the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 May 2026
Elemental and isotopic constraints on zooplankton-mediated carbon fluxes in Ryder Bay, Western Antarctic Peninsula
Abstract. Zooplankton faecal pellets are recognised as a key pathway for particulate organic carbon export in the Southern Ocean, yet their elemental and isotopic composition remains poorly characterised, limiting understanding of how zooplankton-mediated processing and particle type shape the chemical signatures of sinking material. Here, we present moored sediment trap observations from Ryder Bay, Western Antarctic Peninsula, quantifying particulate organic carbon and nitrogen fluxes for total sediment trap material and isolated faecal pellets, alongside measurements of POC:PN ratios and the stable isotopic compositions of carbon and nitrogen. Cylinder faecal pellets dominated summer particulate organic carbon flux, reaching up to ~ 100 % of the total flux, which was concentrated in a brief but intense pulse consistent with episodic changes in zooplankton community composition. POC:PN ratios in both the total sediment trap material and cylinder faecal pellets were substantially elevated above Redfield, reaching values of up to 27, well beyond the range previous reported for inshore Western Antarctic Peninsula environments, whereas round pellets retained near-Redfield stoichiometry. This particle-type specific contrast indicates that the elemental composition of exported material is primarily governed by the dominant fluxing particle type rather than directly reflecting system-wide environmental conditions. Stable isotopic signatures are most consistent with post-production modification of organic matter, including preferential nitrogen remineralisation and trophic reworking, including benthic feeding, rather than directly reflecting primary production signals. Together, these findings suggest the organic matter exported across the WAP shelf break may carry a chemically distinct signature, with implications for tracing the contribution of shelf-derived carbon to broader-scale export budgets.
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Status: open (until 22 Jul 2026)
- RC1: 'Comment on egusphere-2026-2459', Anonymous Referee #1, 23 Jun 2026 reply
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RC2: 'Comment on egusphere-2026-2459', Anonymous Referee #2, 16 Jul 2026
reply
The novelty of the study is the unique setting, but more work needs to be done to better contextualize what is found here relative to the large body of work that has been done elsewhere using similar methods and generating similar results.
The two biggest issues with this manuscript are as follows:
1) Very limited sample sizeThe study only has one sample location and only 4 samples from that location. How can significant findings be justified with such a small sample size. Additionally, if it was estimated that thousands of fecal pellets were captured per sample, why were only 15 used for elemental and isotopic analysis. There is so much extrapolation occurring in this manuscript from very limited actual data. All the conclusions are being formed based on a small time-window as well (only from mid-January to mid-March).
2) Very broad and sweeping statements based on significant literature findings but very little actual sample evidence from this study
There is a good amount of literature review occurring for the study site but very little of it can be traced back to results that were found in this study, let alone unique to this study. Very little, at the present iteration of this article, seems to be new about the results that were generated.
Additionally, the methods section needs a lot more work and whether small particles (single micron particles) are captured/considered needs to at the very least be addressed. Also, more qualification in the abstract is required. Much of what is presented in the abstract as novel has been well documented in many of the global ocean regions. There was significant discussion of the possibility of isotopic analysis to tease apart processes but no isotope mixing models or higher order analysis was performed (CSIA, Bayesian statistics, etc.).
Lastly, the phytoplankton section of the discussion was particularly problematic. There was much too much extrapolation. There are too many processes that could be affecting the isotopic compositions of the bulk material to assume it’s only due to changes in primary production. Trophic processing is not discussed in this section at all. Neither is microbial reworking of the fecal pellets (see Wojtal et al. 2023). Additionally, the authors should think about the efficiency of the gut of the zooplankton (see Doherty et al. 2021).
Major questions
- No where is diel vertical migration discussed or explored. Why?
- The Redfield ratio was conceptualized for phytoplankton physiology. Fecal pellets have presumably undergone significant reworking. Why would one expect that the Redfield ratio of zooplankton fecal pellets would match open ocean phytoplankton cellular elemental ratios?
- Why no zooplankton measurements if you talk about carcasses? Why no experiments trying to make zooplankton poop and isolating their fecal pellets to measure for an end member?
- Need to say somewhere that large and small cylinder were grouped together and that oval and spherical were grouped together
- What are error bars on the figures? Just the standard deviation of the two replicates?
Comments
- Line 6 – why is there a ~ above the 1 in 100%? Also, why is the % sign on the next line?
- Line 9 – should be “the Redfield Ratio”?
- Line 10 and 11 – this has been shown repeatedly. Why is it being stated like a new finding?
- Sentence starting at Line 28 – all at the same time?
- What about diel vertical migration? (lines 33 to 36)
- Line 40 – what about Doherty et al. (2021), Colleen Durkin’s work, or Shea et al. (2023)?
- The sentence starting at Line 45 is hard to follow
- Sentence starting at Line 48 is confusing
- Sentence starting at Line 50 – could it not just be recycling?
- Sentence starting at Line 60 – EXPORTS did this
- Look at Siegel et al. (2021) or Siegel et al. (2023) as a place to start (also Ken Buesseler’s work and Margaret Estapa’s work on the EXPORTS project)
- Line 112 – what was the size of the baffle?
- Line 124 – were the zooplankton alive during recovery? What about carcasses?
- Line 125 – how was the material examined under dissecting microscope? Was it on a mesh screen? What was the size of that mesh? What happened to smaller particles
- Line 126 – was material removed? What happened to it? How was it determined that material should be removed?
- Line 131 – how were those fecal pellets selected? Why only 15? What happened if there were more?
- Line 133 – why air dried? What method are you following?
- Line 134 – reference for method following?
- References for method section overall?
- Was EA data correction only done with 1 point calibration? How was data processing done?
- Line 161 – what was the criteria for oval vs. round? Was that only done by one person to keep consistent?
- Line 169 – how was total pellet count determined? Also by scaling?
- Line 171 – I don’t get how you get a proportion of the total POC or PN that is fecal pellet from what is described
- Line 173 – so this was just ignored?
- Line 178 – how do you test for normality with 4 samples?? How do you do any statistics with 4 samples?
- Lines 185-186 – how can you say this if you are ignoring so much of the POC (small particles)?
- Line 188 – reference figure 2
- Line 202 – so FPC and PFN don’t correlate?
- Sentence starting at Line 263 – a very bold claim for a very small sample size
- Line 274 – how do you get week-scale changes from sampling periods that were much longer than that?
- Paragraph starting at Line 327 – was there any evidence of TEP present?
- Line 340 – what is the range for the region??
- Could this be a constructive interference (synergistic) effect?
- Line 381 – what upper and lower traps?
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RC3: 'Comment on egusphere-2026-2459', Anonymous Referee #3, 17 Jul 2026
reply
General comments
The authors present bulk C, N, and isotopic data from bulk sediment trap material and isolated fecal pellets from four time segments during a bottom-moored sediment trap deployment off the Western Antarctic Peninsula. The authors aim to describe the importance of fecal pellets and other sources of particulate organic material to sinking flux and its properties, such as C:N ratio. The overall background of the manuscript is well presented, but the reader currently cannot assess the quality of the data due to several missing methodological details.
Specific comments
Missing methodological or quality-control details:
- The limits of detection and quantification for C & N analyses are not described, specifically in relation to the amount of C & N obtained from the samples. If, for example, the small number of fecal pellets analyzed resulted in measured quantities of N that were close to the limits of detection or quantification, this could explain a high measured C:N ratio.
- Blank corrections are also not described; contribution from a C blank, which is common on pre-combusted GF/Fs, could also account for high measured C:N ratios, particularly in samples with low organic loading on the filter.
- The methods by which the fecal pellet volumes were estimated are described, but the volumes are not reported. Thus, the reporting of fecal pellet C mg/mm^3 cannot be related back to the amount of C in a single fecal pellet, which would both be useful for the reader and important for understanding how much C & N was actually measured on the 15 isolated pellets (related to limit of detection/quantification above).
- Uncertainty in the reported fecal pellet C & N flux is not adequately described. There appear to be at least four sources of uncertainty, and it is unclear which sources have or have not been included in the “95% confidence intervals” reported. The four sources of uncertainty that I can see are: uncertainty in the geometric method for estimating volume, standard deviation in the mean volume assumed in converting from FP volume to C & N (based on means of volume estimated across a large number of pellets), analytical uncertainty in the instrumental method, and variation across the two replicate samples analyzed for each set of FP. Along with ensuring that adequate uncertainty is expressed, reporting proportional fluxes to two decimal places may be inappropriate once all sources of uncertainty are considered (e.g., Lines 193-194). To highlight this point, this analysis by the authors results in the statement in both the abstract and the text that fecal pellets constituted 100% of the flux in some samples. However, the authors should be able to validate this statement directly, as they performed visual inspection of the samples under magnification; in these samples, did they observe that the samples in fact contained absolutely no material other than identifiable fecal pellets?
- Figure 3 displays POC and PN as a fraction of total trap material, but the quantification of total trap material was not described. Is this a weight %? How was total trap material weighed?
Statistics: Averages and standard deviations across all sampling periods are reported throughout the Results section. In some cases, these time-averages for different measurements are statistically compared against each other (e.g., Lines 197-198). Reporting of these averages across sampling periods for the most part is not useful, since there is a high amount of variability across periods in most measurements; this also means that comparison of two such means against one another is not a reasonable comparison.
Other commenters have provided input about missing literature comparisons; I also noted these but will not comment further, as these issues can be found in the other reviews.
Technical comments
- Isotopic data with precision of +/-0.2 per mille or greater should not be reported to two decimal places—please edit to display one decimal place.
- The journal’s style guide may need to clarify, but it is my understanding that percent and per mille signs should not be separated from numbers by a space. This is because these signs are not units, but rather part of the number itself.
- Line 136-137: Were bulk C & N quantities determined in a separate analysis from the isotopic measurement, or the same analysis? The type of detector used for quantification is not mentioned.
- Line 165: What is ImageJ?
- In all cases, how 95% confidence intervals were determined needs to be described.
Citation: https://doi.org/10.5194/egusphere-2026-2459-RC3
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This excellent manuscript suffers from a few issues that should be considered. It cites only publications from Antarctica where little is known from fecal pellets. From polar regions in the north a wealth of data on fecal pellets is available. They are ignored. Often it is claimed that we need polar, not Antarctic and Arctic science. The main goal of the manuscript is not reached because the total exclusion of the rich Arctic and Sub-Arctic literature.
This is a research paper, not a review. The authors apply > 100 references. At times more than 10 references in the text. The reader understands that the authors know their literature, but I think that half of them would be more than sufficient.
The problem of insufficient preservation of fecal pellets in the cups should be considered. Could it be that particulate N leakes out and give rise to high C/N ratios? They were in place for 2 years before recovery
30. The process of coprorhexy and fecal pellet retention is an important one and should at least be mentioned. Also, the the data are from a post-bloom period with zooplankton playing a big role should be stressed.
185 Please revise all numbers. 92.49 +/- 75.17 is ridiculous! Often based upon two replicate aliquots. Statistics!