Mechanisms of northern North Atlantic biomass variability

McKinley, Galen A.; Ritzer, Alexis L.; Lovenduski, Nicole S.

doi:https://doi.org/10.5194/bg-15-6049-2018

Articles | Volume 15, issue 20

https://doi.org/10.5194/bg-15-6049-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-15-6049-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 15, issue 20

Research article

|

16 Oct 2018

Research article |

| 16 Oct 2018

Mechanisms of northern North Atlantic biomass variability

Galen A. McKinley, Alexis L. Ritzer, and Nicole S. Lovenduski

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Final revised paper (published on 16 Oct 2018)
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Preprint (discussion started on 20 Feb 2018)
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Interactive discussion

Status: closed

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

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RC1: 'Review of "Mechanisms of northern North Atlantic biomass variability"', Anonymous Referee #1, 06 Apr 2018
- AC1: 'Response to reviewer 1', Galen McKinley, 09 May 2018
RC2: 'Review of McKinley et al.', Anonymous Referee #2, 10 Apr 2018
- AC2: 'Response to reviewer 2', Galen McKinley, 09 May 2018

Peer-review completion

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

ED: Reconsider after major revisions (29 May 2018) by Gerhard Herndl

AR by Anna Wenzel on behalf of the Authors (12 Jun 2018) Author's response

ED: Referee Nomination & Report Request started (18 Jul 2018) by Gerhard Herndl

RR by Scott C. Doney (30 Jul 2018)

Suggestions for revision or reasons for rejection

Review of:
Mechanisms of northern North Atlantic biomass variability
Galen A. McKinley, Alexis L. Ritzer, and Nicole S. Lovenduski
Biogeosciences, submitted

The study uses a coupled ocean biogeochemical-physical model to diagnose the patterns and inter-annual variability in upper-ocean (surface layer) phytoplankton biomass concentration in the north North Atlantic. After a brief model-data comparison against satellite ocean color data (SeaWiFS and MODIS), the study quantifies model dynamics in terms of small and large phytoplankton biomass, nutrient and light limitation terms, and the physical and biological divergence terms for surface phosphate. A central question in the study is the relative role of lateral versus vertical processes in governing regional biomass patterns in the western and eastern subpolar gyre.

Overall, the study is well constructed and presented. Disentangling causes of interannual variability is no simple task, even in a 3-D model, and the study provides a convincing case that lateral nutrient transport plays a role in regional biogeochemical variability.

After addressing relatively minor comments below, I feel the manuscript is suitable for publication.

Specific Comments

As a general comment, it would be good to clarify in the analysis when a finding reflects model results versus field and satellite observations with caveats like “In the simulation …” or “simulated large phytoplankton …”

Line 25:
“Surface ocean phytoplankton are the single largest biomass pool on Earth”
The surface ocean phytoplankton biomass pool amounts to 1 to 2 Pg C. Terrestrial plant vegetation is several hundred Pg C. I do not think this statement is well supported and needs to be modified.

Line 33:
There has been considerable discussion in the literature over the past decade about Sverdrup’s critical depth hypothesis and it’s underlying assumptions (e.g., papers by Behrenfeld, Boss, Sosik, Mahadevan, etc.). Much of this discussion has been framed for the subpolar North Atlantic, and the Introduction would benefit from an expanded discussion on the interaction of light and mixing.

Line 52:
“that interannual variability in stratification is uncorrelated with that of productivity”
On the other hand, Behrenfeld’s work does show a relationship between winter mixing and biomass inventory. To some extent, this may reflect differences between surface layer concentrations and vertically-integrated inventories, a topic that could be addressed in the Discussion Section. The statement on line 57 is certainly true:
“Thus, there is growing evidence that vertical processes alone are not sufficient to explain productivity
variability.”
But the Introduction seems to underplay work by other research teams showing the importance of local vertical mixing processes. Even the results of this modeling study do show an influence for vertical mixing variability. The paper would benefit from a more balanced presentation.

Line 185:
“Light limitation is calculated monthly in this way and then annually averaged.”
Was biomass-weighting considered when averaging monthly light and nutrient limitation terms to an annual value?

Line 239:
“In these three regions, annual anomalies of biomass are compared to satellite observations from SeaWiFS (1998-2007) and MODIS (2003-2015) (Fig. 2).”
The paper would benefit from more quantitative analysis of observed-model comparisons. For example, what is the correlation and regression slope comparing annual anomalies between the model and the combined observational record?

Line 258:
“The model captures the mean gradients of the phosphate field well, but mean values are 10-20 % too low across most of the subpolar gyre”
Figure 3 shows surface concentrations, not horizontal gradients, so the reader must do their own gradient estimates in their head. A 10% difference in gradient is not easy to determine by eye. It would be good to match the analysis in the text to the figures; if lateral gradients are essential to the analysis, then gradients should be computed and shown explicitly. Also, this addresses the magnitude; presumably it is the vector interaction of the flow field with nutrient gradient vector that is key.

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ED: Publish subject to minor revisions (review by editor) (26 Aug 2018) by Gerhard Herndl

AR by Galen McKinley on behalf of the Authors (09 Sep 2018) Author's response Manuscript

ED: Publish as is (01 Oct 2018) by Gerhard Herndl

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Short summary

Phytoplankton biomass changed significantly in the North Atlantic north of 40° N over 1998–2007. With a physical-ecosystem model, we show that biomass increases in the northwest are due to reduced vertical mixing that partially relieves light limitation of phytoplankton. To the east, these circulation changes lead to fewer nutrients being supplied horizontally from the west. Relationships between these biomass variations and atmosphere and ocean physics are not straightforward.