Grazing behavior and winter phytoplankton accumulation

Freilich, Mara; Mignot, Alexandre; Flierl, Glenn; Ferrari, Raffaele

doi:https://doi.org/10.5194/bg-18-5595-2021

Articles | Volume 18, issue 20

https://doi.org/10.5194/bg-18-5595-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

Biogeochemistry in the BGC-Argo era: from process studies...

https://doi.org/10.5194/bg-18-5595-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 20

Research article

|

18 Oct 2021

Research article |

| 18 Oct 2021

Grazing behavior and winter phytoplankton accumulation

Mara Freilich, Alexandre Mignot, Glenn Flierl, and Raffaele Ferrari

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Final revised paper (published on 18 Oct 2021)
Preprint (discussion started on 01 Dec 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of the manuscript: “An investigation of grazing behaviors that result in winter phytoplankton biomass accumulation” by Freilich et al', Anonymous Referee #1, 15 Dec 2020
- AC1: 'Reply on RC1', Mara Freilich, 06 Feb 2021
- AC5: 'general reply', Mara Freilich, 21 May 2021
RC2: 'Referee comment', Anonymous Referee #2, 05 Jan 2021
- AC2: 'Reply on RC2', Mara Freilich, 06 Feb 2021
- AC4: 'Additional Reply on RC2', Mara Freilich, 21 May 2021
- AC5: 'general reply', Mara Freilich, 21 May 2021
RC3: 'Comments on "An investigation of grazing behaviors that result in winter phytoplankton biomass accumulation"', Anonymous Referee #3, 23 Mar 2021
- AC3: 'Reply on RC3', Mara Freilich, 21 May 2021
- AC5: 'general reply', Mara Freilich, 21 May 2021

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (25 May 2021) by Giorgio Dall'Olmo

ED: Reconsider after major revisions (25 May 2021) by Katja Fennel (Co-editor-in-chief)

AR by Mara Freilich on behalf of the Authors (29 Jun 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (30 Jun 2021) by Giorgio Dall'Olmo

RR by Anonymous Referee #3 (16 Jul 2021)

Suggestions for revision or reasons for rejection

In this new version, the authors have improved the article and properly addressed most questions and suggestions. However, I still have some comments on the manuscript.

General comments:

1) I agree that considering a linear response of phytoplankton growth to light is a valid simplification considering that the focus of the paper is on the wintertime, when Photosynthetic Active Radiation (PAR) levels are low. However, I think that this assumption should be discussed in more detail and treated more carefully through the paper:

a) In L200, it is stated that the assumption has minimal impact on the wintertime period. Until when could this impact be minimal? Until when the PAR experienced by phytoplankton (taking into account MLD and surface PAR) is low enough to assume a linear response in winter-spring? I think this should be better elaborated and justified, combining some numbers with information from the literature about P-I curves (for example, P-I curves for the North Atlantic during winter-spring).

b) Also, although the model captures the annual cycle with this assumption, I still think that a saturating response of phytoplankton growth to light might slows down growth during the spring bloom, contributing to the recoupling between phytoplankton and zooplankton populations. This could improve the match between model predictions and observations (Fig. 3). Although I understand that not including this response simplifies the model parametrization, I still think that it should be discussed in more depth.

2) The choice of some numbers and assumptions should be better justified and/or supported with references. For example, in L112, a reference should be added in relation to the choice of the MLD definition (or a paper that uses a similar criterion). Also, in L227, I miss a reference for the choice of the N pool value of 30 mg N m-3.

Specific comments:

L5: I think this sentence should be better written. For instance, a function decreases or increases depending on the direction we are moving on the x-axis. Also, it is a bit confusing the part “in phytoplankton concentration at low concentrations”. Maybe you could simplify the sentence, something like “...(or more generally non-linear) at low phytoplankton concentrations…”

L24: Maybe introduce the sentence that starts with “Phytoplankton” with an “Also, /Additionally, ” to clarify that this is a complementary hypothesis and mechanism.

L46: Please replace “are modeled” by “can be modeled”.

L75: Similar problem as in L200. What is low irradiance in numbers?

L160: This is the title, but the paragraph starts with the type II functional response. If you mean that "Grazing linear at low phytoplankton concentrations", please add this last part. Grazing was also linear in the previous section (2.1). Could you please add something to the title to make clear the difference with the previous section?

L180: Similar to the previous case. Thus, if you mean that is quadratic in winter when phytoplankton concentrations are low, please specify.

L181: The term stronger is not clear for me. I'd just indicate that is non-linear. Also, please indicate here that this happens at low phytoplankton concentrations. You indicate this later, but should be mentioned from the beginning.

L188: This is actually just one scenario of the “disturbance-recovery hypothesis”, the one described in Behrenfeld (2010), where this hypothesis was initially called the “dilution-recoupling hypothesis”. Later, in Behrenfeld et al. (2013), the hypothesis was extended to any environmental process that disturbs the balance between phytoplankton growth and losses and got its current name. Thus, maybe you just can simplify it to “consistent with the DRH”.

L232: I’d erase “much”.

L245: “observational timeseries”, to make it clear. Also, although then it is mentioned the days of the year when this peak occurs, please include also here the days of the year that more or less delimit the peak and a reference to Fig. 3.

L259-260: Maybe I am missing something here but, why starting the sentence with a "Despite this"? Is it not sensible to think that if there is larger winter phytoplankton concentration using the type III grazing function (compared to type II), this grazing rate is lower?

L285-288: How do you define the bloom onset? When net phytoplankton biomass accumulation is positive or when phytoplankton biomass is above a particular threshold?

L309: Maybe not the best reference as this paper focuses on the “dilution-recoupling hypothesis”, which describes just one scenario of the “disturbance-recovery hypothesis”. Please, use Behrenfeld et al. (2013), Behrenfeld and Boss (2014) or Behrenfeld and Boss (2018). This might also be applied to the Introduction section of the paper.

Figure 2: Color scales for grazing/growth are different among panels. If making the same scale for all panels is not appropriate, at least please indicate that the scales are different in the caption. In the first line of the caption, replace “(e.g” by “; e.g.” or “, e.g.”. Also, include spaces in the units when necessary (e.g., mg C m-3). At the end of the caption, change it by “depends also on phytoplankton concentration in the case of a Holling type II”.

Figure 3: Add in the caption “dashed thin black line”.

Technical corrections:

L212: Add a comma after “Mignot et al. (2018)”.

L230-231: Add spaces around equal signs.

L231: Add a comma after “However”.

L233: Add space before “If”.

L312: Add a comma after “Furthermore”.

L360: Add space after “deepen”.

Bibliography:

Behrenfeld, M. J. 2010. Abandoning Sverdrup's Critical Depth Hypothesis on phytoplankton blooms. Ecology 91: 977-989.

Behrenfeld, M. J., and E. S. Boss. 2014. Resurrecting the ecological underpinnings of ocean plankton blooms. Annual review of marine science 6: 167-194.

Behrenfeld, M. J., and E. S. Boss. 2018. Student's tutorial on bloom hypotheses in the context of phytoplankton annual cycles. Glob. Change Biol. 24: 55-77.

Behrenfeld, M. J., S. C. Doney, I. Lima, E. S. Boss, and D. A. Siegel. 2013. Annual cycles of ecological disturbance and recovery underlying the subarctic Atlantic spring plankton bloom. Global biogeochemical cycles 27: 526-540.

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RR by Anonymous Referee #1 (20 Jul 2021)

ED: Publish subject to minor revisions (review by editor) (20 Jul 2021) by Giorgio Dall'Olmo

ED: Publish subject to minor revisions (review by editor) (20 Jul 2021) by Katja Fennel (Co-editor-in-chief)

AR by Mara Freilich on behalf of the Authors (12 Aug 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (25 Aug 2021) by Giorgio Dall'Olmo

ED: Publish as is (25 Aug 2021) by Ciavatta Stefano (Co-editor-in-chief)

AR by Mara Freilich on behalf of the Authors (01 Sep 2021)

Short summary

Observations reveal that in some regions phytoplankton biomass increases during the wintertime when growth conditions are sub-optimal, which has been attributed to a release from grazing during mixed layer deepening. Measurements of grazer populations to support this theory are lacking. We demonstrate that a release from grazing when the winter mixed layer is deepening holds only for certain grazing models, extending the use of phytoplankton observations to make inferences about grazer dynamics.