Controlling Eutrophication via Surface Aerators in Irregular-Shaped Urban Ponds
, Dong Xia 1,2, Qi Mi 1, Ning Yan 1, Mengyao Su 1, Aishou Lin 3, Min Zhao 1,2 and Yasushi Iseri 1,2,*Round 1
Reviewer 1 Report
The main issue of the reviewed manuscript is application of the surface aerators in order to reduce rate of eutrophication process in artificial water reservoir. The topic obviously is not novel, by Scopus search with phrases eutrophication AND surface AND aerator 15 outputs have been obtained. The oldest reference is from 1973. On the one hand, the topic is definitely not in the mainstream of the scientific interest, on the other it got some niche group of the recipient, as well also some practical implications.
There is a key issue that has to be addressed before the manuscript acceptance. While, in terms of the English correctness, data presentation and statistical analysis reviewed article presents overall satisfactionary quality, the most critical point is the experiment design. May key doubts are as follows:
- Data presented in the article especially in case of the pond located at the Maanchi Park, seems to be very interesting in terms of the characteristics of the water parameters changes through the seasons. However, due to the lack of detailed archival data on the situation in this pond before the installation of aerators, it is difficult to assess actual role of the aerators in terms of eutrophication control. Obviously, there is brief comment about this issue in lines 131 and 136, but presented data are to general and not precise (for instance line 134: the nutrient level of pond water was high especially for ammonium-N (NH4+-N) and total phosphorus (TP) at least until summer of 2016 (>2 mg L−1 and >0.4 mg L−1, respectively – wide range of values, such as 20 mg/ L−1 as well 2.1 mg/ L−1 meets condition >2 mg L−1 – please be accurate or present average values). Moreover, in my opinion one year survey is too short period to provide conclusive output in such kind of the experiments. If authors prolongated the monitoring of water quality in Maanchi Park pond, I strongly recommend to provide results of the successive analyses or to present more detailed archival data summary.
- In terms of the second pond, survey period was also relatively short (only one season) and in fact only two measuring points were presented. Thus, many questions about measuring side selection and time of data collection arise. For instance: Why June was selected to perform measurements? While sampling points number was reduced in relation to the first survey?
- There is also lack of some crucial information in the M&M section.
- Please provide / precise information whether the aerators were operated continuously and with the same efficiency throughout the year, or only periodically.
- Provide more detailed information about aerators: What was their aeration and energetic efficiency in kg. O2/KW.hr as well averaged rotators frequency in r.p.m.?
- Did authors provided measurements of the total suspended solids (TSS) or volatile suspended solids (VSS)? It will be useful approach to better understand the nature of the lower water transparency and to assess whether contamination was biological or physical.
- In terms of the results discussion, especially where average values are presented, please add standard deviation values in the text, to emphasize statistical significance of differences between analyzed measuring points.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript is reworked taking into account most of the recommendations. Figure 1 was supplemented, the order of the ponds was changed and the discussion was streamlined. Overall, the manuscript is now more consistent and easy to read. The content of the results section has remained the same. As before, the conductivity measurements are presented in detail, although it is not an indicator of eutrophication. In the discussion it is mentioned only very briefly that it may be an indicator of groundwater discharge.
I have suggestions for minor changes/additions that would improve the manuscript:
For Maanchi Park, it should be emphasised more strongly in the abstract and in the results that the condition of the pond after two years of aeration is presented here. Therefore, I propose the following change:
Abstract: line 21: “In the other pond with an irregular shape, water quality was monitored after the installation to examine seasonal variation in trophic state and its spatial variation associated with aerator distribution.”
Comment: Change into: “……water quality was monitored two years after the installation ….”
Chapter 3.2, line 218 ff: Comment: Either the heading of this chapter is supplemented by “after two years aeration” or an additional sentence could be included, e.g., “After two years aeration the pond is characterized by the following conditions:”
Chapter 3.2 line 205-207: “Despite the little difference in TN, the dominant form of N was NH4+and NO3 − in ZS-18 and ZS-19, respectively.”
Comment: This result should be emphasised more by rewriting the sentence. E.g. Despite the little difference in TN its composition has been changed. The concentrations of NH4+ decreased by simultaneous elevation of NO3.
Chapter 4. line 359-362: “In this study, water quality was monitored in two urban ponds, where cyanobacterial bloom had occurred and multiple surficial aerators were introduced afterward. A dramatic increase in DO and reduction in concentrations of TP after the aerator installation in ZS suggests that the aerators can greatly improve eutrophic states associated with hypoxic conditions.”
Comment: It should be indicated here that these changes only last as long as there is ventilation. If the ventilation is finished, the old condition will be restored. A sustainable change in eutrophication is only possible if aeration is coupled with further measures such as the removal of biomass by, e.g., filtration.
Chapter 4.1, line 380-387 “We did not observe cyanobacterial bloom for the whole pond during our survey both before and after the installation. However, phytoplankton community was dominated by cyanobacteria Microcystis species in both periods. It is suspected that the high nutrient levels especially phosphorus even after the installation (TP: 0.4 mg L−1, TN/TP ratio: 17) favoured the dominance of Microcystis in ZS [43,45]. Instead, water circulation by surface aerators might have disturbed and prevented the bloom formation, which was implied by an observation of cyanobacterial bloom in mesocosms installed in ZS in our another study (i.e., using same water but free of circulation by aerators [46]).”
Comment: The paragraph should be rewritten to make it more consistent. E.g.:“ Despite, phytoplankton community was dominated by cyanobacteria Microcystis species both before and after the installation, we did not observe cyanobacterial surface blooms in both periods. However, there is a potential for a surface bloom formation implied by an observation of cyanobacterial bloom in mesocosms installed in ZS in our another study (i.e., using same water but free of circulation by aerators [46])………”
Further comments:
The results of the PCR are hardly mentioned in the discussion. In Figure 6, Chla, DO and pH show the same behaviour. During the production of phytoplankton biomass, CO2 is consumed and DO is produced. This changes the pH. Could it be that despite the high DO concentrations by aeration, effects of biological production are visible?
When comparing the two ponds, it is noticeable that the nutrient concentrations in the Maanchi pond are significantly lower than those in the ZS pond. Could it be that with longer aeration more oxygen penetrates the sediment and thus more nutrients are fixed in it (assuming that the situation in the MA pond before aeration was similar to that in the ZS)?
The authors would like to think about these two points
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The Authors have addressed kindly all my previous comments and are aware what should be improved in the following, similar experiments. I am fully support publication of the reviewed paper. Only final English proofreading should be performed prior to publication.
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
The topic of prevention of cyanobacterial blooms as in excessive growth of cyanobacterial species or even limitation or reduction of this phenomena has bee the subject of many studies. In particular urban and non-urban ponds have been a challenge because of their shallow nature on one hand, but also their proximity to the public (e.g. parks and public spaces) and hence potential direct exposure by humans and animals. It is an important problem facing local authorities who are in charge of these systems. Many hypotheses have been put forward leading to attempts to tackle this problem by applying a management solution designed to fix the problem of cyanobacteria growth. Unfortunately, not one solution seems to work all the time. In this case, it was proposed by aeration would lead to cyanobacteria decline and also reduction in overall eutrophication caused by non-cyanobacteria species. The study has some valuable data, but unfortunately is not conclusive and didn't exploit the data enough to provide some useful insights. The paper reads more like technical engineering report rather than a paper designed to share useful knowledge. Unfortunately, the design of the study does not allow the author to make any conclusive statements about the link between aeration and cyanobacteria blooms. There is no cause to effect relationship that could be drawn from this study. The intention of limiting the release of nutrient from the sediment due to increased oxygen level is not even relevant in some of these systems which are shallow enough to mix and as such rarely see anoxia. Furthermore is the release of the nutrient stops, there is plenty of nutrients already and will be circulated through the physical action of the aerators. Probably this process may be more suitable for deeper system to help break the stratification.
Reviewer 2 Report
The main issue of the reviewed manuscript is application of the surface aerators in order to reduce rate of eutrophication process in artificial water reservoir. The topic obviously is not novel, by Scopus search with phrases eutrophication AND surface AND aerator 14 outputs have been obtained. Thus, the study is a case study rather than a presentation of the novel solutions. Nevertheless, manuscript is well prepared, organized and written. In terms of the ponds, implications of the surficial aerators application indeed is not well characterized in the literature.
In general, I am satisfied with the overall manuscript quality and only minor comments and suggestions have to be considered before final acceptance. Also final English proofreading has to be provided.
Abstract
Please provide information about values of the DO and nutrients concentrations, to precisely explain ‘dramatic’ improvement of water quality in the second pond. Also be precise when talking about parameters.
Introduction
In general, introduction section is well written. However, after line 61, I suggest to add some small paragraph reviewing other aeration methods applied for the surface water and after this go to explanation why surficial aeration is the most commonly selected. Another missing point in the introduction section is presentation of the methods/approaches which enables to assess eutrophication state of the particular water reservoir. Are there in China any regulations/recommendations about assessment this issue?
Materials & methods
Did authors provided measurements of the total suspended solids (TSS) or volatile suspended solids (VSS)? It will be useful approach to better understand the nature of the lower water transparency.
Results and Discussion
This chapters are logically organized and greatly supported by well-prepared graphs and tables.
I suggest to provide three letters abbreviations of the Months names in Figure 2 to keep it coherent with sub-figure g.
Are there any data about ichthyofauna or macrophytes in the analyzed ponds? Are these components may play some important role in terms of nutrients circulation and DO consumption in this case?
Reviewer 3 Report
The authors studied two ponds located in parks of a city in China for one year. Because of the high eutrophication associated with toxic cyanobacterial blooms and the odour nuisance caused by outgassing of sulphur water, these water bodies were aerated. With this study, the authors want to find out how aeration affects eutrophication. However, the question remains open what consequences should be drawn from these results and what measures should be taken to improve the condition of the water bodies. However, the work clearly shows that the potential for eutrophication is maintained by aeration and that the water quality is not sustainably improved. This finding is worth publishing.
In order to assess effects of aeration on eutrophication, data prior to aeration are necessary, as was done for the pond in Zhongshan Park. No data are shown for the pond in Maanchi Park. Only an inventory after two years of aeration is given. Effects on eutrophication cannot be deduced from this because substantial data to evaluate eutrophication are missing.
The authors state that no cyanobacterial blooms occurred after aeration. Data are not shown. The question is whether the blooms no longer occur because the organisms are dispersed by the water turbulence (surface blooms are formed only in calm water) or whether the cyanobacteria are no longer present in the pond. A microscopic analysis of the phytoplankton is missing in this paper. Such an analysis could also show whether stocking phytoplankton-eating fish, which reduce phytoplankton concentration in the long term, could be a possible measure to improve the water quality.
The work would be improved if the order in which the two ponds are described were reversed. First, short-term effects of aeration should be shown using the pond in the ZS Park, and in the second step the state after two years of aeration should be described, if similar initial conditions are assumed in both water bodies. However, the lack of data before aeration remains a shortcoming.
The authors measured physical parameters by using probes and determined the Chla and nutrient concentrations (N and P). The "electric conductivity" is not a parameter for assessing eutrophication. It is a measure of ion concentration and, as the authors correctly note, can be altered by elements such as calcium. A detailed description of nitrogen and phosphorus (the most important nutrients causing eutrophication) should appear in the paper and not in the supplement.
Figure 1 should be modified for better understanding by an international readership. It should include: -a map of China marking the study region (Fig.1a is not sufficient). An additional figure for Zhongshan Park as shown in Fig.1b for Maanchi Park. The photos could be included in the supplements.
Further Comments
Abstract Page 3 line 12: „Surface aerators have been introduced in ponds for esthetics ……..”
Comment: Please change into : „Surface aerators have been introduced in urban ponds….”
The conclusion that aeration can foster eutrophication (higher Chla …) contradicts the original aim of the study.
The pond in the Zhongshan Park is not included in the Abstract.
Page 4 line 136:” …….layers at six middle sites using a resin tube…..”
Comment: change into “….at the six middle sites…)
Page 5 Fig.2b and line 198
Comment: Please correct the unit in Fig.2b
Figure 2g is confusing. The results should be shown in two figures. The relationships between parameters should be discussed in more detail. Which parameters are summarised in the months? What conclusions are drawn from them?
Page 6 line 255-256:“ The second axis was positively or negatively correlated with temperature and ORP 255 in many months, and sometimes with turbidity.”
Comment: This kind of PCR result description is not correct in my view. PCR can be used to determine relationships between parameters and not to axes.
Page 10 line 341-343:“ A dramatic increase in DO and reduction in concentrations of ammonia (NH4+) and SRP after the aerator installation in ZS suggests that the aerators can greatly improve eutrophic states associated with hypoxic conditions.”
Comment: This conclusion is not correct. Figure 7d shows that the total N concentrations were not significantly lowered. The N species composition changed, NH4 was transformed into NO3. Both species can be taken up by phytoplankton and bacteria and force their growth and consequently the eutrophication. Only P is depleted both due to fixation in the sediment (this point is mentioned in the discussion) and due to the phytoplankton production (not mentioned by the authors).
What is meant with “organic N and P”? Are these dissolved or particulate nutrients? Information of particulate N and P would improve the estimation of the eutrophication.
