| 12 Jun 2026
From surface flux to seafloor function: vertical carbon export and benthic communities in high-latitude coastal systems
Abstract. The downward flux of carbon from the surface ocean to the seafloor fuels benthic invertebrate biomass and drives carbon sequestration. In polar regions, pelago-benthic carbon coupling is shaped by complex processes influenced by climate change, including glacial retreat, water stratification, and warming. Here we examine three high-latitude coastal systems along the Northeast Greenland coast: Dove Bugt, the Brede–Ardencaple Fjord system, and Kong Oscar Fjord, each differing in bathymetry, glacier influence, and exposure to Atlantic Water at depth. Using short-term sediment-traps, hydrographic profiling, sedimentary carbon analyses, and trait-based macrobenthic community data, we assess how vertical particulate organic carbon (POC) export, sedimentary organic carbon stocks (SOC), and benthic biomass and function are linked across the three coastal systems. We found that vertical POC flux and SOC were highly variable in space and showed no consistent relationship with benthic invertebrate carbon biomass. Our trait analysis of the benthic communities revealed that only a subset of trait strategies, particularly only epifaunal suspension feeders, consistently translated carbon availability into standing biomass. Latent variable modelling, which identifies underlying ecological gradients not explained by measured environmental predictors, revealed an additional axis structuring benthic trait biomass and is most consistent with bathymetric relief and associated horizontal redistribution processes. Thus, with our results showing a temporal decoupling among POC export, SOC, and benthic carbon biomass, we propose that future research investigate lateral transport pathways associated with Atlantic Water intrusion and stratification, alongside benthic production and ecosystem functioning, to better understand carbon processing, retention, and sequestration in Arctic systems.
The manuscript by Armitage et al., is well written and explores an interesting question in terms of how much vertical carbon export and sediment carbon stock affects benthic communities in high latitudes. A great amount of work and expertise has clearly gone into shaping this manuscript. It is an extensive piece of work trying to answer 4 different hypothesis, which makes it sometimes challenging to follow through on all the different strands the authors are aiming to tie together. There are a few aspects which might need some more attention, before the manuscript is ready for application.
I am not totally convinced by the oceanographic aspect of the paper and the finding that high arctic fjords with high sills are influenced by Atlantic water, which is normally characterised by temperatures > 2 degrees and high salinity. I could not find this in the supplementary material (which shows temperatures below 2 degrees) and the authors do not provide thresholds on what they defined as Atlantic water (in Figure 1 this seems to be primarily based on salinity). Based on the figures provided, I do not think bottom waters at 0-2 degrees are an indication for fjords being influenced by warm Atlantic water. This needs to better explained or possibly dropped.
When it comes to the estimation of carbon content from wet-mass, I am not convinced by the very generic conversion that has been employed here. Behrisch and Zwerschke (2026) have shown that a conversion from wet mass to dry or afdm is not always possible with specific taxa, and that all of these conversion factors are highly taxa dependent. Anemones for example may have a large amount of water stored in their wet-weight compared to bryozoans, which would result in an overestimation of their carbon content. Barnes and Sands (2017) conversion to carbon content was based on functional groups and afdw and not a blanket conversion for all taxa. This could very much bias the results when aiming to link carbon standing stock to sediment carbon and vertical carbon transport. Thus, I would urge to reconsider this aspect of the manuscript.
The other aspect is that vertical carbon transport is unlikely to be completely vertical in a system regulated by tides and currents. This means that surface carbon content may not be linked to sediment carbon stock, particularly in areas with high flux, which is not necessarily linked to benthic-pelagic coupling but dictated by the abiotic environment and it would be good to have this acknowledged or considered when calculating the vertical carbon flux.
One aspect that has been neglected in this paper is, that substratum availability, generally dictated by abiotic environments as well, is a great driver of benthic communities. To find large amounts of high biomass sessile suspension feeders, hard substratum tends to be required, which is found in the high energy environments. The authors touch upon this, by linking their LV gradient to bathymetry and steepness, but it would be nice to see a clearer acknowledgement of this.
Although the paper is very well written, there is a tendency to overcomplicate statements, which sometimes makes it hard to follow and figure out what it is the authors are trying to say. E.g a temporal decoupling of POC, SOC and benthic carbon biomass in the abstract, or greater redistribution of carbon across co-occurring feeding modes under advective conditions in the discussion. A simplification and greater precision of language here might help the reader to better understand the point that is being made. This is particularly evident in the discussion, where the authors are making bold statements, which are not necessarily underpinned by the data that is presented.
The figures are very nice and clear but I am not sure it is necessary to have the same figure three times. It makes it difficult to compare between the differences in traits across the latent variable scores. Maybe all the trait-biomass diagrams could be combined into one, based on their z score.
I would also recommend spelling out some of the abbreviations such as surface water or Atlantic water or as a minimum spell them out in a new paragraph/section again, so the reader is reminded what it means.
Some more specific comments
Line 49: modify in which way?
Line 54: Bathymetric highs?
Line 53-56: Check and consider Buydens 2026
Line 82- 86: sentence is being repeated
Line 130-132: was this analysed in different sections, e.g. surface and lower depth, if so how?
Line 154: When was it deeper
Line 165-170: Can you explain what the coeffcient means, for people not familiar with analysis? You speak of it being stable or high or moderate. Is that the intervariability between sites or stations etc etc
Line 180-183: Not clear
Line 242: I would not call this comparable
Line 251-2: Is this linked to primary production
Line 318: this biomass is greater than other stations, such as KOF3
Line 321: Why would the deep stations be more prone to disturbance from deposition and sediment resuspension
Discussion:
It is not always clear whether some of the statements are supported by a statistical analysis, as some of this factors such as particle size have ben measured but there is no indication about a formal analysis here
Line 428-430: unclear
Line 435-438: unclear
Line 455-457: Would this not be a seasonal pattern
Line 470 – 472: Would this not be the other way around
Line 472-475: this contradicts your previous statements
Line 485-486: exposure to what
Line 487-488: like what substratum type, depth, currents
Line 492-3: Well yes because there is more hard substratum, no?
Line 504: unclear what you mean with hydrographic scale
Line 512-514: This is very hard to interpret
Line 516-518: I am not sure I agree here and whether this is clearly supported in your data
Line 525: what processes
Line 525-530: But this is not what you did, no?
Line 535-537: I am not sure you can say that you have shown that
Line 539-541: But you did not measure that