While this paper represents new data on POM elemental and stable isotope composition (which is always welcome) and the interpretation given appears fairly senseful, though largely speculative. But because the data set is limited to ‘classical’ parameters of POM and does not provide more specific data about POM composition (e.g. isotopic composition of specific components) the paper falls short in substantially improving our insights in the fate of POM in the oceanic water column with regard to existing literature.

A further major shortcoming of this paper is the one-dimensional (surface to deep) approach used when interpreting the data, despite the apparent complexity of ocean currents and counter currents in the studied area. No use is made of T-S, nutrient data to inform on mixed layer depth, DCM position and to identify major water masses and possible impacts of advection processes on observed profiles.

The method section should be more detailed, since no information on sample preservation, standards, references used, corrections applied .. is given.

General comments:

This manuscript provides valuable data on the isotopic and elemental composition of particulate organic matter in the tropical Pacific Ocean and utilizes comparisons with the dissolved inorganic pool and dissolved oxygen to infer transformation and degradation of organic matter over depth. However, the authors mainly discuss the POM data in terms of its vertical distribution, despite the data spanning almost 2000 m and there likely being influences of water masses on all the parameters measured. The authors discuss some differences in the water column structure and characteristics of the different stations due to the currents in the study area, but no data is presented demonstrating these differences. I see that water mass information has been added to some figures based on a previous comment, however without reading an updated manuscript version I cannot confirm if discussion has been added about this. Additionally, there is no mention of the possibility of variations in d¹³C-POC values due to photosynthetic variations within the upper water column. As POM is mainly produced by phytoplankton in the upper ocean, this is one other potential explanation for some of the variations observed. Further, many statements are made without evidence and should either be bolstered with evidence/data or altered to clarify that these are inferences from the data available; please see the examples listed in the “specific comments” section below. Lastly, the methods section needs more detail throughout, though I am aware that a previous comment also mentioned this and it has likely been addressed already -- please disregard my comments below where appropriate. Overall, the manuscript presents valuable data, but the discussion of the data and the interpretation (at least in the manuscript version that I have seen) could be improved.

Specific comments:

Methods section: More detail is needed on the dissolved oxygen methods in Lines 105-106. Since the method is very common, just a brief description would be fine. The POC/d13C-POC/PN section should mention blank corrections for POC and d13C-POC values. All sections on chemical measurements (DO, POC/PN, DIC, isotopes, chl-a) should include some mention of instrument calibration and types and sizes of standards used.

Line 112: I believe the per mil should be a percent sign for the POC/PN concentration accuracy (±0.8)

Section 3.1: This section includes important information on the differences in water column characteristics between the five stations. However, more information would be helpful for the reader to follow the authors’ interpretation of the data. Namely, the euphotic depth and mixed layer depth should be reported for each station, if possible. If photosynthetically available radiation (PAR) was not measured via a CTD sensor, some other metric would still be helpful (like the primary productivity zone based on chlorophyll concentrations, if possible; https://doi.org/10.1073/pnas.1918114117) . A figure including the chlorophyll concentration profiles would also help the reader envision the differences/similarities between the different sites.

Section 3.2: All interpretation of the d13C values of POC within this section is related to degradation of POC. However, there is potential for the magnitude of photosynthetic carbon isotope fractionation to vary over depth within the euphotic zone (https://doi.org/10.1073/pnas.2304613121), and thus production (as well as degradation) can affect the d13C values of POC. Separating out Figure 3A by station for just the RSDL (0-300m) would be one way of investigating this possibility – selective degradation of amino acids and carbohydrates, as described by the authors, should present itself as a positive relationship between POC concentration and d13C-POC values, whereas a different relationship or lack of a relationship may be an indicator that something else is driving the variations – potentially something relating to production of POC or different patterns of degradation than what is being described.

Line 150 mentions the POC concentration averaged over “the surface to the deep chlorophyll maximum layer (DCM, 0-150 m)” – this makes it sound like the DCM at all stations is 150 m, which is not true based on the associated dataset. Clarification of why 150 m was chosen would be helpful

Line 191: are there any studies from this region with biochemical measurements that could be used for comparison?

Sections 3.2 and 3.3: As mentioned in the general comments section above, there is a lot of discussion of the data in terms of variation over depth but little consideration of water masses affecting the observations. For example, Lines 200-202 mention a small increase in DIC over depth due to a small amount of POC degradation, but DIC concentrations are also impacted by the temperature of those waters and where in the surface ocean they originated from.

Lines 223-225: smaller variation in DIC vs POC d13C values is also due to the much larger size of the DIC pool in comparison to POC, as the authors previously note in Lines 75-77. This should be added here as well.

Lines 239-240: degradation of POC, regardless of which portions of the POC pool are degraded, should decrease DIC d13C values because of the large difference in the d13C values of POC and DIC due to photosynthetic carbon isotope fractionation during the production of POC (i.e., POC d13C values  <<< DIC d13C values, so even degradation of isotopically heavy POC is going to lower the d13C value of the DIC pool). Discussion within this section should be altered to reinterpret the data within this context.

Some examples of statements made without evidence:

Line 159-161: change “which enhanced” to “which likely enhanced” as heterotrophic microbial activity is not reported here, unless another study (one of the citations?) has shown this in this region. If so, please make this clear in the way the references are presented

Lines 200-202: the wording of this sentence is not accurate to what was measured – there is no evidence that DIC concentration increased “slowly” because no rates were measured

Line 233-234: is there evidence that the synthesis rate of POM is greater than the degradation rate above the DCM? Rates of NPP would be evidence of this but otherwise I would change the wording here.

Lines 246-247: “the photosynthesis rate to slow”, “the respiration rate … was still very fast”; no rate measurements are reported here

Lines 249-250: “the rate of chemosynthesis of organic carbon was lower than the degradation rate of POC” -- again no measurements of POC degradation rates or chemoautotrophic fixation of OC were made

Line 268: no evidence is presented about the proportion of refractory lipids in POC. This is an inference due to suggestions from the presented data, not an observation

Technical corrections:

Throughout the manuscript: d13C values should always be referred to as “d13C values of…” not just “d13C of…”

Line 44: change “the generally accepted…” to “one generally accepted…”

Line 69: change “molecules” to “concentration”

Line 84: change “respiration of heterotrophic organisms” to “respiration by…”

Line 100: change “was” to “were”

Line 150: use nonbreaking space between “150” and “m” so the unit does not separate from the number