The contrasted phytoplankton dynamics across a frontal system in the southwestern Mediterranean Sea
Abstract. Phytoplankton plays a major role in the ocean, being the basis of the marine food web and controlling the biogeochemical cycles. Numerical simulation have shown that finescale structures such as fronts are often suitable places for the generation of vertical velocities, transporting subsurface nutrients to the euphotic zone and thus modulating phytoplankton abundance and community structure. Since several years, observations have concentrated on nutrient fluxes along these structures. Instead, direct in situ estimations of the phytoplankton growth rates are much less numerous; although difficult to obtain, they provide a precious information on the ecosystem functioning. Here, we consider the case of a front separating two water masses characterized by several phytoplankton groups with different abundances, in the southwestern Mediterranean Sea. In order to estimate possible differences in growing rates, we used an adaptive and Lagrangian sampling strategy to measure the phytoplankton diurnal cycle in these two water masses. The use of a size-structured population model was then applied to these data to estimate the growth and division rates for each phytoplankton groups identified by flow cytometry, showing that these two population parameters are significantly different on the two sides of the front, and consistent with the relative abundances. Our results introduce a general method for estimating growth rates at frontal systems, paving the way to in situ exploration of finescale biophysical scenarios.
Roxane Tzortzis et al.
Status: final response (author comments only)
RC1: 'Comment on egusphere-2022-1008', Anonymous Referee #1, 23 Nov 2022
- AC1: 'Reply on RC1', Roxane Tzortzis, 25 Nov 2022
- AC2: 'Reply on RC1', Roxane Tzortzis, 13 Feb 2023
RC2: 'Comment on egusphere-2022-1008', Anonymous Referee #2, 26 Dec 2022
- AC3: 'Reply on RC2', Roxane Tzortzis, 13 Feb 2023
Roxane Tzortzis et al.
PROTEVS-MED field experiments: Very High Resolution Hydrographic Surveys in the Western Mediterranean Sea https://doi.org/10.17882/62352
Roxane Tzortzis et al.
Viewed (geographical distribution)
In the manuscript “The contrasted phytoplankton dynamics across a frontal system in the southwestern Mediterranean Sea”, Tzortzis et al. compare the phytoplankton communities at two different water masses separated by a frontal region.
The work is original. The sampling design to analyze the two water masses separated by a front, and the phytoplanktonic community that characterizes them, is very interesting. Especially having a tool like the CytoBuoy. However, the manuscript needs improvement in many aspects before it can be considered for publication.
In general, it is a disorganized text. The story does not flow, the paragraphs do not focus on clear topics, there are very long and confusing sentences, there are methodological descriptions in the results and results in the discussion... All sections should be carefully reviewed and improved.
Especially the introduction and discussion. The study area is barely described or named, why it is so relevant to focus on that particular front (besides the scope of the satellite)? Moreover, in the first paragraph of the introduction, the authors mention the context of climate change. How will climate change affect the presence and intensity of the fronts? And in turn, how a possible change in the intensity and frequency of the fronts will affect the associated phytoplankton communities? As a reader, I feel like I am being shown an interesting image, but in black and white instead of full color.
The authors describe in ~15 lines the relevance of phytoplankton and ocean fronts. In my opinion, more information is needed. Knowing the abundance and diversity of phytoplankton is important, but its role in the carbon cycle should also be highlighted, which changes depending on whether the community is dominated by small or large species. Moreover, an oceanic front is not defined, the authors describe briefly the physical-biological interaction. I also miss an intro to the study region.
After that brief introduction, the difficulty of an in situ study is described, and without continuing the story fluently, they begin to talk about an oceanographic campaign/project. I think the information is relevant, although the more technical details should be indicated in the methodology.
Finally, the last paragraph is confusing. There is a lot of information, but not all makes sense, and it is kind of disorganized. The last paragraph of the introduction should clearly define the objectives of the study and how the authors would answer them.
In my opinion, the use of "could" makes the facts described less solid. It is established that fine-scale frontal structures induce vertical velocity. Is there any study where no vertical velocity is associated with these structures?
Vertical velocities do not modulate light availability. Vertical velocities move the phytoplankton cells along the water column and depending on the “resulting” depth they will have more or less light.
Materials and methods
I imagine that to convert size into volume you are considering that all the species are spherical. Then, please consider indicating this fact and that you are converting the FWS signal to Equivalent Spherical Diameter.
Please, indicate the units of both measurements. Also for the rest of the variables (t, E, g, µ*,…).
Moreover, the definition of "bar l" (I do not know how to write the loss symbol here) confuses me. If it is the daily average population loss rate, how dt is 1 hour? On the other hand, what do you mean exactly by loss? The number of cells moving from one size class to another, or death?
What is the description of T1day NT0?
I have no experience using this kind of model, but any reader should be able to understand the methodology followed in the study without having to read previous studies. So please, review this section carefully and try to make it as clear as possible.
The details about how every species was differentiated, in my opinion, are not necessary, therefore I propose the authors move it to the supplementary, together with Figure 2.
Conclusions and perspectives