the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
How does the phytoplankton-light feedback affect marine N2O inventory?
Sarah Berthet
Julien Jouanno
Roland Séférian
Marion Gehlen
William Llovel
Abstract. The phytoplankton-light feedback (PLF) depicts how phytoplankton biomass interacts with the downwelling shortwave radiation entering the ocean. Considering the PLF allows differential heating across the ocean water column as a function of the phytoplankton concentration. Only one third of the CMIP6 Earth system models include a complete representation of the PLF. In other models, the PLF is mimicked either thanks to a prescribed climatology of chlorophyll or not represented at all. Consequences of an incomplete representation of the PLF on the marine biogeochemical content haven’t been assessed yet and remain a source of multi-model uncertainty in future projection. Here, we scrutinize with a single modelling set-up how various representation of the PLF can impact ocean physics and ultimately marine production of a major greenhouse gas, the nitrous oxide (N2O). Global sensitivity experiments considering the ocean, sea ice and marine biogeochemistry have been performed at 1-degree of horizontal resolution over the last two decades (1999–2018). We show that the representation of the PLF has significant consequences on the ocean heat uptake and temperature of the first 300 meters of the tropical ocean. Temperature anomalies due to an incomplete PLF representation drive perturbations of the ocean stratification, dynamics and oxygen concentration. Different projection pathways for N2O production result from the choice of the PLF representation. Considering an incomplete representation of the PLF overestimates the oxygen concentration in the North-Pacific oxygen minimum zone and underestimates the local N2O production. This leads to important regional differences of sea-to-air N2O fluxes: fluxes are enhanced by up to 24 % in the south Pacific and south Atlantic subtropical gyres, but reduced by up to 12 % in oxygen minimum zones of the northern hemisphere. Our results based on a global ocean-biogeochemical model at CMIP6 state-of-the-art thus shine a light on a current uncertainty of the modelled marine nitrous oxide budget in that climate models.
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Sarah Berthet et al.
Status: closed
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RC1: 'Comment on esd-2022-28', Rémy Asselot, 25 Aug 2022
The comment was uploaded in the form of a supplement: https://esd.copernicus.org/preprints/esd-2022-28/esd-2022-28-RC1-supplement.pdf
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AC1: 'Reply on RC1', Sarah Berthet, 23 Nov 2022
The comment was uploaded in the form of a supplement: https://esd.copernicus.org/preprints/esd-2022-28/esd-2022-28-AC1-supplement.pdf
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AC1: 'Reply on RC1', Sarah Berthet, 23 Nov 2022
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RC2: 'Comment on esd-2022-28', Anonymous Referee #2, 16 Sep 2022
OVERVIEW
The authors study the effect of light absorption by phytoplankton on the heat content of the ocean and subsequently on N20. They run simulations with full/partial representation of this process and study its effect. They demonstrate that indeed the biooptical coupling influences N20 and the heat content of the ocean. I propose some modification to this manuscript prior to publication, which are given below.
GENERAL COMMENTS
- The model was spun up considerably longer than the simulation runs. Please elaborate as to why not use longer simulation runs?
- The overall text seems to be missing a thread and has too many shorthand notations. While familiar to the authors and others in the field, this hampers the flow of the paper substantially. I suggest reducing the usage of shorthand notation and move some of the technical parts to the supplement.
- Given the paper deals with the effect of the biooptical coupling I suggest the authors add exact mathematical expressions which describe this process. It is described somewhat shortly in lines 218 to 224. For example: please state the exact equations which model how phytoplankton affects light penetration. Subsequently, how does this affect the rate of phytoplankton growth, rate of heating at depth, and so on. I consider this to be of high value for non-experts. Also, it would make the model more easily reproducible.
- Please state the governing equations for phytoplankton dynamics: in particular the growth term of phytoplankton as a function of light, nutrients and temperature. What limits growth? How is the loss term parametrized? Does light penetration feedback onto the growth rate? Does temperature effect the growth rate? Is the model time step adequate to resolve this?
- Please add a figure displaying the model structure, highlighting the part related to biooptical coupling and the link to nitrous oxide, as it is the core of the paper.
- The effect of phytoplankton on the mixed layer depth is mentioned in line 447 (I assume the authors mean heating due to the biooptical feedback). Here the effect of wind is stated to alter the mixed layer depth more strongly than phytoplankton. However, it is known since Platt et al. (2003) that the phytoplankton change their biomass so that the Critical depth (Sverdrup, 1953) matches the mixed layer depth. This occurs due to the biooptical coupling that the authors explore. Please elaborate more on this.
REFERENCES
Sverdrup, H. U. (1953). On conditions for the vernal blooming of phytoplankton. Journal du Conseil International Pour lExplorationde la Mer, 18: 287–295.
Platt, T., Broomhead, D. S., Sathyendranath, S., Edwards, A. M., Murphy, E. J. (2003). Phytoplankton biomass and residual nitrate in the pelagic ecosystem. Proceeding of the Royal Society A, 459: 1063–1073.
SPECIFIC COMMENTS
L14 Please rephrase the sentence starting with “Considering…”.
L65 Change “into” to “through”.
L67 Please remove “(because the same)”.
L72 – L109 Suggested references on this effect from the literature:
Edwards, A. M., Wright, D. G., Platt, T. (2004) Biological heating effect of a band of phytoplankton. Journal of Marine Systems, 49, 89-103. doi: 10.1016/j.jmarsys.2003.05.011.
Edwards, A. M., Platt, T., Wright, D. G. (2001) Biologically induced circulation at fronts. Journal of geophysical research, 49, 89-103. doi: 10.1016/j.jmarsys.2003.05.011.
L152 Please rephrase the sentence starting with “In that perspective…”.
L209 Table 1 should be on top of the table. Suggest renaming chl_inter, clim_zcst, clim_zvar so as to drop the “_”.
L231 – L317 Please move to the Introduction.
Citation: https://doi.org/10.5194/esd-2022-28-RC2 -
AC2: 'Reply on RC2', Sarah Berthet, 23 Nov 2022
The comment was uploaded in the form of a supplement: https://esd.copernicus.org/preprints/esd-2022-28/esd-2022-28-AC2-supplement.pdf
Status: closed
-
RC1: 'Comment on esd-2022-28', Rémy Asselot, 25 Aug 2022
The comment was uploaded in the form of a supplement: https://esd.copernicus.org/preprints/esd-2022-28/esd-2022-28-RC1-supplement.pdf
-
AC1: 'Reply on RC1', Sarah Berthet, 23 Nov 2022
The comment was uploaded in the form of a supplement: https://esd.copernicus.org/preprints/esd-2022-28/esd-2022-28-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Sarah Berthet, 23 Nov 2022
-
RC2: 'Comment on esd-2022-28', Anonymous Referee #2, 16 Sep 2022
OVERVIEW
The authors study the effect of light absorption by phytoplankton on the heat content of the ocean and subsequently on N20. They run simulations with full/partial representation of this process and study its effect. They demonstrate that indeed the biooptical coupling influences N20 and the heat content of the ocean. I propose some modification to this manuscript prior to publication, which are given below.
GENERAL COMMENTS
- The model was spun up considerably longer than the simulation runs. Please elaborate as to why not use longer simulation runs?
- The overall text seems to be missing a thread and has too many shorthand notations. While familiar to the authors and others in the field, this hampers the flow of the paper substantially. I suggest reducing the usage of shorthand notation and move some of the technical parts to the supplement.
- Given the paper deals with the effect of the biooptical coupling I suggest the authors add exact mathematical expressions which describe this process. It is described somewhat shortly in lines 218 to 224. For example: please state the exact equations which model how phytoplankton affects light penetration. Subsequently, how does this affect the rate of phytoplankton growth, rate of heating at depth, and so on. I consider this to be of high value for non-experts. Also, it would make the model more easily reproducible.
- Please state the governing equations for phytoplankton dynamics: in particular the growth term of phytoplankton as a function of light, nutrients and temperature. What limits growth? How is the loss term parametrized? Does light penetration feedback onto the growth rate? Does temperature effect the growth rate? Is the model time step adequate to resolve this?
- Please add a figure displaying the model structure, highlighting the part related to biooptical coupling and the link to nitrous oxide, as it is the core of the paper.
- The effect of phytoplankton on the mixed layer depth is mentioned in line 447 (I assume the authors mean heating due to the biooptical feedback). Here the effect of wind is stated to alter the mixed layer depth more strongly than phytoplankton. However, it is known since Platt et al. (2003) that the phytoplankton change their biomass so that the Critical depth (Sverdrup, 1953) matches the mixed layer depth. This occurs due to the biooptical coupling that the authors explore. Please elaborate more on this.
REFERENCES
Sverdrup, H. U. (1953). On conditions for the vernal blooming of phytoplankton. Journal du Conseil International Pour lExplorationde la Mer, 18: 287–295.
Platt, T., Broomhead, D. S., Sathyendranath, S., Edwards, A. M., Murphy, E. J. (2003). Phytoplankton biomass and residual nitrate in the pelagic ecosystem. Proceeding of the Royal Society A, 459: 1063–1073.
SPECIFIC COMMENTS
L14 Please rephrase the sentence starting with “Considering…”.
L65 Change “into” to “through”.
L67 Please remove “(because the same)”.
L72 – L109 Suggested references on this effect from the literature:
Edwards, A. M., Wright, D. G., Platt, T. (2004) Biological heating effect of a band of phytoplankton. Journal of Marine Systems, 49, 89-103. doi: 10.1016/j.jmarsys.2003.05.011.
Edwards, A. M., Platt, T., Wright, D. G. (2001) Biologically induced circulation at fronts. Journal of geophysical research, 49, 89-103. doi: 10.1016/j.jmarsys.2003.05.011.
L152 Please rephrase the sentence starting with “In that perspective…”.
L209 Table 1 should be on top of the table. Suggest renaming chl_inter, clim_zcst, clim_zvar so as to drop the “_”.
L231 – L317 Please move to the Introduction.
Citation: https://doi.org/10.5194/esd-2022-28-RC2 -
AC2: 'Reply on RC2', Sarah Berthet, 23 Nov 2022
The comment was uploaded in the form of a supplement: https://esd.copernicus.org/preprints/esd-2022-28/esd-2022-28-AC2-supplement.pdf
Sarah Berthet et al.
Sarah Berthet et al.
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