the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Jun 2025
Dynamic upper-ocean processes enhance mesopelagic carbon export of zooplankton fecal pellets in the southern South China Sea
Abstract. Zooplankton are key contributors to the marine biological pump by converting phytoplankton-derived organic carbon into fast-sinking fecal pellets. Despite the established role of upper ocean dynamics in regulating epipelagic biogeochemistry and plankton communities, their impact on mesopelagic fecal pellet carbon export remains poorly constrained. Here, we present time-series sediment trap mooring observations of fecal pellet fluxes at 500 m from August 2022 to May 2023 in the southern South China Sea. Zooplankton fecal pellet fluxes display distinct seasonal patterns, with average numerical and carbon fluxes of 7.39 × 104 pellets m-2 d-1 and 1.27 mg C m-2 d-1, respectively. Fecal pellets account for 10.0 to 42.6 % (average 21.6 %) of particulate organic carbon export, exceeding most oligotrophic regions. Mesopelagic fecal pellet fluxes are strongly correlated with upper-ocean dynamic processes, including winter mixing, tropical cyclones, and mesoscale eddies. Two tropical cyclones increase regional fecal pellet carbon export by more than 10 % of the annual carbon flux. One spring peak contributes more than 60 % of the total flux, likely driven by the combined effects of winter mixing, cold eddy activity, and spring zooplankton blooms. Our results highlight the critical role of upper-ocean dynamics in fecal pellet carbon export in deep water layers.
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Status: open (until 24 Sep 2025)
- RC1: 'Comment on egusphere-2025-2864', Anonymous Referee #1, 06 Aug 2025 reply
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RC2: 'Comment on egusphere-2025-2864', Anonymous Referee #2, 14 Sep 2025
reply
Review Comments
The study by Wu et al., uses time-series data collected from a sediment trap deployed in the southern South China Sea to provide a year-round assessment of fecal pellet–mediated carbon fluxes and to evaluate how these fluxes respond to upper-ocean dynamics. The findings highlight that, in addition to the seasonal monsoon-driven pattern, other physical processes—such as typhoons, eddies, and episodes of high wind speed—also contribute significantly to the variability of deep fecal pellet carbon fluxes.
Overall, the manuscript is well written and logically organized. The figures are presented effectively, and the results strongly support the main points. I do not have major concerns. My suggestions are mainly minor, relating to figure design, methodological description, and expansion of the discussion to improve clarity and depth. Please see my detailed comments below:
Specific Comments
Line 40: Please add relevant references after each study approach cited, to demonstrate how these methods have improved our understanding of fecal pellet–mediated carbon fluxes.
Line 75: The abbreviation SCS has already been defined in a previous section; repetition is unnecessary.
Lines 165–176: Cite the corresponding figures or tables when describing results.
Line 130: Provide more details (including model data source, configuration, and validation) for the models used to derive the biogeochemical parameters (e.g., Global Ocean Eddy-Resolving Reanalysis, CMEMS Global Ocean Low and Mid Trophic Levels). Many biogeochemical models are available; please justify why these particular models were selected and comment on their performance in the SCS.
Figure 1: Consider embedding a small map showing the relative location of the SCS within the broader Pacific Ocean. This would help readers unfamiliar with the region.
Line 80: The description that the mixed layer depth (MLD) is “relatively deep” in the SCS may not be appropriate. As shown in your results, there is a significant seasonal cycle, and the maximum MLD remains shallower than 60 m.
Line 90: It is difficult to believe that diatoms contribute ~70% of the phytoplankton biomass in the open-ocean upper layer. Do you mean in the sunlit surface water, or are you referring to aggregates/sinking particles in the mesopelagic zone? Please clarify.
Line 120: Please add methodological details on how POC flux was quantified from the sediment trap, since the FPC:POC flux ratio is discussed extensively later. In addition, the observation that the FPC:POC ratio remains relatively constant despite increases in total POC flux is intriguing. Does this reflect a true ecological signal—i.e., that different components contributing to POC flux increase proportionally—or could it be an artifact of the methodological approach? Some clarification on this point would strengthen the interpretation.
Line 125: The a in Chl a should be italicized. Please modify throughout the manuscript.
Line 140: The p in p-value should be italicized. Please modify throughout the manuscript.
Figure 3: Use different color schemes to denote the two monsoon seasons to improve visualization.
Figure 5a: Please clarify which direction the positive values represent.
Figure 5h: Report depth-integrated NPP in mg C m⁻² d⁻¹ to align with the units of POC flux. Also, the time unit is missing.
Precipitation data may be redundant and contribute little to your analysis; consider removing it.
Figures 6g–6h: Place surface nitrate on the x-axis, since the aim is to examine how zooplankton respond to upper-ocean dynamics.
Line 295: Please clarify how you estimated the ~22-day time lag at your study site.
Line 335: Replace CMD with the more standard term deep chlorophyll maximum (DCM).
Line 410: What is the typical fractional contribution of zooplankton fecal pellets to total POC flux in oligotrophic oceans? Additional discussion and comparison with previous studies would strengthen this section. Yibin
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- 1
Fecal pellets act as important particulate organic matter components, and play vital role in marine carbon cycling. This manuscript presented the downward fluxes of zooplankton fecal pellets in the southern South China Sea, based on a long time sediment trap mooring system. The results are very helpful to understand the importance of fecal pellets in organic carbon export and cycling. However, there are some defects through the manuscript. Below are my major advice:
1) The potential impacts of monsoon, typhoon and eddies to fecal pellets export fluxes are discussed. However, through the discussion section, their importance is only qualitatively discussed, and no direct quantitative analysis is done. E.g., in section 4.1, the authors focused on comparing the MLD during monsoon and non-monsoon periods, and then the relationships nitrate and chl a with MLD. I advise the authors quatify the effects of the above events in enhancing the FPN and FPC using statistical analysis/test.
2) The language and logic of the manuscript must be improved before submitting and published to such high quality journal.
3) The manuscript only simply presented the data sources, such as wind field, primary production…. But, how the trap system pretreated before using? In other words, how the bottles cleaned and did any chemicals (e.g. HgCl2 or NaCl solution) added to each bottle? Since the storing durations of the particles in the bottles are different and the degradation percent in each bottle should be considered. In addition, POC is used in Figure 4, but how was it analyzed is not mentioned in the method section.
4) There are some mistakes/typos in certain figures: There is no panel d in Figure 4. The y-axis names are repeated in Figure S7, the right one should be wrong. Base on the contents, Figure 3a and Figure S7d are repeated. Figure 3a and 3b, 3d and 3e are also repeated. Figure 5 and Figure 7 are partially repeated and the logic/order in Figure 5 are bad (they should be presented in the order of physical parameters, then biochemical parmeters, and raw data and then reanalized data). Additionally, the panels should be cited for the first time in the main text sequentially.
5) Please add note for the bold numbers in Table 1. I think they the average values. Please also present the standard deviations.
6) Lines 25-26: “sequestering over 1300 Pg C” Is this the intergrating value？ If yes, what’s the time scale?
7) Lines 37-38: The authors mentioned “These studies”, but did not cite any references, please add the relevant references. In addition, why the contribution of fecal pellets to POC can > 100%?
8) Line 110: Based on the location of the mooring station and depth, terrigenous particles may be not the only component of the fine particles. I mean biological originated organic/inorganic particles, including detritus of phytoplankton, partially degraded organic particles, etc, may contribute more to this fraction.
9) Lines 155-156: Why the authors say spherical pellets are “slightly smaller”? What’s the criteria? Based on the average biovolume, it’s much smaller than cylindrical ones, but larger than the ellipsoidal ones (not similar). Please do statistical significance and present p values.
10) Lines 176-178: I am confused here. The contributions of ellipsoidal, spherical and cylindrical to FPN are 48%, 38% and 15%, respectively, so how can we got > 90% for ellipsoidal + spherical?
11) Lines 244-249: What’s the relationship of these sentences and pellets export? I do not think they are necessary in discuss how phytoplankton blooms. I also suggest to re-organize this section to make it simple and clear.
12) Line 252: “microbial remineralization and degradation”, please delete “and degradation”, because remineralization is generally equal to degradation.