Round 1

Reviewer 1 Report

1. The authors should more clearly indicate the experiment purpose. Do the authors want to explore methods for reducing macrophyte abundance?

2. There is no data on the studied Lake Ge. Why did the macrophytes of this particular lake were used it? Are macrophytes a serious problem for the water quality of this water body? Is brownification had here? Did the authors conduct a relationship study between brownification and DTN, DOC and water color for Lake Ge? Is there a correlation between the real water body and the experimental data?

3. The authors should explain why these particular values of temperature, pH and humic substances additions were chosen.

4. The authors do not show changes in DTN, DOC and water color over time. The authors did not maintain a constant level of these indicators throughout the experiment, maybe the plants were affected by these nutrientsdepletion?

5. The authors should conduct a CCA for DTN, DOC and water color and plant growth and root vigor for CK, LH, MH and HH to identify which parameters affect the vital macrophytes activity.

6. The microbial community importance research is not clear from the paper.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

General Comments:

This paper summarizes interesting results of from small scale mesocosm studies on a single species P. crispus.  The overall results are suggestive indicators of potential “real world” responses to brownification.  The data represents a limited “autecological” approach within very limited growth conditions (i.e. high density planting, constrained, very shallow water column.)

 

Experimental Design:

This paper reports on the use of small, replicated mesocosm systems and extensive analysis of growth responses in P. crispus and associated microbiomes due to varying imposed “brownification” conditions.  The design is statistically good; however the growth “space” is extremely limited (30 cm of water column depth from plant base to surface of the water).  In natural conditions, P. crispus commonly grows from 1 to 4 meter depths.  Also, planting 9 plants in a 15 cm. container introduces intense intraspecies interactions (i.e. self-competition), and this is further complicated since plants were sequentially removed, thus introducing a continuously variable intraspecies component (including rhizosphere and above sediment (shoot) components).

 

These physical growth constraints may have affected the physiological, microbial and general growth responses to varying “brownification” conditions.  The authors need to address this issue and perhaps temper their conclusions as to applying these finding to the real (i.e. natural lakes/ponds) world.  I suggest this data provides a good approach toward field studies- or even more up-scaled (depth/volume) mesocosms with multiple naturally occurring macrophyte species. 

 

Specific Comments:

Lns 37-39:  Macrophytes also compete for light, nutrients and space with neighboring macrophytes (both intra and inter-species competition), and thus differential effects of, and responses to brownification may shift populations.  This paper describes an mesocosm study with one species.

 

Ln 53-77: Authors may want to review Eklof et al. 2021 “Brownification on hold: What traditional analyses miss in extended surface water records”

 (“Water Research 203. 117544-open source), which provides a broad view and suggest multiple drivers /responses to brownification.  The authors may want to modify their “Conclusion” and acknowledge that climate change-alone may not explain these “global” shifts and that submersed aquatic plants exhibit plasticity in their response to low light (elongated shoots, larger internode lengths, leaf shape etc.) all of which may also compensate for at least respond to brownification regardless of species.

 

I suggest the authors  also review published papers on macrophyte interactions (intra- and inter-species competition relative to plant canopy structure) and at least discuss how their results may have limitations. 

Lns 110- etc.  Methods: Please provide the surface irradiance (PAR) on the water columns and the daylength from during the culture period.

 

Lns  128- and 214- etc. (growth)
Did these plants produce turions by 42 day?  If so, were there different responses to the brownification conditions?  Since turion formation is a critical phase in the plants phenology it would be interesting to know if high brownification affected this.

 

Figure 1: Assuming a 30 cm distance from plant pot sediment to the water surface, these (length) values suggest that the plant were occupying the entire mesocosm surfaces and producing significant self-shading (in addition to any imposed brownification)- Please explain. 

Figure 5: Again, this shows a single plants (single species) but does not reflect natural conditions where multispecies interactions with brownification would actually occur.  The diagram shows “Shading effect”, but not from adjacent macrophytes: Again, I suggest the discussion of this figure emphasize the “idealistic” model presented and its limitations. 

 

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

This is a very well prepared manuscript. The studies were properly planned and conducted, and the results obtained were well presented and interpreted. After reading, I have only a few comments regarding mistakes, shortcomings and typos.

The methodology says that apical shoot segments (line 100-101) were used for the experiment. Shoots are usually rootless. Meanwhile, from Fig. 1 shows that on day zero the root biomass was approx. 100 mg. So, were you sure that only shoots were planted, or were the whole young plants?

Line 229. One “the” in brackets is unnecessary.

Line 243. A 10-fold increase is overkill. Fig. 1 shows that the biomass increased 4-fold and the height 5-fold.

Line 244. Probably not “simulate” but “stimulate”.

Line 253 and 273. Chlorophyll should be written in lower case.

In Table 1, HH and MH seem to be reversed because the DOC in the HH column is smaller than in the MH column.

As above.

Author Response

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Author Response File: Author Response.docx

Reviewer 4 Report

This study conducted an indoor mesocosm experiment during 42 days with a common submerged macrophyte, Potamogeton crispus, along an increasing gradient of brownification. They suggest that the shading effect of a large number of epiphytic biofilms under brownification may further aggravate the low light stress on macrophytes. This paper deals with relevant and interesting topic and is potentially a valuable contribution to studies about aquatic plant ecology. However, there are numerous problems that need to be addressed. The following is my concerns.

1. The authors need to better explain the objective 2 (examining the growth, physiological and photosynthetic responses to different degrees of brownification by a gradient of humic substances addition). Is this about the macrophyte?

2. The authors need to provide the testable hypothesis and prediction we can follow.

3. The authors need to better describe about the statistical analyses used for this study. For example, the authors used one-way ANOVA with Duncan’s test; however, they should explain why this statistical analysis is appropriate for this study.

4. The authors need to add more citations that are related to this study in the final paragraph of the discussion.

5. The conservation/management suggestions are limited in this study. I recommend that the authors write the more specific conservation/management strategies based on your results.

6. There are grammatical mistakes in this manuscript. Please check English grammar throughout the manuscript.

 

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

It is desirable that the authors study in more detail the actual lake state and the relationship between brownification  and macrophytes in the future. Experiments cannot take into account the full complexity of real ecosystems.

Reviewer 4 Report

I think that the authors appropriately respond to my comments. I recommend that the authors check the manuscript again to make sure if there are minor mistakes in the manuscript. Thank you for giving me the opportunity for reviewing this manuscript.