A Critical Review of Methods for Analyzing Freshwater Eutrophication

Round 1

Reviewer 1 Report

Review of “A critical review of methods for analyzing freshwater eutrophication”

This review is important, but would be more useful if improvements were made in the organization and presentation. I found the review to be difficult to follow and interpret.  The diversity of water quality evaluation methods requires a better level of organization of presentation.  Can the authors group methods that use similar data and evaluations and separate out those that stand alone?

The simplistic definition of eutrophication presented at the beginning of the paper is not carried throughout.  Far more parameters than nitrogen and phosphorus are considered for all the evaluation methods and then the definition is expanded later in the manuscript.

It might be helpful to place section 4 earlier in the review. The authors may choose to better emphasize the methods that are most utilized or current, given the changing challenges to water quality. The question of which methods are more suitable for wetlands vs rivers vs lakes should also be addressed earlier given the chemical, physical and biological properties differ among the environments.  Also, which methods are most feasible from an economic perspective – methods that can best be adopted with limited funds? The higher technology methods are great for places that can afford them, but are not options for many.

Suggest closing the abstract with a statement more similar to “The aim of this review is to guide future studies on the most appropriate methods available for assessing and reporting water eutrophication.”

Offer a more complete definition early on and Figure depiction and description of eutrophication.

“While this method is simple, the evaluation pattern is usually one-sided, it does not provide adequate measures of water eutrophication [20].” This statement requires a greater level of explanation.

For table 1, is it more accurate to identify this parameter as documented eutrophication, instead of history of eutrophication?

Line 113: “However, as the evaluation criteria and the values of each level of each evaluation factor are artificially divided, the method can become subjective.” Again, this broad statement requires an example or explanation.

The authors made judgements on the first two evaluation methods they presented, yet did not for 3.3. The algal dominant species evaluation method. The methods are not presented in a consistent manner. A few are described in greater detail (are all the equations necessary? Put in supplementary material?). It would be useful to have this type of assessment for each method: “Satellite imagery was originally obtained from Landsat Thematic mapper (TM), spot satellite images of France, and NOAA/AVHRR. Satellite remote sensing (HJ1B-CCD and GF-2 PMS2) is currently used to research inland water quality [61]. The eutrophication of Pamvotis Lake in Ioannina, Greece, was studied using the application of Chl-a detection algorithms based on Sentinel-2 satellite image data. The results showed Pamvotis Lake is a eutrophic lake, and the highest Chl-a concentration was located in the east and south-east of the lake [62].”

Line 188: “The Carlson index evaluates the nutritional status based on the SD of lake water. It was first proposed by Carlson in 1977 [34,35]. This evaluation method overcomes the limitations of a single factor evaluation of eutrophication.” Then, “It integrates various parameters and combines the simplicity of a single variable with the accuracy of a multivariable comprehensive assessment.” This is confusing.

Line 208: “This model uses a BP algorithm as the learning algorithm of the network and does not need to establish mathematical equations.” Then, “The main formula of the BP neural network is Equation 20:..”

In section 4, the authors sub-title the sections: “4.1. The evaluation method for lake eutrophication,” which suggest one accepted method.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors have compiled methods for assessing the eutrophication status of inland waters and crystallised the method that is best suited for the respective water body type. In doing so, they aim to standardise the assessment of water body status in order to achieve better comparability of water bodies. This is a very good idea. However, the manuscript has numerous shortcomings that need to be addressed before it can be published.

The manuscript seems to be based mainly on investigations done in China, as about 75% of the references  are from Chinese authors. However, water monitoring and assessment has also been done in other countries for a long time. The "European Water Framework Directive" specifies a uniform assessment system and uniform objectives for improvement of the water quality of lakes and rivers. A similar system exists in the USA (National Environmental Protection Agency).   Information on this can be found on the internet and should be taken into account by the authors.

The manuscript is difficult to read because it contains a lot of abbreviations. This cannot be changed in part. However, the abbreviations should be mentioned under all tables so that the reader remembers them and does not always have to search in the manuscript for what it means.

The manuscript contains a number of inaccurate formulations. The caption of Figure 1 is not correct and the table 1 caption is also inadequate. Figure 1 does not show the "Formation of eutrophication". It shows sources of nutrient inputs and the cycling of nutrients in the water body. To demonstrate eutrophication and its effects in the water body, a second figure should be created additionally where the arrows from the sources of input are thicker and changes in the water are shown. These are, for example, increased algal blooms and anoxic depth areas where the bottom fauna and macrophytes are absent. Examples can be found in the literature.

Figure 2 shows the status of water bodies at the global level. However, it is not mentioned how many water bodies are included in the presentation for each continent. If I use the numbers given on page 2, lines 66-67, then that are only about 4 lakes, 2 rivers, 2-3 reservoirs and 1-2 wetlands per continent. This is not sufficient for a generalisation for the respective continent. The authors should search for individual countries. At least for European countries more information can be found. The colours green and blue are difficult to distinguish. Other colours must be chosen here.

Table 1 gives examples where which assessment method was applied. This is very informative, but the selections seems to be more or less arbitrary. Not all methods mentioned in the text are integrated. The table should be completed. In this table values for TN, TP and Chla are mentioned.  It needs to be explained what these values are? Are they annual mean values, do they apply to the whole water body or only to the euphotic layer? What are these values given for? What are they supposed to say?

Table 2 shows the status classes for different methods with the respective parameters and concentrations.  However, the content of this table is not discussed. The large differences in concentration for the same trophic level between the methods are striking. This needs to be commented on. The method „ Empirical frequency“ is not mentioned in the text.

In the title it is said " ... freshwater eutrophication".  However, the Baltic Sea is a marginal sea and a brackish water system.  The authors should refer only to inland waters and therefore not integrate the HELCOM classification. To be more stringent, also wetlands should not be included. Lakes, reservoirs  and river systems are complicated enough. Lakes should also be considered in a differentiated way, whether they are deep and thermally stratified or shallow and without any stratification.

 Spezific comments:

Abstract line 23-25: „ Rather, a comprehensive assessment of methods can guide the most appropriate  selection of methods to avoid future eutrophication events, and to assist in developing management  and remediation options.“

Please rewrite the sentence: „ Rather, a comprehensive assessment of methods can guide the most appropriate  selection of methods to develope management  and remediation options avoiding future eutrophication events.“

Page 1 line 40 „TLI“ the full name of an abbreviation should be given at the first use. This should be generally done.

Page 6 lines 131, 139, and 143: please give the full name for the abbreviations.

Figure 3: The title is not correct. The figure shows: Evaluation methods applied to different waters  (lakes,reservoirs, river…). The best methods identified in this work is written in red color.

 Page 13, 325-338: „for the MNCM method is still uncertain, and requires further study [71]; Theoretically, ….the BP neural network method is a more practical  method for the evaluation of lake eutrophication [72], but the method requires further study.“, „the TLI is currently the most suitable method for the evaluation of lake eutrophication.“

These statements must be substantiated. Why is the method assessed in this way and in what direction must further investigations go.

I recommend major revision including renew search for more water bodies (or water body classification), and their integration into the manuscript.

Some  additional literature that  should be read for this manuscript:

Vollenweider R (1968) Scientific fundamentals of the eutrophication of lakes and flowing waters, with particular reference to nitrogen and phosphorus as factor of eutrophication OECD Rep. DA 5/SCI/8, 27, Paris

VOLLENWEIDER, R. A. (1982): Eutrophication of waters - Monitoring, assesment and control. OECD, Paris, 155 p. ·  ISBN-10 : 9264122982 ·  ISBN-13 : 978-9264122987

Eutrophication in Fresh Waters: An International Review, by Malcolm Farley

DOI: https://doi.org/10.1007/978-1-4020-4410-6_79

The authors have compiled methods for assessing the eutrophication status of inland waters and crystallised the method that is best suited for the respective water body type. In doing so, they aim to standardise the assessment of water body status in order to achieve better comparability of water bodies. This is a very good idea. However, the manuscript has numerous shortcomings that need to be addressed before it can be published.

The manuscript seems to be based mainly on investigations done in China, as about 75% of the references  are from Chinese authors. However, water monitoring and assessment has also been done in other countries for a long time. The "European Water Framework Directive" specifies a uniform assessment system and uniform objectives for improvement of the water quality of lakes and rivers. A similar system exists in the USA (National Environmental Protection Agency).   Information on this can be found on the internet and should be taken into account by the authors.

The manuscript is difficult to read because it contains a lot of abbreviations. This cannot be changed in part. However, the abbreviations should be mentioned under all tables so that the reader remembers them and does not always have to search in the manuscript for what it means.

The manuscript contains a number of inaccurate formulations. The caption of Figure 1 is not correct and the table 1 caption is also inadequate. Figure 1 does not show the "Formation of eutrophication". It shows sources of nutrient inputs and the cycling of nutrients in the water body. To demonstrate eutrophication and its effects in the water body, a second figure should be created additionally where the arrows from the sources of input are thicker and changes in the water are shown. These are, for example, increased algal blooms and anoxic depth areas where the bottom fauna and macrophytes are absent. Examples can be found in the literature.

Figure 2 shows the status of water bodies at the global level. However, it is not mentioned how many water bodies are included in the presentation for each continent. If I use the numbers given on page 2, lines 66-67, then that are only about 4 lakes, 2 rivers, 2-3 reservoirs and 1-2 wetlands per continent. This is not sufficient for a generalisation for the respective continent. The authors should search for individual countries. At least for European countries more information can be found. The colours green and blue are difficult to distinguish. Other colours must be chosen here.

Table 1 gives examples where which assessment method was applied. This is very informative, but the selections seems to be more or less arbitrary. Not all methods mentioned in the text are integrated. The table should be completed. In this table values for TN, TP and Chla are mentioned.  It needs to be explained what these values are? Are they annual mean values, do they apply to the whole water body or only to the euphotic layer? What are these values given for? What are they supposed to say?

Table 2 shows the status classes for different methods with the respective parameters and concentrations.  However, the content of this table is not discussed. The large differences in concentration for the same trophic level between the methods are striking. This needs to be commented on. The method „ Empirical frequency“ is not mentioned in the text.

In the title it is said " ... freshwater eutrophication".  However, the Baltic Sea is a marginal sea and a brackish water system.  The authors should refer only to inland waters and therefore not integrate the HELCOM classification. To be more stringent, also wetlands should not be included. Lakes, reservoirs  and river systems are complicated enough. Lakes should also be considered in a differentiated way, whether they are deep and thermally stratified or shallow and without any stratification.

 Spezific comments:

Abstract line 23-25: „ Rather, a comprehensive assessment of methods can guide the most appropriate  selection of methods to avoid future eutrophication events, and to assist in developing management  and remediation options.“

Please rewrite the sentence: „ Rather, a comprehensive assessment of methods can guide the most appropriate  selection of methods to develope management  and remediation options avoiding future eutrophication events.“

Page 1 line 40 „TLI“ the full name of an abbreviation should be given at the first use. This should be generally done.

Page 6 lines 131, 139, and 143: please give the full name for the abbreviations.

Figure 3: The title is not correct. The figure shows: Evaluation methods applied to different waters  (lakes,reservoirs, river…). The best methods identified in this work is written in red color.

 Page 13, 325-338: „for the MNCM method is still uncertain, and requires further study [71]; Theoretically, ….the BP neural network method is a more practical  method for the evaluation of lake eutrophication [72], but the method requires further study.“, „the TLI is currently the most suitable method for the evaluation of lake eutrophication.“

These statements must be substantiated. Why is the method assessed in this way and in what direction must further investigations go.

Some  additional literature that  should be read for this manuscript:

Vollenweider R (1968) Scientific fundamentals of the eutrophication of lakes and flowing waters, with particular reference to nitrogen and phosphorus as factor of eutrophication OECD Rep. DA 5/SCI/8, 27, Paris

VOLLENWEIDER, R. A. (1982): Eutrophication of waters - Monitoring, assesment and control. OECD, Paris, 155 p. ·  ISBN-10 : 9264122982 ·  ISBN-13 : 978-9264122987

Eutrophication in Fresh Waters: An International Review, by Malcolm Farley

DOI: https://doi.org/10.1007/978-1-4020-4410-6_79

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The manuscript reads much better after the revision and is easier to understand. The different number/ compilation of methods in chapters 3 and 4, in tables 1 and 2 and in figure 3 is still confusing for the reader. I have  summarised here which method occurs where in the text.(the attached table)

From my point of view, chapter 4 lists all methods that have been or are being used globally to assess the state of eutrophication. Chapter 3 deals with the methods that are the best methods for the respective water body. If this is the case,  the uncertainties can be resolved by changing the chapter headings:.

Heading for Chapter 4 : Globally applied methods for determining the eutrophication status of waters.

4.1: Methods based on mathematical calculations

4.2 Methods based on models

Chapter 3.  contains the methods that are most suitable to describe different water bodies.

Possible heading: Methods best suited to describe the degree of eutrophication.

It should be made clear at the beginning of the text why or according to which criteria these methods were selected.

It should also be considered whether the order of chapters 3 and 4 should be changed again.

It is also unclear according to which criteria methods are grouped together in the tables and in Figure3 . Either the text and tables and Figure3 are made consistent or it is mentioned in the heading according to which criteria the compilation was made.

I would replace OECD  by OECD classification in Table 1. It should be  discussed in the text.

4.1.2. Formula scoring method. Please add the abbreviation (SCO) here.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf