The manuscript of Mangolte et al. constitutes an interesting analysis on plankton communities dynamics in response to a frontal region in the California Current. As explain by the authors, although fronts are well known to be ecological hotspots with high plankton diversity due to the injection of nutrients into the euphotic layer, the mechanisms between nutrient supply and higher trophic levels is still poorly understood. Furthermore, the impacts of the fronts on plankton distribution and on the behavior of the plankton communities have been scarcely studied in situ. Since several years, researchers have fully realized the significance of this problem, but limited by the observation means (especially biological parameters), the current understanding is very limited.



Based on these backgrounds, I think this work is a very good attempt.

In this study, in situ datasets were achieved during several cruises lead in the California Current between 2008 and 2017 with a similar sampling methodology consisting to perform CTD stations spaced at high resolution along transects across frontal structures. These fronts have been identified thanks to satellite observations and the use of a MVP. Plankton samples were analyzed post cruises with 3 methods : flow cytometry, HPLC and ZooScan. This methodology is clear. The identification of frontal structures is well supported by supplementary information in Appendix.

The authors used these large datasets to study the response of plankton communities (in term of biomass and taxonomy) to frontal dynamics. Generally, the results are fairly well presented and are interpreted appropriately in the discussion.

AC1 : We thank the reviewer for their kind remarks and their constructive criticism, which will help improve the manuscript. 

In the conclusion, the authors are well aware of the limitations of their method and in particular they highlighted the importance of leading Lagrangian cruises with instruments able to sample at high temporal and spatial resolution the physical and biological variables. This part could be improved. Indeed, it lacks a bit of connection with the context and the scientific questions developed in the introduction.

AC1 : We propose to rephrase the first paragraphs of the conclusion as follows, to better connect the perspectives (specifically the use of a Lagrangian perspective) to the general questions raised in the introduction. 

“In this study, we describe the taxonomic structure and fine scale spatial organization of plankton communities across ten fronts in the California Current Ecosystem upwelling region. The hypothesis of frontal nutrient injections explains the predominance of diatoms at fronts, but needs to be supplemented by other processes (biotic interactions and transport) to explain the differential responses of the other plankton groups and the cross-frontal patchiness. Although the high horizontal and the taxonomic resolution of our dataset allowed us to gain a more complete view of fronts as complex structures driven by the coupling of physical and biological processes, the exploration of these processes was limited by methodological constraints.

The understanding of the role of fronts on marine ecosystems by empirical means can be improved in two ways. Firstly, frontal communities should be sampled at appropriate horizontal, vertical, taxonomic and temporal resolution. While many instruments can achieve excellent performance at one scale, the challenge lies in simultaneously increasing all the dimensions of resolution, which can be achieved by strategically combining multiple shipboard and autonomous instruments. Secondly, the quantification of the role of the various processes at fronts requires the adoption of a Lagrangian perspective to follow the evolution of the community as it is advected by the currents, which can be achieved by measuring the in-situ biological (growth and grazing) and transport rates.”

The appendix section is very supplied with figures. Although these results are interesting, the authors should consider if all of them are really relevant. Indeed, in my opinion the total number of additional figures is excessive.

AC1 : We acknowledge that all the supplementary figures might not be of interest to every reader, but prefer to keep a published record of as much information as possible.

Finally, for me the grammar is ok, but I am not a native English speaker, that is why I let the English editing service check.

I recommend the manuscript for minor revisions prior to publication in Biogeosciences.

Specific comments

AC1 : We thank the reviewer for taking the time to give such detailed feedback, it is greatly appreciated ! All minor editorial suggestions will be included in the final manuscript, and we have answered in more detailed below where needed.



1. Introduction

After a concise presentation of the context of this study (i.e., fronts and their potential impact on plankton), the authors have well exposed their hypothesis based on the work of Lévy et al., 2018 as well as their scientific questions. Then, the authors provided a short description of the region studied and introduced the CCE-LTER program and cruises during which the data were acquired.



However, the end of the Introduction (Ln 52-63) is a little confusing, with a lot of information. The information is relevant, although the more technical details should be indicated in the methodology. I think this last paragraph should be simplified. Based on the objectives that the authors have previously defined, they can just briefly explain how they would answer them.

AC1 : We agree with the reviewer that the last paragraph of the introduction is very dense and contains information that is repeated and developed in the method and results sections, but we believe it is necessary to give an overview of the dataset because these technical details constrain and justify our approach.

Ln 12 : I understand that it is an introductory sentence for open your Introduction, but can you provide some references.

AC1 : We propose to add the following reference : Hoskins BJ (1982), The mathematical theory of frontogenesis, Annu.Rev. Fluid Mech., 14, 31–151, doi: 10.1146/annurev.fl.14.010182.001023



Ln 43 : Do not write in italics “California Current Ecosystem”. We will remove the italics.

Ln 48 : Define the acronym SSH as Sea Surface Height.

Ln 52 : Here there is a change of topic, please, start a new paragraph.

Ln 52-53 : “The empirical measurements [...]” : This sentence should be in the methodology section.

Ln 54 : Move “(with a spacing of 1 to 5 km between stations)” in the methodology section (Ln 68).

Ln 55-61 : “We identified [...], We first investigate [...], We find [...], We examine [...]” : I think here the authors provided too much detail which should be in the Results section.

Ln 61-63 : “We find that [...]” : This sentence should be in the Results section.

AC1 : The comments on the last paragraph are addressed above.

1. Methods

My main remark concerns the section 2.2. Indeed, in this section the authors explained how the fronts have been identified (supported by supplementary information in Appendix). I understand why the authors have organized their manuscript like this. However, they should add a sentence at the beginning of this part to clearly indicate that all the characteristics of these frontal structures are described in the Appendix. (I think it is difficult for the reader to well understand this part without read before the Appendix).



Furthermore, this section 2.2 is closely related to the section 2.5 named ‘regional context’ concerning the satellite products used in addition to the MVP data to identify these fronts. That is why, I suggest to move this section ‘regional context’ just after the section 2.2.

AC1 : We thank the reviewer for noticing this oversight. We propose to move the section “regional context” before “identification of the fronts” add the following sentence at the end of the first paragraph of the latter :

“[...] This led to a total of ten segments, each containing one front (Fig. 2). In this section, we describe the common procedure we applied to identify these ten fronts; their individual characteristics and their regional context (up to a few months before and after each transects) are presented in detail in the supplementary text.”

Ln 65 : Remove “California Current Ecosystem Long Term Ecological Research”. You have previously defined the acronym CCE-LTER in the Introduction (Ln 43)

Ln 67-68 : “[...] stations regularly spaced at high resolution” : Provide here the resolution.

Ln 119 : Add a reference to the section ‘regional context*’ after ‘SSH’. *section 2.5 in the manuscript or 2.3 in the revised manuscript (if you follow my previous suggestion).



Ln 130-134 : “The distribution of density [...]” : This sentence should be in the Results section.

AC1 : We believe that including this element in the method section helps to understand the rationale behind the procedure we chose to use to define the fronts.



Ln 135 : Do not start a new paragraph.

Ln 175 : Remove “Sea Surface Temperature” and “Sea Surface Height”. You have previously defined these acronyms.



Ln 182 : Add references after FSLE.

AC1 : We propose to add the following references : d'Ovidio, Francesco, et al. "Mixing structures in the Mediterranean Sea from finite‐size Lyapunov exponents." Geophysical Research Letters 31.17 (2004).; Fifani, Gina, et al. "Drifting speed of Lagrangian fronts and oil spill dispersal at the ocean surface." Remote Sensing 13.22 (2021): 4499.

1. Results

Ln 190 : Replace “then” with “secondly”.



Ln 195-196 : “In particular front C3 [...]” : I understand that you referred here to Fig. 5. But please indicate that for help the reader. It is also necessary for all this section (Ln 198-205).

Ln 207-221 : Here also please indicate at which figures you referred for help the reader.

Ln 231-233 : Please keep the same name (calanoids or copepods) in the text as in Fig. 6. (See also my comments about Fig. 6 in the Technical corrections).

Ln 236 : “wide physical fronts (such as E1 and F3)” and “narrow fronts (such as A and C2)”. Can you provide an order of magnitude to highlight why fronts E1 and F3 are wider than fronts A and C2 ?

1. Discussion

This discussion contains rich information and the organization in scientific questions is very appreciable.

Ln 279 : “This dataset” refers to our dataset or the dataset used in the works that you cited previously ? Please clarify this sentence.

AC1 : We will change the sentence to “The dataset used in the present study provides snapshots of the planktonic ecosystem during each overnight transect but cannot capture their time evolution over several days.”



Ln 292 : Concerning the use of the SPASSO software during cruises, you can also add the following references : Rousselet et al. (2018) (https://doi.org/10.5194/bg-15-2411-2018  ) and Barrillon et al. (2023) (https://doi.org/10.5194/bg-20-141-2023).

AC1 : thank you for the suggestions, we will include these references.



Ln 293 : Add references

AC1 : We propose to add the following references : Barth J. A., Pierce, S. D., & Smith, R. L. (2000). A separating coastal upwelling jet at Cape Blanco, Oregon and its connection to the California Current System [Article]. Deep-Sea Research Part Ii-Topical Studies in Oceanography, 47(5-6), 783-810. https://doi.org/10.1016/s0967-0645(99)00127-7; Kosro P. M., & Huyer, A. (1986). CTD and velocity surveys of seaward jets off northern California, July 1981 and 1982, JULY 1981 AND 1982 [Article]. Journal of Geophysical Research: Oceans, 91(C6), 7680-7690. https://doi.org/10.1029/JC091iC06p07680; Zaba K. D., Franks, P. J. S., & Ohman, M. D. (2021). The California Undercurrent as a Source of Upwelled Waters in a Coastal Filament [Article]. Journal of Geophysical Research: Oceans, 126(2), 13, Article e2020JC016602. https://doi.org/10.1029/2020jc016602

Ln 317 : Add references

We propose to add the following references : Aleksandra M. Lewandowska, Maren Striebel, Ulrike Feudel, Helmut Hillebrand, Ulrich Sommer, The importance of phytoplankton trait variability in spring bloom formation, ICES Journal of Marine Science, Volume 72, Issue 6, July/August 2015, Pages 1908–1915, https://doi.org/10.1093/icesjms/fsv059; Eiane, Ketil, and Mark D. Ohman. "Stage-specific mortality of Calanus finmarchicus, Pseudocalanus elongatus and Oithona similis on Fladen Ground, North Sea, during a spring bloom." Marine Ecology Progress Series 268 (2004): 183-193.; Bouquet, Jean-Marie, et al. "Increased fitness of a key appendicularian zooplankton species under warmer, acidified seawater conditions." PLoS One 13.1 (2018): e0190625.; Kotori, Moriyuki. "Life cycle and growth rate of the chaetognath Parasagitta elegans in the northern North Pacific Ocean." Plankton Biology and Ecology 46.2 (1999): 153-158.

1. Conclusion

This part is a little short and can be improved (see my general comments). 

AC1 : We addressed this in the general comments above.

Technical corrections

AC1 : We thank the reviewer for the careful attention with which they read our manuscript. All the suggested corrections will be made in the final manuscript.

Ln 28 : remove one “between”



Ln 44 : Write “Table 1” not “Tab. 1” (the word "Table" is never abbreviated and should be capitalized when followed by a number, following the Biogeosciences guidelines).



Ln 67 : Add parenthesis after “(Fig. 1”



Ln 78, 80, 84, 86 : Add a space between “100” and “m”.



Ln 92 : Add a space between “100” and “%”.



Ln 93 : Add a space before “The”.



Ln 129 : Add a space between “50” and “m”.



Ln 263 : Add a space between “100” and “m”.



Ln 268 : Write “Fig. 6” not “fig. 6”. See also Ln 271 and legend of figure 7. (Check I all the manuscript).



Ln 275 : Add a space between “10” and “m”.



Ln 277 : Add a space between “20” and “m”.



Ln 281 : Write in capitals “First”

In all the manuscript for Appendix figures do not write “sup. Fig Ax” or “Sup. Fig. Ax” just “Fig. Ax”.

In all the manuscript, units should be written exponentially (e.g. m s -1 not m/s) following the Biogeosciences guidelines. (See Ln 93, 105, 294 in the text and check also the units in the figures).

Please, consider changing the written of the coordinates in your maps, with a degree sign and a space when naming the direction (e.g. 36 ° N, 120 ° W), following the Biogeosciences guidelines.

Check the units on the legend of your figures. There are a lot of plots were units are missed. 

There are some figures (for instance Fig. 3, 4, 5, A5, A11, A12, A13) where the typography is very small. I understand that sometime you have constraints for making figures, but where it is possible please increase the size of typography for help the reader.

AC1 : The Biogeosciences template for the draft required figures to be only 12 cm wide, but the published version should be larger.

Figure 1

AC1 : We will correct all figures as suggested.

Add “Latitude” and “Longitude” on the axis.



On the SST panel, the red crosses are not very visible on this colormap. Please, consider changing the color of the crosses.



On the Chl-a panel, the contrast of Chl-a is not very visible. Maybe, you can modify the color scale. The scale is already logarithmic and shows that here is only a very thin band of high values at the coast.



Correct the legend as : “Transect names (A, C2, C3, E1, E2, F1, F2, F3) ”

Figure 3

Legend : Define acronyms PRO as Prochlorococcus and SYN as Synechococcus.

Figure 6

Although calanoids is an order of copepods, please keep the same name in the figure and in the legend.

Figure 8

Nice figure !

Figures A1, A2, A3, A4

Add “Latitude” and “Longitude” on the axis.



Indicate in the legend that the black dots correspond to the CTD stations. In my opinion there are not all necessary and you can just keep the ones that indicate the transects with the frontal stations.



As for Fig. 1, the red crosses are not very visible on this colormap.



Add units of FSLE. Is it day -1 ?

Figure A6

Correct the legend as : “Prochlorococcus on the top row, Synechococcus on the bottom row [...]”.



HPLC (left column) and Flow Cytometry (right column).

Figures A11, A12, A13

Please consider if all these figures are really relevant (see my general comments). My suggestion is to choose only a few fronts that can be considered representative of all observed.



Furthermore on these figures, there are a lot of panels and the typography is minuscule. I suggest to present a limited number of variables.

Mangolte et al. perform a meta-analysis of the nearly 10 years of data collected in the CCE-LTER cross-front studies to try to identify the physical and biological processes driving plankton community distribution. This is a worthy effort, and the authors do a good job wrestling with data that is undoubtedly patchy, complex, and representing snapshot views on time-evolving fronts.

1. One of the major challenges of extracting process-based information out of snapshots of time-evolving physical-biological features is that you often do not sample enough stages of the feature to get a sense of how the biological processes lead into one another. I found this paper overall good but the analysis is a bit descriptive as far as marine ecology/biology papers are concerned, e.g., there are no statistical analyses to distinguish between the various drivers of the plankton community dynamics (more below). Nonetheless – descriptive papers are okay – but if you’re going to go the descriptive route there does seem to be a missed opportunity here to rearrange and/or present the data in terms of the frontal age. In the manuscript, all the data are presented in chronological order in terms of the years in which the cruises were conducted. What if you reordered the data with C3, F1, F2 at the beginning and “older” fronts later in the plots? There certainly seems to be some taxa (e.g., some gelatinous carnivores) where the negative FEF tend to be present in the younger fronts. I wonder if more patterns will emerge if you take the frontal age as a key organizing factor. (Also, I acknowledge that in the Southern CCE there are some persistent fronts where the older vs. younger definition does not apply, but certainly there seem to be a gradient with some of these frontal systems).
2. Regarding statistical analyses, this is what I would expect out of a classical meta-analysis. There are 24 functional groups, a measure of their aggregation (via the FEF), and a variety of potential physical and biogeochemical quantities measured – what if the authors were to create a statistical model (either classical multivariate, like a generalized linear mixed model, or machine learning based, like a random forest model or something similar) with the species-specific FEF as response variable and the relative gradients of T, S, density, nitrate, phosphate, silicate, and their prey and predator FEF as explanatory variables? Or, instead of the FEF one could also focus on the size of the plankton biomass peaks and distance to center as the response variable. This may help elucidate a dominant mechanism that can then be summarized in a nice schematic like Fig. 8. As such, Fig. 8 shows a generic set of processes whereby two plankton community meet at a front and the sampling cuts across the communities and the front. This is fine – but just from inspection there are some interesting patterns that do not seem to be captured in the schematic, e.g., Fig 7 suggests on average, the biomass peaks occur on the cold side of the front, and that there is a declining effect by trophic level. The potential with this dataset is quite high, and it often takes some carefully crafted statistical models to cut through some of the fine-scale variability to arrive at a set of overarching mechanisms.

AC2 : We thank the reviewer for their fair comments, and we will better justify our approach in the revision. We conducted an initial statistical analysis, but we were not able to extract significant relationships from preliminary tests with a PCA. In fact, this dataset is not well-suited for statistical analyses because the dataset is too small (only ten fronts) and covers a wide variety of situations, as we show. Similarly, while it is true that the median position of the peaks is shifted to the cold side of the fronts, this results is not  statistically significant because of the high level of variability between fronts.

With the given constraints imposed by the relatively small number of available data, and by their synopticity, our aim is necessarily restricted to emphasize the multiplicity of the potential driving processes, which is reflected in the generic schematic we included. Statistical studies of the kind suggested by the reviewer would require more fronts covering the full seasonal cycle, and ideally, time-evolution over some fronts. Another limitation of our data set, which makes it difficult to interpret in terms of the processes, is the lack of measurements of growth and grazing rates, as well as nutrient fluxes.

Regarding the “frontal age”, we agree with the reviewer that the notion is appealing, and we had attempted to characterize it, but we found no satisfactory way to do so from satellite observations  : we tried to characterize it from the intensification of the SST gradient and from the appearance of the fronts from SLA data but were limited by the cloud cover and the fact that the surface gradient is not always representative of the vertical structure of the front. Moreover, more than the “frontal age”  it is the time elapsed since the input of nutrients that is relevant to the composition of the plankton community, and here we faced two difficulties. First, modeling studies have shown that the vertical nutrient injections at fronts are both patchy spatially and intermittent in time (see for instance Mangolte et al., 2022). Secondly, and more importantly, there is a very fast horizontal current along the density gradient which decouples the site of the nutrient injection from the site where a biological response might be observed. In an upwelling region such as the CCE, this is further complicated by the fact that nutrients are also supplied by the coastal upwelling itself, and probably in much larger quantities than the frontal ageostrophic circulation. The frontal age, even assuming we could determine it for all ten fronts, and assuming that it is a proxy of the earliest ageostrophic nutrient injection, would thus explain very little of the plankton community structure observed at the transect sites. Instead, our results and other recent studies show that, at least in this region, the investigation of horizontal along-front transport is an important step to understand the overall impact of fronts on biological processes. 

Therefore, we decided to simply organize the fronts chronologically and to focus the results of the paper on the description of the data. We included a detailed “Discussion” section to address the issue of the driving processes. We do agree with the reviewer’s comment that there tends to be less zooplankton in the “younger fronts”, which we noted in the discussion l.321-322, but don’t believe that this observation is enough to draw robust conclusions. Instead, we thought it would be more interesting for the scientific community to highlight the potential role of passive and reactive processes.

We will add the above discussions on the statistical analysis and frontal age in the discussion, as negative results. 

My minor comments are mostly focused on the figures:

• I particularly found the overlapping y-axes quite difficult to read, especially when they started to overlap with adjacent panels. E.g., Figure 4, C2-C3 columns, I had no way to read the yaxis labels on the phytoplankton and zooplankton panels. Also the plot title labels in Figures A11-13 are overlapping with each other and the axes labels, which make things hard to read. If these plots are made in python (sort of looks to be the case?) then try using plt.tight_layout() to fix this. Otherwise, increase the vertical and/or horizontal spacing between panels. 
• Figure 2. Need to add some indication in the figure legend as to what the small black dots refer to.
• Figure 5. “arraw” should be “array”? or “panel”?

AC2 : Thank you for bringing this to our attention, the overlap of the labels and the other issues will be fixed in the final version.