Variability of light absorption coefficients by different size fractions of suspensions in the southern Baltic Sea

Meler, Justyna; Litwicka, Dagmara; Zabłocka, Monika

doi:https://doi.org/10.5194/bg-2022-233

Preprints

https://doi.org/10.5194/bg-2022-233

Preprints

12 Dec 2022

| 12 Dec 2022

Status: a revised version of this preprint was accepted for the journal BG and is expected to appear here in due course.

Variability of light absorption coefficients by different size fractions of suspensions in the southern Baltic Sea

Justyna Meler, Dagmara Litwicka, and Monika Zabłocka

Abstract. Measurements of light absorption coefficients by particles suspended in seawater (a_p (λ)), by phytoplankton (a_ph(λ)) and detritus (a_d (λ)) were carried out in the Baltic Sea waters. Measurements were performed for the original (unfiltered) seawater samples and the four selected size fractions: pico-particles with diameters (0.2–2 μm), ultra-particles with diameters (2–5 μm), nano-particles with diameters (5–20 μm) and micro-particles (20–200 μm). Chlorophyll a (Chla) concentrations and total suspended particulate matter (SPM) concentrations were determined. The proportions of particles from the size classes (micro, nano, ultra and pico) in the total light absorption by particles, phytoplankton and detritus were determined. Particles with sizes < 5 μm (i.e. pico and ultra-particles) had the largest contribution to the total particles absorption – an average of 38 % and 31 %. Particles of 5–20 μm accounted for approximately 20 % of all particles and phytoplankton and 29 % of the detritus. The contribution of large particles > 20 µm averaged 5–10 %.

The average chlorophyll-specific and mass-specific light absorption coefficients, i.e. light absorption coefficients normalized to Chla and to SPM concentration, were determined for all size fractions. The determined average chlorophyll-specific light absorption coefficients a_p^(Chla)(λ), a_d^(Chla)(λ) and a_ph^(Chla)(λ), along with standard deviations, do not allow clear separation of the individual fractions. For mass-specific light absorption coefficients, a_p^(SPM)(λ), a_d^(SPM)(λ) and a_ph^(SPM)(λ), it is possible to distinguish between large particle fractions (microplankton – 20–200 μm) and small and medium particle fractions (0.2–20 μm).

Received: 02 Dec 2022 – Discussion started: 12 Dec 2022

Justyna Meler et al.

Status: closed

RC1:
'Comment on bg-2022-233', Anonymous Referee #1, 02 Feb 2023

Meler et al. present a descriptive study documenting measurements of particulate, phytoplankton, and detrital absorption coefficients obtained from size-fractionated water samples collected from nearshore and offshore waters of the Baltic Sea. The water samples comprise a significant range in biogeochemical properties based on SPM and Chla observations, although the sampling locations were confined to a southern sub-region of the Baltic sea. The dataset contains 38 elements. The results do not indicate significant differences between size fractionated samples in terms of absorption properties. Further, the results indicate differences in mass-specific (but not Chla-specific) light absorption coefficients between larger (micro) versus smaller (nano and pico) organic and inorganic particles.
Although the general topic could potentially be compliant with the journal's scope, the manuscript does not satisfy the journal's citeria to merit publication, as follows:

Scientific Significance: The study is not sufficiently comprehensive based on insufficient number of observations (n=38) and small spatial extent of the sampling area, compared with the variety of oceanic conditions, physical forcings, biological conditions, and the terrestrial, riverine, and anthropogenic inputs to the Baltic Sea. The key-finding (that Chla-specific absorption properties of size-fractionated samples are not significantly different from eachother within the authors' n=38 dataset from southern Baltic Sea waters near Poland) would be more compelling if the study was more expansive, or if the authors could better extablish the significance of their null results. The study may also not be generalizable outside of the Baltic Sea, and becuase the observations span a small geographical sub-region of the Baltic sea (the southern waters around Poland) the results may also not be representative of the optical properties elsewhere in the Baltic Sea; and

Presentation Quality: Comprehension of the manuscript is inhibited by low presentaiton quality. In particular, the authors' combination of the Results and Discussion materials into a single section significantly detracts from the presentaiton of each, and at times made comprehension of the manuscript difficult, or resulted in ambiguity in elements of the methods or results. I suggest that the authors separate the results and discussion in order to add clarify.
Additional (General) Comments:
The authors do not adequately demonstrate the other dimensions of variability in their dataset, e.g., due to seasonal factors, site-specific differences like onshore vs offshore, biomass, or total particle content. One way that the authors could have helped with this would be to color the markers in the scatter plots to indicate other parameters, e.g., by seasons or by whether the site was nearshore or offshore.
Comparing the overlap in mean +/- std between data points is most useful when uncertainties due to environmental or methodological variability are well described (uncertain measurements of moderately disimilar parameters can easily overlap). The authors do not convey uncertainty in their absorption, Chla, or SPM measurements, which would help to identify the extent to which overlap in absorption properties is or is not meaningful.
The authors did not identify differences in Chla-specific optical properties between size fractionated samples. I'd suggest that the authors investigate or discuss what other factors (e.g., distance from shore, biomass, wind-driven mixing, contribution of inorganic particles) may have been associated with the variability in observed Chla-specific absorption properties within size fractions.
Minor (Specific) Comments:
Table 1: Is the section "Nano+ultra particles (2-20um)" intended to be Pico + nano particles (based on the sampling difficulty of the first 14 samples; L200-202)?
Lines 303-312 and figures 5-7: I'd suggest that log scale R2 values are reported as well. These datasets are mostly log-normally distributed in both axes, and R2 calculated on the linear axes is strongly influenced by the points in the upper-right corner of the plot. For exmaple, consider the high R2 despite low association of points in Fig 6 panel G.

Citation: https://doi.org/10.5194/bg-2022-233-RC1
- AC1: 'Reply on RC1', Justyna Meler, 07 Mar 2023
  
  We would like to thank the Reviewer for extremely valuable, critical comments on our work. Taking these comments into account leads to a significant improvement of the manuscript over the original version. Please find detailed responce to each comment in the attached file.
  
  Citation: https://doi.org/10.5194/bg-2022-233-AC1
RC2:
'Comment on bg-2022-233', Anonymous Referee #2, 08 Feb 2023
The manuscript by Meler et al. investigated the size-fractionated absorption spectra of particles, phytoplankton, and non-algae particles (NAP) in the southern Baltic Sea. They also conducted the measurement of total and size-fractionated suspended particulate matter (SPM) and Chlorophyll (Chl) a concentrations and then examined the relationships between the absorption coefficients, SMP, and Chl a concentrations for each size fraction. They found that the SPM-specific absorption coefficients are a useful parameter to distinguish between large and small plus medium particle fractions. The data presented in this study is informative. However, this manuscript requires considerable alteration along the lines I have suggested below.

[Major comments]
The description of total and size-fractionated Chl a-specific NAP absorption needs more detail. It is possible to understand the meaning for calculating the absorption coefficients of particles (a_p) and phytoplankton (a_ph) normalized by Chl a and SPM concentrations to see the contribution of each size component to the spectral shape and magnitude. However, I am not sure the significance of the Chl a-specific NAP absorption spectra and coefficient at 443 nm as shown in Figures 6a – e, and 9c, 10c.

I agree with the author’s assertion that the data obtained by this study could improve the model to retrieve the inherent optical properties (IOPs) in the Baltic Sea (Lines 464 – 465). However, it is not clear that which of the results or relationships examined in this study would contribute to the improvement of the IOPs models and how to expand the results into the models for estimating the size parameters. Given that many cases have already been reported in the literature (as cited by the authors themselves in the Conclusion section), it would be advisable to explain specific information on the improvement of IOP models.

The large part of the sentences in the Introduction reviews the previous literatures. Therefore, it seems to me that it is hard from reading the Introduction to understand why this study is needed. To better organized the introduction and objectives, I would encourage the authors to rewrite the section. Similarly, abstract and most parts of results and discussion sections, especially 3.2, 3.3, and 3.4, are not well organized. It is descriptive and is like a data report, making difficult to follow what is the new findings described in this study. However, I believe that the authors can elaborate.

Although average Chl a-specific absorption coefficients of phytoplankton generally decrease with increasing cell size because of self-shading, the authors showed the opposite trends as compared with previous work of Ciotti et al. (2002). Therefore, I feel that the package effect (as mentioned by the authors themselves in Line 410) may be open to further discussion.

[Minor comments]
Names of observed stations are missing in Figures 1, which make it difficult to refer to Figures 2 and 8 and SF04 and SF13 in Lines 417 – 427. The information will help readers understand the results more easily.

Figure 2a showed the results of size-fractionated “SPM” in each sampling station. A more appropriate legend would be required for Figure 2a to better reflect the investigation of SPM.

I would suggest that the results of Figure 4, 9, or 10 be presented in a different way; for example, a box plot at satellite ocean colour bands with average spectra could be used. I think that this make it easier for the readers to understand the importance of them. For example, please refer to Brunelle et al. (2012, doi: 10.1029/2011JC007345).
Citation: https://doi.org/10.5194/bg-2022-233-RC2
- AC2: 'Reply on RC2', Justyna Meler, 07 Mar 2023
  
  We would like to thank the Reviewer for extremely valuable, critical comments on our work. Taking these comments into account leads to a significant improvement of the manuscript over the original version. Please find detailed responce to each comment in the attached file.
  
  Citation: https://doi.org/10.5194/bg-2022-233-AC2

Status: closed

RC1:
'Comment on bg-2022-233', Anonymous Referee #1, 02 Feb 2023

Meler et al. present a descriptive study documenting measurements of particulate, phytoplankton, and detrital absorption coefficients obtained from size-fractionated water samples collected from nearshore and offshore waters of the Baltic Sea. The water samples comprise a significant range in biogeochemical properties based on SPM and Chla observations, although the sampling locations were confined to a southern sub-region of the Baltic sea. The dataset contains 38 elements. The results do not indicate significant differences between size fractionated samples in terms of absorption properties. Further, the results indicate differences in mass-specific (but not Chla-specific) light absorption coefficients between larger (micro) versus smaller (nano and pico) organic and inorganic particles.
Although the general topic could potentially be compliant with the journal's scope, the manuscript does not satisfy the journal's citeria to merit publication, as follows:

Scientific Significance: The study is not sufficiently comprehensive based on insufficient number of observations (n=38) and small spatial extent of the sampling area, compared with the variety of oceanic conditions, physical forcings, biological conditions, and the terrestrial, riverine, and anthropogenic inputs to the Baltic Sea. The key-finding (that Chla-specific absorption properties of size-fractionated samples are not significantly different from eachother within the authors' n=38 dataset from southern Baltic Sea waters near Poland) would be more compelling if the study was more expansive, or if the authors could better extablish the significance of their null results. The study may also not be generalizable outside of the Baltic Sea, and becuase the observations span a small geographical sub-region of the Baltic sea (the southern waters around Poland) the results may also not be representative of the optical properties elsewhere in the Baltic Sea; and

Presentation Quality: Comprehension of the manuscript is inhibited by low presentaiton quality. In particular, the authors' combination of the Results and Discussion materials into a single section significantly detracts from the presentaiton of each, and at times made comprehension of the manuscript difficult, or resulted in ambiguity in elements of the methods or results. I suggest that the authors separate the results and discussion in order to add clarify.
Additional (General) Comments:
The authors do not adequately demonstrate the other dimensions of variability in their dataset, e.g., due to seasonal factors, site-specific differences like onshore vs offshore, biomass, or total particle content. One way that the authors could have helped with this would be to color the markers in the scatter plots to indicate other parameters, e.g., by seasons or by whether the site was nearshore or offshore.
Comparing the overlap in mean +/- std between data points is most useful when uncertainties due to environmental or methodological variability are well described (uncertain measurements of moderately disimilar parameters can easily overlap). The authors do not convey uncertainty in their absorption, Chla, or SPM measurements, which would help to identify the extent to which overlap in absorption properties is or is not meaningful.
The authors did not identify differences in Chla-specific optical properties between size fractionated samples. I'd suggest that the authors investigate or discuss what other factors (e.g., distance from shore, biomass, wind-driven mixing, contribution of inorganic particles) may have been associated with the variability in observed Chla-specific absorption properties within size fractions.
Minor (Specific) Comments:
Table 1: Is the section "Nano+ultra particles (2-20um)" intended to be Pico + nano particles (based on the sampling difficulty of the first 14 samples; L200-202)?
Lines 303-312 and figures 5-7: I'd suggest that log scale R2 values are reported as well. These datasets are mostly log-normally distributed in both axes, and R2 calculated on the linear axes is strongly influenced by the points in the upper-right corner of the plot. For exmaple, consider the high R2 despite low association of points in Fig 6 panel G.

Citation: https://doi.org/10.5194/bg-2022-233-RC1
- AC1: 'Reply on RC1', Justyna Meler, 07 Mar 2023
  
  We would like to thank the Reviewer for extremely valuable, critical comments on our work. Taking these comments into account leads to a significant improvement of the manuscript over the original version. Please find detailed responce to each comment in the attached file.
  
  Citation: https://doi.org/10.5194/bg-2022-233-AC1
RC2:
'Comment on bg-2022-233', Anonymous Referee #2, 08 Feb 2023
The manuscript by Meler et al. investigated the size-fractionated absorption spectra of particles, phytoplankton, and non-algae particles (NAP) in the southern Baltic Sea. They also conducted the measurement of total and size-fractionated suspended particulate matter (SPM) and Chlorophyll (Chl) a concentrations and then examined the relationships between the absorption coefficients, SMP, and Chl a concentrations for each size fraction. They found that the SPM-specific absorption coefficients are a useful parameter to distinguish between large and small plus medium particle fractions. The data presented in this study is informative. However, this manuscript requires considerable alteration along the lines I have suggested below.

[Major comments]
The description of total and size-fractionated Chl a-specific NAP absorption needs more detail. It is possible to understand the meaning for calculating the absorption coefficients of particles (a_p) and phytoplankton (a_ph) normalized by Chl a and SPM concentrations to see the contribution of each size component to the spectral shape and magnitude. However, I am not sure the significance of the Chl a-specific NAP absorption spectra and coefficient at 443 nm as shown in Figures 6a – e, and 9c, 10c.

I agree with the author’s assertion that the data obtained by this study could improve the model to retrieve the inherent optical properties (IOPs) in the Baltic Sea (Lines 464 – 465). However, it is not clear that which of the results or relationships examined in this study would contribute to the improvement of the IOPs models and how to expand the results into the models for estimating the size parameters. Given that many cases have already been reported in the literature (as cited by the authors themselves in the Conclusion section), it would be advisable to explain specific information on the improvement of IOP models.

The large part of the sentences in the Introduction reviews the previous literatures. Therefore, it seems to me that it is hard from reading the Introduction to understand why this study is needed. To better organized the introduction and objectives, I would encourage the authors to rewrite the section. Similarly, abstract and most parts of results and discussion sections, especially 3.2, 3.3, and 3.4, are not well organized. It is descriptive and is like a data report, making difficult to follow what is the new findings described in this study. However, I believe that the authors can elaborate.

Although average Chl a-specific absorption coefficients of phytoplankton generally decrease with increasing cell size because of self-shading, the authors showed the opposite trends as compared with previous work of Ciotti et al. (2002). Therefore, I feel that the package effect (as mentioned by the authors themselves in Line 410) may be open to further discussion.

[Minor comments]
Names of observed stations are missing in Figures 1, which make it difficult to refer to Figures 2 and 8 and SF04 and SF13 in Lines 417 – 427. The information will help readers understand the results more easily.

Figure 2a showed the results of size-fractionated “SPM” in each sampling station. A more appropriate legend would be required for Figure 2a to better reflect the investigation of SPM.

I would suggest that the results of Figure 4, 9, or 10 be presented in a different way; for example, a box plot at satellite ocean colour bands with average spectra could be used. I think that this make it easier for the readers to understand the importance of them. For example, please refer to Brunelle et al. (2012, doi: 10.1029/2011JC007345).
Citation: https://doi.org/10.5194/bg-2022-233-RC2
- AC2: 'Reply on RC2', Justyna Meler, 07 Mar 2023
  
  We would like to thank the Reviewer for extremely valuable, critical comments on our work. Taking these comments into account leads to a significant improvement of the manuscript over the original version. Please find detailed responce to each comment in the attached file.
  
  Citation: https://doi.org/10.5194/bg-2022-233-AC2

Justyna Meler et al.

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Short summary

We present a variability of absorption properties by different size fractions of particles suspended in the Baltic Sea waters. The light absorption coefficient by all suspended particles (a_p) and, by detritus (a_d) and phytoplankton (a_ph) was determined for 4 size fractions: pico-particles, ultra-particles, nano-particles, and micro-particles. We have showed the proportions of particles from the size classes (micro, nano, ultra and pico) in the total a_p, a_d, and a_ph.


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