the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jun 2025
Budgets of particulate organic carbon in the mesopelagic layer across contrasting North Atlantic ocean biomes: a model study with PISCESv2_RC
Abstract. Biogeochemical and physical processes in the mesopelagic layer regulate the long-term storage of photosynthetic carbon in the ocean interior. However, persisting uncertainties in the budgets of particulate organic carbon (POC) underscore our limited understanding of mesopelagic ecosystem functioning in relation to the biological carbon pump. This study examines the drivers of POC variability in the top 1000 m of the North Atlantic Ocean over a climatological seasonal cycle. Budgets of detrital POC are comprehensively analyzed using the NEMO4-PISCESv2 model, which features two classes of detritus with different sinking speeds, a variable reactivity scheme for POC decay, and diverse modes of zooplankton detritivory and particle aggregation-disaggregation processes. Results reveal a latitudinal shift in detrital POC supply and removal dynamics. In the subtropical area, PISCES depicts relatively simple budgets where gravitational supply is mostly balanced by microbial degradation. By contrast, higher latitudes exhibit marked seasonal succession in supply and removal processes. From February through April, POC diffusion by vertical mixing dominates export fluxes, supplementing gravitational export (by 37 % annually in the subpolar area). During bloom demise in summer, consumption and fragmentation of large aggregates by mesozooplankton explain up to half of the flux attenuation. Interestingly, the lowest mesopelagic transfer efficiency (11 %) occurs in midlatitudes, the most productive area. Optimal detritus removal at midlatitudes results from opposed latitudinal gradients in temperature and particle lability, concurrent with high zooplankton activity. Our results prompt more explicit representation of suspended and slow-sinking particle dynamics and detritus-organism interactions in biogeochemical models.
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-2162', Anonymous Referee #1, 22 Sep 2025
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RC2: 'Comment on egusphere-2025-2162', Anonymous Referee #2, 07 Nov 2025
General appreciation:
The manuscript deals with Carbon budget in the mesopelagic layers at three latitudes in the Norther Atlantic. It is a 3D modeling study using a recent version of PISCES model that include more realism in the mesopelagic layers. The study aims to understand the main drivers of flux attenuation in the mesopelagic trying to understand which component of the pelagic ecosystem is more important: ocean mixing,
surface community composition of phytoplankton and initial ballasting, temperature controlled bacterial remineralisation, zooplankton feeding. Main results is that latitude affects all of them in a way that all processes are locally important.Overall this is a very good and complete study which many questions of the control of the flux in a rigorous manner, providing the mechanistic insights that model can. The question is if the model include all important players and processes. This aspect is discussed in section "study limitation" but could be improved (see below). Hence the manuscript deserves publication with minor changes
Improvement could be made in the section :
- "model evaluation" by using more up to date data sourcing from imaging systems that become available in the recent years: zooplankton biomass (Drago et al., 2022), rhizarians respiration (Laget et al., 2024), and marine particle size distribution (Kiko et al., 2022) and zooplankton size distribution (Dugenne et al., 2023), and vertical export flux (Clement et al., 2023). It would be nice to see how the model is able to fit to these datasets.
- "study limitation": recent works have showed that a major player, Rhizaria (Drago et al., 2022) are important in the mesopelagic carbon budget (Stuckel et al., 2029; Laget et al., 2024) accounting for 9% of POC attenuation (globally, a little less in the Atlantic). Would the addition of this group fit within the budget. Recognizing that maybe not all players are yet known could be mention in this section.
- Fragmentation by zooplankton
The fact that fragmentation was mediated by zooplankton was proposed earlier. The flow field associated with swimming or feeding zooplankton has been observed to frag ment aggregates several millimeters long by Dilling and Alldredge (2000). They suggested that aggregate breakup by vertically migrating euphausiids was responsible for their observed day–night variation in marine snow size distribution in the upper 100m depth. This type of fragmentation was then shown to have less impact than the fragmentation by zooplankton feeding in the upper mesopelagic which release small particles during flux feeding on large particle (Stemmann et al., 2004b). This particle fragmentation during flux feeding was important to slow down the sinking of particles which could then be consume by bacteria. An important parameter in Stemmann et al., 2004b was the size of feeding area. Where does this value comes from. Has some sensitivity analysis been carried on this term in the study?
here are the references:
Clements, D. J., S. Yang, T. Weber, A. M. P. McDonnell, R. Kiko, L. Stemmann, and D. Bianchi. 2023. New Estimate of Organic Carbon Export From Optical Measurements Reveals the Role of Particle Size Distribution and Export Horizon. GLOBAL BIOGEOCHEMICAL CYCLES 37. doi:10.1029/2022GB007633Dilling, L., and A. L. Alldredge. 2000. Fragmentation of marine snow by swimming macrozooplankton: A new process impacting carbon cycling in the sea. Deep-Sea Research Part I-Oceanographic Research Papers 47: 1227–1245.Drago, L. and others. 2022. Global Distribution of Zooplankton Biomass Estimated by In Situ Imaging and Machine Learning. FRONTIERS IN MARINE SCIENCE 9. doi:10.3389/fmars.2022.894372Dugenne, M. and others. 2023. First release of the Pelagic Size Structure database: Global datasets of marine size spectra obtained from plankton imaging devices. Earth System Science Data Discussions 2023: 1–41.Kiko, R. and others. 2022. A global marine particle size distribution dataset obtained with the Underwater Vision Profiler 5. EARTH SYSTEM SCIENCE DATA 14: 4315–4337. doi:10.5194/essd-14-4315-2022Laget, M. and others. 2024. Global census of the significance of giant mesopelagic protists to the marine carbon and silicon cycles. Nature Communications 15: 3341.-
Specific comments:
Citation: https://doi.org/10.5194/egusphere-2025-2162-RC2
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- 1
Summary:
The objective of this study is to analyze simulated POC dynamics in the mesopelagic layer (defined here as below the euphotic zone and above 1,000 m) in the North Atlantic from a NEMO-PISCES simulation, compare the model results to observations, and derive a budget. The model has a relatively complicated treatment of mesopelagic particle dynamics. My hope, after reading the abstract, was that this study would drive forward ongoing efforts to better constrain the mesopelagic POC dynamics in numerical models, since these are poorly constrained by observations and prone to big uncertainties. However, the comparison to observations presented in thus manuscript is cursory (in Figures 2 and 3 in section 2) and doesn’t add any new insight. Presented instead (in section 3) is a detailed analysis of the output from the somewhat complicated model for different subregions. I fail to see how this adds any valuable insight. Then, in section 4, the authors seem to jump from the model analysis to drawing conclusions about the real world (line 617: “This study provides a comprehensive analysis of POC dynamics in the top 1000 m of the North Atlantic, linking POC distribution and seasonality with transport and transformation processes.”) even though we know that the model is prone to big uncertainties. The obvious flaw that the model results do not equal the real world is not even mentioned. In section 5, the authors claim to have “obtained mechanistic insights into POC distribution, export patterns, and biological carbon pump efficiency across three North Atlantic regions” (line 832-834). No insights about the real world were obtained. In my view, the basic approach to this manuscript flawed. I do not recommend publication.
Perhaps more of a side note: The authors give an inaccurate definition of the biological carbon pump that is, unfortunately, common but should not be perpetuated further. When stating “The ensemble of the biology-mediated processes that transfer POC, PIC, and DOC to the deep ocean is known as the biological C pump (Volk & Hoffert, 1985; Legendre, 2024)” (line 74-76) the authors neglect to mention that biologically derived inorganic carbon is moved from the deep ocean to the sea surface through ocean circulation. That the biological pump is the balance of the organic carbon that is moved downward (in different forms and via different mechanisms) and the resulting inorganic carbon that is moved back to the sea surface and outgasses. See Frenger et al. (2024).
Frenger, I., Landolfi, A., Kvale, K., Somes, C. J., Oschlies, A., Yao, W., et al. (2024). Misconceptions of the marine biological carbon pump in a changing climate: thinking outside the “export” box. Glob. Chang. Biol. 30:e17124. doi: 10.1111/gcb.17124