
- SC1: 'Reply to Interactive comment II on “Geophysical and geochemical controls on the megafaunal community of a high Arctic cold seep” by Arunima Sen et al.', Emmelie Åström, 09 Apr 2018
  - AC2: 'Response to additional comment by first reviewer (Paul Dando)', Arunima Sen, 26 Apr 2018
RC3: 'comment bg-2017-540', Anonymous Referee #2, 25 Apr 2018
- AC3: 'Response to second reviewer', Arunima Sen, 27 Apr 2018

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (29 Apr 2018) by Jack Middelburg

AR by Arunima Sen on behalf of the Authors (29 May 2018)

ED: Referee Nomination & Report Request started (06 Jun 2018) by Jack Middelburg

RR by Paul Dando (24 Jun 2018)

Suggestions for revision or reasons for rejection

This manuscript is a useful addition to the literature on the fauna at Arctic seeps, although it is regrettable that it does not include information on the background sediment and drop-stone fauna, at these depths, this would have better revealed the seep fauna characteristics. The revised manuscript is greatly improved and it is now a lot clearer how the study was organized. However, there are still some areas that need further clarification, reconsideration and and re-wording.

The main finding was that Oligobrachia were absent from Pingo 5 that showed no visible gas flares and had lower methane concentrations below 50 cm sediment depth. However, since isolated tubes could not be detected it is not possible to prove the species was not present. Supplementary Fig. 1 now explains how the frenulates were observed in the camera images. The authors still need to explain how the “mottled sediment” in this Figure was identified as frenulate tubes, since it is not clear, even on enlarging the supplied figure, that the blotches are frenulate tubes. If the tubes were apparent on enlargement of the original images then an example of this should be shown in the supplementary figure. Alternatively, one of the cores containing Oligobrachia may have been placed into this mottled seabed to allow identification and this should be described.
It is not stated whether the Supplementary Figure is from a tow-cam or a ROV camera image. Since the ROV camera was of lower resolution than the tow-cam it would be useful to have images, showing the frenulate clusters, from both cameras in the Figure, especially since there is not an explicit statement that Oligobrachia clusters could be seen in the ROV images from Pingo 3. This is needed since, unlike the pingos with active gas flares, pingo 5 was only imaged using the ROV camera and the larger Nothria polychaetes were not seen in the ROV images (P7). On P15 it is stated that “GHP5 was surveyed with the ROV before mosaic based imaging was conducted with the explicit purpose of locating siboglinid worms, since they were considered to be representative of locations with active seepage. Despite these efforts, no aggregations of these animals were seen.” However, there is still no statement that siboglinids were seen using the ROV on other pingos. It would also be useful to list the cores in which frenulates were recovered.

An absence of Oligobrachia from Pingo 5, if real, is difficult to explain since, as the authors state, the sulphide gradient in all the cores is similar and O. haakonmosbiensis obtains nutrition from the endosymbiotic sulphur-oxidising symbionts. The authors propose that methane “serves as a carbon source for frenulate worms. The pingo currently lacking a large sub-surface gas source and lower methane concentrations likely has lower sulfide flux rates and limited amounts of carbon, insufficient to support frenulates.” There is no evidence in the literature that DIC is limiting for any frenulate and, although DIC was measured in the present study, no comparative DIC depth profiles are shown. (In any case the frenulates will produce DIC through respiration and this could lessen the demand.) Similarly neither sulphide flux rates nor sulphate reduction rates are presented. Sulphide flux rates/sulphate reduction rates will influence frenulate density but are unlikely to be a reason why they are absent from pingo 5, given the available sulphide data and the presence of bacterial mats on part of pingo 5, indicating active seepage. Sulphate reduction rates in the O. haakonmosbiensis zone on the HM mud volcano are extremely variable (Felden et al. 2010), suggesting that high fluxes are not necessary for the species. The rates peaked at 50-75 cm depth, consistent with the penetration depth of the frenulates there (de Beer et al. 2006).
The discussion on the possible role of DOM uptake as a requirement for the frenulates (P17-18) is interesting but ignores the most probable explanation for the large 13C depletion in O. haakonmosbiensis, i.e. that the DIC utilized by the endosymbionts is mainly derived from methane oxidation, as shown by the 13C depletion of the authigenic carbonates (Hong et al. 2017). This DIC would be further depleted by the frenulate symbionts: see the discussion in Spiro et al. (1986).

Both Nothria and sipunculids were absent from Pingo 5, was this due to lack of resolution of the ROV images or to some other cause?

The authors provide a large number of references to justify their use of the term megafauna (i.e. very large fauna), in place of macrofauna. It is equally possible to provide references for the use of macrofauna for the range of species described, including a recent paper by three of the co-authors who refer to similar fauna as “macrobenthos” (Åström et al. 2016). In the present manuscript megafauna is defined as “animals large enough to be seen with the naked eye”, this definition would include the larger meiofauna (commonly used to refer to animals passing through a 0.5 mm sieve) and therefore, apparently, leaving no animals in the macrofauna. On P19 the authors state that “Details on the macrofaunal community composition is currently being compiled” and mention thyasirid bivalves as having being found, these are certainly “visible to the naked eye”. How do the authors therefore define “macrofauna” since there does not appear to be a consistency in their terminology? Later in the manuscript Amon et al. (2017) is cited that the smallest megafauna should have a minimum dimension of 1 cm.

P3 “this study is particularly useful for teasing apart the factors affecting the large-scale distribution of chemosynthesis based species, since these animals are directly reliant on seeping chemicals.” On continental shelves and slopes chemosynthesis-based species are frequently found in non-seep reducing sediments, indeed most of the frenulate species are not found at seeps, the reducing chemicals coming from the breakdown of organic matter in the upper sediment. The authors show that they appreciate this in the Discussion on P13. However, we still do not know whether O. haakonmosbiensis is a seep obligate species or also occurs in lower densities in reducing sediments.

P4 “no such free gas emissions were seen over GHP5 during repeat on-site observations over 3 years (2013-2016) across different seasons.” Since the observations were not continuous it would be useful to add the amount of time spent on-site at pingo 5.

P8 L10 “The detection limit for this method is 40 μM.” This should read: “ The detection limit for the variant of the method used was 40 µM”, since the original method (Cline, 1969) allows detection of sulphide as low as 1.0 µM. A limit of 0.1µM is achievable using reduced reagent amounts.

Figure 4a: the colours for GHP5 920 and GHP5 1048 are not easily distinguished. Methane concentration – is the volume sediment or pore water, since whole sediment samples were taken?

Figure 5: change the symbols to be consistent with Fig. 4

de Beer, D., Sauter, E. J., Niemann, H., Kaul, N., Foucher, J.-P., Witte, U., et al. (2006). In situ fluxes and zonation of microbial activity in surface sediments of the Håkon Mosby Mud Volcano. Limnology and Oceanography, 51(3), 1315-1331.
Felden, J., Wenzhöfer, F., T., F., & Boetius, A. (2010). Transport and consumption of oxygen and methane in different habitats of the Håkon Mosby Mud Volcano (HMMV). Limnology and Oceanography, 22, 2366-2380.
Spiro, B., Greenwood, P. B., Southward, A. J., & Dando, P. R. (1986). 13C/12C ratios in marine invertebrates from reducing sediments: confirmation of nutritional importance of chemoautotrophic endosymbiotic bacteria. Mar. Ecol. Prog. Ser., 28, 233-240.

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ED: Publish subject to minor revisions (review by editor) (24 Jun 2018) by Jack Middelburg

AR by Arunima Sen on behalf of the Authors (05 Jul 2018) Author's response Manuscript

ED: Publish subject to technical corrections (06 Jul 2018) by Jack Middelburg

AR by Arunima Sen on behalf of the Authors (06 Jul 2018) Manuscript

Short summary

Diverse benthic communities populate a site of methane seepage on the Arctic shelf. Despite a likely reliance on sulfide as an energy source, Oligobrachia worm distributions did not correlate with sulfide concentrations. We suggest that sulfide and carbon generation linked to microbial activity and high methane fluxes determines their presence or absence. We discuss the site and our results within the context of Arctic ecology and economy as well as the biology of seafloor hydrocarbon seeps.