Spatial Distribution of Phytoplankton Community Composition and Their Correlations with Environmental Drivers in Taiwan Strait of Southeast China

Round 1

Reviewer 1 Report

The authors presented data of environmental variables and phytoplankton composition obtained during two research cruises performed in different phases of the seasonal cycle. The objective was researching the factors that trigger the seasonal blooms of dinoflagellates that occur frequently in the study area. The authors conclude that temperature and salinity increasing and DIN decreasing are the main factors that control the growth of dinoflagellates. This conclusion is apparently well supported by the data. The results are not too much novel and the dataset is limited (a number reduced of samples was analyzed); however, the manuscript is well written and I think that the work would be useful for publication if some changes are performed. Particularly, I find the next main flaws that must be improved:

-The data and samples were taken in the upper layer of the water column (surface). However, commonly, the phytoplankton presents an non-homogenous vertical distribution that is more conspicuous when the thermal stratification is intensive. This dissimilar distribution would affect both diatoms and dinoflagellates; consequently, it is possible that differences in relative abundance between both groups in surface is not representative of the whole euphotic layer. The authors should present some data (or at least, information) showing the vertical structure of the water column during the samplings and that it does not modify their findings.

-The authors recognize in the Introduction that the N:Si ratio would be important in determining the succession patterns of diatoms and dinoflagellates; however, surprisingly, data of Si are not presented in the manuscript. This fact should be justified.

-I think that the results of the RDA should be presented in the main text instead of in the form of supplementary information. This analysis gives an overall view of the main variation patterns of the environmental variables and the phytoplankton community which, to me, is more informative that the multiple correlations presented in Fig. 5. However, information about the statistical significance of the RDA is missed; the constrained and unconstrained variances should be also shown as it indicates how much variation in the response variables may be accounted for the explanatory variables.

-From the presented data, I am not sure that a bloom of dinoflagellates was actually sampled during the surveys. I miss a clear definition of bloom (in quantitative terms). Furthermore, the distribution patterns of diatoms are almost uncommented.

 

Other specific comments:     

Line 46. The last part of the sentence: 'ultimately necessary for prediction ...' requires to be revised.

Line 53. It is appears that some words are missed in 'play important factors'.

Line 56-57. I am not sure that the authors mean with 'formation of thin layers'. I suppose that they are mentioning the effect produced by stratification of the water column. Please, to be clearer.

Lines 74. This sentence is confused. I guess that high N:Si ratios favor replacement of diatoms by dinoflagellates.

Lines 80-82. It is obvious that dinoflagellate blooms are produced 'within limited ranges of temperature, salinity and nutrients'. A more specific hypothesis should be formulated.

Lines 85-87. Please, to indicate when during the daily cycle the samplings were performed as some measured variables can be strongly affected by the daily phytoplankton metabolism (i.e. pH and oxygen).

Lines 106-114, line 124. It is unclear how values of biomass were calculated from these data. I think that the authors obtained relative contribution of the different groups to total chlorophyll a, which is not equivalent to biomass contribution. Additionally, I wonder how photoacclimation status of the communities can affect these calculations (specially for surface communities that modify their pigment content to cope with high radiation doses).

Lines 127. The selected response and explanatory variables used for the RDA should be listed. I also wonder if the variables were standardized. 

Line 185. I guess that '+' means presence of the given species better than 'distribution'.

Lines 194-198. Cell abundance data of diatoms should be presented. Otherwise, it is difficult to compare the results presented with those ones published.

Line 217. Units for y-axis in Fig. 5 are missed.

Lines 218-221. Probably, this Figure 5 is unnecessary if the results of RDA are presented in the main text.

Lines 223-229. Without a calculation of the biomass attributable to diatoms and dinoflagellates in the samples this hypothesis ('diatoms are dominant phytoplankton group in terms of the more important biomass') is too speculative. Probably, the authors will find data of bio-volume for the identified species that can be used for calculating biomass. I suggest the next article that would be useful for this proposal: Mercado et al. 2008; Marine Ecology Progress Series, 2008, 359, pp. 51-68).

Lines 244-245. This sentence contains an apparent contradiction as it appears to explain that low temperature seawater flow leads to 'temperature increases'. Please, reword.

 Lines 255-256. It is obvious that there are optimal values of temperature for growth rate and photosynthesis and that the metabolism rates decrease beyond this optimum. This sentence can be deleted.

Lines 284-290. I think that this discussion about the DO values is not relevant for the objective of the work. Similarly, discussion about pH is not important in this context. In fact, I wonder if it is necessary to analyze these two variables in this work as they did not contribute to explain the factors affecting the phytoplankton composition (quite the contrary they are affected by the phytoplankton).

 Lines 298-304. I am a bit confused. From what I know, diatoms growth is stimulated by high nitrate concentrations; additionally, the fact of dinoflagellates can grow by using reduced nitrogen forms gives them competitive advantages for growing in environments with low concentration of inorganic nutrients. Please, to revise this paragraph taking into account this background that is an paradigm assumed broadly.

Line 313. I think that the growth is 'disfavored' better than 'inhibited'.  

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Page 1; Line 29: Delete ‘within the test range’

Page 1; Line 30: Does ‘dinoflagellate’ refer to P. donghaiense?

Page 1; Line 29-31: This is an exaggerated conclusion.

Are there any characteristics that represent this environment other than this specific conditions?

Can these environmental conditions always represent this season? Or is it an unusual event?

Page 2; Line 61: Please check the species name.

‘Shin, H. H., Li, Z., Mertens, K. N., Seo, M. H., Gu, H., Lim, W. A., ... & Matsuoka, K. (2019). Prorocentrum shikokuense Hada and P. donghaiense Lu are junior synonyms of P. obtusidens Schiller, but not of P. dentatum Stein (Prorocentrales, Dinophyceae). Harmful algae, 89’

Page 3; Line 102, 104: Please provide accurate information about standard solutions. Apply the same as below according to this journal form.

Page 5; Figure 2: Change the DO unit to mg/L.

Page 7; Table 2. Datyliosolen -> Dactyliosolen. Please check the whole paper.

Page 8; Page 208-209 It is necessary to discuss the opposite hypothesis.

Why does dinoflagellate tend to dominate when the water temperature is high? Perhaps because of the ability to do DVM. This is probably the result of the dominance of P. donghaiense, which is suitable in May.

Page 9; Figure 5: The environmental conditions are clearly different in May and July. If the correlation analysis is performed together, the data may be misinterpreted. Please revise the analysis.

Please cite a paper that studies the Yangtze River area that low salinity is advantageous.

Change the DO unit to mg/L.

Why is there an F value?

Page 10; Line 227-229: Correct the sentence.

Page 10; Figure 6: The interpretation was exaggerated compared to the data. There is not enough evidence. It is necessary to add data and discussions, or revise the content.

Page 12; Line 288-290 No discussion. delete?

Page 12; Line 291-304 This discussion is different from general information. It is recommended to delete it.

Page 12; Line 315-326 What is the basis for the model? It need evidence based on data.

Page 12; Line 328 Which dinoflagellate? Please specify in detail.

Page 12; Line 331 Is there a link between the growth of dinoflagellate and pH??

Page 12; Line 331-334 More explanation is needed in the discussion to reach this conclusion.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I thank the authors for considering my previous comments and modifying the manuscript accordingly. In my previous revision, I pointed out several flaws that limited the interest of the manuscript. The authors have replied satisfactorily these queries, although I still suggest some changes that would improve the manuscript (hopefully):

(1) My main concern in the previous revision was about the possible dissimilar vertical distribution of diatoms and dinoflagellates and how it might affect the conclusions. In the new version, the authors added some information regarding to this point that indicates that the impact of cell vertical distribution in the results would be low. Additionally, some data of diatom abundance are now presented in the text.

(2) The authors have chosen deleting any comment regarding to the possible role of the N:Si ratio in determining the succession patterns of the phytoplankton. I would prefer that the authors mention in the text the lack of Si data is a limitation of their work.

(3) RDA is now presented in the main text instead of supplementary information. However, I still miss information about the overall statistical significance of this analysis as well as the constrained and unconstrained variances. These data are necessary to interpret the results of the analysis. Furthermore, in the Figure 5, a title for the axis should be added (including the amount of variability explained by them). The scores for each sample should be also shown in the plot; they are useful to analyze the seasonal and horizontal variability.

(4) Regarding to the occurrence of a bloom during the sampling, I will expect that the authors justify with some data, results or literature cite that " P. donghaiense blooms occurred at the end of May 2019 when we took samples "

(5) Every specific comments have been responded satisfactorily.

I find that the new version of the manuscript is useful for publication provide the thess minor changes are performed.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Thank you for your sincere reply.

Other comments have been fully answered and revised.

Please revise only the small questions below.

 

“Page 1; Line 29-31: This is an exaggerated conclusion. Are there any characteristics that represent this environment other than this specific conditions?”

We already know that this data was based on May.

What I'm curious about is are these environmental conditions a unique event in this sea area?

Or is it a result that can represent the general May or spring water environment?

 

“Page 9; Figure 5: The environmental conditions are clearly different in May and July. If the correlation analysis is performed together, the data may be misinterpreted. Please revise the analysis.”

S1 May and July are definitely different conditions.

Correlation analysis with these two data can lead to misinterpretation of the data.

If necessary, perform a separate correlation analysis or perform another analysis.

 

 

Author Response

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Author Response File: Author Response.docx