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Open AccessArticle
Peer-Review Record

CDOM Absorption Properties of Natural Water Bodies along Extreme Environmental Gradients

Water 2019, 11(10), 1988; https://doi.org/10.3390/w11101988
Reviewer 1: Anonymous
Reviewer 2: Anonymous
Reviewer 3: Anonymous
Reviewer 4: Anonymous
Water 2019, 11(10), 1988; https://doi.org/10.3390/w11101988
Received: 11 July 2019 / Revised: 6 September 2019 / Accepted: 15 September 2019 / Published: 24 September 2019
(This article belongs to the Section Aquatic Systems—Quality and Contamination)

Round 1

Reviewer 1 Report

This a well presented study and well written manuscript.  This has been one of few studies that compared on a global scale the variability in CDOM absorption across several distinct environments.  My suggestion would be to develop a stronger link between your conclusion between variation CDOM absorption properties, high UV radiation levels and altitude change.

Author Response

This a well presented study and well written manuscript.  This has been one of few studies that compared on a global scale the variability in CDOM absorption across several distinct environments.  My suggestion would be to develop a stronger link between your conclusion between variation CDOM absorption properties, high UV radiation levels and altitude change.

Our response: We have rewritten the last two sentences as follows (see Lines 425-429 in the revised manuscript): “Photobleaching of CDOM is prominent in LN, indicated by reduced absorption towards shorter wavelengths in the UV spectrum, resulting in very high S280-500 [nm-1] values and low aCDOM (320) [m-1] values. For the other water bodies only modest photobleaching was found. We conclude that extremely high altitudes, implying high UV radiation levels, and oligotrophic water conditions are important to make a significant change in CDOM absorption properties.”

Reviewer 2 Report

1) The study lacks a statistical analysis to determine true differences in absorption and slope. With the exception of high latitude Lake Victoria, almost all of the sites overlap. The authors need to determine whether the sites are actually statistically different from one another.

2) Looking at comparing the sites, it may be that season is actually a better metric to distinguish CDOM characteristics rather than location.

3) With no accessory data such as absorption or DOC it is hard to make any definitive determination of reasons for differences between sites.

4) The authors have reviewed a lot of the previous literature and made comparisons with their data. This is a nice review of the existing data.

5) How was KdPAR calculated, i can not find this in the methods?



Author Response

Comments and Suggestions for Authors

1) The study lacks a statistical analysis to determine true differences in absorption and slope. With the exception of high latitude Lake Victoria, almost all of the sites overlap. The authors need to determine whether the sites are actually statistically different from one another. 

Our response: We are so sorry that we did not manage to understand the meaning of this comment, could you please give us some more specific information regarding this comment, and we will carry out analysis according to your comment.

2) Looking at comparing the sites, it may be that season is actually a better metric to distinguish CDOM characteristics rather than location.

Our response: We have inserted the following text in Lines 118-120 in the revised manuscript: “Due to few and variable sampling times at the various sites during the year, and the fact that LV, which is situated close to the Equator, has small seasonal variations, we decided not  to include seasonal variations in our analysis. “

3) With no accessory data such as absorption or DOC it is hard to make any definitive determination of reasons for differences between sites.

Our response: We do not have DOC data but all CDOM absorption data we have are included in supplemental material.

4) The authors have reviewed a lot of the previous literature and made comparisons with their data. This is a nice review of the existing data.

5) How was KdPAR calculated, i can not find this in the methods?

Our response: We have added the following text in Lines 204-207 in the revised manuscript :”The kd(PAR) shown in Table 1 for LN, BS, and RCW were calculated using the relationship Kd=1.44/ZSD derived by Holmes [46], where ZSD is the Secchi disk depth and Kd is the vertical diffuse attenuation coefficient for the downward irradiance. For SF, LF and LV it was calculated from 400-700 nm PAR sensor measurements between surface and lower part of euphotic zone.”

Reviewer 3 Report

The manuscript “CDOM absorption properties of natural waters along extreme altitudinal, latitudinal and trophic state gradients: The search for environmental signatures” presents an interesting data base from a range of diverse locations. I believe that with these data an interesting research paper can be written. However, at present, it lacks any statistical analysis of the data, and methodology is confusing. The methodology section presents data that can be considered results, and for some of these results there is no methodological explanation. I am detailing the recommendations that authors should follow, in my opinion, to rewrite the manuscript. More specific comments are not included, because I think that the paper needs a lot of work before a more detailed review.

OBJECTIVES

The authors aimed at (line 104) improving the knowledge on CDOM absorption properties for a wide range of water types, ranging from limnic to marine, and representing extreme gradients both in environmental conditions and geographical location.

They state that (lines 100 to 103) “Previous studies on CDOM absorption properties for natural water bodies have been carried out in restricted areas and with differences in the methods used [10,26,31]. Thus, few studies … by using the same method (i.e. the choice of wavelength interval for fitting as well as the fitting method).” However, in their manuscript in the methodology section (line 136) they don’t explain the methodology for LV, SF, and LF and just include citations of other works, while the methodology used in the other study areas is explained in this section.

If the methodology used is not the same, authors should reconsider their objectives. If the methodology is the same, it should clearly state that is the same in all studies areas. At present, is not clear at all. I suggest that one objective could be comparing different methodologies for places with similar characteristics.

METHODOLOGY

This section is very confusing, mixes methodology and results. Specific comments are:

(1) Line 136 Methods for measuring CDOM absorption in (Table 1) are described in [29], [11], and [36], respectively. The methodology used in the rest of locations is explained. Authors should be consistent and explain and compare all methodologies taking into account their objectives.

(2) Figure 2 shows the mean CDOM absorption spectrum for each study site obtained by averaging over all samples. Among the six study sites, the highest mean absorption coefficients for the wavelength range 280–500 nm were found in LV and the second highest values in BS. My first concern is that there is no statistical analysis of the results. We cannot know if differences are statistically significant or not. Authors must do a statistical analysis. In addition, I don’t agree with comparing only 8 surface samples from LV with much more numerous samples from other locations and sample at different depths. Depth is an important factor that should be taking into account when analyzing CDOM differences. One cannot attribute differences to inputs when comparing different depths.

(3) Table 1, present Chl-a data and Kd(PAR)data. How were they calculated? There is no methodology described for these parameters. In addition, these are results and Table 1 appears in methodology section

(4) Lines 162 to 164 “comparison of the CDOM spectral slopes derived by applying these two methods (…) (data not shown), revealed that the derived by applying nonlinear and linear fitting agree quite well, but with slightly higher values obtained from nonlinear fitting”. These type of analysis should be presented as result. If authors focus on methodology, discussing different methologies is a result of their research

RESULTS

(5) Some examples of expressions that don’t reveal statistical significance of the differences found

Line 184 “were quite similar”

Line 203 “They were much higher”


Author Response

The manuscript “CDOM absorption properties of natural waters along extreme altitudinal, latitudinal and trophic state gradients: The search for environmental signatures” presents an interesting data base from a range of diverse locations. I believe that with these data an interesting research paper can be written. However, at present, it lacks any statistical analysis of the data, and methodology is confusing. The methodology section presents data that can be considered results, and for some of these results there is no methodological explanation. I am detailing the recommendations that authors should follow, in my opinion, to rewrite the manuscript. More specific comments are not included, because I think that the paper needs a lot of work before a more detailed review.

Our response: We present some preliminary results on CDOM absorption in the methodology section in order to justify our choice of nonlinear fitting instead of linear fitting for derivation of the spectral slope S.

OBJECTIVES

The authors aimed at (line 104) improving the knowledge on CDOM absorption properties for a wide range of water types, ranging from limnic to marine, and representing extreme gradients both in environmental conditions and geographical location.

They state that (lines 100 to 103) “Previous studies on CDOM absorption properties for natural water bodies have been carried out in restricted areas and with differences in the methods used [10,26,31]. Thus, few studies … by using the same method (i.e. the choice of wavelength interval for fitting as well as the fitting method).” However, in their manuscript in the methodology section (line 136) they don’t explain the methodology for LV, SF, and LF and just include citations of other works, while the methodology used in the other study areas is explained in this section. 

If the methodology used is not the same, authors should reconsider their objectives. If the methodology is the same, it should clearly state that is the same in all studies areas. At present, is not clear at all. I suggest that one objective could be comparing different methodologies for places with similar characteristics.

Our response: We have rewritten the methodology part of the manuscript as shown in sections 2.2, 2.3, and 2.4.

METHODOLOGY

This section is very confusing, mixes methodology and results. Specific comments are:

(1) Line 136 Methods for measuring CDOM absorption in (Table 1) are described in [29], [11], and [36], respectively. The methodology used in the rest of locations is explained. Authors should be consistent and explain and compare all methodologies taking into account their objectives.

Our response: We have rewritten the methodology part of the manuscript as shown in sections 2.2, 2.3, and 2.4.

 (2) Figure 2 shows the mean CDOM absorption spectrum for each study site obtained by averaging over all samples. Among the six study sites, the highest mean absorption coefficients for the wavelength range 280–500 nm were found in LV and the second highest values in BS. My first concern is that there is no statistical analysis of the results. We cannot know if differences are statistically significant or not. Authors must do a statistical analysis. In addition, I don’t agree with comparing only 8 surface samples from LV with much more numerous samples from other locations and sample at different depths. Depth is an important factor that should be taking into account when analyzing CDOM differences. One cannot attribute differences to inputs when comparing different depths.

Our response regarding only surface samples in LV (see Lines108-111 in the revised manuscript): “At the shallow sampling stations (7-9 m) in LV the water column was homogenously mixed with respect to temperature, nutrients, CDOM, and Chl-a [40]. Therefore, surface samples are representative for all depths for these parameters.”

(3) Table 1, present Chl-a data and Kd(PAR)data. How were they calculated? There is no methodology described for these parameters. In addition, these are results and Table 1 appears in methodology section.

Our response: We have added the methodology for determination of Chl-a concentration in Lines 197-202 in the revised manuscript: ”In [44] a detailed description is given of how to determine the Chl-a concentration. A brief summary is as follows. “Water samples were filtered on the day of collection, and sample filters were stored in a freezer with dry ice before and during transportation. Next, all samples were stored in a freezer at -78oC or -80oC. Both High Pressure Liquid Chromatography (HPLC) and spectrophotometry were used to determine the Chl-a concentrations. For details, see [44].”

 

Our response: We have added the following text in Lines 204-207 in the revised manuscript: ”The kd(PAR) shown in Table 1 for LN, BS, and RCW were calculated using the relationship Kd=1.44/ZSD derived by Holmes [46], where ZSD is the Secchi disk depth and Kd is the vertical diffuse attenuation coefficient for the downward irradiance. For SF, LF and LV it was calculated from 400-700 nm PAR sensor measurements between surface and lower part of euphotic zone.”

 (4) Lines 162 to 164 “comparison of the CDOM spectral slopes derived by applying these two methods (…) (data not shown), revealed that the derived by applying nonlinear and linear fitting agree quite well, but with slightly higher values obtained from nonlinear fitting”. These type of analysis should be presented as result. If authors focus on methodology, discussing different methologies is a result of their research.

Our response: We present some preliminary results on CDOM absorption in the methodology section in order to justify our choice of nonlinear fitting instead of linear fitting for derivation of the spectral slope S.

 

RESULTS

(5) Some examples of expressions that don’t reveal statistical significance of the differences found

Line 184 “were quite similar”

Our response: By this we mean that the CDOM absorption curves for the fjord water from LF and SF overlap for all wavelengths, except for a small difference between 280 and 320 nm (Fig. 2). Thus, we claim these curves to be more similar than different.

Line 203 “They were much higher”

 Our response: For the spectral region 320-280 nm, the CDOM absorption for the LV and BS water bodies increases much more strongly than for the other water bodies. At 280 nm, it reaches 5 and 5.5 m-1 for LV and SF, respectively, and these values that are distinctly higher than the corresponding values of 3 and 3.5 m-1 for SF and LF, respectively (Fig. 2).  

Reviewer 4 Report

Manuscript Review 

Manuscript Title

‘CDOM absorption properties of natural waters along extreme altitudinal, latitudinal and trophic state gradients: The search for environmental signatures’

 

Authors

Nima et al.,

 

General comments

The authors present an interesting dataset of CDOM observations from a wide range of meridional and zonal waters. After a few reads in my opinion the results have potential value to the science community. However, the current version of the manuscript and presentation need a thorough revision to make the manuscript of great impact.

 

Major Issues

Point 1.

I assume it is a simple mistake that the authors state the names of other countries (Norway and China) and yet identify Lake Victoria by its continent Africa?

- is Uganda is in Africa and Lake Victoria is shared by Tanzania, Uganda and Kenya

 

Point 2.

-Introduction is very long and reads like a review manuscript yet this is a research article

 

Point 3.

-There are way too many sentences starting with ‘CDOM….’

-it becomes very hard to follow the manuscript flow

 

Point 4.

-The title is rather long can it be made shorter?

-is it really necessary to include ‘The search for environmental signatures’?

 

 

Point 5

-It is not clear why the authors use two different wavelengths and ranges to describe the slope values S280-500 and S350-500 and absorption coefficients

-stick to one reference slope and absorption coefficient e.g. 275nm or 350nm that make comparing locations in low to high concentration regions as well as those from other papers

 

-it is confusing that different A_cdom are being reported 320, 440, 355, 375 nm can the authors just report one wavelength and one slope because Equation 1 can then be used for back calculating A_cdom for any wavelength

 

-why not use slope ratio SCDOM (275 – 295) nm : SCDOM (350 – 400) nm as originally proposed (Helms et al., 2008)?

-from the current dataset was it not possible to study (a) humification index calculated at an excitation wavelength of 254 nm HIX 254 = ?? = ?(???? ??????? 435−480 ??)/?(???? ??????? 300−345 ??)

biological index (BIX) and the recent produced material index (REPIX).?

 

Point 6

-If the authors insist on using the two coefficients and slopes, it would be important to also show the detection limit of the instrument at each which I doubt would be easy given the time series is from a long time and different work groups

-why detection limit… because it would give you confidence in the lower limit values like the stated a_440nm = 0.063 1/m (line 6)

 

Point 7

-Of all the parameters measured it is strange that the authors did not measure salinity which is a vital variable collected along with CDOM

-salinity can also be used to distinguish water bodies so why was it not measured here?

 

Point 8

-why did the authors not look at the temporal variations in the dataset given it was collected over long time and it is not clear if same areas were sampled (Table 1)

 

 

Point 9

-in the methods section references are given to publications cited as 11 and 29 which are not open-access and citation 36 provides a further reference to Højerslev, N.K., Aas, E., 1998. Which a coneference proceeding and not easy to find

-why did the author not explain the steps taken to prepare the samples and analyze them in this manuscript

-it is become tedious for the reader to be searching for a paper and then also be referenced to another paper which is even hard to find or is not in open-acces

 

-can the authors make it clear that 303 spectra collected were processed in the same way and measured by the same instrument and the results were processed in a single approach?

 

In my opinion, these missing major aspects need to be clarified or justified!

 

Specific comments(Line by line)

Abstract

Line 1

–the opening sentence is too long please revise and make it consise

-what about ‘We present optical properties of Colored Dissolved Organic Matter (CDOM) sampled from lakes, fjordal and estuarine systems in China, Norway as well as Uganda.’

-the next sentence can then go into the names of the places if that is important to the authors

 

Line 6

-why provide the coefficient values at 440nm and 320nm instead of a single value e.g. at 320 nm?

 

 

Introduction

Line 19

-Should this not be ‘1. Introduction’?

 

Line 22

-suggest rephrasing ‘light over a broad range of visible and ultraviolet (UV) wavelengths’ to something like ‘sunlight over a broad wavelength range from the ultraviolet to visible spectrum’

-consider adding classic papers like (Bowers et al., 2004; Kalle, 1937; Warnock et al., 1999)

 

Line 30-31

-since authors mention for decades maybe include other major studies (Del Castillo and Coble, 2000; Kalle, 1937; Kowalczuk, 1999; Warnock et al., 1999)

 

Line 31

-rephrase starting from ‘Fortunately, in the context of..’ what exactly are you trying to explain?

 

Line 47

-consider the following publications (Zepp et al., 1998)

 

Line 48

-is this not a repeat of line 22 ‘CDOM absorbs light mainly in the UV and short-wavelength visible regions [1].’

 

Line 49

-please explain what is ‘short-wavelength visible regions’ ? what numerical values are these ranges

 

Line 57

-CDOM absorption alone does not ‘diminish’ primary production but it play a role because primary production is driven by other factors such as nutrients and presence of grazers

-suggest the authors revise the sentence

 

Line 71

-provide literature references

 

Line 73-88

-what is the message in this paragraph?

 

Line 73

-‘ as a simple means’ suggest change to ‘indicator’

-see Helms paper SR slope ratio is used as a source indicator. ‘The SR term, because of its independence from DOC concentration, may therefore be useful for differentiating open ocean waters from those of nearshore coastal or estuarine origin.’

 

Line 96-98

-here it is not clear what the authors are trying to explain

-this is a well know problem across marine science e.g. chl-a can be determined by the same instrument in different labs but issues can arise from instrument calibration, sensor drift, sample handling even if researchers do the best possible to get good results

-same as getting remote sensing reflectance different teams use different correction approaches and yet for decades comparisons of dataset have been made

-revise and be consice

 

Line 112

-add ‘303 CDOM samples’

 

Line 113

-why not be consistent and use the water types (Table 1) column 5

 

 

Line 136

‘Methods for measuring CDOM absorption in LV, SF, and LF (Table 1) are described in [29], [11],

and [36], respectively.’

-why not provide a brief description of the steps because a reference to a paper not in open-access is made and these methods cannot be checked.

-citation 11, citation 29 is not open-access

 

-‘ The bottles were first rinsed three’ replace with ‘The bottles were pre-rinsed three’

 

-duplicate sample… where are the statistics for uncertainty analysis? Can this be presented

 

-how did you store the samples after collection because some work done by Stedmon indicated microbial activity might affect samples

-was the storage time the same for all the samples?

 

Table 1

-the salinity and temperature is missing would have been useful to classify the water bodies

-why sampling ‘‘0’, ‘hs’, and ‘s’ in the third column refer to measurements at the surface, at half the Secchi

depth, and at the full Secchi depth, respectively.’ Of this format what was the question to be answered here?

 

Line 142 -151

-this is like a repetition of text above Table 1?

-what exactly was performed by the authors need to be made clear not a review of what can be done to prepare the data

 

Line 153-174

-this reads like a review can the authors simply explain what method was used to process the 303 CDOM samples

-provide statistics for error budget

-provide information how Slope ratio was determined and be uniform with literature Helms paper provides wavelength ranges to use

-was data smoothed?

 

Line 199

-The bump observed could this be an effect of not smoothing the data considering it was sampled at 2 nm intervals

-see the bumps also in Figure 2a SF and LF (spectra 3 and 4) around 300 nm Figure 2 b LF(spectra 4 from bottom) around 420 and 460

 

Figure 2

-maybe use different lines with different symbols or styles and change range in figure 2b

-what is the message in the log plot?

-where is the ‘N’ mentioned in the caption?

 

Line 212

-‘Chinese eutrophic lakes,’ based on what parameters? Or is this speculation or can the authors prove this with the current dataset?

 

Line 222

-‘ fall into the category oligotrophic’ according to which classification provide literature reference?

 

Figure 3

-not sure what this explains

 

Line 265

-well the study by Babin was done outside your study areas and sources of CDOM are completely difference this is one explanation

-fjordal and lake waters will have different sources compared to estuarine systems especially looking at Babin study was in the Mediterranean and Baltic sea North sea definitely now comparable

-time of the year samples were collected and when each dataset for each region was gathered should be a factor e.g. 57 samples in the Baltic between 22-25 sept..

 

Figure 4

-just too much information what is the message?

-why not just use one reference wavelength and slope range

 

Suggested References (optional)

Bowers, D. G., Evans, D., Thomas, D. N., Ellis, K., and Williams, P. J. l. B., 2004, Interpreting the colour of an estuary: Estuarine, Coastal and Shelf Science, v. 59, no. 1, p. 13-20.

Del Castillo, C. E., and Coble, P. G., 2000, Seasonal variability of the colored dissolved organic matter during the 1994–95 NE and SW Monsoons in the Arabian Sea: Deep Sea Research Part II: Topical Studies in Oceanography, v. 47, no. 7–8, p. 1563-1579.

Helms, J. R., Aron, S., Jason, D. R., Minor, E. C., Kieber, D. J., and Mopper, K., 2008, Absorption spectral slopes and slope ratios as indicators of molecular weight, source, and photobleaching of chromophoric dissolved organic matter: Limnology and Oceanography, v. 53, no. 3, p. 955-969.

Kalle, K., 1937, Meereskundliche chemische Untersuchungen mit Hilfe des Zeißchen Pulfrich-Photometers, VI. Mitt., Die Bestimmung des Nitrats und des Gelbstoffes (in German): Annalen der Hydrographie und maritimen Meteorologie, v. 65, p. 276-282.

Kowalczuk, P., 1999, Seasonal variability of yellow substance absorption in the surface layer of the Baltic Sea: Journal of Geophysical Research, v. 104, no. C12, p. 30047-30058.

Warnock, R. E., Gieskes, W. W. C., and van Laar, S., 1999, Regional and seasonal differences in light absorption by yellow substance in the Southern Bight of the North Sea: Journal of Sea Research, v. 42, no. 3, p. 169-178.

Zepp, R. G., Callaghan, T. V., and Erickson, D. J., 1998, Effects of enhanced solar ultraviolet radiation on biogeochemical cycles: Journal of Photochemistry and Photobiology B: Biology, v. 46, no. 1-3, p. 69-82.

 

Author Response

Major Issues

Point 1.

I assume it is a simple mistake that the authors state the names of other countries (Norway and China) and yet identify Lake Victoria by its continent Africa?

- is Uganda is in Africa and Lake Victoria is shared by Tanzania, Uganda and Kenya

Our response: We agree and have rewritten this as follows in Lines 1-5 in the revised manuscript “We present absorption properties of Colored Dissolved Organic Matter (CDOM) sampled in six different water bodies along extreme altitudinal, latitudinal, and trophic state gradients. Three sites are in Norway: the mesotrophic Lysefjord (LF), Samnangerfjord (SF), and Røst Coastal Water (RCW); two sites are in China: the oligotrophic Lake Namtso (LN) and the eutrophic Bohai Sea (BS); and one site is in Uganda: the eutrophic Lake Victoria (LV).”

Point 2.

-Introduction is very long and reads like a review manuscript yet this is a research article 

Our response: Yes, we agree and have changed and moved “Fortunately, in the context of……are preferred [5]” to the method section in Lines 150-163 to shorten the introduction.

 Point 3.

-There are way too many sentences starting with ‘CDOM….’

-it becomes very hard to follow the manuscript flow

Our response: By moving the text discussing equation (1) from the introduction to section 2.2, we have improved the logical flow of the manuscript.

Point 4.

-The title is rather long can it be made shorter?

Our response: We agree and have shortened the title to “CDOM absorption properties of natural water bodies along extreme environmental gradients”.

-is it really necessary to include ‘The search for environmental signatures’?

Our response: We agree and have shortened the title to “CDOM absorption properties of natural water bodies along extreme environmental gradients”.

 Point 5

-It is not clear why the authors use two different wavelengths and ranges to describe the slope values S280-500 and S350-500 and absorption coefficients

-stick to one reference slope and absorption coefficient e.g. 275nm or 350nm that make comparing locations in low to high concentration regions as well as those from other papers

-it is confusing that different A_cdom are being reported 320, 440, 355, 375 nm can the authors just report one wavelength and one slope because Equation 1 can then be used for back calculating A_cdom for any wavelength 

Our response: We reported all these wavelengths to make comparison with literature data easier (absorption of CDOM at different wavelengths is reported in the literature) . Also, from the Figure 2(b),  it follows that the CDOM absorption spectra curves are not strictly straight or follow the exponential in Eq. (2) with a single spectral slope, especially the CDOM absorption spectra obtained for LN. Further, the use of  Eq.(2) with the wrong spectral slope to calculate the absorption coefficient at a certain wavelength can cause a large error, as shown in Figure 2(b) and Figure 4 (b) in our paper Nima et al. 2015. Therefore, two sets of CDOM spectral slopes, i.e. S280-500 and S350-500, are given in this paper.

Nima, C.; Hamre, B.; Frette, Ø.; Erga, S.R.; Chen, Y.C.; Zhao, L.; Sørensen, K.; Norli, M.; Lu, D.R.; Xing, Q.G.; Ma, Y.M.; Gelsor, N.; Stamnes, J.J. Impact of particulate and dissolved material on light absorption  properties in a High-Altitude Lake in Tibet, China. Hydrobiologia 2016, 768, 63–79.

 

-why not use slope ratio SCDOM (275 – 295) nm : SCDOM (350 – 400) nm as originally proposed (Helms et al., 2008)?

Our response:  SR in figure 5 is the ratio of S280-295 and S350-400 not S350-500. We made a mistake in the text. The correct formula is SR=S280-295/S350-400, corrections have been made, as shown in Lines 367-388 in the revised manuscript.   Since we do not have CDOM absorption data below 280nm,  we could not use the slope ratio SCDOM (275 – 295) nm : SCDOM (350 – 400).

-from the current dataset was it not possible to study (a) humification index calculated at an excitation wavelength of 254 nm HIX 254 = ?? = (???? ??????? 435−480 ??)/(???? ??????? 300−345 ??)

Our response: This is not possible since we do not have CDOM absorption data below 280 nm and did not measure CDOM fluorometrically.

biological index (BIX) and the recent produced material index (REPIX).?

Our response: This is not possible since we did not measure CDOM fluorometrically.

 

Point 6

-If the authors insist on using the two coefficients and slopes, it would be important to also show the detection limit of the instrument at each which I doubt would be easy given the time series is from a long time and different work groups

Our response: We have added the following text in Lines 126-127 in the revised manuscript: “And the detection limit for abs in the Shimadzu MPS 2000 spectrophotometer is ±0.003.” and in Lines 143-144 in the revised manuscript: “ The detection limit for abs in the Shimadzu spectrophotometer UV-1800 is ± 0.008.”

-why detection limit… because it would give you confidence in the lower limit values like the stated a_440nm = 0.063 1/m (line 6)

Our response: Same as above.

 

Point 7

-Of all the parameters measured it is strange that the authors did not measure salinity which is a vital variable collected along with CDOM

-salinity can also be used to distinguish water bodies so why was it not measured here?

Our response: We have added the following text in Lines 242-250 in the revised manuscript: “Among the marine study sites, salinity varied between 10-34 and 15-33, respectively, in the SF and LF for the depth interval 0-30 m [10, 41]. Monitoring of coastal water around Røst (Institute of Marine Research) at the times of our samplings show salinity values in the range 30.1-33.4 for the depth interval 1-30 m (http://www.creativecommons.no/), while typical salinities for surface and bottom water of BS are within the range 24-31.5[58]. The tendency of an inverse relationship between CDOM absorption and salinity can be seen at our near shore stations (SF, LF, and BS), which are strongly influenced by freshwater outlets, and this tendency is in accordance with Aksnes et al.[59], who concluded that Norwegian coastal water has “darkened” due to freshening during the period 1935-2007.”

Point 8

-why did the authors not look at the temporal variations in the dataset given it was collected over long time and it is not clear if same areas were sampled (Table 1)

Our response: We have added the following text in line 118-120 in the revised manuscript: “Due to few and variable sampling times at the various sites during the year, and the fact that LV, which is situated close to the Equator, has small seasonal variations, we decided not  to include seasonal variations in our analysis. “

Point 9

-in the methods section references are given to publications cited as 11 and 29 which are not open-access and citation 36 provides a further reference to Højerslev, N.K., Aas, E., 1998. Which a coneference proceeding and not easy to find

-why did the author not explain the steps taken to prepare the samples and analyze them in this manuscript

-it is become tedious for the reader to be searching for a paper and then also be referenced to another paper which is even hard to find or is not in open-access

-can the authors make it clear that 303 spectra collected were processed in the same way and measured by the same instrument and the results were processed in a single approach?

In my opinion, these missing major aspects need to be clarified or justified!

Our response: This has been taken into account and we have changed this part of manuscript to read as follows:

     Samples collected from SF and LF (Table 1) were analyzed as follows. Water samples were filtered through a 0.2 mm Nucleopore filter and stored in opaque glass bottles at 40oC for a few days before analysis in the laboratory. CDOM absorbance spectra were measured at wavelengths within the interval between 250 and 650 nm with 2 nm increments using a 10 cm quartz cuvette and a Shimadzu MPS 2000 spectrometer [11]. And the detection limit for abs in the Shimadzu MPS 2000 spectrophotometer is ±0.003.

     Water samples collected from LV (Table 1) were filtered through Nuclepore membrane filters with 0.2 mm pore size and the CDOM samples were stored in acid cleaned glass bottles in a refrigerator until analysis. The CDOM absorption was determined for wavelengths between 200 and 900 nm using a dual-beam Perkin Elmer Lambda 40P spectrophotometer at the Norwegian Institute for Water Research (NIVA). The measurements were performed relative to distilled water [34].

      Samples collected from LN, BS, and RCW (Table 1) were analyzed as follows. Water samples were filtered through Whatman Polycarbonate filters (diameter 47 mm, pore size 0.22 mm) at low vacuum pressure on the day they were collected, and the filtrate was collected in pre-washed bottles. The bottles were pre-rinsed three times with filtered sample water to minimize the possibility of contamination. At some locations and depths duplicate samples were prepared. The CDOM samples were stored in a dark refrigerator until analysis. Prior to analysis, the CDOM samples and Milli-Q water were acclimated to room temperature. The absorbance spectra were recorded at wavelengths in the range from 200 to 900 nm with 2 nm increments using a dual-beam Shimadzu spectrophotometer (UV-1800) equipped with a 10 cm quartz cuvette. The detection limit for abs in the Shimadzu spectrophotometer (UV-1800) is  ± 0.008. Baseline data were obtained by filling Milli-Q water both in the sample and reference cells, and baseline correction was obtained by subtracting the offset from each sample spectrum.

                                                                                        

 Specific comments(Line by line)

Abstract

Line 1

–the opening sentence is too long please revise and make it consise

-what about ‘We present optical properties of Colored Dissolved Organic Matter (CDOM) sampled from lakes, fjordal and estuarine systems in China, Norway as well as Uganda.’

Our response: This has been taken into account and we have changed the sentence to (see Lines 1-2 in the revised manuscript): “We present absorption properties of Colored Dissolved Organic Matter (CDOM) sampled in six different water bodies along extreme altitudinal, latitudinal, and trophic state gradients.”

-the next sentence can then go into the names of the places if that is important to the authors

Our response : This has been taken into account and we have changed the sentence to (see Lines 2-7 in the revised manuscript): “Three sites are in Norway: the mesotrophic Lysefjord (LF), Samnangerfjord (SF), and Røst Coastal Water (RCW); two sites are in China: the oligotrophic Lake Namtso (LN) and the eutrophic Bohai Sea (BS); and one site is in Uganda: the eutrophic Lake Victoria (LV). The site locations ranged from equatorial to subarctic regions, and they included water types from oligotrophic to eutrophic and altitudes from 0 m to 4,700 m.”

Line 6

-why provide the coefficient values at 440nm and 320nm instead of a single value e.g. at 320 nm?

Our response: As stated in Lines 38-41 in submitted manuscript: ”The absorption coefficient of CDOM at 440 nm is commonly used to characterize the CDOM concentration (see Table 3.2 in Kirk [1]), since this wavelength corresponds approximately to the mid-point of the blue waveband peak that most classes of algae have in their photosynthetic action spectrum [1].”

 

Introduction

Line 19

-Should this not be ‘1. Introduction’? 

Our response: Yes, It have been changed.

Line 22

-suggest rephrasing ‘light over a broad range of visible and ultraviolet (UV) wavelengths’ to something like ‘sunlight over a broad wavelength range from the ultraviolet to visible spectrum’

-consider adding classic papers like (Bowers et al., 2004; Kalle, 1937; Warnock et al., 1999)

Our response: This has been taken into account and we have changed the sentence to (see Lines 22-23 in the revised manuscript): “It exists in all natural water bodies and absorbs light over a broad range of visible and ultraviolet (UV) wavelengths [1–4]”. Also, we have added the references to Bowers et al., 2004 and Warnock et al., 1999 , and Kalle 1937.

Line 30-31

-since authors mention for decades maybe include other major studies (Del Castillo and Coble, 2000; Kalle, 1937; Kowalczuk, 1999; Warnock et al., 1999)

Our response: This has been taken into account and we added these references after the sentence (see Lines 28-29 in the revised manuscript) ‘Absorption properties of CDOM have been reported in numerous studies of marine water bodies [3,4,7–13]. ‘

Line 31

-rephrase starting from ‘Fortunately, in the context of..’ what exactly are you trying to explain?

Our response: We have changed and moved “Fortunately, in the context of……are preferred [5]” from the introduction section to the method section in Lines 151-164 in the revised manuscript.

 

Line 47

-consider the following publications (Zepp et al., 1998)

Our response: In order to make the introduction section shorter, we removed this sentence, and therefore we did not add this reference.

Line 48

-is this not a repeat of line 22 ‘CDOM absorbs light mainly in the UV and short-wavelength visible regions [1].’

Our response: We agree and have removed the sentence ‘CDOM absorbs light mainly in the UV and short-wavelength visible regions [1].’

 Line 49

-please explain what is ‘short-wavelength visible regions’ ? what numerical values are these ranges

Our response: Same as above.

 

 Line 57

-CDOM absorption alone does not ‘diminish’ primary production but it play a role because primary production is driven by other factors such as nutrients and presence of grazers

-suggest the authors revise the sentence  

Our response: We rewritten the sentence to read as (see Lines 43-46 in the revised manuscript): “Simultaneously, CDOM absorption in the visible spectral region reduces the availability of Photosynthetically Active Radiation (PAR) for photosynthesis, and hence can contribute to reduced primary production [22] provided there are no other limiting factors, such as nutrients and grazing.”

 Line 71

-provide literature references 

Our response: We have added the reference  “Mannino et al. (2014)” in Line 52 in the revised manuscript.

 Line 73-88

-what is the message in this paragraph?

Our response: The paragraph “The spectral slope S of CDOM can be used as a simple means to characterize CDOM in natural water bodies and to reveal their origin [18,20,21]. …….Song et al. [28] discovered that the humic-like materials in CDOM tended to decrease with altitude in lakes.”  is included in order to address the importance of analyzing the spectral slope S of CDOM in this paper.

 Line 73

-‘ as a simple means’ suggest change to ‘indicator’

Our response: We changed the sentence to read (see Lines 54-55 in the revised manuscript):  ‘The absorption spectral slope S of CDOM can be used as an indicator to characterize CDOM in natural water bodies and to reveal their origin [24-26].’

-see Helms paper SR slope ratio is used as a source indicator. ‘The SR term, because of its independence from DOC concentration, may therefore be useful for differentiating open ocean waters from those of nearshore coastal or estuarine origin.’

 

 Line 96-98

-here it is not clear what the authors are trying to explain

-this is a well know problem across marine science e.g. chl-a can be determined by the same instrument in different labs but issues can arise from instrument calibration, sensor drift, sample handling even if researchers do the best possible to get good results

-same as getting remote sensing reflectance different teams use different correction approaches and yet for decades comparisons of dataset have been made

-revise and be consice

Our response: We have replaced the sentence with (see in Lines 77-78 in the revised manuscript): “Therefore, to obtain comparable and reliable data from large geographical areas the same method should be used.”

 Line 112

-add ‘303 CDOM samples’

Our response: The sentence has been changed to  (see in Lines 92-94 in the revised manuscript):”In this study, we analyzed and compared absorption properties of 303 CDOM samples from 6 different water bodies, including 2 lakes, 2 fjords, and 2 coastal water bodies, having wide differences in optical properties, environmental conditions, and trophic states.”

 

 Line 113

-why not be consistent and use the water types (Table 1) column 5 

Our response: Same as above.

 Line 136

‘Methods for measuring CDOM absorption in LV, SF, and LF (Table 1) are described in [29], [11],

and [36], respectively.’

-why not provide a brief description of the steps because a reference to a paper not in open-access is made and these methods cannot be checked.

-citation 11, citation 29 is not open-access

Our response: We have changed this part of manuscript as follows:

Our response: This has been taken into account and we have changed this part of manuscript to read as follows:

     Samples collected from SF and LF (Table 1) were analyzed as follows. Water samples were filtered through a 0.2 mm Nucleopore filter and stored in opaque glass bottles at 40oC for a few days before analysis in the laboratory. CDOM absorbance spectra were measured at wavelengths within the interval between 250 and 650 nm with 2 nm increments using a 10 cm quartz cuvette and a Shimadzu MPS 2000 spectrometer [11]. And the detection limit for abs in the Shimadzu MPS 2000 spectrophotometer is ±0.003.

     Water samples collected from LV (Table 1) were filtered through Nuclepore membrane filters with 0.2 mm pore size and the CDOM samples were stored in acid cleaned glass bottles in a refrigerator until analysis. The CDOM absorption was determined for wavelengths between 200 and 900 nm using a dual-beam Perkin Elmer Lambda 40P spectrophotometer at the Norwegian Institute for Water Research (NIVA). The measurements were performed relative to distilled water [34].

      Samples collected from LN, BS, and RCW (Table 1) were analyzed as follows. Water samples were filtered through Whatman Polycarbonate filters (diameter 47 mm, pore size 0.22 mm) at low vacuum pressure on the day they were collected, and the filtrate was collected in pre-washed bottles. The bottles were pre-rinsed three times with filtered sample water to minimize the possibility of contamination. At some locations and depths duplicate samples were prepared. The CDOM samples were stored in a dark refrigerator until analysis. Prior to analysis, the CDOM samples and Milli-Q water were acclimated to room temperature. The absorbance spectra were recorded at wavelengths in the range from 200 to 900 nm with 2 nm increments using a dual-beam Shimadzu spectrophotometer (UV-1800) equipped with a 10 cm quartz cuvette. The detection limit for abs in the Shimadzu spectrophotometer (UV-1800) is  ± 0.008. Baseline data were obtained by filling Milli-Q water both in the sample and reference cells, and baseline correction was obtained by subtracting the offset from each sample spectrum.

 

- The bottles were first rinsed three’ replace with ‘The bottles were pre-rinsed three’

Our response: The sentence have been changed to (see in Lines 138-139 in the revised manuscript) ‘The bottles were pre-rinsed three times with filtered sample water to minimize the possibility of contamination.’

 -duplicate sample… where are the statistics for uncertainty analysis? Can this be presented

Our response regarding Duplicate samples:

 In total, 65 duplicate pairs of CDOM samples were taken from  RCW, BS, and LN. The spectral variation of the ratio of the absolute difference between duplicate samples [abs(aCDOM2-aCDOM1)]  and the mean value [aCDOMmean] of each pair of duplicate samples in the wavelength range of 280–500 nm are shown in Figure below together with the spectral average of the ratio over all samples.  In most cases, the values of the ratio [abs(aCDOM2-aCDOM1)/aCDOMmean] tends to increase with wavelength, which may be attributed to the low CDOM absorption at longer wavelengths, as seen in Fig. 2 for  wavelengths between 280 and 500 nm. The average of [abs(aCDOM2- aCDOM1)/aCDOMmean] over all samples varies between 0.09 at 280 nm and 0.21 at 500 nm.

figure of duplicate samples is with the supplemental materials.

 -how did you store the samples after collection because some work done by Stedmon indicated microbial activity might affect samples was the storage time the same for all the samples?

Our response:  As stated in Lines 139-140 in the revised manuscript: “ The CDOM samples were stored in a dark refrigerator until analysis.”  Also, 0.22 µm filtration removes all bacteria.

 Table 1

-the salinity and temperature is missing would have been useful to classify the water bodies

Our response: Please see above response on salinity for point 7.  Unfortunately, we did not measure the temperature during these cruises.

-why sampling ‘‘0’, ‘hs’, and ‘s’ in the third column refer to measurements at the surface, at half the Secchi

depth, and at the full Secchi depth, respectively.’ Of this format what was the question to be answered here?

Our response: To save space in Table 1, we used ‘0’, ‘hs’, and ‘s’ to indicate measurements at the surface, at half the Secchi depth, and at the full Secchi depth, respectively. ‘‘0’, ‘hs’, and ‘s’ in Table 1 are only used to indicate depths at which samples were taken.

 Line 142 -151

-this is like a repetition of text above Table 1?

Our response: This is not a repetition of text above Table 1. The text above Table 1 explains how to measure the CDOM absorption spectrum, whereas the text in Lines 142-151 (in the submitted manuscript) explains how to carry out the scattering correction for the CDOM absorption spectrum.

-what exactly was performed by the authors need to be made clear not a review of what can be done to prepare the data  

Our response: We have deleted the first three sentences to avoid repetition and make clear what we have done (see Lines 166-171 in the revised manuscript): “To correct for scattering due to residual particles (< 0.22µm) in CDOM samples, we used a method similar to that of Babin et al. [10] and Matsuoka et al. [31], according to which the measured CDOM absorption coefficient is corrected for scattering due to residual particles at all spectral values by subtracting the measured absorption coefficient averaged over a 5-nm interval around 685 nm. For the samples collected from SF, the absorption spectra were measured between 250 and 650nm [11], hence no scattering correction was performed.”

 

 Line 153-174

-this reads like a review can the authors simply explain what method was used to process the 303 CDOM samples 

Our response: This is not a review. It is a description of our testing of two different fitting methods, linear and nonlinear, in order to determine which will be the best to use. All samples were analyzed by both methods before a decision was made.

 -provide statistics for error budget

Our response: We are so sorry that we did not manage to understand the meaning of this comment, could you please give us some more specific information regarding this comment, and we will carry out analysis according to your comment.

-provide information how Slope ratio was determined and be uniform with literature Helms paper provides wavelength ranges to use

Our Response:  We have added the following text in Lines 195-196 in the revised manuscript: “To obtain the slope ratio SR, we first used nonlinear fitting to derive S280-295 and S350-400, and then we calculated SR as SR=S280-295/S350-400. “

-was data smoothed?

Our response: No, data were not smoothed.

 Line 199

-The bump observed could this be an effect of not smoothing the data considering it was sampled at 2 nm intervals

Our response: No, this bump is too big for being an effect of not smoothing.

-see the bumps also in Figure 2a SF and LF (spectra 3 and 4) around 300 nm Figure 2 b LF(spectra 4 from bottom) around 420 and 460 

Our response: These are minor bumps compared with the one at 340 nm for LN.

 Figure 2

-maybe use different lines with different symbols or styles and change range in figure 2b

Our response: We have changed line styles as much as possible in Figure 2. We did not use different symbols because then the curves would overlap even more, and it would be difficult to see the difference between them.  The range in Figure 2b has also been changed.

-what is the message in the log plot?        

Our response:  Plotting the Figure 2(b) is to show that CDOM absorption spectra curves are not strictly straight lines or follow the exponential in Eq. (2) with a single spectral slope. This is obvious for the CDOM absorption spectra for LN. Also, the use of Eq.(2) with the wrong spectral slope to calculate the absorption coefficient at a certain wavelength can cause a large error, as shown in Figure 2(b) and Figure 4 (b) in our paper Nima et al. 2015. Therefore, two sets of CDOM spectral slopes, i.e. S280-500 and S350-500, are given in this paper.

Nima, C.; Hamre, B.; Frette, Ø.; Erga, S.R.; Chen, Y.C.; Zhao, L.; Sørensen, K.; Norli, M.; Lu, D.R.; Xing, Q.G.; Ma, Y.M.; Gelsor, N.; Stamnes, J.J. Impact of particulate and dissolved material on light absorption  properties in a High-Altitude Lake in Tibet, China. Hydrobiologia 2016, 768, 63–79.

-where is the ‘N’ mentioned in the caption?

Our response: This has been deleted.

 

Line 212

-‘Chinese eutrophic lakes,’ based on what parameters? Or is this speculation or can the authors prove this with the current dataset? 

Our response: We have inserted the following text in Lines 255-261 in the revise manuscript: “The trophic state of an aquatic system can be described in terms of concentrations of the dissolved nutrients nitrogen and phosphorus. When the concentrations of these nutrients are low (oligotrophic), they limit algal growth, and when they are high (eutrophic), they trigger algal growth, resulting in respectively low and high concentrations of total phosphorus (TP) and Chl-a, and large and small Secchi depths, respectively [62]. Based on these criteria and with reference to lake regions in China, typical aCDOM(355) mean values for oligotrophic, mesotrophic (medium level of nutrients) and eutrophic water bodies are 1.47, 2.75 and 4.21 m-1, respectively [15].”    

 Line 222

-‘ fall into the category oligotrophic’ according to which classification provide literature reference

Our response: We have replaced the text in Lines 266-269 in the revised manuscript as follows:  “In a trophic state perspective (see definitions above of oligotrophic, mesotrophic, and eutrophic), the use of CDOM absorption properties to classify water bodies does not seem to be appropriate in our case, since even LV will fall into the category oligotrophic  (aCDOM(355) =  1.27 m-1) and so will Norwegian fjords.”

 Figure 3

-not sure what this explains

 Our response: This has been explained in Lines 272-288 in the revised manuscript.

 

Line 265

-well the study by Babin was done outside your study areas and sources of CDOM are completely difference this is one explanation

Our response: We have inserted the following text in Lines 315-318 in the revised manuscript : “This could be due to different CDOM sources, but it should be noted that in the study of Babin et al. [10], the mean value of S350−500 for the North Sea sites, which is closest to our study sites in Norway, was lower (0.0167 nm-1) than the other mean values in their study.”

-fjordal and lake waters will have different sources compared to estuarine systems especially looking at Babin study was in the Mediterranean and Baltic sea North sea definitely now comparable

Our response: Same as above.

-time of the year samples were collected and when each dataset for each region was gathered should be a factor e.g. 57 samples in the Baltic between 22-25 sept..

Our response: We have inserted the following text in Lines 318-320 in the revised manuscript: ”The North Sea values were obtained from 96 samplings conducted during the phytoplankton growth season in April, May, and September, and they compare well with our number of samplings and sampling period.”

 

 Figure 4

-just too much information what is the message?

Our response: Both reference wavelengths and spectral slope ranges are included to enable one to compare with literature data, and to discuss how the linear relationships vary among sites for different reference wavelengths and spectral slope ranges.

 

-why not just use one reference wavelength and slope range 

Our response: Same as above.

Round 2

Reviewer 3 Report

The authors have improved the manuscript according to the reviewers suggestions

Author Response

Our response: thanks. We made some small change to improve English language in our manuscript. 

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