Dependence of Long-Term Dynamics of Zooplankton in the Ob River on Interannual Changes in Hydrological and Hydrochemical Parameters
Round 1
Reviewer 1 Report
Since the first submission, the manuscript was very improved. Nevertheless, the narrative, mainly concerning results and discussion, needs further enhancement. I suggest joining “Results” and “Discussion” in the same item to avoid repetitions of ideas, allowing more synthetically and understandable text. Please find my suggestions in the manuscript body.
Comments for author File: 
Comments.pdf
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Reviewer 2 Report
The manuscript has improved but I have pointed out a few minor questions that authors should correct/change in the manuscript itself (highlighted in yellow and with my comments in speech bubbles.
Comments for author File: Comments.pdf
Author Response
Many thanks to the referee for the attention and interest shown to our article. Please see the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Now, as far as I am concerned, the manuscript has the standard quality to be published. Nevertheless, it is necessary to read carefully the manuscript (some scientific species names are not in italics, there are some orthographic mistakes)
Best regards
Author Response
Dear Reviewer! Dear Editor!
We have carefully read the manuscript for correcting of orthographic and formatting mistakes. We are sending the edited version.
We hope that in this version the article will be acceptable for publication.
Sincerely. Authors
Author Response File: 
Author Response.docx
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
Two items.
1. Please revise the English language usage throughout the MS. See some suggestions that follow.
REVISED / Suggested new AbstractHere we summarize a long-term study on qualitative and quantitative composition of zooplankton (Cladocera, Copepoda, Rotifera) in the Ob River. We carried out these investigations at 13 sampling stations of the Middle and Low Ob in the years 1994, 1996, 1999, 2001, 2002, and 2009. Species richness of all zooplankton and abundance of cladocerans and rotifers were correlated to low temperatures observed during the month preceding sampling. Among abiotic factors we found that pH were important variables in this system. Dissolved oxygen and oxidizable organic substances (BOD5) were positively correlated to copepod population levels; on the other hand, an increase in compounds refractory to oxidization (COD) inhibited their development. During this study, we found that high water levels had a negative influence on zooplankton richness probably due to being downstream from Ob River floodplain lakes.
- Introduction REVISED / Suggested new Parts of the Introduction
In contrast to lentic freshwater systems understanding the influences of hydrological and chemical characteristics of streams on zooplankton is difficult as the environmental conditions of the flow change rapidly. This challenge is all the more difficult due to the expanding need of protein from wild-caught fish, a problem that is exacerbated by climate change. Thus, we need practical strategies for water use that include policies for the protection of water resources on the worldwide scale, including little studied waters such as those in the far north. One such large waterway is the Ob River of western Siberia.
Unlike fish and other large river organisms, zooplankton, including Cladocera, Copepoda, and Rotifera, comprise a diverse group of heterotrophic organisms transported passively by flowing waters (Bertani et al. 2010; Lair, 2006; Tóth et al. 2020 // Add More References). Zooplankton feed on phytoplankton, bacterioplankton, detritus, and each other, thus they are critical elements in both the classical and microbial loop food web. Their nutrients and energy are transfer to higher trophic levels, including fish and insects. They also contribute to purification of water bodies by removing suspended dead organic matter. The composition of the zooplankton community has been used to assess water quality and is often used as a pollution indicator (Ejsmont-Karabin, 2012; Sládeček, 1983 // Add More References).
Cited References
Bertani, I., H. Segers & G. Rossetti, 2010. Biodiversity down by the flow: new records of monogonont rotifers for Italy found in the Po River. Journal of Limnology 69(2):321–328. doi:10.3274/JL11-70-2-N3.
Ejsmont-Karabin, J., 2012. The usefulness of zooplankton as lake ecosystem indicators: Rotifer trophic state index. Polish Journal of ecology 60(2):339–350.
Lair, N., 2006. A review of regulation mechanisms of metazoan plankton in riverine ecosystems: aquatic habitat versus biota. River Research and Applications 22:567–593.
Sládeček, V., 1983. Rotifers as indicators of water quality. Hydrobiologia 100:169–201. doi:10.1007/BF00027429.
Tóth, F., K. Zsuga, É. Kerepeczki, L. Berzi-Nagy, L. Körmöczi & G. L. Lövei, 2020. Seasonal differences in taxonomic diversity of rotifer communities in a Hungarian lowland oxbow lake exposed to aquaculture effluent. Water 12(5). doi:10.3390/w12051300.
2. The taxonomy of some of the species of rotifer is out of date: i.e., some of the taxa that they identify are not recognized by the List of Available Names (LAN) for the Rotifera as recognized by the International Code of Zoological Nomenclature (Jersabeck et al. 2018). I have highlighted these in Yellow for Table S1. For example,
Brachyonus calyciflorus f. amphiceros Ehrenberg, 1838. is not an available names; it is Brachionus calyciflorus Pallas, 1766
Jersabek, C. D., W. H. De Smet, C. Hinz, D. Fontaneto, C. G. Hussey, E. Michaloudi, R. L. Wallace & H. Segers, 2018. List of Available Names in Zoology, Candidate Part Phylum Rotifera, genus-group names established before 1 January 2000. International Code of Zoological Nomenclature: Available at: https://archive.org/details/LANCandidatePartSpeciesRotifera.
Comments for author File: Comments.docx
Reviewer 2 Report
This manuscript by Nadezdha et al. deals with zooplankton abundance, biomass and species richness and composition variations in the Ob River (Western Siberia) and their relationship to hydrological variables and several chemical features of water. The data are interesting because the comparison spans a 15 yr period, although only 6 years data are available. However, the manuscript requires major revision in order to be in an acceptable form.
The major drawbacks are the following:
- Lack of clarity in many sentences describing the results
- Discrepancies between data described in text and shown in figures/tables, or statements which are not supported by their data:
-Range of water temperatures (page 5)
-Water level in July did not coincide with highest temperature in Aleksandrovskoye according to the figure (page 5)
-I cannot see in figure that precipitations were highest in Aleksandrovskoye in August (page 6)
-Authors state “In the high-water years (2001, 2002), a shift to the alkaline region is noted (from the Tom mouth to the Vasyugan one (almost to 9))” in page 6, but it cannot be seen if it is a shift without data from previous years.
-I cannot see in Figure 5e high concentrations of nitrate at the mouth of the Tom River.
- In text it is stated that BOD5 values 0.83 to 3.80, but in Figure 5f highest values exceed 4 mgO2/L.
-Authors state “BOD5 values usually decreased downstream”, but they have no sufficient data to make such a statement with only 3-year data and one of the years clearly showing the opposite pattern. More years would be needed to provide solid evidence for such a statement.
-Authors state “By the COD values, the content of oxidized substances was several times higher downstream (Fig. 5g).”, however, they should mention an exception in 2002, when the uppermost stations’ values were the highest.
- There are big discrepancies between values of 1999 shown in Figure 6 and Table 1. Which are the correct ones? Change accordingly.
- The discussion is quite poor, having used only two other works (citations) to support it. The discussion should be expanded, so that results from the present study are put into a broader context of studies of zooplankton in rivers, being discussed in the light of many more works in the literature and interpretations being based on ample literature evidence.
Some interpretations of results seem awkward/contradictory:
Page 14, lines 115-121: How can authors state that a de-eutrophication occurred, if they are also saying that summer phytoplankton biomass increased with heavy rains?
Explanations between the relationships between variables are not clear. Authors should state clearly the relationship between precipitation rate, organic matter concentration, development of phytoplankton, eutrophication and saprobity index in 2009.
Page 15 (Conclusions) Here authors are saying the opposite, that an intensification of eutrophication process occurred but, in any case, it is difficult to observe if such an intensification of eutrophication has occurred during the period of study, because authors have only shown inorganic nutrient concentrations for 3 years and no information on plant/algal biomass is provided.
On the other hand, how can zooplankton cope with eutrophication in 2009, but not in 1999?
Other issues
Material and Methods
It seems that different measurements were conducted in different years/months, so in the methods section authors should state clearly what was measured when.
Samples were not taken in the last decade! Authors should state in the same paragraph sampling years and months. In results they are showing data from July too. Which variables were measured in July and August and which ones only in August? Be clear and specific.
Which multivariate method did they use? In some places they say it was a PCA in others a CCA. In addition, they should specify clearly how the analysis was conducted (which variables were used as supplementary and why, etc.).
In the results section they show correlation analysis results, so they should state clearly which correlation test was performed in the methods section.
In Figure 1, authors should mark the location of the Novosibirsk dam, the Ket’ River, the Tym River, and the Vasyugan River, and delimit the areas of permafrost, steppe, forest-steppe, and taiga.
Results
Authors describe in text which are the dominant and subdominant species each year at each station, but they do not provide the data. They should show data on % contribution to total abundance and biomass, instead of just + or – in Table S1, and state clearly what percentages they have used as subdominant.
Table 1. I suggest to create 3 heat maps (one for abundance, another for biomass and another for species richness) instead of this table, to better visualize the information.
In addition, I recommend a Bray-Curtis dissimilarity test of zooplankton communities in different years and stations, to better visualize if there were temporal or spatial patterns.
Discussion
Results from the multivariate analysis and correlation tests should be described in the Results section, not in Discussion. The discussion section should be devoted to the interpretation of those and other results.
Authors should explain why waters being less basic can stimulate zooplankton growth
They should also explain why COD can affect copepod abundance.
Other minor questions and suggestions can be found in the manuscript document itself (highlighted in yellow with comments/questions in speech bubbles).
Comments for author File: Comments.pdf
Reviewer 3 Report
Dear Authors.
I was expectant when I begin to review this paper: Indeed studies concerning zooplankton in large rivers are being still rare and novel approaches are always welcome. Data were collected every August in 13 sampling sites in 1994, 1999, 2001, 2002 and 2009 in 3 different points: in the middle of the river and in the respective margins. Therefore, there is enough data to do a very fine analysis. However, the present manuscript needs a very deep revision before being able to be published. Introduction, results (including data analysis), and discussion are written in a very messy way and do not facilitate the reader's comprehension. Besides literature review and discussion are very poor. Discussion should confront our results with those obtained by other authors and in the present manuscript, the discussion is a repetition of the obtained results. Therefore, in the present format, I reject the manuscript. Nevertheless, I hope that the authors reformulate the manuscript and proceed to a novel submission. Please, find the comments in the manuscript.
Comments for author File: Comments.pdf
