Optical properties of size fractions of suspended particulate matter in littoral waters of Québec

Mohammadpour, Gholamreza; Gagné, Jean-Pierre; Larouche, Pierre; Montes-Hugo, Martin A.

doi:https://doi.org/10.5194/bg-14-5297-2017

Articles | Volume 14, issue 23

https://doi.org/10.5194/bg-14-5297-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-14-5297-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 14, issue 23

Research article

|

29 Nov 2017

Research article |

| 29 Nov 2017

Optical properties of size fractions of suspended particulate matter in littoral waters of Québec

Gholamreza Mohammadpour, Jean-Pierre Gagné, Pierre Larouche, and Martin A. Montes-Hugo

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Final revised paper (published on 29 Nov 2017)
Preprint (discussion started on 01 Jun 2017)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of manuscript bg-2017-159', Anonymous Referee #1, 15 Jun 2017
- AC1: 'reply to reviewer #1', Martin Montes, 25 Jul 2017
RC2: 'Review of Optical Properties of size and chemical fractions of suspended particulate matter in littoral waters of Quebec', Anonymous Referee #2, 29 Jun 2017
- AC2: 'reply to reviewer #2', Martin Montes, 25 Jul 2017

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (03 Aug 2017) by Emmanuel Boss

AR by Martin Montes on behalf of the Authors (07 Aug 2017) Manuscript

ED: Referee Nomination & Report Request started (15 Aug 2017) by Emmanuel Boss

RR by Anonymous Referee #1 (23 Aug 2017)

Suggestions for revision or reasons for rejection

Main comments

The manuscript has been re-organized and significantly improved since the first version. Some valuable work analyses are presented and discussed, based on shipborne measurements made on water samples collected in complex estuarine environments. At least part of the results obtained are (potentially) interesting for a better understanding of relationships between the optical, physical and biogeochemical properties of particles in suspension in natural waters. The discussion at the end of the study is rather speculative, notably due t the lack of complementary measurements (organic carbon, phytoplankton pigments, iron) and/or theoretical computations.
I still have many detailed comments concerning:
1. the section ‘Data and methods’ in which (i) the different parameters must be better defined, with appropriate physical units, (ii) the processing of optical and biogeo-chemical measurements must be clarified (see detailed comments below)
2. the section ‘Results’ in which could be partly re-organized for better logics
3. the section ‘Discussion’ which too speculative and could be shortened.

Therefore, I recommend a major revision of the manuscript.

Detailed comments

Page 5 lines 20-25
The absorption-beam attenuation meter (ac-s, WetLabs) is already calibrated by Wetlabs in pure water, so that it measures light absorption and attenuation on top of pure water contributions. Therefore, you do not have to subtract pure water absorption and attenuation coefficients from your measurements. Please correct in the text.
Also why not applying the “proportional” method recommended by Wetlabs, that developed by:
J. R. V. Zaneveld, J. C. Kitchen, and C. C. Moore, “Scattering error correction of reflecting tube absorption meters,“ Proc. SPIE 2258, 44-55 (1994).

Page 6, line 16
You should introduce/define here the Junge exponent often used to describe the size distribution of marine particles
Why 35.17um?

Page 6, Line 27
‘the spectral slope of the particulate attenuation coefficient is negatively related with the mean particle size for particles smaller than 20 mm.’
What is the reference for that??

Page 7, lines 4-6
Untrue: if SPM includes a significant contribution of phytoplankton (chlorophyll-a) to light absorption, which kas absorption peaks at440 nm and 675 nm.
Your Equation (5) only applies in the case of non-algal particles.
Please correct this.

Equations (6-7): units?
Lines 10-15: totally unclear whereas you define the mass-specific absorption and scattering coefficients (in m2 /g) of the mass-specific absorption cross sections for mineral and organic fractions of SPM??? And what is the difference between these coefficients. Please clarify these definitions and always provide the appropriate physical units.
where m is the mass in g m-3 for each size class i….? g m-3 is a mass per unit of volume, which means a concentration of SPM…… not surprising I do not understand the definitions of your parameters!!

>> Please rewrite sections 2.5, 2.6 and 2.7 and clarify the definitions of the parameters you compute then use to describe how the SPM size and chemical composition influence the SPM optical properties. As it I found it very confusing.

Page 8, section 3.1
Please use either decimal numbers (e.g., 0.0111) or percentages (11%) to report the contributions f small/large and organic-rich/poor particles on the measured optical properties. This should make the text easier to read.

Line 10
‘The uncertainty of x calculations, as estimated from 2 standard errors, varied between 1.6 and 10.2%’
>> To be computed and reported in the section ‘Data and Methods‘

page 8
‘3.1 Spatial variability of microphysical properties of SPM’
What do you mean exactly by ‘microphysical’?

Overall I do not clearly understand the logics with the other two Results sections entitled: ‘3.2 Mass-specific optical properties of SPM’ and ‘3.4 Optical proxies’ (should be 3.3)
I would rather suggest to 1) report the mass-specific coefficients and corresponding spectral slopes, analyse their spatial variations and compare them to values already published in similar environments, 2) analyse the influence of SPM size on the mass-specific absorption and scattering coefficients, then 3) analyse the influence of SPM composition (PIM, POM) on the mass-specific absorption and scattering coefficients.

page 8, section 3.2
Are your mass-specific optical properties of SPM in good agreement with values reported in similar environments? Please discuss this first.
Why finally presenting results for the 400-650 nm spectral interval?

Line 23
Here I do not understand why you report values in 1/m. Based on the definition of this parameter (Equation 6, i.e. absorption coefficient (in 1/m) divided by a concentration of SPM (in g m-3) we would expect values in m2 g-1….??? Please check and clarify all this.

Line 25
‘Similar to ai*, highest bi* values (up to 5.7 m2 g-1 for l’
and here you report values in m2 g-1…..

Line 26
‘corresponded with size fractions having particles with the smallest and the largest diameter’
What do you mean?? Please rewrite

Lines 27-28
‘In general, the spectral slope of bi* was very variable in all size fractions (-6 10-5 to 6.28 10-3 nm-1) with the greatest spectral changes associated to particulates greater than 10 mm.’
??? Why suddenly talking about the spectral slope of the SPM scattering coefficient? This spectral slope was not even defined!

Page 9, line 22
‘Also for SPM fraction having the largest particulates’
?? largest grain size?

Line 30
‘chemical fractions of SPM fractions’

pages 10-11, section 4.1
It is fair to report and discuss the errors associated to ac-s measurements, but then other measurement techniques (e.g., PSICAM) should have been considered? Or your results should be presented only at wavelengths corresponding to minimum measurement errors?

Page 12, lines 1-10
I cannot see the optical signature of chla in the SPM absorption spectra you report on Figs. 2 and 3 (no absorption peak at 440 and 675 nm). Therefore, the ration chla/SPM is not expected to explain the variations you observed.
Regarding the ratio POC/SPM you could use POM/SPM as a proxy.
Packaging effects mainly concerns phytoplankton cells.

Pages 12-13-14, sections 4.3 to 4.5
The discussion is interesting in itself but unfortunately often speculative as you do not have no measurement of chlorophyll-a, POC, particulate iron concentrations on your samples. Several speculative parts could be shortened.

Page 15 ‘Conclusions’
A rather short conclusion as there are no many striking new results presented in the study.

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RR by Anonymous Referee #2 (28 Aug 2017)

Suggestions for revision or reasons for rejection

Review 2 (note to authors: I tried to put my latest comments in italics but the italics disappeared upon copying. I hope my latest comments are recognizable)

Optical Properties of size and chemical fractions of suspended particulate matter in littoral waters of Quebec

by

Mohammadpour, Gagné, Larouche, and Montes-Hugo

The authors are making an honest, full-faith effort to respond to my comments on their paper. Much of my comments were on emphasis and they have for the most part considered this well. Part of the difficulty I have is they are doing much that is “in the literature” or considered a “standard approach.” I feel that much of what is in the literature is simply poorly done and does not belong in scientific investigations of the suspended matter of natural waters. That is, the field of SPM investigations has many problems that have hitherto not been recognized by the research community. The only solution to this problem is, of course, more research that confronts these problems. My new comments in the following are in italics.

Page 2, lines 1-2 of original manuscript. Algorithms based only on CSPM will never have the accuracy required for optical inversions because SPM is undefined optically, an unknown mixture of
inorganic and organic matter. Therefore partition of SPM into at least major chemical
composition classes (PIM and POM) and estimation of size distribution are required
independently for optically-based remote sensing algorithmsof primary productivity and
suspended mineral dynamics of “disappearing shorelines” etc.

It is a relative questioning. Depends on the level of accuracy you are interested.
Many studies have been proposed for estimating CSPM based on remote sensing
methods. Adding remotely sensed PIM and POM to calculate SPM will also have a
large error due to the addition of two errors linked to PIM and POM algorithms

True, many studies have been proposed for estimating CSPM but they all have the same limitation; these are estimates of an unknown suspended mixture of differing complex refractive indices and therefore they are limited to regional interpretation, the regions from which the algorithm was derived, because of the systematic error involved. Estimating SPM from summing PIM and POM estimates has at least a definable and reproducible measurement error. Basically there is a confusion here between systematic errors and measurement errors. The ubiquitous (and wrongly proposed in my opinion) algorithms based on just SPM will provide precisely calculated inaccurate estimates.

Author's response for Page 3, lines 15-20 of original manuscript:

No, different filters were used in this study to fractionate 0.4-0.7 and 0.7-10 micron
fractions
This sentence was clarified as:

Size fractionation of SPM into four size classes (>10 μm, 0.7-10 μm, 0.4-0.7 μm, and
0.2-0.4 μm) was done after sequentially filtering the original samples through preweighted
membranes having a diameter of 47 mm and a pore size of 10 μm
(Whatman, polycarbonate), 0.7 μm (GF/F, Whatman, glass fiber), 0.4 μm
(Whatman, polycarbonate), and 0.2 μm (Nucleopore, polycarbonate), respectively.

Also, we add the whatman GF/F was used for PIM and POM determinations.

What is unclear here is why the glass fiber GF/F filters were used for both “size fractionation” and retrieval of “total SPM.” If the GF/F filters are removing all the SPM then sequential fractionation should not yield anything removed by filters smaller than the 70 micron nominal pore size of the GF/F filters.

Page 7, line 19 of revised manuscript. The authors should quote Richter and Stavn (2014) which actually gives instruction on accomplishing Model II multiple regression utilizing R code

Page 13, lines 12 and 19, Page 14, lines 29 and 30 of revised manuscript. Don't the p values quoted in these lines indicate that the correlations quoted are not significant?

Page 11, lines 20-21 of original manuscript. Suggested wording: These relationships will be useful in
investigating local and regionally-limited relationships and properties of SPM. Without separate independent studies of true optical properties of PIM and POM, and of PSD, these relationships will remain problematical.

We added the first sentence to the end of the conclusions paragraph

I recommend both sentences at the end of the conclusions.

Reference:

Richter, S.J. and R.H. Stavn. 2014. Determining functional relations in multivariate oceanographic
systems: Model II multiple linear regression. Journal of Atmospheric and Oceanic Technology, 31:
1663-1672.

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ED: Publish subject to minor revisions (Editor review) (11 Sep 2017) by Emmanuel Boss

AR by Martin Montes on behalf of the Authors (19 Sep 2017) Author's response Manuscript

ED: Publish subject to minor revisions (Editor review) (28 Sep 2017) by Emmanuel Boss

AR by Martin Montes on behalf of the Authors (09 Oct 2017) Manuscript

ED: Publish subject to minor revisions (Editor review) (12 Oct 2017) by Emmanuel Boss

AR by Martin Montes on behalf of the Authors (19 Oct 2017) Manuscript

ED: Publish subject to technical corrections (24 Oct 2017) by Emmanuel Boss

AR by Martin Montes on behalf of the Authors (25 Oct 2017) Author's response Manuscript

Short summary

The mass-specific absorption coefficients of total suspended particulate matter (a_SPM*) had relatively low (high) values in areas of of the St. Lawrence Estuary influenced by marine (freshwater) waters and dominated by large-sized (small-sized) and organic-rich (mineral-rich) particulates. The inorganic content of particulates was correlated with size-fractionated a_SPM* values at a wavelength of 440 nm and the spectral slope of a_SPM* as computed within the spectral range 400–710 nm.