Timing of Ice Retreat Determines Summer State of Zooplankton Community in the Ob Estuary (the Kara Sea, Siberian Arctic)

Round 1

Reviewer 1 Report

The article raises an important issue within the framework of the recent climate change – timing of the ice retreat in Arctic and its influence on the aquatic communities. Such investigations are scarce in latitudes above 70° N. The work has been accomplished at high scientific level. All proposed hypotheses have been addressed in work, and conclusions are reasonable. The article merits the publication in the journal only with minor corrections.

Remarks (file of article with remarks in text is attached):

Line 78 - "in a retarded phase of zooplankton seasonal development" - better "... in a retardation of plankton seasonal development"

Line 79 - Why feeding rate appeared here? It had not been mentioned earlier in Introduction. Please, substantiate this expectation.

Line 93 - Two stations sampled twice - too little to speak about variability. Maybe "to study differences between day and night"?

Line 108 - "staged" - is there such term?

Line 197 - "independent variables" = predictors

Line 225 - "differences were observed at the repeated stations" - better "differences between samplings?"

Line 251 - "estuary stations" - "estuarine stations"

Line 265 - Why threshold is set at level 78%?

Figure 6 - Is situation at the station 6241 the same on both visits there? It is not clear from the text.

Line 353 - I would say, 2019 was later than typical. 

Line 378 - "mush" - "much"?

Lines 383 and 395 - "brackish" - "brackish-water". Species can not be brackish!

Lines 390-391 - Maybe it is better to mention here only the lowest threshold (7 degC)?

Line 397 and onward - An earlier development of the populations in the OFZ may have additional explanation: stations in the OFZ had cleared of ice much earlier, therefore, are at  the later stage of the zooplankton development. Phytoplankton was also at later phase of spring succession - the spring peak had been probably passed by the time of study.

Line 430 - missprint: "considerable" = "considerably"

Line 487 - "favored by the population" = "favourable for population"

Line 492 - Incorrect form of citation.

Line 503 - "trends in extended ice-free period" = "trends towards extension of ice-free period"

Line 506 - "(but in 2015-2016)" - Unclear!

Lines 507-509 - It was the advanced ice retreat, which was discussed as the reason of the population success of L.m. in 2016, not extension of ice-free period! Why the latter is stressed here?

Comments for author File: Comments.pdf

Author Response

Please see the attachement

Author Response File: Author Response.docx

Reviewer 2 Report

The estuaries of high latitudes are coastal areas of great interest affected strongly by the environmental variability. The zooplankton from arctic estuaries, such as the Ob estuary, may be affected by the ice retreat resulting in a marked seasonal composition of species. The population dynamic of the dominant copepods are studied using a development index to evaluate the percentage of copepodite stages. Data of biomass and feeding rates were also given. The transect design and the pore 180 mesh size employed are adequate to analyze the gradient from brackish to marine species.  It is a nice manuscript, and I have only few comments that would improve it which are the following:

-   Be care with the vertical sampling, because probably the abundance of zooplankton would be underestimated. Do you have other studies performed with oblique trawls for comparison? In this sense, also the biomass would be underestimated.

-  Regarding the time of sampling, despite in table 1 the hours are mentioned, it would be interesting to add if there is a day/night sampling. For example, 15:20 hs is a day sampling? And 20.05 hs?

-   I notice that there are vertical samplings and I think that you can explore better this information hypothesized why the organisms would be migrate through the water column. Are they migrating to evade the predators? To feed on surface layers? Is an ontogenetic migration? Please discuss in more detail this section.

-  Regarding the vertical layers, why in some stations there are 1 depth layer (eg st 6254 ) and in others there are 2 or 3 depth layers (eg st 6242 or st 6250)? Which is the reason?

-    Looking at the biomass estimation of appendicularians I am not sure that the estimation of Hopcroft et al 1998 for tropical waters it would be entirely appropriated for this arctic ecosystem. If you haven´t another reference for this, clarify this in the methodology section.

Author Response

Please see the attachement

Author Response File: Author Response.docx

Reviewer 3 Report

Comments on the manuscript

General comments:

This is a nice contribution targeting the river plume effect on copepod communities and the population structure of the dominant copepods.  Most of the authors are senior and I have no concerns about their conclusions.  At one point, my concern is with the presentation styles of this manuscript.  It is very traditional and some modifications will help readers to easily understand the meaning of figures and tables.  Following, I will make comments based on figures and tables.  After consideration of the comments, I want to recommend the manuscript for publication.  

Detailed comments:

Fig. 1 and Table 1: Station names.  Is it need to provide four-digit for each station?  Since the station location is arranged from coast to ocean along one line (see Fig. 1), I recommend the authors apply St. A to J, which easily understood where the stations are located.  Another point is at the location of the station separation should be written in Fig. 1.  While later, the authors divided stations into two regions (Fresh Water Zone [FWZ] and Ob Frontal Zone [OFZ]) (see Fig. 8, Tables 2 and 3), the readers would hard to understand which stations included for each region.  It should be marked in Fig. 1.  Thus, my recommendation is to show where the hydrographic separation is available for stations in Fig 1.  

Fig. 2: Legends on longitudes are overlapped with the lower letters.  Such overlap should be avoided.  Thus, the longitudes should be written at the upper part of the panels.  

Fig. 3: Numbers for temperature, salinity, and turbidity are too small to read.  It should be written in larger-size fonts.  For the help of the readers, the location of two hydrographic locations (Fresh Water Zone [FWZ] and Ob Frontal Zone [OFZ]) should be marked in the figure panels.  It might be better to be shown by the dashed lines or something.  

Fig. 4: Where is the boundary between Fresh Water Zone [FWZ] and Ob Frontal Zone [OFZ] located?  It should be written in the panel.  For the caption, I could not find an explanation on P.e. and Var.  It should be mentioned in the figure captions. 

Fig. 5: Add to cluster analyses, the readers want to know the locations or distributions of each group.  Please add a map which varied symbols with the group.  Adding such a map to show the horizontal distribution of each group may help readers to understand the horizontal distribution of each group. 

Fig. 8: Colors of CII and CIV are difficult to distinguish please use another color which easily identifies.  For the order of mentioning OFZ and FWZ, the authors applied OFZ first and FWZ second in Fig. 8.  But later, the authors applied FWZ first and OFZ second in Tables 2 and 3.  It is confusing.  Please use the same order (first and second mentioned) throughout the manuscript.  Which may help the readers to avoid confusion.  

Tables 2 and 3: See the above comment (the mentioned order for OFZ and FWZ should be in the same order throughout the manuscript).  For P. spp. CI-CIV in the middle of Table 2, their digit 0.1 should be the same for the other data (0.10).  

Fig. 9: The location of the separating boundary between FWZ and OFZ should be shown by a dashed line or arrow in the panel. 

Fig. 10: For Y-axis (days from the beginning of the year) should be expressed by Julian day, and the differences in months might better show it.  By showing in a month, the readers may easily understand when the ice retreat of Ob Estuary occurs and their inter-annual changes.  

Fig. 11: The colors of CII and CIV are difficult to distinguish.  Please use a different color.  I could not follow the message of this figure.  Is it to show the population structure of L.m. greatly varied from the other species?  If it is so, I think that the figure is fine.  

Author Response

Please see the attachement

Author Response File: Author Response.docx