the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Sinking fate and carbon export of zooplankton fecal pellets: insights from time-series sediment trap observation in the northern South China Sea
Hanxiao Wang
Jiaying Li
Baozhi Lin
Yulong Zhao
Xiaodong Zhang
Junyuan Cao
Jingwen Zhang
Hongzhe Song
Wenzhuo Wang
Abstract. The sinking of zooplankton fecal pellets is a key process in the marine biological carbon pump, facilitating the export of particulate organic carbon (POC). Here, we analyzed zooplankton fecal pellets collected by two time-series sediment traps deployed on mooring TJ-A1B in the northern South China Sea (SCS) from May 2021 to May 2022. The results show a seasonal variability in both fecal pellet numerical (FPN) flux and fecal pellet carbon (FPC) flux, with peaks in November to April and June to August. It implies that the fecal pellet flux is largely regulated by the East Asian monsoon system. Vertical analysis further shows that FPN and FPC fluxes are higher at 1970 m than at 500 m water depth, with larger pellets occurring in the deeper water, indicating a significant influence of mesopelagic/bathypelagic zooplankton community and lateral transport on deep-sea fecal pellet carbon export. However, the biovolume of amorphous pellets decreases significantly from 500 m to 1970 m water depth, implying that these fecal pellets are broken and fragmented during the sinking process, possibly due to zooplankton grazing and disturbance by deep-sea currents. The contribution of fecal pellets to total POC export in the northern SCS is in average 3.4 % and 1.9 % at 500 m and 1970 m water depths, respectively. This study highlights that the sinking fate of fecal pellets is regulated by marine surface productivity, deep-dwelling zooplankton community, and deep-sea currents in the tropical marginal sea, thus providing a new perspective for exploring the carbon cycle in the world ocean.
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Hanxiao Wang et al.
Status: open (until 20 Oct 2023)
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RC1: 'Comment on bg-2023-112', Anonymous Referee #1, 31 Jul 2023
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Review of a manuscript by H. Wang et al. entitled: “Sinking fate and carbon export of zooplankton fecal pellets: insights from time-series sediment trap observation in the northern South China Sea” submitted to Biogeosciences.
This is an interesting manuscript, which should become acceptable for publication after minor English editing. Suggestions for editing are listed below by line number.
2: in the title, change “time-series sediment trap observation” to “time-series
sediment-trap observations” and hyphenate all other double-word adjectives
11: hyphenate “fecal-pellet-numerical (FPN) flux” and “fecal-pellet-carbon (FPC) flux”
and hyphenate all other more-than-one-word adjectives
80: change “Hydrological system” to “The hydrological system”
84: change “Combination” to “A combination”
165: delete “of”
Figures 4 and 5: the micrographs are too dark, with no contrast. If these had been taken
using film and printed in a darkroom (as in the past) I would have used a different F-stop to take the micrographs, and different F-stops and contrast filters to print these photos. I do not know how to do this now, using digital photography, but these micrographs need to be re-taken.
239: change “Besides,” to “However,”
243: change “5 Discussions” to “5 Discussion”
246: change “The FPC flux” to “The FPC flux was”
263: change “though in the oligotrophic seas” to “although in oligotrophic seas”
278: change “this additional nutrient” to “these additional nutrients”
297: change “literatures,” to “literature,”
309: change “number” to “amount”
317: change “fecal pellet” to “fecal pellets”
322: hyphenate “laterally transported”
323: hyphenate “highly-adaptable”
334: change “may play” to “and may play”
336: change “at K2 station” to “at Station K2”
352: change “form the larger aggregate.” to “form larger aggregates.”
353: change “basin” to “basins”
358: hyphenate “hydrodynamically-induced”
394: change “twice higher” to “twice as high”
478: italicize “Oithona”
478, 480, 484, 490: change “Gonzalez” to “González”
503: change “Paffenhofer” to “Paffenhöfer”
Citation: https://doi.org/10.5194/bg-2023-112-RC1 -
AC1: 'Reply on RC1', Zhifei Liu, 16 Aug 2023
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Reply: Thank you for your effort reviewing our manuscript and your positive feedback, which have helped to improve the quality of the manuscript. We have read through all the comments carefully and have made related modifications. We highly appreciate your time and consideration. The notes (in blue) explain how and where each point of comments has been addressed.
Comment 1:
2: in the title, change “time-series sediment trap observation” to “time-series sediment-trap observations” and hyphenate all other double-word adjectives
Reply: We thank you for this correction. We have corrected the sentence in Line 2 and hyphenated all other double-word adjectives in the revised manuscript, like in Lines 62, 64, 322, 333, and 358.
Comment 2:
11: hyphenate “fecal-pellet-numerical (FPN) flux” and “fecal-pellet-carbon (FPC) flux” and hyphenate all other more-than-one-word adjectives
Reply: Thank you for this comment. We have hyphenated “fecal-pellet-numerical (FPN) flux”, “fecal-pellet-carbon (FPC) flux”, and all other more-than-one-word adjectives in the revised manuscript, like in Lines 2, 62, 64, 120–121, 124–125, 190, 322, 333, and 358.
Comment 3:
80: change “Hydrological system” to “The hydrological system”
Reply: Done.
Comment 4:
84: change “Combination” to “A combination”
Reply: Done.
Comment 5:
165: delete “of”
Reply: Done.
Comment 6:
Figures 4 and 5: the micrographs are too dark, with no contrast. If these had been taken using film and printed in a darkroom (as in the past) I would have used a different F-stop to take the micrographs, and different F-stops and contrast filters to print these photos. I do not know how to do this now, using digital photography, but these micrographs need to be re-taken.
Reply: Thank you for pointing out this problem. We have adjusted the brightness and contrast of the photographs in Figures 4 and 5, and we hope this correction meet your requirements.
Comment 7:
239: change “Besides,” to “However,”
Reply: Done.
Comment 8:
243: change “5 Discussions” to “5 Discussion”
Reply: Done.
Comment 9:
246: change “The FPC flux” to “The FPC flux was”
Reply: We are so sorry for the incorrect writing here. We have changed the sentence into “The FPC flux was” in Line 246.
Comment 10:
263: change “though in the oligotrophic seas” to “although in oligotrophic seas”
Reply: Done.
Comment 11:
278: change “this additional nutrient” to “these additional nutrients”
Reply: Done.
Comment 12:
297: change “literatures,” to “literature,”
Reply: Done.
Comment 13:
309: change “number” to “amount”
Reply: Done.
Comment 14:
317: change “fecal pellet” to “fecal pellets”
Reply: Done.
Comment 15:
322: hyphenate “laterally transported”
Reply: Done.
Comment 16:
323: hyphenate “highly-adaptable”
Reply: We have hyphenated “highly-adaptable” in Line 333 of the revised manuscript.
Comment 17:
334: change “may play” to “and may play”
Reply: We feel sorry for our carelessness. We have changed the sentence into “and may play” in Line 334.
Comment 18:
336: change “at K2 station” to “at Station K2”
Reply: Done.
Comment 19:
352: change “form the larger aggregate.” to “form larger aggregates.”
Reply: Done.
Comment 20:
353: change “basin” to “basins”
Reply: Done.
Comment 21:
358: hyphenate “hydrodynamically-induced”
Reply: Done.
Comment 22:
394: change “twice higher” to “twice as high”
Reply: Done.
Comment 23:
478: italicize “Oithona”
Reply: Done.
Comment 24:
478, 480, 484, 490: change “Gonzalez” to “González”
Reply: Thank you very much for pointing out the error here. We have changed “Gonzalez” to “González” in Lines 478, 480, 484, and 490 of the revised manuscript.
Comment 25:
503: change “Paffenhofer” to “Paffenhöfer”
Reply: We are very grateful to you for reviewing the manuscript so carefully. We have changed “Paffenhofer” to “Paffenhöfer” in Line 503 of the revised manuscript.
Citation: https://doi.org/10.5194/bg-2023-112-AC1
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AC1: 'Reply on RC1', Zhifei Liu, 16 Aug 2023
reply
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RC2: 'Comment on bg-2023-112', Anonymous Referee #2, 28 Sep 2023
reply
This is all in all a well written and interesting article, providing new insights into fecal pellet fluxes and their contribution in POC export in the northern SCS. Very nice figures, photographs of pellets, and generally thorough and very well explained text. I have minor comments and adjustments that should be addressed before publication.
Title: This is the only place in the manuscript that “time-series” sediment traps are mentioned. Sediment trap details (Producer, etc.) missing from the Materials and Methods section.
Line 19: “by marine surface productivity” - this paper does not outline directly marine surface productivity, especially as shallowest trap is at 500 m. Recommend to focus on the papers findings in the abstract.
Line 22: change “a process” with “a collection of processes” or similar. The BCP is not one process but a collection of many processes (correctly used in line 24).
Line 27: Sentence starting with “As a key process” - consider rewording. Is it the zooplankton fecal pellets that reduced dissolution and degradation, or the zooplankton themselves that do by packaging the material into feral pellets that are harder to degrade? Also, fecal pellets themselves are not a process, so consider wording.
Line 32: first mention of amorphous pellets – define or mention that these are mostly fragmented pellets.
Line 36: define or explain coprorhexy, coprophagy and coprochaly if mentioned.
Line 40: “may affect the POC export” - remove “the”; define/explain efficiency of the BCP otherwise it is difficult for reader to understand.
Line 41: you introduce studies focusing on biogeochemical mechanisms but the rest of the paragraph describes FP fluxes with depth, which is not a biogeochemical mechanism. Consider rewording.
Line 42: avoid use of the word “obvious”. Reword to “clear” or other words that are less definite. Please consider changing or removing this word throughout the manuscript (e.g. lines 281 and 285).
General introduction: Perhaps mention active flux / zooplankton mediated injection pump (Boyd et al. 2019 Nature).
Line76/ “2. Study Area”: Nice summary, but this should be part of the Methods (2.1) instead of its own section?
Section 2.1 Sediment trap deployment: Information lacking about the sediment traps. No make/producer. Also, why was a different concentration of fixative added to the different trap depths? If this is common practice, please provide reference.
Section 4. Results: Hydrological results missing from the results section. Since it is outlined in methods and included in figures in the discussion, a brief results section is missing. Lines 248–253 could be considered as results instead of discussion.
Line 154: “significantly different between two depths” - how so? Which depth was darker? Or Refer to Fig. 3
Lines 176–184: Where there any size/biovolume changes across seasons (i.e. were they smaller or bigger on average during certain times of the year?)
Lines 191–192: Which months had minimum and maximum? “… from a minimum of 216 pellets m-2 d-1 in __(month)__, to a max..__(month). Was it the same for both depths?
Line 246: “of the total POC flux”, consider changing to “of the total annual POC flux”
Line 278: “this additional nutrient” - terrestrial OM is not just bring in one additional nutrient so this sentence is slightly confusing to read. Consider rewording.
Line 283: “southwestern Taiwan have been transported” - how sure are you of their origin? Perhaps change to “are likely transported” or similar.
Line 287: “Role of zooplankton repackage in fecal pellet export” - Repackage? Reworking? The grammar here doesn’t make sense and hard to know what you mean. Consider rewording.
General section 5.2: Consider adding one sentence about uncertainty in using the same carbon conversion factor for the whole year and for all fecal pellet shapes and zooplankton producers. This could change the FPC export quite a bit, so worth a mention.
Line 288: should read “repackaging by deep-sea dwelling zooplankton”
Line 344: “zooplankton grazing” - do you mean grazing here or reworking/fragmentation?
Line 350: change “variable” to “various”
Line 350: Here you use “euphotic zone” but in Figure 12 you use “Epipelagic”. Be consistent.
Line 355-356: “consumed and reworked by zooplankton grazing and strong hydrodynamic activities” seems to imply hydrological activities consume and rework the pellets. Consider changing to “consumed and reworked by zooplankton grazing and fragmented by strong hydrodynamic activities”
Line 357: “repackaging”, change to “repackaging and fragmentation”
Line 366: Remove “However, “ and switch sentence around to read “The contribution of fecal pellets to the total annual carbon flux was lower in the northern SCS compared to the southern SCS, possibly due to…”
Line 369: Replace “biospheric” with “biogenic”
Citation: https://doi.org/10.5194/bg-2023-112-RC2
Hanxiao Wang et al.
Hanxiao Wang et al.
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