Vertical Boundary Mixing Events during Stratification Govern Heat and Nutrient Dynamics in a Windy Tropical Reservoir Lake with Important Water-Level Fluctuations: A Long-Term (2001–2021) Study

Round 1

Reviewer 1 Report

The authors present a paper using empirically measured and modeled components to assess how varying water levels affect thermal and nutrient dynamics in an impounded reservoir with inflows from multiple rivers in a high-altitude and tropical environment. Limited information is available for tropical climate lakes and reservoirs relative to northern temperature systems and the authors’ research is valuable to lake and reservoir managers that require this information to provide informed decisions to best manage limited freshwater resources. In general, the paper is well written, but it would be beneficial to provide some clarification of terms to minimize confusing the reader.

Although the authors refer to the study site as a lake in the manuscript title, the system is better described as a reservoir since there is a man-made structure (dam) built to impound flow from at least five different rivers. The authors mention this several times in the paper (Lines 84, 118, 128, 147, 172, etc.) primarily in the Methods section. Because of this, it is recommended that throughout the manuscript the study system should be specifically referred to as a reservoir or lake reservoir. Where the authors specifically refer to lake conditions (see: Introduction section), this should also be specified to include the term reservoir (such as ‘lakes and reservoirs’ seen in line 113, 289, 318) when applicable and appropriate. This suggested change would minimize confusion for the reader.

The authors frequently refer to nutrient cycling within the reservoir as external loading from hypolimnion to epilimnion compartments. However, the appropriate term is internal loading as the nutrients are not being transported from outside the reservoir (from upland, riparian, or riverine sources), but rather internally transported from reservoir sediments to the hypolimnion and then the epilimnion (internal loading) during varying water level fluctuations during the study period. It is suggested that the authors refer to these nutrient transport mechanisms as internal loading and not external loading.

Also, somewhere between the last sentence in Materials and Methods subsection 2.5 but before the Results section, the authors must include a Data Analysis sub-section with a complete description of the data analyses. It’s clear, beginning in the Results section, that the authors conducted several statistical analyses including linear and non-linear regressions, but currently there is no description of this. This is required for publication and the authors must include this information prior to acceptance for publication.

Please see the attached MS Word document for specific comments for this manuscript.

Comments for author File: Comments.pdf

Author Response

Dear Water Editor,

We have uploaded the revised manuscript with ID: WATER-1358180, now entitled “Vertical boundary mixing events during stratification govern heat and nutrient dynamics in a windy tropical reservoir lake with high water-level fluctuations: a long-term (2001-2021) study”.

 

We have fully revised the manuscript following and addressing each of the comments and suggestions from the reviewers.

 

We have also updated our database to the newest available results from 2021, because this year the reservoir lake level has decreased to a new record, so this additional data extends to 21 years of our long-term series and it also expands the range of water level fluctuations it includes.

 

We find that the reviewer´s comments and this revision have improved significantly the manuscript.

 

Please find below a detailed list of our responses, point by point, to the reviewer´s comments.

 

Best regards,

Martín Merino-Ibarra

Corresponding author WATER-1358180

 

 

REVIEWER´S COMMENTS AND RESPONSES:

 

Reviewer 1  WATER-1358180 Review General Comments

The authors present a paper using empirically measured and modeled components to assess how varying water levels affect thermal and nutrient dynamics in an impounded reservoir with inflows from multiple rivers in a high-altitude and tropical environment. Limited information is available for tropical climate lakes and reservoirs relative to northern temperature systems and the authors’ research is valuable to lake and reservoir managers that require this information to provide informed decisions to best manage limited freshwater resources. In general, the paper is well written, but it would be beneficial to provide some clarification of terms to minimize confusing the reader.

We agree with the reviewer and we share the aim to minimize confusing the reader as much as possible, so we have tried to clarify all terms as requested throughout the manuscript.

Although the authors refer to the study site as a lake in the manuscript title, the system is better described as a reservoir since there is a man-made structure (dam) built to impound flow from at least five different rivers. The authors mention this several times in the paper (Lines 84, 118, 128, 147, 172, etc.) primarily in the Methods section. Because of this, it is recommended that throughout the manuscript the study system should be specifically referred to as a reservoir or reservoir lake. Where the authors specifically refer to lake conditions (see: Introduction section), this should also be specified to include the term reservoir (such as ‘lakes and reservoirs’ seen in line 113, 289, 318) when applicable and appropriate. This suggested change would minimize confusion for the reader.

We have corrected the text using the term “reservoir lake” in the title and throughout the manuscript, as recommended by the reviewer.

The authors frequently refer to nutrient cycling within the reservoir as external loading from hypolimnion to epilimnion compartments. However, the appropriate term is internal loading as the nutrients are not being transported from outside the reservoir (from upland, riparian, or riverine sources), but rather internally transported from reservoir sediments to the hypolimnion and then the epilimnion (internal loading) during varying water level fluctuations during the study period. It is suggested that the authors refer to these nutrient transport mechanisms as internal loading and not external loading.

We apologize for the confusion caused by our reference to external loadings. We were in fact referring to external loads (from outside the reservoir) that we have previously measured and published in an earlier paper (Ramirez-Zierold et al 2010), and we were using them only for comparison with the nutrient fluxes measured in this manuscript, which we agree correspond to internal loading. To avoid the readers any confusions, we have added in the Study Area section a paragraph describing in detail the previously measured external loading to VB, their consequences on the reservoir lake, and also the nutrient concentrations in the reservoir lake previously reported in another paper (Merino et al., 2008). We have also carefully revised all our mentions to external loads throughout the text to avoid any confusions by readers.

Specific Comments:

Lines 3-4: If only one reservoir was studied, revise title to reflect this as follows: ‘...nutrient dynamics in a windy tropical reservoir with high water-level fluctuations...’

We have changed the title as suggested.

Line 38: The authors mention ‘...external loads...’ into the reservoir. But nutrient loads transferred from the hypolimnion to the epilimnion, likely from reservoir bed sediments (although this is not specifically referenced in the abstract), is not external loading into the reservoir. External loading into the reservoir comes from upland, riparian, or inflow riverine sources. The authors are referring to internal loading via nutrient cycling processes resulting from WLF and should be referred to as such. It is recommended that the term ‘external loads’ be revised as ‘...internal loads...’.

We apologize for the confusion caused by this sentence; we were comparing the vertical nutrient fluxes with the external loads, which we have measured and published in an earlier paper (Ramirez-Zierold et al 2010). We have eliminated this mention in the abstract to avoid any other possible confusion.

Line 121: The system is described as located in both a high-altitude environment and tropical climate. This is interesting as higher altitudes tend to have different climates than low-altitude tropical ones. It is suggested that the authors provide a sentence or two describing the specific climate conditions for this reservoir lake in this environment. What are the annual rainfall patterns for this system? What are the annual temperature patterns? Is there more climate fluctuation at higher altitudes than lower altitudes at this latitude? This information would be valuable to the reader to better understand factors affecting reservoir lake system dynamics.

We agree with the reviewer that there was a lack of information on the climate conditions and rainfall patterns in the manuscript. We have added in the Study Area section the sentences required to describe the climate conditions for this reservoir, in particular the rainfall pattern. VB has very pronounced dry and rainy seasons, with 86% of the annual rain (947 mm) falling during the rainy months (June to October, around the hemispheric summer, as expected from being in tropical latitudes with trade winds). Together with the wind regime, rainfall pattern is the main climatic aspect of relevance for this manuscript, so we have not extended on variations with altitude, since such analysis is out of the scope of the manuscript. The information added will be very valuable for the readers, and addresses the interest expressed by both reviewers on the seasonal rain pattern in VB. This is the text that was added: " VB is located in the tropical latitudinal zone with trade winds, rainy from spring to autumn and mostly dry the rest of the year. Climate plays an important role for VB, mainly due to rain and wind patterns. It is sub-humid, warm to temperate with very pronounced dry and rainy seasons. Mean annual precipitation is 947 mm, most of which (809 mm, 86%) fall in the rainy months (June–October), while barely 14% (138 mm) of the rain falls distributed throughout the six months (November–May) of the dry season. The mean temperature in VB is 18.7°C, the minimum annual temperatures (mean 9.0°C) occur during January, and the maximum (mean 28.0°C) during May, before the onset of the rainy season."

Line 135: Figure 1 (and Methods section) clearly indicates the study site is a reservoir. It is suggested to the authors that they refer to the system as a reservoir or reservoir lake and not just as a lake. This should be reflected in the title and throughout the manuscript to avoid confusing the reader.

We accept this correction, we have modified the text, referring to the system throughout the revised manuscript as a reservoir lake as suggested here and in previous comments.

Lines 147-148: Please provide the reader with more reservoir hydrology data. Were there regular drawdowns of the reservoir water level? What was the outflow at the dam spillway? How much water is released annually during the study period?

We have added the information requested in the Study Area section: "there are no spill outflows, but water is regularly withdrawn from VB by the National Water Agency (CONAGUA); on average, 221 * 106 m3 are withdrawn annually from this reservoir."

Line 182: ‘(2)’ should refer to equation 1 (equation 1). Please make appropriate changes.

Corrected as requested (note that because we added a new figure, we reorganized the results mentioning stability before, so the methods section also changed, and the equations numbering also changed -now it is equation 3-).

Line 187: before ‘as’ include a reference to equation 2. For example: ‘...equation 2 as...’

We have made changes to attend this request. Reference to equation 2 is now made after the start of this sentence “was calculated…” in original line 184 (see note on additional data and figure, and the associated rearrangement of the methods section).

Lines 197-199: One sentence is not a paragraph.

We agree, to address this we have put this and another sentence within a paragraph

Line 201: after ‘depth’ include references to equations 4 and 5. For example: ‘...(equations 4, 5)...’

Done as requested (see note above).

Line 210: revise from ‘...in spite of the...’ to ‘...despite...’ Line 211: revise from ‘...notoriously...’ to ‘...notably...’

Corrected as requested.

Line 222: after ‘...DIN (kg m-3)...’ include a reference to equation 7. For example: ‘...(equation 7)...’

Done as requested.

Lines 222-223: Somewhere between the last sentence in Materials and Methods subsection 2.5 but before the Results section, the authors must include a Data Analysis sub-section with a complete description of the data analyses. It’s clear, beginning in the Results section, that the authors conducted several statistical analyses including linear and non-linear regressions, but currently there is no description of this. This is required for publication and the authors must include this information prior to acceptance for publication.

Done as requested. We have included a new subsection “2.6. Data statistical analysis” with the description of the data analyses we did.

Lines 225-234: Because water level in the study reservoir is strongly influenced by rainfall, please provide the reader with either monthly rainfall data for the study period (perhaps as supplemental material) or as a citation for the reader to find this information. Currently the reader does not know how much rainfall occurs during either the rainy season or the dry season. Please clarify.

We have added the information requested on rainfall: mean annual precipitation is 947 mm, most of which (809 mm, 86%) fall in the rainy months (June–October), while barely 14% (138 mm) of the rain falls distributed throughout the six months (November–May) of the dry season.

Line 237: Figure 3 caption, place a space between ‘...(daily resolution),’ and ‘(b)’

Corrected

Lines 259-260: The reviewer notes that ‘R2’ is a coefficient of determination (for regression analyses) and not a correlation coefficient. The authors should revise their statement from ‘correlation coefficients’ to ‘coefficients of determination’. For future reference, depending upon the type of correlation analysis, the authors should use either ‘r’ for Pearson correlation coefficients or ‘ρ’ for Spearman Rank correlation coefficients. Please correct.

We apologize for this involuntary mistake. We have corrected both the text and the figures throughout the manuscript to address this observation.

Line 290: Why is ‘molecular conductivity’ by ‘[6] MacIntyre et al. 1999’ included in this table? This information does not seem to belong here, it is not mentioned in the Table 2 caption, and is confusing for the reader. Please clarify and/or correct.

We included ‘molecular conductivity’ in the vertical mixing rates table as a reference for the reader of the lower limit of vertical mixing, the Kz values at which mixing is null, and only molecular movements occur. We have added in the Table 2 Caption an explanation to guide the readers and help them benefit on this reference.

Line 302: Table 3 includes redundant information that is reported twice in the same table: SRP 1.97, Ammonia 12.3, DIN 30.5, Valle de Bravo (mean), This paper
SRP, 5.99, Ammonia 63.1, DIN, 97.1, Valle de Bravo (lowest water level), This paper Please revise the table such that the above information is reported only once in the table.

We have modified Table 3 as requested.

Line 306: Figure 6 needs to include references for 6a and 6b since the authors are reporting two graphs within the figure. Please correct.

We have corrected figure 6 as requested

Lines 310-311: Revise from ‘presented’ to ‘was’ Line 316: Revise from ‘perceives’ to ‘utilize’ Line 319: Delete ‘of’; revise ‘this’ to ‘these’

We have corrected these lines following the request of the reviewer. We are thankful, because it helped us to improve the text.

Lines 341-358: While the authors do a good assessment of reporting variations of these variables across different tropical and temperate lakes and reservoirs, there is a lack of comparison with other high-altitude reservoirs. Are there any available published reports for these types of lacustrine systems? Is the current study unique in this regard? If not, please provide comparisons with other comparable high-altitude lake/reservoir systems.

To our knowledge there are not any other published reports on vertical mixing and the associated nutrient fluxes in high-altitude tropical reservoirs, so -in that regard- the current study unique. Of course, there are other studies on high-altitude tropical reservoirs, but they deal on their limnological description or other aspects, but do not focus on the vertical mixing or report any Kz values to which we could compare.

Line 359: Revise from ‘...it should be born in mind...’ to ‘...it should be noted...’ Lines 395-396: Insert space between ‘series’ and ‘[14]’.
Line 401: Revise from ‘notorious’ to ‘noted’
Line 418: Delete ‘...an important factor...’ as this is redundant.

Corrections made as requested.

Line 420: The authors measured SRP in VB and not TP. Why is TP reported here? How does this compare with the authors’ measured SRP? Please clarify.

After comparing with SRP vertical fluxes in other reservoirs and lakes, we also wanted to compare our estimates with the reservoir lake´s P budget, which was published previously in terms of TP (Ramirez-Zierold et al [15]. However, we fully understand and acknowledge the reviewer´s observation, and we have revised the manuscript, extending the Study Area section with the information on the published nutrient budget and external loading to VB to better inform the readers and avoid any confusions.

 

 

Reviewer 2

The long-term studies are absolutely needed to understand changes in nutrient dynamics. Then your study is a great opportunity. But I was surprised when I didn't see in your discussion what you announce in your very good introduction. You will find my detail comments in the reviewed manuscript.

The first main point is to revise your explanation of what is the equation to link Mix(meta-hypo) and Kz.  By your actual explanation, I understood that Kz is in m/s (and not m²/s). And further, if we applied your calculation method, in the table 3 I got a difference of one order in your values. Then check this issue, since Kz is your main discussed variable. 

We have carefully revised this crucial aspect of the manuscript following the reviewer´s comment. We understand the mismatch between our data and the calculations of the reviewer. Thanks to this observation we have detected there was a term missing on the explanation of the link between Mix(meta-hypo) and that it was incorrect. We apologize for this. We have rewritten this text explaining the link, including all the details as requested, and we have added an equation on the calculation of Kz to avoid any other mistakes or confusions for the readers. So the units for Kz are correct (m²/s) and consistent with the equation. We have also revised all the Kz values including the missing term, and fortunately the changes are minor. We are very thankful to the reviewer for detecting this mistake.

The order of magnitude difference obtained by the reviewer with his calculations was due to a typing error in the number written for the exchange area in the manuscript in line 200. It should read 1.2 * 10⁷ m² instead of 1.2 * 10⁶ m². This is why the reviewer obtained a difference of one order when he calculated our Kz values. We have corrected the area value in the text, and are very thankful because the reviewer´s effort and comment allowed detecting this typing mistake.

And here is the point. Why you didn't present more your results on nutrients ? In line 161-162, you wrote: " Our approach is based on the increasing evidence [6,22–24] that turbulence generated by internal waves breaking in boundary mixing events can transport nutrients and gases vertically through the metalimnion in systems where a source of external energy". So what about the transport of nutrients ? What is the correlation between the monthly evolution of Kz and the monthly concentration in nutrients ? 

Our results on the transport of nutrient are now included in Table 1. We apologize for not including our data numerically in the previous manuscript. We also understand and share the interest of presenting more results on nutrients, so we have expanded the discussion on the transport of nutrients as much as possible. Nevertheless, because nutrients are non-conservative and have intense and fast interactions with the surface layer´s community, the changes in their concentration -caused by the vertical mixing processes we are assessing- likely occur on a much shorter time scale, and are not detected at the monthly scale of our samplings. Because vertical nutrient transport due to mixing does not therefore translate onto sustained changes in nutrient concentrations in the surface layer –with, perhaps the very visible exception of large-scale upwelling events- is why we enrolled in the adventure of this manuscript, to asses indirectly this vertical transport that is not revealed by direct changes in nutrient concentrations. To attend this very valuable and shared interest, we have discussed all the indirect evidences we have found consistent with the nutrient transport we are assessing. Also, to guide the reader into this research gap we also added different reports of chlorophyll change or heat exchange that suggest nutrient vertical transfer occur, but only few attempts have addressed these processes.

Second, you discussed only a comparison with other lakes in the tropical world. But you have a wonderful database in your hands, we would prefer to have a discussion about your data. Moreover, take care, everyone know very well that it's always very imprudent to compare mean result of nutrients from different studies (see the review paper of Johnson et al, Water, 2016) on that issue. 

We are happy that our database is appreciated, and have expanded both the database itself (including additional data for the years 2019, 2020 and 2021), a new figure showing the vertical temperature variations, and have also extended the discussion on our data. After this modification, we think it will be easier for the reader to actually see hypolimnetic warming and its relationship with low water levels.

So based on what you have written in your discussion, you should change the title and the introduction. But it would be a pity. I would prefer you add more results on monthly nutrient concentration. And then you will discuss the long-term comparative evolution of Kz and nutrient concentration. Of course, for that, you should introduce also monthly data on rainfall and maybe on the water-uptakes from the management decision. 

We have included in the revised manuscript data on the rainfall and water-uptakes (withdrawal), expanding the study area section. Nutrient data are also included there now. We share the interest on nutrient concentrations expressed here and in the previous comments. We hope our above explanation on reasons why we have not found -and there likely cannot be found- a relation between the evolution of mixing intensity and the surface layer nutrient concentrations without a very high frequency sampling availability (as explained before it is probably visible when the magnitude of the flux is as intense as in some marine upwelling events). Stratification paradigmatically represents a barrier to the vertical transport of oxygen and nutrients; however, this static appreciation is unable to explain surface layer dynamics, in which vertical transport of mass and energy occur frequently. Mass fluxes carry nutrients, which have a non-conservative signal and instead, have intense and fast interactions with surface layers biota. Probably the only case in which vertical nutrient transport has been directly detected is in upwelling events. For other cases, vertical nutrient flux is a fast-disappearing signal due to the biota (cf. e.g. Bootsma and Hecky 1999, Palter et al 2005, Díaz-Torres et al 2021, Lin et al 2021 now cited in the manuscript, among others) that has to be assessed indirectly. This is in fact, the motivation of our manuscript.

Considering these arguments on the non-conservative nature of nutrients, the complexity of analyzing the nutrient concentrations for the 21 years here reported, and the very short time (10 days) the Water Editorial Office has given us to review the manuscript, makes unfeasible the extension of this manuscript with such amount of information.

Nevertheless, we agree that the readers will benefit from the inclusion of some nutrient data in the manuscript, so we have expanded the study area section, including the nutrient concentrations in the epilimnion and the hypolimnion previously published (Merino-Ibarra et al 2008). Additionally, we will be working on a nutrients budget manuscript that will help shed light on the nutrient processes in this kind of processes, but that relies on the vertical mixing and nutrient fluxes published in this present manuscript to obtain the budgets. We have also added a figure with the vertical and temporal distribution of temperature, which allows seeing the possible variations on the epilimnion thickness and also on the hypolimnetic warmings, which are the core of the manuscript. We find this addition, driven by the reviewer´s comment, has helped the contribution of the manuscript to its objectives.

At last, something seems to have occurred in the lake from 2009. You didn't discussed that. Why?

We agree that 2009 was a very particular year, and it is so because it was the lowest level year (the highest WLF) in our initially reported period (2001-2018). Our results showed that vertical mixing and nutrient flux to the epilimnion were highest precisely on this year.

To better attend this inquire and enhance the manuscript, we have made an effort to process the information we have been gathering, and have added additional data from the years 2019, 2020 and 2021 (to date), because 2021 has now become the year with the lowest water level in VB, expanding the range of our long-term observations. The new data included show a similar behavior in 2021 than in 2009, supporting its association with the extremely low water levels of these years. We have extended the discussion on the variations among years, and have added labels for each of the years in all the figures to ease their identification to the readers.

And finally, you should on the wind efficiency for the nutrient entrainment, as you wrote in line 111. 

Perhaps we may have generated a confusion, because it was not within the scope of this manuscript to assess specifically the “wind efficiency for the nutrient entrainment”, but the “effects of the water level fluctuations (WLF) on the intensity of this water mixing and nutrient entrainment” as written on line 111 of the manuscript. Assessing the wind efficiency would require additional data and would likely a specific paper on this subject. Nevertheless, we understand and share the interest in the importance of wind for the mixing events and nutrient transport we are trying to assess, so we have further discussed the importance of wind for these processes.

Etc... See your reviewed manuscript. 

I engage you to refer to :

Ishikawa et al, Inland Waters 2021;

De Crop et al, J African Earth Sc 2019 ;

Liu et al, STOTEN 2019;

Herrera et al, J Great Lakes Res 2021;

Lau et al, Nature 2020. 

 

We are thankful for the suggested references, we have found them very useful, and we have referred to most of them in the revised manuscript where it was pertinent for the discussion.

 

Additional comments:

Review of “Vertical boundary mixing events during stratification govern heat and nutrient dynamics in windy tropical lakes with high water-level fluctuations: a long-term (2001–2018) study”

Line 68:

Recently, [8] found that the synchronization of IW can even provoke enhanced shear in the hypolimnion and vertical diffusivity (Kz) reaching up to 10−4 m2 s −1.

.......I know Water requires you to format references like this, but it is hard to read. I would prefer it to be written

Recently, Auger et al [8] found that the synchronization of IW can even provoke enhanced shear in the hypolimnion and vertical diffusivity (Kz) reaching up to 10−4 m2 s −1.

Similar for all in text references

We fully agree with the reviewer that references in that style are hard to read, and would like to write them as the reviewer suggests if the journal editorial office approves it.

Figure 2: what do the red and orange BM symbols mean (the spirals?)

BM and the spiral symbols represent the boundary mixing events that occur due to the interaction of the thermocline with the bottom and margins of the lake. We have included an explanation of this in the legend of Figure 2 to make it perfectly clear to the readers.

Line 236: This seasonal pattern (Fig. 3b) shows that stratification was never broken during any of the stratification periods between 2001 and 2018. Nevertheless, the stability during the stratification periods diminished significantly as the water level decreased (Fig. 3c)

Corrected as requested.

Line 247: Although linear fits yielded quite high correlation coefficients for all the stratification periods (R2 ranging from 0.965 to 0.999) some of the stratifications showed a sigmoidal trend (e.g. 2016, 2012, 2018, 2005), that could be related to the transitions between stratification and circulation periods.

R2 isnt a correlation coefficient? I think you are quoting here R2, the coefficient of determination from the linear regressions in Fig. 4a, which I don’t think is the same thing as a correlation coefficient (although they are of course related). Please check this, or else rephrase: Although linear regression yielded high coefficients of determination for all the stratification periods (R2 ranging from 0.965 to 0.999)

We totally agree with the reviewer, apologize for this typing mistake and are very thankful for pointing it out. It has been corrected as requested.

Line 285

The data presented here are derived from long-term observations over two decades of monthly monitoring at VB, where strong winds, eutrophication, and water extraction for the Mexico City interact in a system where explosive phytoplankton blooms are frequent [11,15].

Corrected as requested.

Line 293 (delete “of”)
Despite the high WLF recorded in VB during this 18 years

Corrected as requested.

Table 3: Lake Malwi/Nyasa, Africa should be Lake Malawi

Corrected as requested.

Line 413
“Our model allows”
What model? I don’t think you did any modelling?

We agree, we have corrected this sentence, replacing “Our model” by “Our results”

Other things:

Are all the calculations based on data collected at the deepest point? If so, why even bother to mention all the additional data points between 2001 and 2003? And why include them in the map? If you don’t use that data, don’t put it on the map.

The calculations are based on averages of the stations sampled. As stated in the manuscript, the monitoring proceeded with all the stations shown in figure 1 during the first three years of sampling, then, as we did not find important spatial variations, the number of stations sampled was reduced in order to make viable the long-term monitoring that has proceeded since then.

Id quite like to see a heat map with a varying water level, showing how the depth of the epilimnion and hypolimnion change over the years? It would be interesting for the reader to see whether the epilimnion depth or the hypolimnion depth change over the years. Does the hypolimnion warm up more in some years, because the epilimnion is much shallower in those years?

We fully understand the interest for the reviewer, and for the readers, in seeing whether the epilimnion depth or the hypolimnion depth change over the years. Therefore, despite that it will extend the length of the manuscript, we have added a new figure with the thermal variations of VB throughout the study period, where both the depth of epilimnion and the hypolimnetic warming can easily be visualized by the reader. This figure shows that the depth of the epilimnion does not change significantly over the years, and that the hypolimnion warm up is not related to a shallower epilimnion, but to the decrease on the overall water level in the reservoir, also shown in the figure. We find this figure, and the information it shows has improved notably the manuscript and its clarity for readers, without exceeding the scope of the paper.

Were there worse algal blooms in years where nutrient flux into the epilimnion was higher?

We have also been monitoring chlorophyll concentrations, which exhibits very high variations, and so far, we have found no correlation of this parameter with any other variable measured. Coccoid cyanobacteria are the main bloom-forming organisms in VB, and a non-linear relationship has been largely described in the literature between cyanobacteria cell counts and chlorophyll (e.g., Ahn, C. Y., Joung, S. H., et al (2007). Alternative alert system for cyanobacterial bloom, using phycocyanin as a level determinant. Journal of microbiology, 45(2), 98-104). Moreover, it seems that the occurrence of algal blooms is multifactorial, and we suspect that most likely it is associated to extraordinary and/or unbalanced (in terms of N:P proportion) inputs of nutrients. However, some there are some relations with production and respiration monitoring that we have also done and published (Guimarais-Bermejo et al 2018) for part of the period here reported, and we have discussed relations with the lake metabolism in the revised manuscript. Additionally, some of the blooms that taken place in VB in the past are related to mucilaginous cyanobacteria, described in other reports as low pigment bloomers. This provides another source of variation for bloom occurrence.

Id like to see a figure with hypo and epi-limnetic nutrients over the sampling period as well.

We agree on the interest on hypo and epi-limnetic nutrient concentrations, and have included a paragraph describing the contrasting concentrations (previously published) among the epilimnion and the hypolimnion in VB. Please consider our previous arguments on the non-conservative nature of nutrients and also on the complexity of including a figure with nutrient concentrations over the 21 years here reported, the unfeasibility of extending more this manuscript, and the very short time (10 days) the Water Editorial Office has given us to review the manuscript.

There is a lot of reference to previous studies (which is good – I like to see work building on from other work), but it does make it difficult to ascertain what is the new data presented in this paper. Could you add a sentence at the end of the introduction with an objective or aim, and outlining what this particular paper brings to the larger body of work.

This is the first time we are publishing internationally this data on vertical mixing and nutrient flux. Previous papers were on other aspects of VB: limnology, external loading, metabolism, phytoplankton and zooplankton communities. As required, we have revised the last paragraph, to clearly outline what this particular paper brings to the larger body of work, and also what it does not.

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear Authors, 

The long-term studies are absolutely needed to understand changes in nutrient dynamics. Then your study is a great opportunity. But I was surprised when I didn't see in your discussion what you announce in your very good introduction. You will fin my detail comments in the reviewed manuscript. 

The first main point is to revise your explanation of what is the equation to link Mix(meta-hypo) and Kz.  By your actual explanation, I understood that Kz is in m/s (and not m2/s). And further, if we applied your calculation method, in the table 3 I got a difference of one order in your values. Then check this issue, since Kz is your main discussed variable. 

And here is the point. Why you didn't present more your results on nutrients ? In line 161-162, you wrote: " Our approach is based on the increasing evidence [6,22–24] that turbulence generated by internal waves breaking in boundary mixing events can transport nutrients and gases vertically through the metalimnion in systems where a source of external energy". So what about the transport of nutrients ? What is the correlation between the monthly evolution of Kz and the monthly concentration in nutrients ? 

Second, you discussed only a comparison with other lakes in the tropical world. But you have a wonderful database in your hands, we would prefer to have a discussion about your data. Moreover, take care, everyone know very well that it's always very imprudent to compare mean result of nutrients from different studies (see the review paper of Johnson et al, Water, 2016) on that issue. 

So based on what you have written in your discussion, you should change the title and the introduction. But it would be a pity. I would prefer you add more results on monthly nutrient concentration. And then you will discuss the long-term comparative evolution of Kz and nutrient concentration. Of course, for that, you should introduce also monthly data on rainfall and maybe on the water-uptakes from the management decision. 

At last, something seems to have occurred in the lake from 2009. You didn't discussed that. Why ? 

And finally, you should on the wind efficiency for the nutrient entrainment, as you wrote in line 111. 

Etc... See your reviewed manuscript. 

I engage you to refer to : Ishikawa et al, Inland Waters 2021; De Crop et al, J African Earth Sc 2019 ; Liu et al, STOTEN 2019; Herrera et al, J Great Lakes Res 2021; Lau et al, Nature 2020. 

Kind regards

 

 

 

 

 

Comments for author File: Comments.pdf

Author Response

Dear Water Editor,

We have uploaded the revised manuscript with ID: WATER-1358180, now entitled “Vertical boundary mixing events during stratification govern heat and nutrient dynamics in a windy tropical reservoir lake with high water-level fluctuations: a long-term (2001-2021) study”.

 

We have fully revised the manuscript following and addressing each of the comments and suggestions from the reviewers.

 

We have also updated our database to the newest available results from 2021, because this year the reservoir lake level has decreased to a new record, so this additional data extends to 21 years of our long-term series and it also expands the range of water level fluctuations it includes.

 

We find that the reviewer´s comments and this revision have improved significantly the manuscript.

 

Please find below a detailed list of our responses, point by point, to the reviewer´s comments.

 

Best regards,

Martín Merino-Ibarra

Corresponding author WATER-1358180

 

 

REVIEWER´S COMMENTS AND RESPONSES:

 

Reviewer 1  WATER-1358180 Review General Comments

The authors present a paper using empirically measured and modeled components to assess how varying water levels affect thermal and nutrient dynamics in an impounded reservoir with inflows from multiple rivers in a high-altitude and tropical environment. Limited information is available for tropical climate lakes and reservoirs relative to northern temperature systems and the authors’ research is valuable to lake and reservoir managers that require this information to provide informed decisions to best manage limited freshwater resources. In general, the paper is well written, but it would be beneficial to provide some clarification of terms to minimize confusing the reader.

We agree with the reviewer and we share the aim to minimize confusing the reader as much as possible, so we have tried to clarify all terms as requested throughout the manuscript.

Although the authors refer to the study site as a lake in the manuscript title, the system is better described as a reservoir since there is a man-made structure (dam) built to impound flow from at least five different rivers. The authors mention this several times in the paper (Lines 84, 118, 128, 147, 172, etc.) primarily in the Methods section. Because of this, it is recommended that throughout the manuscript the study system should be specifically referred to as a reservoir or reservoir lake. Where the authors specifically refer to lake conditions (see: Introduction section), this should also be specified to include the term reservoir (such as ‘lakes and reservoirs’ seen in line 113, 289, 318) when applicable and appropriate. This suggested change would minimize confusion for the reader.

We have corrected the text using the term “reservoir lake” in the title and throughout the manuscript, as recommended by the reviewer.

The authors frequently refer to nutrient cycling within the reservoir as external loading from hypolimnion to epilimnion compartments. However, the appropriate term is internal loading as the nutrients are not being transported from outside the reservoir (from upland, riparian, or riverine sources), but rather internally transported from reservoir sediments to the hypolimnion and then the epilimnion (internal loading) during varying water level fluctuations during the study period. It is suggested that the authors refer to these nutrient transport mechanisms as internal loading and not external loading.

We apologize for the confusion caused by our reference to external loadings. We were in fact referring to external loads (from outside the reservoir) that we have previously measured and published in an earlier paper (Ramirez-Zierold et al 2010), and we were using them only for comparison with the nutrient fluxes measured in this manuscript, which we agree correspond to internal loading. To avoid the readers any confusions, we have added in the Study Area section a paragraph describing in detail the previously measured external loading to VB, their consequences on the reservoir lake, and also the nutrient concentrations in the reservoir lake previously reported in another paper (Merino et al., 2008). We have also carefully revised all our mentions to external loads throughout the text to avoid any confusions by readers.

Specific Comments:

Lines 3-4: If only one reservoir was studied, revise title to reflect this as follows: ‘...nutrient dynamics in a windy tropical reservoir with high water-level fluctuations...’

We have changed the title as suggested.

Line 38: The authors mention ‘...external loads...’ into the reservoir. But nutrient loads transferred from the hypolimnion to the epilimnion, likely from reservoir bed sediments (although this is not specifically referenced in the abstract), is not external loading into the reservoir. External loading into the reservoir comes from upland, riparian, or inflow riverine sources. The authors are referring to internal loading via nutrient cycling processes resulting from WLF and should be referred to as such. It is recommended that the term ‘external loads’ be revised as ‘...internal loads...’.

We apologize for the confusion caused by this sentence; we were comparing the vertical nutrient fluxes with the external loads, which we have measured and published in an earlier paper (Ramirez-Zierold et al 2010). We have eliminated this mention in the abstract to avoid any other possible confusion.

Line 121: The system is described as located in both a high-altitude environment and tropical climate. This is interesting as higher altitudes tend to have different climates than low-altitude tropical ones. It is suggested that the authors provide a sentence or two describing the specific climate conditions for this reservoir lake in this environment. What are the annual rainfall patterns for this system? What are the annual temperature patterns? Is there more climate fluctuation at higher altitudes than lower altitudes at this latitude? This information would be valuable to the reader to better understand factors affecting reservoir lake system dynamics.

We agree with the reviewer that there was a lack of information on the climate conditions and rainfall patterns in the manuscript. We have added in the Study Area section the sentences required to describe the climate conditions for this reservoir, in particular the rainfall pattern. VB has very pronounced dry and rainy seasons, with 86% of the annual rain (947 mm) falling during the rainy months (June to October, around the hemispheric summer, as expected from being in tropical latitudes with trade winds). Together with the wind regime, rainfall pattern is the main climatic aspect of relevance for this manuscript, so we have not extended on variations with altitude, since such analysis is out of the scope of the manuscript. The information added will be very valuable for the readers, and addresses the interest expressed by both reviewers on the seasonal rain pattern in VB. This is the text that was added: " VB is located in the tropical latitudinal zone with trade winds, rainy from spring to autumn and mostly dry the rest of the year. Climate plays an important role for VB, mainly due to rain and wind patterns. It is sub-humid, warm to temperate with very pronounced dry and rainy seasons. Mean annual precipitation is 947 mm, most of which (809 mm, 86%) fall in the rainy months (June–October), while barely 14% (138 mm) of the rain falls distributed throughout the six months (November–May) of the dry season. The mean temperature in VB is 18.7°C, the minimum annual temperatures (mean 9.0°C) occur during January, and the maximum (mean 28.0°C) during May, before the onset of the rainy season."

Line 135: Figure 1 (and Methods section) clearly indicates the study site is a reservoir. It is suggested to the authors that they refer to the system as a reservoir or reservoir lake and not just as a lake. This should be reflected in the title and throughout the manuscript to avoid confusing the reader.

We accept this correction, we have modified the text, referring to the system throughout the revised manuscript as a reservoir lake as suggested here and in previous comments.

Lines 147-148: Please provide the reader with more reservoir hydrology data. Were there regular drawdowns of the reservoir water level? What was the outflow at the dam spillway? How much water is released annually during the study period?

We have added the information requested in the Study Area section: "there are no spill outflows, but water is regularly withdrawn from VB by the National Water Agency (CONAGUA); on average, 221 * 106 m3 are withdrawn annually from this reservoir."

Line 182: ‘(2)’ should refer to equation 1 (equation 1). Please make appropriate changes.

Corrected as requested (note that because we added a new figure, we reorganized the results mentioning stability before, so the methods section also changed, and the equations numbering also changed -now it is equation 3-).

Line 187: before ‘as’ include a reference to equation 2. For example: ‘...equation 2 as...’

We have made changes to attend this request. Reference to equation 2 is now made after the start of this sentence “was calculated…” in original line 184 (see note on additional data and figure, and the associated rearrangement of the methods section).

Lines 197-199: One sentence is not a paragraph.

We agree, to address this we have put this and another sentence within a paragraph

Line 201: after ‘depth’ include references to equations 4 and 5. For example: ‘...(equations 4, 5)...’

Done as requested (see note above).

Line 210: revise from ‘...in spite of the...’ to ‘...despite...’ Line 211: revise from ‘...notoriously...’ to ‘...notably...’

Corrected as requested.

Line 222: after ‘...DIN (kg m-3)...’ include a reference to equation 7. For example: ‘...(equation 7)...’

Done as requested.

Lines 222-223: Somewhere between the last sentence in Materials and Methods subsection 2.5 but before the Results section, the authors must include a Data Analysis sub-section with a complete description of the data analyses. It’s clear, beginning in the Results section, that the authors conducted several statistical analyses including linear and non-linear regressions, but currently there is no description of this. This is required for publication and the authors must include this information prior to acceptance for publication.

Done as requested. We have included a new subsection “2.6. Data statistical analysis” with the description of the data analyses we did.

Lines 225-234: Because water level in the study reservoir is strongly influenced by rainfall, please provide the reader with either monthly rainfall data for the study period (perhaps as supplemental material) or as a citation for the reader to find this information. Currently the reader does not know how much rainfall occurs during either the rainy season or the dry season. Please clarify.

We have added the information requested on rainfall: mean annual precipitation is 947 mm, most of which (809 mm, 86%) fall in the rainy months (June–October), while barely 14% (138 mm) of the rain falls distributed throughout the six months (November–May) of the dry season.

Line 237: Figure 3 caption, place a space between ‘...(daily resolution),’ and ‘(b)’

Corrected

Lines 259-260: The reviewer notes that ‘R2’ is a coefficient of determination (for regression analyses) and not a correlation coefficient. The authors should revise their statement from ‘correlation coefficients’ to ‘coefficients of determination’. For future reference, depending upon the type of correlation analysis, the authors should use either ‘r’ for Pearson correlation coefficients or ‘ρ’ for Spearman Rank correlation coefficients. Please correct.

We apologize for this involuntary mistake. We have corrected both the text and the figures throughout the manuscript to address this observation.

Line 290: Why is ‘molecular conductivity’ by ‘[6] MacIntyre et al. 1999’ included in this table? This information does not seem to belong here, it is not mentioned in the Table 2 caption, and is confusing for the reader. Please clarify and/or correct.

We included ‘molecular conductivity’ in the vertical mixing rates table as a reference for the reader of the lower limit of vertical mixing, the Kz values at which mixing is null, and only molecular movements occur. We have added in the Table 2 Caption an explanation to guide the readers and help them benefit on this reference.

Line 302: Table 3 includes redundant information that is reported twice in the same table: SRP 1.97, Ammonia 12.3, DIN 30.5, Valle de Bravo (mean), This paper
SRP, 5.99, Ammonia 63.1, DIN, 97.1, Valle de Bravo (lowest water level), This paper Please revise the table such that the above information is reported only once in the table.

We have modified Table 3 as requested.

Line 306: Figure 6 needs to include references for 6a and 6b since the authors are reporting two graphs within the figure. Please correct.

We have corrected figure 6 as requested

Lines 310-311: Revise from ‘presented’ to ‘was’ Line 316: Revise from ‘perceives’ to ‘utilize’ Line 319: Delete ‘of’; revise ‘this’ to ‘these’

We have corrected these lines following the request of the reviewer. We are thankful, because it helped us to improve the text.

Lines 341-358: While the authors do a good assessment of reporting variations of these variables across different tropical and temperate lakes and reservoirs, there is a lack of comparison with other high-altitude reservoirs. Are there any available published reports for these types of lacustrine systems? Is the current study unique in this regard? If not, please provide comparisons with other comparable high-altitude lake/reservoir systems.

To our knowledge there are not any other published reports on vertical mixing and the associated nutrient fluxes in high-altitude tropical reservoirs, so -in that regard- the current study unique. Of course, there are other studies on high-altitude tropical reservoirs, but they deal on their limnological description or other aspects, but do not focus on the vertical mixing or report any Kz values to which we could compare.

Line 359: Revise from ‘...it should be born in mind...’ to ‘...it should be noted...’ Lines 395-396: Insert space between ‘series’ and ‘[14]’.
Line 401: Revise from ‘notorious’ to ‘noted’
Line 418: Delete ‘...an important factor...’ as this is redundant.

Corrections made as requested.

Line 420: The authors measured SRP in VB and not TP. Why is TP reported here? How does this compare with the authors’ measured SRP? Please clarify.

After comparing with SRP vertical fluxes in other reservoirs and lakes, we also wanted to compare our estimates with the reservoir lake´s P budget, which was published previously in terms of TP (Ramirez-Zierold et al [15]. However, we fully understand and acknowledge the reviewer´s observation, and we have revised the manuscript, extending the Study Area section with the information on the published nutrient budget and external loading to VB to better inform the readers and avoid any confusions.

 

 

Reviewer 2

The long-term studies are absolutely needed to understand changes in nutrient dynamics. Then your study is a great opportunity. But I was surprised when I didn't see in your discussion what you announce in your very good introduction. You will find my detail comments in the reviewed manuscript.

The first main point is to revise your explanation of what is the equation to link Mix(meta-hypo) and Kz.  By your actual explanation, I understood that Kz is in m/s (and not m²/s). And further, if we applied your calculation method, in the table 3 I got a difference of one order in your values. Then check this issue, since Kz is your main discussed variable. 

We have carefully revised this crucial aspect of the manuscript following the reviewer´s comment. We understand the mismatch between our data and the calculations of the reviewer. Thanks to this observation we have detected there was a term missing on the explanation of the link between Mix(meta-hypo) and that it was incorrect. We apologize for this. We have rewritten this text explaining the link, including all the details as requested, and we have added an equation on the calculation of Kz to avoid any other mistakes or confusions for the readers. So the units for Kz are correct (m²/s) and consistent with the equation. We have also revised all the Kz values including the missing term, and fortunately the changes are minor. We are very thankful to the reviewer for detecting this mistake.

The order of magnitude difference obtained by the reviewer with his calculations was due to a typing error in the number written for the exchange area in the manuscript in line 200. It should read 1.2 * 10⁷ m² instead of 1.2 * 10⁶ m². This is why the reviewer obtained a difference of one order when he calculated our Kz values. We have corrected the area value in the text, and are very thankful because the reviewer´s effort and comment allowed detecting this typing mistake.

And here is the point. Why you didn't present more your results on nutrients ? In line 161-162, you wrote: " Our approach is based on the increasing evidence [6,22–24] that turbulence generated by internal waves breaking in boundary mixing events can transport nutrients and gases vertically through the metalimnion in systems where a source of external energy". So what about the transport of nutrients ? What is the correlation between the monthly evolution of Kz and the monthly concentration in nutrients ? 

Our results on the transport of nutrient are now included in Table 1. We apologize for not including our data numerically in the previous manuscript. We also understand and share the interest of presenting more results on nutrients, so we have expanded the discussion on the transport of nutrients as much as possible. Nevertheless, because nutrients are non-conservative and have intense and fast interactions with the surface layer´s community, the changes in their concentration -caused by the vertical mixing processes we are assessing- likely occur on a much shorter time scale, and are not detected at the monthly scale of our samplings. Because vertical nutrient transport due to mixing does not therefore translate onto sustained changes in nutrient concentrations in the surface layer –with, perhaps the very visible exception of large-scale upwelling events- is why we enrolled in the adventure of this manuscript, to asses indirectly this vertical transport that is not revealed by direct changes in nutrient concentrations. To attend this very valuable and shared interest, we have discussed all the indirect evidences we have found consistent with the nutrient transport we are assessing. Also, to guide the reader into this research gap we also added different reports of chlorophyll change or heat exchange that suggest nutrient vertical transfer occur, but only few attempts have addressed these processes.

Second, you discussed only a comparison with other lakes in the tropical world. But you have a wonderful database in your hands, we would prefer to have a discussion about your data. Moreover, take care, everyone know very well that it's always very imprudent to compare mean result of nutrients from different studies (see the review paper of Johnson et al, Water, 2016) on that issue. 

We are happy that our database is appreciated, and have expanded both the database itself (including additional data for the years 2019, 2020 and 2021), a new figure showing the vertical temperature variations, and have also extended the discussion on our data. After this modification, we think it will be easier for the reader to actually see hypolimnetic warming and its relationship with low water levels.

So based on what you have written in your discussion, you should change the title and the introduction. But it would be a pity. I would prefer you add more results on monthly nutrient concentration. And then you will discuss the long-term comparative evolution of Kz and nutrient concentration. Of course, for that, you should introduce also monthly data on rainfall and maybe on the water-uptakes from the management decision. 

We have included in the revised manuscript data on the rainfall and water-uptakes (withdrawal), expanding the study area section. Nutrient data are also included there now. We share the interest on nutrient concentrations expressed here and in the previous comments. We hope our above explanation on reasons why we have not found -and there likely cannot be found- a relation between the evolution of mixing intensity and the surface layer nutrient concentrations without a very high frequency sampling availability (as explained before it is probably visible when the magnitude of the flux is as intense as in some marine upwelling events). Stratification paradigmatically represents a barrier to the vertical transport of oxygen and nutrients; however, this static appreciation is unable to explain surface layer dynamics, in which vertical transport of mass and energy occur frequently. Mass fluxes carry nutrients, which have a non-conservative signal and instead, have intense and fast interactions with surface layers biota. Probably the only case in which vertical nutrient transport has been directly detected is in upwelling events. For other cases, vertical nutrient flux is a fast-disappearing signal due to the biota (cf. e.g. Bootsma and Hecky 1999, Palter et al 2005, Díaz-Torres et al 2021, Lin et al 2021 now cited in the manuscript, among others) that has to be assessed indirectly. This is in fact, the motivation of our manuscript.

Considering these arguments on the non-conservative nature of nutrients, the complexity of analyzing the nutrient concentrations for the 21 years here reported, and the very short time (10 days) the Water Editorial Office has given us to review the manuscript, makes unfeasible the extension of this manuscript with such amount of information.

Nevertheless, we agree that the readers will benefit from the inclusion of some nutrient data in the manuscript, so we have expanded the study area section, including the nutrient concentrations in the epilimnion and the hypolimnion previously published (Merino-Ibarra et al 2008). Additionally, we will be working on a nutrients budget manuscript that will help shed light on the nutrient processes in this kind of processes, but that relies on the vertical mixing and nutrient fluxes published in this present manuscript to obtain the budgets. We have also added a figure with the vertical and temporal distribution of temperature, which allows seeing the possible variations on the epilimnion thickness and also on the hypolimnetic warmings, which are the core of the manuscript. We find this addition, driven by the reviewer´s comment, has helped the contribution of the manuscript to its objectives.

At last, something seems to have occurred in the lake from 2009. You didn't discussed that. Why?

We agree that 2009 was a very particular year, and it is so because it was the lowest level year (the highest WLF) in our initially reported period (2001-2018). Our results showed that vertical mixing and nutrient flux to the epilimnion were highest precisely on this year.

To better attend this inquire and enhance the manuscript, we have made an effort to process the information we have been gathering, and have added additional data from the years 2019, 2020 and 2021 (to date), because 2021 has now become the year with the lowest water level in VB, expanding the range of our long-term observations. The new data included show a similar behavior in 2021 than in 2009, supporting its association with the extremely low water levels of these years. We have extended the discussion on the variations among years, and have added labels for each of the years in all the figures to ease their identification to the readers.

And finally, you should on the wind efficiency for the nutrient entrainment, as you wrote in line 111. 

Perhaps we may have generated a confusion, because it was not within the scope of this manuscript to assess specifically the “wind efficiency for the nutrient entrainment”, but the “effects of the water level fluctuations (WLF) on the intensity of this water mixing and nutrient entrainment” as written on line 111 of the manuscript. Assessing the wind efficiency would require additional data and would likely a specific paper on this subject. Nevertheless, we understand and share the interest in the importance of wind for the mixing events and nutrient transport we are trying to assess, so we have further discussed the importance of wind for these processes.

Etc... See your reviewed manuscript. 

I engage you to refer to :

Ishikawa et al, Inland Waters 2021;

De Crop et al, J African Earth Sc 2019 ;

Liu et al, STOTEN 2019;

Herrera et al, J Great Lakes Res 2021;

Lau et al, Nature 2020. 

 

We are thankful for the suggested references, we have found them very useful, and we have referred to most of them in the revised manuscript where it was pertinent for the discussion.

 

Additional comments:

Review of “Vertical boundary mixing events during stratification govern heat and nutrient dynamics in windy tropical lakes with high water-level fluctuations: a long-term (2001–2018) study”

Line 68:

Recently, [8] found that the synchronization of IW can even provoke enhanced shear in the hypolimnion and vertical diffusivity (Kz) reaching up to 10−4 m2 s −1.

.......I know Water requires you to format references like this, but it is hard to read. I would prefer it to be written

Recently, Auger et al [8] found that the synchronization of IW can even provoke enhanced shear in the hypolimnion and vertical diffusivity (Kz) reaching up to 10−4 m2 s −1.

Similar for all in text references

We fully agree with the reviewer that references in that style are hard to read, and would like to write them as the reviewer suggests if the journal editorial office approves it.

Figure 2: what do the red and orange BM symbols mean (the spirals?)

BM and the spiral symbols represent the boundary mixing events that occur due to the interaction of the thermocline with the bottom and margins of the lake. We have included an explanation of this in the legend of Figure 2 to make it perfectly clear to the readers.

Line 236: This seasonal pattern (Fig. 3b) shows that stratification was never broken during any of the stratification periods between 2001 and 2018. Nevertheless, the stability during the stratification periods diminished significantly as the water level decreased (Fig. 3c)

Corrected as requested.

Line 247: Although linear fits yielded quite high correlation coefficients for all the stratification periods (R2 ranging from 0.965 to 0.999) some of the stratifications showed a sigmoidal trend (e.g. 2016, 2012, 2018, 2005), that could be related to the transitions between stratification and circulation periods.

R2 isnt a correlation coefficient? I think you are quoting here R2, the coefficient of determination from the linear regressions in Fig. 4a, which I don’t think is the same thing as a correlation coefficient (although they are of course related). Please check this, or else rephrase: Although linear regression yielded high coefficients of determination for all the stratification periods (R2 ranging from 0.965 to 0.999)

We totally agree with the reviewer, apologize for this typing mistake and are very thankful for pointing it out. It has been corrected as requested.

Line 285

The data presented here are derived from long-term observations over two decades of monthly monitoring at VB, where strong winds, eutrophication, and water extraction for the Mexico City interact in a system where explosive phytoplankton blooms are frequent [11,15].

Corrected as requested.

Line 293 (delete “of”)
Despite the high WLF recorded in VB during this 18 years

Corrected as requested.

Table 3: Lake Malwi/Nyasa, Africa should be Lake Malawi

Corrected as requested.

Line 413
“Our model allows”
What model? I don’t think you did any modelling?

We agree, we have corrected this sentence, replacing “Our model” by “Our results”

Other things:

Are all the calculations based on data collected at the deepest point? If so, why even bother to mention all the additional data points between 2001 and 2003? And why include them in the map? If you don’t use that data, don’t put it on the map.

The calculations are based on averages of the stations sampled. As stated in the manuscript, the monitoring proceeded with all the stations shown in figure 1 during the first three years of sampling, then, as we did not find important spatial variations, the number of stations sampled was reduced in order to make viable the long-term monitoring that has proceeded since then.

Id quite like to see a heat map with a varying water level, showing how the depth of the epilimnion and hypolimnion change over the years? It would be interesting for the reader to see whether the epilimnion depth or the hypolimnion depth change over the years. Does the hypolimnion warm up more in some years, because the epilimnion is much shallower in those years?

We fully understand the interest for the reviewer, and for the readers, in seeing whether the epilimnion depth or the hypolimnion depth change over the years. Therefore, despite that it will extend the length of the manuscript, we have added a new figure with the thermal variations of VB throughout the study period, where both the depth of epilimnion and the hypolimnetic warming can easily be visualized by the reader. This figure shows that the depth of the epilimnion does not change significantly over the years, and that the hypolimnion warm up is not related to a shallower epilimnion, but to the decrease on the overall water level in the reservoir, also shown in the figure. We find this figure, and the information it shows has improved notably the manuscript and its clarity for readers, without exceeding the scope of the paper.

Were there worse algal blooms in years where nutrient flux into the epilimnion was higher?

We have also been monitoring chlorophyll concentrations, which exhibits very high variations, and so far, we have found no correlation of this parameter with any other variable measured. Coccoid cyanobacteria are the main bloom-forming organisms in VB, and a non-linear relationship has been largely described in the literature between cyanobacteria cell counts and chlorophyll (e.g., Ahn, C. Y., Joung, S. H., et al (2007). Alternative alert system for cyanobacterial bloom, using phycocyanin as a level determinant. Journal of microbiology, 45(2), 98-104). Moreover, it seems that the occurrence of algal blooms is multifactorial, and we suspect that most likely it is associated to extraordinary and/or unbalanced (in terms of N:P proportion) inputs of nutrients. However, some there are some relations with production and respiration monitoring that we have also done and published (Guimarais-Bermejo et al 2018) for part of the period here reported, and we have discussed relations with the lake metabolism in the revised manuscript. Additionally, some of the blooms that taken place in VB in the past are related to mucilaginous cyanobacteria, described in other reports as low pigment bloomers. This provides another source of variation for bloom occurrence.

Id like to see a figure with hypo and epi-limnetic nutrients over the sampling period as well.

We agree on the interest on hypo and epi-limnetic nutrient concentrations, and have included a paragraph describing the contrasting concentrations (previously published) among the epilimnion and the hypolimnion in VB. Please consider our previous arguments on the non-conservative nature of nutrients and also on the complexity of including a figure with nutrient concentrations over the 21 years here reported, the unfeasibility of extending more this manuscript, and the very short time (10 days) the Water Editorial Office has given us to review the manuscript.

There is a lot of reference to previous studies (which is good – I like to see work building on from other work), but it does make it difficult to ascertain what is the new data presented in this paper. Could you add a sentence at the end of the introduction with an objective or aim, and outlining what this particular paper brings to the larger body of work.

This is the first time we are publishing internationally this data on vertical mixing and nutrient flux. Previous papers were on other aspects of VB: limnology, external loading, metabolism, phytoplankton and zooplankton communities. As required, we have revised the last paragraph, to clearly outline what this particular paper brings to the larger body of work, and also what it does not.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

None

Author Response

We thank reviewer 1 for his important contribution to improving our manuscript.

Best regards

Reviewer 2 Report

Dear Authors, 

Thanks a lot for your effort and excellent explanation to my previous review. I red this new version with pleasure, interest and easy understanding. But some key points are still under questions. Mainly the discussion is not enough structured, maybe you should add subtitles. There are not enough links between the result part and the discussion : ou don't use your result to confirm your hypothesis etc... It's why I propose to you to move the figure 5 and so on to the discussion. On the same way, what about the Kz ? I miss you didn't provide any figure ? any relationship ? what about Kz and Nitrate ? How is the evolution of the deltaQhypomix ? 

And your equations to fix the vertical mixing calculation are not yet very clear. 

You will see in my review my detail comments.  I report here below the most important: 

Line 224-229: really not clear. Do we need so many equations ? What is the simple expression of Mixmeta-hypo by definition ? From which eqution the equation 5 is coming ? What is the logical link between eq 3, 4 and 5 ?

And (Tepi-Thypo) is what you call deltaThypo in the eq 4. So I will write the definition of the deltaThypo in one equation then in all the other equation I will use deltaThypo.

Line 230: No, in this equation (5), it's m3/h  (and not m3/d). And in your table 3 and fig 5, the values are in m3/h !

Line 231: How do you measure the mean hypolimnetic density ? Is it constant along the year or dependant to the Temparature ? In fact, precise when it's a constant or not: same question for c. 

Line 277: You could precise the cycle duration of the stability is one year whatever the WLF.

Line 282: You didn't say any word about the good relationship between S and WL. You should do it. And why you calculate the relationship with WL and not with WLF ??

Line 297: It seems a confusion between Smax and Smean ? Fig3c is about Smean. I will delete this sentence since you already say before (in line 277) that the stratification cycle keeps its frequency of 1 year whatever the WLF. 

Line 322-325: I will move this paragraph to discussion part.

Line 330: why with the annual minimum ???  And why not with the WLF ? It would be more significant and coherent with your study. I feel that annual minimum WL is directly linked to the WLF, no ?

Line 330: On fig 5, you could look for the curve break. And at this singularity, there is a change of behavior of your system. It seems to be around (1824 m asl; 40 000 m3/h)

Line 332: To demonstrate that, you should make a plot. I will do it to replace the Fig 5 that I will put in the discussion part to discuss more the relationship between WL and Vertical mixig intensity. It's a key point of your paper since in the introduction you target to provide recommendation of WL management.

Line 355: I will move this nice figure 6 in the discussion part.

Line 357: The discussion part will be improved through sub-titles, since your discussion is not enough set up on your results & data.

Line 366-368: I agree, so now you should do it: which recommendation do you provide to the managers ?

Line 370: Your discussion is too much based on a list of assertion. You should name the variables or the relationships you used to prove or to describe the lake functioning. 

Line 385: From which results ? 

Line 392-393: I did not understand the link between these 2 sentences ? I didn't understand the interest of the comparison here. Why you didn't do later on in the paper ? First, you explain the thermocline functioning of the VB lake, it would be good that there you discuss the impact of dam water uptakes and of dry year (low annual rainfall)....

Line 484: explain which kind of loads ? sediment ? wastewaters ? ... ?

Line 494-498: so, what is your advice for the managers of VB ? Which WLF should they avoid ? More than 10 m ? 8 m ? ...

Line 521: The conclusion could be more attractive. Here it's more a resume of the paper. No, please, try to say what we have to keep in mind in few words. Thanks

Kind regards

 

 

 

 

 

  

 

 

Comments for author File: Comments.pdf

Author Response

Response to Report 2 from Reviewer 2

Dear editor,

Thank you for the second report from reviewers on our revised manuscript Water- 1358180.

We are very happy to learn that reviewer 1 has readily accepted our previous revision of the manuscript.

Following the additional comments and suggestions from reviewer 2 on its 2nd report, we have fully revised the manuscript again.

We find that the manuscript has improved significantly through this process, and are very thankful with the two reviewers, and we now invite them to accept their names appear as reviewers of the manuscript, so they can get credit for it.

We have uploaded the revised manuscript to the Water page, also our detailed responses to this second report uploaded in the page.

Best regards,

Martin Merino-Ibarra

Corresponding author Water- 1358180

 

Responses to Report 2 from Reviewer 2

Dear Authors,

Thanks a lot for your effort and excellent explanation to my previous review. I red this new version with pleasure, interest and easy understanding. But some key points are still under questions. Mainly the discussion is not enough structured, maybe you should add subtitles. There are not enough links between the result part and the discussion : ou don't use your result to confirm your hypothesis etc... It's why I propose to you to move the figure 5 and so on to the discussion. On the same way, what about the Kz ? I miss you didn't provide any figure ? any relationship ? what about Kz and Nitrate ? How is the evolution of the deltaQhypomix ? And your equations to fix the vertical mixing calculation are not yet very clear.

We appreciate the reviewer´s attention and very detailed review, we are happy he found the revised manuscript interesting and easier to read. In our second revision of the manuscript and in our responses below we address the key points pointed out.

We have added subtitles to further structure the discussion. We have moved figures 5 and 6 to the discussion as proposed.

As requested, we have added a new figure on Kz evolution among years (in Figure 5), including also the relationship with water level. We have also revised the text, equations and units, to improve clearness to the readers as much as possible in a mathematical presentation.

You will see in my review my detail comments.  I report here below the most important:

Line 224-229: really not clear. Do we need so many equations ? What is the simple expression of Mixmeta-hypo by definition ? From which eqution the equation 5 is coming ? What is the logical link between eq 3, 4 and 5 ?

We have thoroughly revised these lines and found a small text mistake in line 227 that referred to the wrong equation (equation 5, when it should have referred to equation 3). We are happy this mistake was found thanks to the reviewer, and we have revised the text in these lines to ensure their clearness, adding the names of the terms described. We do need all the equations to describe correctly the method used. Equation 3 presents the heat budget considered for our calculations, which relates total heat transport to heat transport due to mixing. Equation 4 shows the way total vertical heat transport was calculated from hypolimnetic warming (time change of temperature). In turn, Equation 5 shows the way the vertical mixing volume was calculated from the heat transport due to mixing and the vertical thermal difference. We consider that finding this small mistake has improved clearness for readers, and we consider equations should remain as they are now, so that a logical and complete presentation of our calculations is offered to the readers.

And (Tepi-Thypo) is what you call deltaThypo in the eq 4. So I will write the definition of the deltaThypo in one equation then in all the other equation I will use deltaThypo.

These are two different terms: Tepi-Thypo is the difference between the mean metalimnetic temperature the mean hypolimnetic temperature (i.e. the vertical gradient). DeltaThypo (DT_hypo ) is the time difference of the hypolimnion between samplings (and DT_hypo/Dt is in fact the hypolimnetic warming rate). So this are two different terms; DT_hypo  is used to calculate the vertical heat transport, while Tepi-Thypo is used to calculate the mixing volume from this heat transport. This is why all the equations are needed.

Line 230: No, in this equation (5), it's m3/h  (and not m3/d). And in your table 3 and fig 5, the values are in m3/h !

We totally agree with the reviewer. We found it will be very very useful for the reader to use the same time units, and have homogenized all the units of Vmix to m³ d^-1 , changing these units in table 3 and fig 5, to make it also consistent with the nutrient fluxes we calculated, since nutrient fluxes are reported in the literature in mg m^-2 d^-1. We have also revised the volume units to 10⁶ m³ for the mixing rates, so that they are in the same units than the maximal volume of the reservoir (i.e. 391 x 10⁶ m³, see study area) or the annual extracted volume (i.e. 221 x 10⁶ m³, see also study area), and can therefore be compared directly and easily to them.

 

Line 231: How do you measure the mean hypolimnetic density ? Is it constant along the year or dependant to the Temparature ? In fact, precise when it's a constant or not: same question for c.

The hypolimnetic density is calculated from temperature, as it is a function of it. To obtain mean values among the two samplings for each calculation of Vmix and Kz, we averaged vertically our registers (every meter) within the hypolimnion, and then between the two samplings to obtain the mean for the period between two samplings, as are also the epilimnetic and hypolimnetic temperatures used in equation 5.

We have revised and extended the paragraph on lines 230-232 to make this clear and explicit, it now reads:

“Where c is the calorific capacity of water (cal g^-1 ºC^-1), r_hypo is the mean hypolimnetic density (g cm^-3), calculated as the mean of the density values measured in the hypolimnion, T_meta is the mean metalimnetic temperature (°C) and T_hypo is the mean hypolimnetic temperature (°C), all three calculated averaging the measurements from the two samplings used for each period calculation.”

Line 277: You could precise the cycle duration of the stability is one year whatever the WLF.

We agree with the reviewer and have added, on the sentence of this line, the precision that the cycle duration of the stability is one year, without losing the meaning we intended to give it.

Line 282: You didn't say any word about the good relationship between S and WL. You should do it. And why you calculate the relationship with WL and not with WLF ??

We have extended this paragraph to outline the good relationship between stability (S) and the water level (WL), as indicated by the reviewer. The reason we use the water level (WL) for relationships with other parameters and not water level fluctuations (WLF) is described in detail three comments ahead, where this point is mentioned again by the reviewer.

Line 297: It seems a confusion between Smax and Smean ? Fig3c is about Smean. I will delete this sentence since you already say before (in line 277) that the stratification cycle keeps its frequency of 1 year whatever the WLF.  

Corrected as pointed out. We apologize for this “maximal” term out of place, we have deleted and the sentence now refers to mean stability, as pointed out by the reviewer and shown in figure 3c.

Line 322-325: I will move this paragraph to discussion part.

This paragraph is part of the description of our results, and is related to the following sentences (on Kz) of that description, so we do not consider convenient to move it to the discussion. We accept, though, the movement of figures 5 and 6 to discussion as also suggested by the reviewer.

Line 330: why with the annual minimum ???  And why not with the WLF ? It would be more significant and coherent with your study. I feel that annual minimum WL is directly linked to the WLF, no ?

Water level is the parameter in relation to which vertical mixing and nutrient transport are studied. Water level fluctuations (WLF) refers to the feature of some water bodies where water level changes. It is a condition, not a parameter. To parameterize it, one would have to use the water level time derivative, which would have no meaning, at least in terms of the approach of this manuscript. The water level is a critical number that reservoir managers should take into account numerically, different to the water level fluctuations condition that implies water level variations and their ecosystem effects. We used the annual minimum, because it showed the best relationships with the hypolimnetic warming rate, with the mixing volume, and also with the nutrient fluxes.

Please see the attached figure showing the absence of a relationship between temperature and water level variations (best coefficient of determination for a polynomial regression, r²=0.259

Line 330: On fig 5, you could look for the curve break. And at this singularity, there is a change of behavior of your system. It seems to be around (1824 m asl; 40 000 m3/h)

We appreciate the reviewer´s attention and detailed review. It is very attractive to find a critical threshold, where there would be a change of behavior of the system. We think this likely will occur in terms of the ecology of the system, due to the vulnerability/resilience of biological systems. However, this most likely will depend on multiple parameters (not exclusively on water level), and it will also depend on the duration of critical conditions. To fully address this issue, another manuscript, dedicated to tipping point, would be needed. The results here included show that, although the effects of lowering water levels increase non-linearly as a function of the water level, a continuous equation fit well with the data, so it is important to consider the full range of variation as a function of water level as a very powerful tool for the prediction of changes in the mixing conditions and to fully use for management recommendations, along with the multiple other considerations related to management. Nevertheless, to address the reviewers request, we have included in the manuscript a reference water level (1822 m asl, 8 m below capacity level), below which mixing is certainly above the variability found in the high water level years (~1.5 x 10⁶ m³ d^-1).

Line 332: To demonstrate that, you should make a plot. I will do it to replace the Fig 5 that I will put in the discussion part to discuss more the relationship between WL and Vertical mixig intensity. It's a key point of your paper since in the introduction you target to provide recommendation of WL management.

Following on our arguments on the previous response, we think that showing the full range of variation as a function of water level provides a very powerful tool for prediction of mixing and nutrient flux conditions that can be made using our model and considered together to multiple other aspects and priorities (including social issues) that must be considered for integrated management decisions.

We have also revised the introduction, to eliminate the objective of providing recommendations for water level management, restricting it to contribute with understanding useful for management.

While the hydrodynamics trend (mixing intensity vs. WL) is non-linear, biological systems are resilient and their responses might be different. Moreover, the suggestion of lowering the level to change hydrology and biogeochemistry might be beneficial for the system in certain aspects, it might also have deep social impacts, both in terms of the water supply to Mexico City and in terms of the impact on all the nautical infrastructure that has been constructed around the reservoir.

We have also revised the introduction, to eliminate the objective of providing recommendations for water level management, restricting it to contribute with understanding useful for management.

Line 355: I will move this nice figure 6 in the discussion part.

We have kept this figure where it was, because it is mentioned in the results. But we have expanded the discussion on it, particularly in section “4.2. Vertical mixing increases up to five-fold as the water level decreases”, following the idea suggested by the reviewer.

Line 357: The discussion part will be improved through sub-titles, since your discussion is not enough set up on your results & data.

We have included subtitles, and we have also revised and reordered the discussion to improve its structure and to make it easier to follow for the readers. We have also increased links between to the results and figures following the ideas suggested by the reviewer.

Line 366-368: I agree, so now you should do it: which recommendation do you provide to the managers ?

We only intend to provide a useful tool for managers, so that they can include the water level and its impact on mixing and nutrient fluxes among the pool of variables, processes and interests they have to outweigh for management decisions. We have better specified this recommendation in the text.

Line 370: Your discussion is too much based on a list of assertion. You should name the variables or the relationships you used to prove or to describe the lake functioning.

We have revised the text, naming the variables and the relationships where they were missing.

Line 385: From which results ?

We refer to our temperature observations. We have specified it so in the text pointed out by the reviewer.

Line 392-393: I did not understand the link between these 2 sentences ? I didn't understand the interest of the comparison here. Why you didn't do later on in the paper ? First, you explain the thermocline functioning of the VB lake, it would be good that there you discuss the impact of dam water uptakes and of dry year (low annual rainfall)....

These two sentences summarize our approach and the characteristics of VB that made it feasible.

Line 484: explain which kind of loads ? sediment ? wastewaters ? ... ?

We are referring to nutrient external loadings, which were directly measured in the rivers as TP and TN in the paper by Ramirez-Zierold et al 2010 cited, and are described there. We have added a new reference (Calderón-Cendejas et al 2021 [60]) where urban wastewater and agricultural land uses in the VB basin are identified as the main sources. We have revised this sentence, including this information and the new reference.

Line 494-498: so, what is your advice for the managers of VB ? Which WLF should they avoid ? More than 10 m ? 8 m ? ...

We have added the advice to managers to consider that below a reference water level (1822 m asl, 8 m below the reservoirs capacity level) below which mixing is certainly above the variability found in the high water level years (~1.5 x 10⁶ m³ d^-1), additionally to the water level (1818 m asl, 12 m below the reservoirs capacity level) where the increase is multiplied we have previously included in the abstract and text. However, this decision on whether avoiding a certain water level is more complex. Lowering the level to increase mixing and intensify nutrient cycling and might be beneficial for the system in terms of reducing the presence of toxic blooms, but it might also have deep social impacts, both in terms of affecting the reservoir of the water supply to Mexico City and in terms of the impact on all the nautical infrastructure that has been constructed around the reservoir. Therefore, our results will be useful for managers, but management decisions must also consider other ecological and social aspects and comply with the priorities of the involved communities.

Line 521: The conclusion could be more attractive. Here it's more a resume of the paper. No, please, try to say what we have to keep in mind in few words. Thanks.

We have revised the conclusions to make them more specific and attractive, including in fewer words the main aspects the reader has to keep in mind.

Response to Additional comments made on the manuscript .pdf file.

Line 38: Corrected as requested

Line 64: Corrected as requested

Line 66: Corrected as requested.

Line 75: Done as requested.

Line 89: Corrected as requested

Line 99: Corrected as requested.

Line 109: We sustain our redaction

Line 119: Corrected as requested.

Line 142: Corrected as requested.

Line 179 to 181: Corrected as requested.

Line 188: Done as requested.

Line 199: Corrected.

Line 205: Done as requested, we have added a paragraph describing all the variables and parameters in these equations.

Line 223: Corrected as requested.

Line 224-229: We have thoroughly revised these lines, corrected a small text mistake and added the precise terms to improve clarity. See detailed explanation on the equations and other questions on the same comment above.

Line 230: Yes, we agree with the reviewer, and to correct this issue we have unified all the time units of Vmix to d^-1 , so the time units are also the same as for the nutrient fluxes. We have changed the corresponding values of Vmix to these units, and corrected accordingly the text, Table 1 and Figure 5. We have also revised the volume units to 10⁶ m³ for the mixing rates, so that they are in the same units than the maximal volume of the reservoir (i.e. 391 x 10⁶ m³, see study area) or the annual extracted volume (i.e. 221 x 10⁶ m³, see also study area), and can therefore be compared directly and easily to them.

Line 231: It is calculated from temperature and dependent on it. Se detailed explanation in the comment with the same question above.

Line 233: Done as requested.

Line 235: Corrected as requested: “it” was deleted

Line 277: We apologize for the confusion, we simplified and extended the sentence to “Despite these high WLF, stability maintained marked annual cycles that show that the stratification was not broken during any of the stratification periods of 2001 - 2021 (Fig. 3b).”

Line 282: We have extended this paragraph to outline the good relationship between stability (S) and the water level (WL), as indicated by the reviewer.

Line 297: We have corrected this sentence, deleting the word “maximal”.

Line 332: We have commented on the difference between the feature of water level variations and the water level parameter itself, and explained why we used the minimal water level in the response to the similar comment above.

We have revised the introduction, to eliminate the objective of providing recommendations for water level management, restricting it to contribute with understanding useful for management. Please see detailed response above.

Line 355-356: We have added subtitles as requested by the reviewer.

Line 366: We only intend to provide a useful tool for managers, so that they can include the water level and its impact on mixing and nutrient fluxes among the pool of variables, processes and interests they have to outweigh for management decisions. We have better specified this recommendation in the text.

Line 370: We refer to our temperature observations. We have specified it so in the text pointed out by the reviewer

Line 385: We have specified in the revised manuscript that we refer to our stability results.

Line 392: These two sentences summarize our approach and the characteristics of VB that made it feasible

Line 468: Corrected as suggested.

Line 474: Corrected as suggested.

Line 478: Corrected as suggested.

Line 484: We have added information and a new reference on the nature of the external loading.

Line 494: See response to the comment above. We have provided a reference water level below which the increase in mixing should be considered among the management considerations

Line 521: We have revised the conclusions to make them more specific attractive and to include in a few words the main aspects the reader has to keep in mind. 

 

 

Author Response File: Author Response.pdf