Some Observations on Phytoplankton Community Structure, Dynamics and Their Relationship to Water Quality in Five Santiago Island Reservoirs, Cape Verde

Round 1

Reviewer 1 Report

This is an interesting manuscript and quite well presented. Apart from some minor corrections marked on the text, my major concern is with the length of the manuscript. If the authors could reduce the text (30%) and the number of references cited (50%), I think it would greatly improve the readability. 

Comments for author File: Comments.pdf

Author Response

Reviewer #1

1. This is an interesting manuscript and quite well presented. Apart from some minor corrections marked on the text, my major concern is with the length of the manuscript. If the authors could reduce the text (30%) and the number of references cited (50%), I think it would greatly improve the readability. 

Response: The suggestions presented in the pdf file were included in the revised MS. Except for the comment on line 25: Are temperature and precipitation not included in the physicochemical variables? In this case we considered that air temperature and precipitation are not water physicochemical variables, but meteorological, therefore, we kept them separately, but we included “air temperature” in the abstract to improve the clarity.

We followed the suggestion to reduce the text and references cited and some parts of the text were deleted, namely, as suggested, lines 1151-1174, including the references number 147-150, 152-154. Additionally, the text in lines 436-441 was also deleted.

Reviewer 2 Report

The authors present a phycological study of five reservoirs on Santiago Island, Cape Verde. The paper is well-written and has relevant figures, tables and references. 

The major questions relate to the sampling regime and data analysis. Regarding the sampling regime, it would appear that samples were obtained somewhat seasonally over a five-year period between 2016 and 2020; however, there was no regular sampling and the sampling that was carried out could be described as episodic. Given that some reservoirs actually dried up during this period, the irregular sampling dates perhaps could be explained. That said, the data then seem to have been "homogenized" with all samples considered as the output of a single sampling period. This appears to obscure the seasonality and results in some data being omitted, such as the N:P ratios, which should be important indicators relating to the cyanbacterial blooms. This is pointed out in the text, but it does not appear that the N:P ratios are tabulated in the paper.

A larger-scale map would also be helpful in showing the geographic relationships between the impoundments. Figures 1 and 2 are good to show the location of Cape Verde and the orientation of the island, but a better graphic of reservoir location would be very useful as the narrative does not do the locational information justice and Figure 2 is too small to clearly show the reservoir locations.

Presenting the algal data and water chemistry data as time series also could be helpful in elucidating the discussion. 

In terms of the discussion, the section between lines 991 and 1000 seem to describe a source-limited scenario and transport-limited scenario: in the first, the torrential rains had washed off the available nutrients, while in the latter, drier periods the rainfall was insufficient to mobilize the available nutrients. Some discussion of this would be helpful, especially in the context of semi-arid waterbodies. Likewise, the section between lines 1072 and 1085 touches on nutrient limitation; while P-limitation may be assumed, it seems that there might be episodes of both N and P limitation? Also, tropical systems would seem to generally have rapid cycling of P, which makes the cyanobacterial ability to fix atmospheric N a factor in their relative success.

The conclusion (lines 1239ff) mentions ecosystem services and citizen engagement, but these issues do not seem to have been addressed elsewhere.

The paper is an extremely useful contribution to the knowledge of eutrophication in small island developing states and should be published, but perhaps disaggregating the data would make for a clearer presentation (even considering that there are relatively few data points). Perhaps the title should include "Some Observations on..."?

Author Response

Reviewer #2

The authors present a phycological study of five reservoirs on Santiago Island, Cape Verde. The paper is well-written and has relevant figures, tables and references. 

1. The major questions relate to the sampling regime and data analysis. Regarding the sampling regime, it would appear that samples were obtained somewhat seasonally over a five-year period between 2016 and 2020; however, there was no regular sampling and the sampling that was carried out could be described as episodic. Given that some reservoirs actually dried up during this period, the irregular sampling dates perhaps could be explained. That said, the data then seem to have been "homogenized" with all samples considered as the output of a single sampling period.

This appears to obscure the seasonality and results in some data being omitted, such as the N:P ratios, which should be important indicators relating to the cyanobacterial blooms. This is pointed out in the text, but it does not appear that the N:P ratios are tabulated in the paper.

Response: In fact, the sampling campaigns were realized between 2016 and 2020, not regularly, but whenever possible. The study of the Cape Verde reservoirs was not financed, having been carried out due to the interest of the authors and the relevance of the theme for Cape Verde. We did the best and most complete possible, taking into account the economic constraints. We agree that the samplings performed were episodic, thus as suggested this specification was added to the methodology.

As suggested N:P ratio was calculated, presented on Results (point 3 Figure 8) and further discussed and related with cyanobacteria.

2. A larger-scale map would also be helpful in showing the geographic relationships between the impoundments. Figures 1 and 2 are good to show the location of Cape Verde and the orientation of the island, but a better graphic of reservoir location would be very useful as the narrative does not do the locational information justice and Figure 2 is too small to clearly show the reservoir locations.

Response: A larger-scale map was included in Figure 1, with the location of the reservoirs and the river basins included. Additionally, Figure 2 was updated with the symbols and colours of all studied reservoirs. Also, the order of Figure 9b was corrected to be the same as Figure 9a. Furthermore, the axis legends of Figures 4 and 5 were changed to “Mean of monthly PPT in axis 1 and, Mean of air Temp in axis 2. In the PCA figure (Figure 6), Carb was replaced by CO₃, and Bicarb was replaced by HCO₃.

3. Presenting the algal data and water chemistry data as time series also could be helpful in elucidating the discussion. 

Response: The temporal evolution of algal data is already presented in Figure 9, in terms of Relative abundance of phytoplankton groups, in density (a) and biovolume (b). Additionally, in Supplementary material the cell density of each taxon per reservoir and sampling campaign is presented. Water chemistry data are now also presented as a time series, in the new figures 7 and 8, but only to the parameters that significantly differentiate reservoirs (Figure 7) and sampling campaigns (Figure 8). Include all the others physicochemical parameters would enlarge the manuscript without adding anything very relevant.

4. In terms of the discussion, the section between lines 991 and 1000 seem to describe a source-limited scenario and transport-limited scenario: in the first, the torrential rains had washed off the available nutrients, while in the latter, drier periods the rainfall was insufficient to mobilize the available nutrients. Some discussion of this would be helpful, especially in the context of semi-arid waterbodies.

Response: The concepts of transported-limited denudation and source-limited denudation were introduced on the discussion, especially in Lines 939-940, 954, 956-957, 976-980.

5. Likewise, the section between lines 1072 and 1085 touches on nutrient limitation; while P-limitation may be assumed, it seems that there might be episodes of both N and P limitation? Also, tropical systems would seem to generally have rapid cycling of P, which makes the cyanobacterial ability to fix atmospheric N a factor in their relative success.

Response: Despite the negative correlation between TP and phytoplankton density, and the high N:P ratios observed in same situations, we cannot consider that TP is limiting phytoplankton because the observed TP concentrations were above 0,035 mg/L in all the situations (the limit of a eutrophication situation). We must not forget that the N:P ratio is an indicator dependent on the initial concentrations of N and P, which means that it can only be considered a limiting potential.

Considering this, we modified the sentence to better explain the observed relation between TP and phytoplankton density.

Additionally we calculated the N:P ratio and discussed the obtained values taking in count the cyanobacteria dominance

6. The conclusion (lines 1239) mentions ecosystem services and citizen engagement, but these issues do not seem to have been addressed elsewhere.

Response: We agree with the reviewer, ecosystem services were not the focus of our study. We only mention in the last 9 lines of the Conclusion, the importance of understanding the relationship between LULC and reservoirs services (Ecosystems Service consensus) to improving the ecosystem’s health and sustainability. We believe that to achieve this, is necessary to involve local inhabitants and stakeholders. We can continue studying reservoirs, but if we do not involve citizens, we will never be able to implement adequate management. We would like to maintain or encouragefuture studies that involve local inhabitants and stakeholders.

7. The paper is an extremely useful contribution to the knowledge of eutrophication in small island developing states and should be published, but perhaps disaggregating the data would make for a clearer presentation (even considering that there are relatively few data points). Perhaps the title should include "Some Observations on..."?

Response: We agree with the reviewer and the title was modified accordingly.

Reviewer 3 Report

The manuscript entitled "Phytoplankton community structure, dynamics and their relationship to water quality in five Santiago Island reservoirs, Cape Verd" refers to interesting aspect concerning the functioning of phytoplankton communities in five reservoirs in Santiago Island, Cape Verde, i.e. in a region affected by water availability. The database covers five campaigns, from 2016 to 2020, with data on phytoplankton and main environmental variables. The results confirmed that environmental condition are connected with reservoir differentiations including Cyanobacteria dominated phytoplankton and high risk levels from potential producers of different toxins. However, the corrections should be done. The section of Results should not include the parts of area description according to other authors.

Detailed main comments

1. Table 2 and 3 should be improved because of hard readability.
2. Lines 436-444: This text is rather as a part of study area which should be placed in Materials and Methods.
3. Figure 5. Extension of the dry period (2016–2020). Mean of accumulated precipitation and temperature data obtain from the five meteorological stations that better characterize each reservoir – please specify which line is for temperature and which is for precipitation.
4. Figure 6 – please give the explanation of codes (physicochemical parameters) used on the graph.
5. Figure 7 – please add the letters A, B, C and D to each graph.
6. Figure 8 – the same as in Figure 7.

Author Response

Reviewer 3

The manuscript entitled "Phytoplankton community structure, dynamics and their relationship to water quality in five Santiago Island reservoirs, Cape Verd" refers to interesting aspect concerning the functioning of phytoplankton communities in five reservoirs in Santiago Island, Cape Verde, i.e. in a region affected by water availability. The database covers five campaigns, from 2016 to 2020, with data on phytoplankton and main environmental variables. The results confirmed that environmental condition are connected with reservoir differentiations including Cyanobacteria dominated phytoplankton and high risk levels from potential producers of different toxins. However, the corrections should be done. The section of Results should not include the parts of area description according to other authors.

 Detailed main comments:

1. Table 2 and 3 should be improved because of hard readability.

Response: The font size of both Tables was increased.

2. Lines 436-444: This text is rather as a part of study area which should be placed in Materials and Methods.

Response: We agree with the reviewer that this is part of the study area, however, since this information is not discussed elsewhere in the MS, and following the advice of reviewer #2, this part of the text was deleted (from line 437-442).

3. Figure 5. Extension of the dry period (2016–2020). Mean of monthly accumulated precipitation and temperature data obtain from the five meteorological stations that better characterize each reservoir – please specify which line is for temperature and which is for precipitation.

Response: We agree with the reviewer, this information is now included in the figure 5 caption, as “Extension of the dry period (2016–2020). Mean of accumulated precipitation (blue line) and temperature (red line) data obtain from the five meteorological stations that better characterize each reservoir.”

4. Figure 6 – please give the explanation of codes (physicochemical parameters) used on the graph.

Response: The figure caption was complemented with the information about the explanation of the physicochemical parameters codes: “Reservoirs are represented by different shapes and colors, and the codes include S = Surface or B = Bottom samples and the sampling date Month/Year. The codes for the physicochemical parameters are: T – temperature (◦C); PPT – monthly accumulated precipitation (mm); TN – total nitrogen; NO3 – nitrates; NH4 – ammonium; TP – total phosphorus; Na – sodium; K – potassium; Ca – calcium; Cl – chlorides; SO4 – sulfates; HCO3 – bicarbonates; CO3 – carbonates; Mg – magnesium; Si – silica; and DO – Dissolved Oxygen “

5. Figure 7 – please add the letters A, B, C and D to each graph.

Response: Following the advice of reviewer #2 we modified the graphs. However, the letters were included in each new graph

6. Figure 8 – the same as in Figure 7.

Response: Following the advice of reviewer #2 we modified the graphs. However, the letters were included in each new graph.

Reviewer 4 Report

This thorough study will undoubtedly interest researchers of freshwater phytoplankton. It also has humanitarian significance as it touches on the problem of environmental safety.

I have no critical comments on the paper.

Author Response

Reviewer 4

This thorough study will undoubtedly interest researchers of freshwater phytoplankton. It also has humanitarian significance as it touches on the problem of environmental safety.

I have no critical comments on the paper.

Response: We thank the reviewer for the positive comments on the manuscript.

Round 2

Reviewer 2 Report

The authors present observations on the algal populations of several Cape Verde reservoirs. Their manuscript has been thoroughly revised, with appropriate figures, references and tables. The refined text addresses previous concerns and is recommended for publication.