| 27 Feb 2026
Krill defecation at depth reduces carbon flux attenuation in the Weddell Sea euphotic zone
Abstract. The Weddell Sea, Southern Ocean, is a highly productive location of deep-water formation and a globally important site of carbon sequestration. Here, the biological carbon pump is dominated by particulate processes (e.g. zooplankton faecal pellets and phytoplankton detritus). However, climate driven changes in sea ice have the potential to disrupt these processes, highlighting a need for contemporary observations. This study quantified the flux of particulate organic carbon (POC) and nitrogen (PON) across three depths (50, 100 ,150 m) at five locations (including shelf, off shelf, ice covered and ice-free environments) in the western Weddell Sea using a drifting sediment trap. POC and PON fluxes were greater on shelf than off-shelf, likely reflecting increased nutrient supply and productivity on shelf. No strong patterns between sea ice and ice-free stations were present, likely because the ice pack was constantly shifting, with most sites influenced by sea ice. The POC flux remained stable or increased with depth at most stations, ranging from 42.5–364.1 mg C m-2 day-1 (mean of 123.2 mg C m-2 day-1). Krill faecal pellets represented 98 % of all pellets, which contributed an estimated 17–99 % (median of 48 %) of the POC flux. The faecal pellet flux peaked at 100 m across the shelf, suggesting krill defecating at depth during daily migrations effectively counteracted attenuation in the upper ocean. Our findings emphasise the importance of zooplankton mediated processes in determining the particle flux and the benefits of resolving the vertical flux at a resolution which incorporates their ecology. It is unclear how changing sea ice dynamics will impact zooplankton, so a process-driven understanding of biogeochemical fluxes is integral for predicting the future of carbon cycling in the Southern Ocean.
Review for Atherden et al.
The study summarises the results of drifting sediment traps in February 2024 at different sites in the Weddell Sea. The study adds urgently needed data on particulate organic carbon and nitrogen flux in a remote, and therefore understudied, region and highlights the role of krill faecal pellets in the carbon export.
It’s a good manuscript overall, but could do with a bit more data and statistics to support the statements.
In general, it would be good to know if you think the voyage happened during a bloom or post-bloom. The highest value of chl-a you found is 0.8 ugL-1 – is that a lot or rather typical for the region and time of year?
The sediment trap deployments are rather short and therefore favour the collection of fast-sinking particles, which, of course, would be dominated by fast-sinking krill faecal pellets. This potential bias needs to be discussed.
It would be great if you could add a data availability section.
Points by line:
12-13: add if it’s a downward migration, plus on a daily basis. Inject instead of defecate makes it even clearer.
20: Passively sinking of carbon-rich particles? Particulate processes not clear
33: Zooplankton-mediated
Introduction
40: cool waters? Why? Mention higher solubility of co2 in cooler waters (=Solubility pump)
42: cite another paper that is focused on the BCP in the Southern Ocean in particular besides the general BCP paper by Volk and Hoffert, 1985. For example, Boyd et al. 2024 (https://www.nature.com/articles/s43017-024-00531-3)
45: What is etc? Remove
45: If you need another carcass ref: Halfter et al. 2022 https://doi.org/10.1002/lno.11971
66: primary productivity?
70: remove “helping to”, edit fuel to “fuelling”
90-93: Reference to add, which reports on the slowdown of AABW: Zhou et al. 2023 https://doi.org/10.1038/s41558-023-01695-4
96: downward fluxes?
102: with set you mean determined? Maybe spell out what you mean. “Capturing the flux both at the ocean surface, where the upper limit of carbon flux is determined, and at depths of high carbon flux attenuation.”
Methods
111: how far did the other deployed sediment traps drift?
117: Any preservative in the sediment trap?
132-133: Check if only CTD profile data was analysed in R.
136: Any zooplankton swimmers that were picked out beforehand?
149: Make sure sections and subsections are consistently labelled in the final manuscript.
152: add how many blanks you used.
162: correct equation for flux, but earlier you mentioned that the traps were drifting between 11.5 and 20.5 hours, so less than a day. Maybe add half a sentence on how this was accounted for to estimate flux per day. It is also worth noting that with these “short” deployment times, you’d mostly get the fast-sinking particles. For example, if a sediment trap is open for let’s say 12 hours, deployed at 50 m depth, the sinking speed of a particle produced at the surface must be at least 100 m d-1 to be caught in the trap. This is something to consider in the discussion section.
172: Derived using what organisms?
178: This is the first time you mention microplankton. Would be useful to have it mentioned in the introduction and in the aims of the papers. Also, add a definition of microplankton. What happened to the microplankton in the trap?
Results
In general, did you conduct any statistics on potential drivers? This study is rather descriptive. Any of the CTD parameters that could be used to explain the difference in fluxes.
203: Maybe more oceanographic?
215: Where do you describe the chlorophyll-a measurements? Add a clearly marked section to the methods.
213: Interesting! Very shallow MLD, and low chl-a peak, even at the most productive station!
Figure 3: what’s the dotted line? MLD? Add to description. Also, add labels A-E to caption. Did you measure Chl-a or is it rather fluorescence? Add to the methods how you treated fluorescence data to get to Chl-a data in ug L-1.
227: Highest instead of greatest.
Figure 4: Less numbers on the x-axes of A, as it looks crowded. Did you produce the figures in R as well? Earlier, you wrote that you use R to generate CTD profiles. Maybe just say that all data analyses (unless stated otherwise) are performed in R (version xyz). As you arrange stations on a gradient from shelf to off shelf, maybe it would be good to arrange Figure 3 in the same way.
Figure 5: I’d merge this with Figure 4 for easier comparison.
255: delete “overwhelming”
Figure 6A: I can’t really see any orange in that graph. There were no round faecal pellets? POC flux: x-axis looks crowded. Reduce labels.
Figure 7: Reduce tick marks on the x-axis. Could you merge with Figure 6A?
Discussion
284: What is “its”? The Weddell Sea? Spell out.
292: day -1. Check unit.
302-304: do you have any information on nutrient concentrations from water sampling with the CTD on this voyage, that could support this statement?
321-322: Maybe add “sinking” to particulate material to distinguish from fresh phytoplankton (which would also appear as particulate material).
335: briefly mentioned the low numbers of ovoid and round faecal pellets and the absence of their producers, just to be comprehensive.
346: Define export in this context.
354-357: temporal mismatches should be relatively small due to the shallow depths and also the short deployment times. However, this could be a good spot to mention that you might not catch all sinking particles, only faster sinking ones.
360-361: What times of day were the traps deployed? Does it make sense to propose DVM (noting that there could be inverse migration too). I think deployment dates, duration and times could be put in a table in the methods.
366-368: defecate at depth, or faecal pellets just sink out very fast?
373: Any information on nets or acoustics from the voyage that would support the hypothesis of migrating krill?
Conclusions
403: Daily or diel? Consistent across the paper.