Plant Nutrient Uptake in Full-Scale Floating Treatment Wetlands in a Florida Stormwater Pond: 2016–2020

Round 1

Reviewer 1 Report

The paper describes challenging full scale study of N and P uptake by wetland plants in the form of FTWs. I found this work very interesting and I will suggest the publisher to consider its publication.

There are some minor remarks which should be answered:

Line 168: what was the share of volunteer species in the total biomass harvested

Line 189: I hope that the fuel level in the truck was included?

Table 2 and figure 3  should be shifted to the results section

Table 2, and others, line 248 : it is not fully clear, why there is so big difference between overall N removal (g/m2/year) in mixed and non-mixed variants. Please explain

I hope, that despite water quality was not the point of the study, it was monitored in terms of nutrients during this period, and Author will be able to write another interesting paper focused on the FWTs efficiency.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

The goal of this study was to quantify the mass of nutrients removed by plants in full-scale floating treatment wetlands (FTWs). The total mass of nutrients removed by partial harvest (shoots) and complete harvest (shoots and roots) were quantified over four years. The intense plant sampling protocols used during the third year of monitoring are the primary focus of this article was to better permit quantification of nutrient uptake and removal from the pond contributed by individual plants, volunteer species, and the FTWs as a whole.

I strongly support a publication of this interesting and comprehensive data set. Indeed, the author (and colleagues??) did an impressing effort for gathering and compiling the data for this study. However, I very much regret that water quality information are more or less completely lacking (just one water sampling for the whole period) which not only complicates a comparison with other studies but also hamper the interpretation of the data presented here in particular of the water mixing experiment. Another weakness of the study is that biomass and nutrient content data are not presented on the basis of dry mass. Please clarify also if N/P values shown in this study are mass or molar ratios and I would also suggest to include shoot/root biomass ratios. All this can be easily done of course but I wonder if some of the presented data are correctly calculated in this study. For example, the nutrient uptake for the period 2016 to 2018, but also for 2019 (Table 4) and also the proportion of dry mass/solids of fresh plant tissues seems to be much too high?! These weaknesses of missing water quality data and data accuracy apart the work is well structed and mostly easy to read and understand. Tables and figures are nicely drafted but not always fully self-explaining. This and other flaws are addressed more detailed in following.

Page 2, Lines 64-72. Suggest to remove this paragraph, interesting information but out of the scope of the study.

Page 3, Line 105. For better comparison with literature I would suggest to refer/convert biomass yield to dry mass. I would recommend to do it throughout the manuscript.

Page 3, Line 110. I would assume that these values are 10-fold to high (for example in comparison to Walton et al. 2020; Table 4). An explanation could be that P content of plant material was referring to dry mass so that the multiplication with fresh biomass led to this overestimate? As suggested before plant biomass data and nutrient contents should always refer to dry mass which is the common way in science and practice.

Table 1. Usually the water content of plants varies between 50 and 90%. According to these data shown here the water content would be in average less than bout 20%. How was dry mass determined in this study? Please indicate if nutrient contents are related to fresh or dry mass here?

Page 5. Line 183. Suggest to remove “before 16:00“

Table 2. Please indicate the investigation period for data shown in this table. Otherwise data of first columns remain uncertain. Again, the proportion of dry mass or solids respectively seems to be much  too high for me.

Page 6, Line 234 “absorbed” is not favorable wording. Please check also rest of text (e.g. Line 237, …, 336,….)

Page 8, Line 249. Delete “whether for a yearly or daily uptake rate“ (of course it makes no difference if the year or the day is considered), the same Lines 284-285.

Table 4. Please check the data on nutrient uptake again. It seems to be much too high by about factor 10 both for N and P?!

Page 11, first para. In general, I would not exclude that mixing the water column alter the nutrient uptake of the plants, the more so as other studies found such relation. However, can we really exclude completely that here a local effect was of higher importance, i.e. a different nutrient level in the western arm compared to the northern arm also without mixing. I agree that measurements the year before or after applying the mixing can reject this “negative hypothesis”, but again can we exclude bigger differences for the nutrient levels in the pond between the four years? It is indeed a pity that no water quality measurements were conducted to support the one or the other idea!

Page 11, Line 328. Reword “Trends were similar trends for P data [19]”

Page 11, Line332-33. For me it this sentence reads awkward as well: “the N and P removed via FTW harvest were similar if not higher for non-mixed FTWs than mixed FTWs”

Page 11, Lines 340-344. Consider rewording this section. It seems to be a “scenario” described for another system? What is meant with increasing the flow rate with the mixer. Does the mixer increase the water inflow of the pond; by which factor? As I understand the mixer is mixing the water column but not increasing the water exchange/water residence time in the ponds?! I would suggest to delete this part anyway.

Page 11, Lines 344-348. I assume that N/P mass ratios are shown here which imply that before mixing the plant growth was P limited and with mixing N limited. This would indeed support the idea that P uptake by plants was supported by the water mixing. But according to Table 4 there was no difference of N/P ratios of plants from non-mixed and mixed treatments?!

Page 12, Lines 375-385. Strongly suggest to delete this section and report the data on dry mass basis here. Referring to dry mass is a common practice both in wetland and freshwater research independent on the species diversity. Municipalities and water authorities are not necessarily interested in nutrient contents of plants anyway if reported before or after drying the plants but require removal rates or nutrient stocks (per area). Still it looks to me that determination of dry mass was somehow biased in this study?!

Page 12, Lines 386-390. I cannot support this statement. The majority of constructed wetland studies on nutrient removal by plants also consider the water quality. In this context I strongly recommend to include the valuable FTW studies of Barco et al. 2020 and DeStefani et al. 2011 in this work.

Page 12, Lines 390-399. This part is widely OK but in the context of lacking water quality data it becomes a strange reading! Would it not be better to start in this way: “Unfortunately water quality data could not be obtained in this study …..! Is there any chance at least to categorize the pond systems, probably eutrophic to polytrophic conditions?

I wonder why this work is not acknowledging the support from other people. According to the fig. 2 it seems that some people were supporting this big and impressing effort behind this study?! 

References

Barco, A.; Bona, S.; Borin, M. Plant species for floating treatments wetlands: A decade of experiments in North Italy. Sci. Total Environ. 2020, 751.

De Stefani, G., D. Tocchetto, M. Salvato & M. Borin, 2011. Performance of a floating treatment wetland for in-stream water amelioration in NE Italy. Hydrobiologia. doi:10.1007/s10750-011-0730-4

Walton, C. R., Zak, D., Audet, J., Petersen, R. J., Lange, J., Oehmke, C., Wichtmann, W., Kreyling, J., Grygoruk, M., Jabłońska, E., Kotowski, W., Wiśniewska, M. M., Ziegler, R., & Hoffmann, C. C. (2020). Wetland buffer zones for nitrogen and phosphorus retention: Impacts of soil type, hydrology and vegetation. Science of the total Environment, 727, [138709]. https://doi.org/10.1016/j.scitotenv.2020.138709

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

I would suggest to change "Biological Oxygen Demand" to "Biochemical Oxygen Demand" in the whole manuscript.

Author Response

Changed at line 439 (the only instance we found throughout).

Reviewer 4 Report

The authors presented the investigation of nutrient uptake by plants in a long-term operated, large scale constructed wetland.  The paper is interesting and can be accepted after revision:

1. The abstract and also the introduction started with the influence of nutrient contaminants on the coastal regions. These statement confused the readers because this manuscript is about freshwater floating treatment wetlands. Besides, nutrient contaminants are also of concerns to non-coastal communities because of the same reason the authors listed in the introduction (line 26-27). These part should be revised.

2. Line 53-57, the author mentioned different research on contrivution of nutrient uptake by plants. Please provide some specific examples on how much the plant contributed to the nutrient removal. Besides, the differences of plant contribution may be due to the different plant species used.  Please also provide some information on this.

3. Please provide the intensity of plants in the studied site.

4. What is the meaning of "A", "CD", "BC", etc. in Figure 3 & Figure 4? 

5. In the methodology section, there are three periods of the investigation: 2016-2018, 2018-2019, and 2019-2020, but these information was not used when comparing the N and P removal by different plants.

6. The total removal of N and P by the floating wetland should be mentioned to clarify the contribution of plant uptake.

7. The conclusion of this manuscript is missing and the significance of the investigation should be addressed in the conclusion.

Author Response

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Reviewer 5 Report

First of all, I want to congratulate the author for their efforts in this paper. The authors performed a long term analysis of N and P removal by plants in a water pond. The topic is interesting and fits with the journal. Some aspects must be improved to maximize the quality of the paper:

• Authors should avoid using the same words in the title and in the keywords.
• The abstract must be improved, and authors must include the comparison of the observed performance of different species in the absorption of N and P in their different parts (roots and shoots). Thus, they can highlight their results.
• In tables and graphics substitute the family of the species by their names. Agrostys à alba. Use italics in these cases.
• The tables and figures must be cited in the text before they appear. I suggest analyzing the data after the table is presented. Currently, the data is analyzed 4 pages after data is presented. And this can confuse the readers, who might not understand the relevance of this data before arriving at page 10.
• Authors must uniform the nomenclature. They used the words “volunteer sp.” and “weed plants”. They must select just one name. Furthermore, they have to define the species or at least the dominant ones.
• Figure 3 and Figure 4 must be checked. There are some missing data.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Dear author,

I was happy to find this revised version. Of course I am also able to tolerate that data still refer to fresh mass the more so as it really seems that there went something wrong with the determination of dry mass! However it will surely hamper the comparison with other studies!

At the other hand I would really encourage you to check the data of Table 4 again. Indeed it seems that nutrient contents of plants are OK. Beside this, the mentioned dry mass issue alone would not explain this exceptionally high P and N removal rates by plants reported here I have never seen before also not in heavily nutrient-enriched sites as reported in the Walton paper. Thus, would it be possible that there is a mistake with fresh biomass data reported in Table 4 (column 2). If I understand it correctly biomass data shown here are related to the harvest of the either two islands from the mixed zone or the unmixed zone. The area of each island was 225 square meter so 500 square meter for each treatment, Taking the results from April 2019 it would refer to a biomass yield of 290 000 kg fresh mass/ha  which is not realistic?! 

I mean in the text the N uptake was shown to be 28.2 ± 1.96 mg/m2/day and 12.4 ± 1.96 mg/m2/day (mixed vs non-mixed; Lines 246-247) and the P uptake 9.17 ± 0.60 mg/m2/day and 3.86 ± 0.60 mg/m2/day (mixed vs non-mixed; Lines 284-285). Also the data in Table 3 seems to be OK. These data are well covering what I found in other wetland types but this 10-fold higher values in Table 4 cannot be truth to my mind! The only explanation I have now is that the first column in Table 4 shows biomass data which refer to 1 ha, this was multiplied with the P content and eventually instead of dividing by 10000 square meter the number was divided by 500 square meter?!

Author Response

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