the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Iron “Ore” Nothing: Benthic iron fluxes from the oxygen-deficient Santa Barbara Basin enhance phytoplankton productivity in surface waters
De’Marcus Robinson
Anh L. D. Pham
David J. Yousavich
Felix Janssen
Frank Wenzhöfer
Eleanor C. Arrington
Kelsey M. Gosselin
Marco Sandoval-Belmar
Matthew Mar
David L. Valentine
Daniele Bianchi
Abstract. The trace metal iron (Fe) is an essential micronutrient that controls phytoplankton productivity, which subsequently affects the cycling of macronutrients. Along the continental margin of the U.S. West Coast, high benthic Fe release has been documented, in particular from deep anoxic basins in the Southern California Borderland. However, the influence of this Fe release on surface primary production remains poorly understood. In the present study from the Santa Barbara Basin, in-situ benthic Fe fluxes were determined along a transect from shallow to deep sites in the basin. Fluxes ranged between 0.23 and 4.9 mmol m-2 d-1, representing some of the highest benthic Fe fluxes reported to date. To investigate the influence of benthic Fe release from the oxygen-deficient deep basin on surface phytoplankton production, we combined benthic flux measurements with numerical simulations using the Regional Ocean Model System coupled to the Biogeochemical Elemental Cycling model (ROMS-BEC). For this purpose, we updated existing Fe flux parameterization to include new benthic fluxes from the Santa Barbara Basin. Our simulation suggests benthic iron fluxes support surface primary production creating positive feedback on benthic Fe release by enhancing low oxygen conditions in bottom waters. However, the easing of phytoplankton Fe limitation near the coast may be partially compensated by increased nitrogen limitation further offshore, reducing the efficacy of this positive feedback.
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De’Marcus Robinson et al.
Status: open (until 28 Mar 2023)
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AC1: 'Comment on bg-2022-237', Tina Treude, 17 Dec 2022
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Shortly after the submission of our manuscript, we became aware of a recent publication related to our work:
Wallmann, K., José, Y.S., Hopwood, M.J., Somes, C.J., Dale, A.W., Scholz, F., Achterberg, E.P. and Oschlies, A., 2022. Biogeochemical feedbacks may amplify ongoing and future ocean deoxygenation: a case study from the Peruvian oxygen minimum zone. Biogeochemistry, 159(1), pp.45-67.
We plan to implement and discuss this study in our revised manuscript.
Citation: https://doi.org/10.5194/bg-2022-237-AC1 -
RC1: 'Comment on bg-2022-237', Christopher Somes, 13 Jan 2023
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This paper by Robinson et al. uses a high-resolution regional ocean-biogeochemical model (ROMS-BEC) to investigate the impact of benthic iron input from sediments on ocean net primary productivity (NPP) and oxygen levels in the California Current System (CCS). They also present new field data of benthic iron fluxes from the near-shore system in the Santa Barbara Basin (SBB) channel that show high benthic iron fluxes in a sill (~500 meters) that correspond with low bottom water oxygen concentrations, which are among the highest benthic iron fluxes ever measured. The included model sensitivity simulations modify the amount of iron that is released with respect to bottom water oxygen concentration to demonstrate its importance on NPP in the CCS. The main sensitivity experiments include capping the benthic iron release at oxygen thresholds (i.e. Low Oxygen Thresholds 100 and 65 µM) in order to test a scenario without high benthic fluxes under low bottom water oxygen concentrations. Another high benthic flux sensitivity experiment (High-Flux) is included where higher benthic iron fluxes are included under very low oxygen conditions, which are supported by their new measurements. The authors argue that including this high benthic iron release under low oxygen conditions supports higher nearshore NPP in this region, which could develop into a positive feedback driving lower oxygen conditions, higher benthic iron flux, and additional NPP. However, nitrogen limitation ultimately dampens this potential positive feedback, resulting in virtually no change in NPP to the overall region. I will focus my review on the modeling effort since that is where my expertise lies, noting that I do find the new field data an exciting aspect to the paper.
Overall I find this to be a useful and interesting study. The paper is nicely structured and well written. The model experiments performed are relevant and well designed. The model experiments certainly show many interesting features demonstrating how benthic iron fluxes are important drivers of NPP in the model, and thus may well be in the real ocean as well. However, I have some reservations about how comprehensively the dynamical numerical model experiments are discussed and support the positive versus dampening feedback mechanisms, from which many of the main conclusions are drawn upon. The most important aspects of the numerical model experiment “High-Flux” are described in only one brief paragraph (Section 3.4). Therefore, I am rather critical of the manuscript in its current form and I think a more robust description of this High-Flux model experiment in particular is necessary before I would endorse it for publication.
-Christopher Somes
GEOMAR Helmholtz Centre for Ocean Research Kiel
Major Comments/Questions:
line 362-363: “patchwork changes in NPP” Why are the changes in NPP so much patchier in the High-Flux experiment compared to dFe and NO3 changes (Figure 7)?
It is not well explained why the highest hotspots of increased NPP in High-Flux are so far offshore (~36°N, 124°W and 33.5°N,119°W), which does not correspond with the highest increase in dFe around the nearshore zones e.g. Point Conception and the Santa Barbara Channel. For example, is the NPP hotspot immediately west of San Nicolas Island (~33.5°N,119°W) driven by the high dFe from the Santa Barbara basin and horizontal transport or is it derived from benthic dFe release from the deeper Santa Cruz basin and more local vertical transport/mixing? Figure 2 shows much higher benthic dFe release at greater depths in the Santa Barbara Basin. Does this trend continue in the deeper basins of the Southern California Bight in the model? I would be curious to see some type of vertical plot (or description at least) of dFe from one of these deeper basins (e.g. Santa Cruz basin near this high NPP hotspot) to see if a significant amount of benthic-derived dFe can avoid being scavenged and make it to the surface ocean in non-coastal settings.
lines 364-365: “These patterns are opposite in sign to the changes observed in the Low Oxygen Threshold experiments, although more intense, and can be explained by similar dynamics”.
I suppose this is the reasoning for such a short section 3.4, which in my opinion should be one of the most important in the paper. Figures 6 and 7 do not appear strictly “opposite” to me, although it is difficult to interpret much from Figure 6 with its color bar scale. For example, the main dFe increase in High-Flux (Figure 7) is centered around Point Conception, whereas there appears to be no significant change there in Low Oxygen Threshold-100 (Figure 6).
line 366: “Nearshore, where Fe is more frequently limiting, higher Fe availability releases Fe limitation and drives the higher NPP and more intense NO3 drawdown.”
But when I look at one of the nearshore regions with the highest increases in dFe in High-Flux, which is centered off Point Conception (Figure 7c), there is very little change if not a slight decrease in NPP there. Thus this claim of widespread Fe limitation in nearshore waters is not very convincing to me. Because if this were true, I would expect a higher correlation between increased nearshore dFe and increased NPP. Thus I think some additional description on this model behavior is required.
lines 369-370: “… localized increase in Fe fluxes from the deep SBB has cascading effects on NPP across a much larger region in the CCS.”
Can the general increase in NPP across the entire Southern California Bight mainly be attributed to the benthic flux from the SBB alone? I would guess increased benthic dFe would occur across the entire Southern California Bight/Borderland region but this is not discussed or shown.
Model-Data Comparison (Figure 4)
The Control simulation already overestimates offshore dFe concentrations as mentioned in lines 308-309. Since offshore dFe increases even more in High-Flux (Figure 7a), I wonder if surface dFe distribution in High-Flux is improved (or not). The benthic flux measurements indicate that aspect of the model is improved in High-Flux, but could it be that too much dFe is being transported offshore where much of the higher NPP occurs in High-Flux?
Spatial Complexities in the Feedback loop
I think the language regarding the positive versus dampening feedback loop could be more specific. For example, Table S1 shows that the High-Flux simulation has exactly (to four significant digits) the same amount of NPP as Control over the full model domain despite that surface dFe is higher on average. Doesn’t that suggest that the dampening effect completely compensates the potential positive feedback over the entire model domain?
I find it pretty remarkable how similar these general results are over the entire model domain compared to my global biogeochemical modeling study where I performed a conceptually similar set model experiments, which tested different benthic and atmospheric Fe fluxes, and also found no significant increase to global NPP despite substantially higher benthic Fe fluxes (Somes et al., 2021). One notable difference is that I increased my scavenging rate constants with source fluxes to prevent my model from overestimating the extent of high dFe concentrations, which also helped my model reproduce the strong offshore dFe gradient. Since it is mentioned that scavenging becomes more rapid when dFe is above the constant 0.6 nM ligand concentration, I wonder if more effective scavenging at high dFe concentrations in High-Flux may contribute to the lack of additional NPP over the entire model domain similar to my global simulations.
On the other hand, it appears that a positive feedback may have developed in the Southern California Bight region, perhaps due to a wider continental shelf/borderland and thus higher dFe fluxes that could more effectively reach the surface there? I think it would be really interesting to have more insights about how some of these features operate differently throughout the model domain and contribute to the patchy, non-uniform NPP changes in Figure 7c.
Summary
Although I have been critical regarding the discussion of the model results, I want to reemphasize that I think this is a quality study has the potential to make a really nice paper. I think my comments can be resolved in a straightforward manner by expanding section 3.4 to describe more of the complex dynamics and spatial patterns (as mentioned above). Additionally, more insights on how the dynamic numerical model operates regarding the positive versus dampening feedback mechanisms should be provided (perhaps in section 4.2). For example, it seems that the dampening effect dominates across the entire domain, whereas perhaps this positive feedback is more important in some more localized regions.
Reference:
Somes, C. J., Dale, A. W., Wallmann, K., Scholz, F., Yao, W., Oschlies, A., Muglia, J., Schmittner, A., and Achterberg, E. P.: Constraining Global Marine Iron Sources and LigandâMediated Scavenging Fluxes With GEOTRACES Dissolved Iron Measurements in an Ocean Biogeochemical Model, Global Biogeochemical Cycles, 35, 10.1029/2021gb006948, 2021.
Citation: https://doi.org/10.5194/bg-2022-237-RC1 -
RC2: 'Comment on bg-2022-237', Anonymous Referee #2, 20 Mar 2023
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The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2022-237/bg-2022-237-RC2-supplement.pdf
De’Marcus Robinson et al.
De’Marcus Robinson et al.
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