RSE-D-06-00261
Response to reviewers’ comments
Note to Editor-in-Chief: We appreciate your suggestion and have changed the paper title to:
“On the accuracy of Southern Ocean chlorophyll a concentrations estimated from
SeaWiFS”
Reviewer #1:
This is an excellent paper. I recommend its publication in its present form. This manuscript
provides important data and conclusions regarding the accuracy of SeaWiFS estimates of
chlorophyll a in the Southern Ocean. The topic and the conclusions are very significant and
relevant to RSE. The manuscript is written concisely and organized well, the analytical and
statistical methods are appropriate and sound, and the figures are well-illustrated. The authors
also do a good job in explaining how pigment methodology (fluorometric vs HPLC) combined
with the likely importance of chlorophyll c in Southern Ocean phytoplankton results in poor
agreement between conventional fluorometric in situ data and remotely-sensed SeaWiFS data.
Reply: We thank the reviewer for his positive review and encouraging comments.
The only very minor comment I have is that the authors might provide some summary general
information on the spacing of the discrete depths sampled in the first optical depth at each station
(Page 7, line 16) so that the reader can appreciate how many near-surface samples were used to
obtain the optically-weighted satellite chlorophyll estimate (equation 2).
Reply: As recommended, we have provided more detail on the vertical spacing of the in situ
samples in the Methods section (now page 8 line 14).
Reviewer #2:
The basic message of this paper is that the SeaWiFS chlorophyll algorithm does not
underestimate chlorophyll a concentration (Ca) in the Southern Ocean as previously believed.
The explanation is that the fluorometric methods, which are the predominant type used in
algorithm parameterization and validation efforts, actually overestimate chlorophyll as
compared with the preferred, more accurate HPLC method. The SeaWiFS chlorophyll agrees
reasonably well with the HPLC chlorophyll measurements. This message will be of interest to
many people involved in ocean color algorithm development, as well as those who use SeaWiFS
data for the Southern Ocean.
I am recommending that this paper be accepted for publication with minor changes (author
discretion). Here I provide some suggestions for improving the paper. They are not as daunting
as they might appear at first glance.
Reply: We thank the reviewer for the thorough and detailed review comments that will certainly
lead to improvement of the manuscript.
1.
The abstract actually contradicts the message stated above. The second sentence states
that the "high-resolution .. daily SeaWiFS Ca data was an accurate measure of in situ
Ca based on both fluorometric and HPLC measurements?" Later the point is made that
the fluorometric method is in error relative to HPLC, and thus disagreement between
fluorometric Ca and SeaWiFS Ca is not a flaw of the satellite algorithm. The second
sentence of the abstract should be reworded to be more consistent with the conclusions
of the paper.
Reply: What we really meant was that we used both fluorometric and HPLC
measurements in the validation. However, we agree that it is slightly misleading.
Therefore, we reworded the sentence to refer to HPLC only.
2.
The explanation given for the disagreement between fluorometric and HPLC
measurements is the "relatively low" and "relatively high" levels of chlorophylls b and
c, respectively. While these explanations may be valid - they are somewhat gratuitous,
and hard to put into perspective, especially for readers of RSE who are not familiar with
methods for measuring algal pigments. When you say that Cb is "relatively low" and Cc
is "relatively high" - the question I have is relative to what? relative to phytoplankton
populations found elsewhere? or relative to each other? Even if you had a pure culture
of a species known to have Cb or Cc, would these ratios of Cb/Ca and Cc/Ca be
considered low or high? I recommend that this explanation be downplayed. Delete the
sentence in the abstract (next to last sentence), and eliminate the two sentences on top of
page 6 that speak about fluorescence emission spectra of degradation products. These
two sentences add little to the message of the paper, and may be confusing to many
readers (such as myself).
Reply: Even though the explanation may seem confusing in the abstract where little
detail is given about the reasoning behind it, this argument is developed further in the
discussion (page13). The relative concentrations of Cb and Cc are compared to those
from other areas. Several other studies that have reported Cb/Ca and Cc/Ca ratios are
cited for comparison. Studies that found that Cb can cause an underestimation of Ca by
the fluorometric method reported ratios of Cb/Ca ranging from 0.16 to 0.5, which are
significantly higher than the ratio of 0.023 that we report. We added the phrase “as
compared with other regions” in the abstract to make it clearer.
3. The statement in the introduction (pg. 3) "? errors in the Ca data products after logarithmic
transformation are about 0.3 or less (Gregg and Casey, 2004), which corresponds to roughly
50% root mean square (RMS) relative error" may be what was reported by Gregg and Casey
(2004), but a log-error of 0.3 does not correspond to a 50% relative error. This statement
can easily be corrected by rewording it as follows: "? errors in the Ca data products after
logarithmic transformation are about 0.2 (O'Reilly et al. 2000), which corresponds to
roughly 50% root mean square (RMS) relative error." The O'Reilly reference is already in
their list, and is the official reference for the OC4v4 algorithm.
Reply: The sentence has been reworded as suggested.
4. The sentence at bottom of pg. 4 is too long and quite awkward. It would be worth breaking it
into 2 sentences and rewording. Suggested change: "For example, Dierssen and Smith
(2000) applied in situ bio-optical data measured between 1991 and 1998 to the OC2v2
algorithm to test its applicability in the Southern Ocean west of the Antarctic Peninsula.
They concluded that Ca derived from the OC2v2 algorithm was 60% lower than the in situ
Ca (ranging from 0.7 to 43 mg m-3, median ~ 1 mg m-3)."
Reply: The sentence has been changed as suggested.
5. Bottom pg. 6. change "Level 1 data collected by all HRPT ground stations, ?" to "Level 1
data collected by ground stations, ?" No need to introduce another acronym (HRPT) that
should then be spelled out.
Reply: Removed “all HRPT’ from the sentence
6. pg. 7, sentence following equation 1. This definition of R makes no sense. It was the notation
used by O'Reilly et al. (2000) only after they defined it. It is better to define R as follows: R
= log10[max(Rrs443, Rrs490, Rrs510))/Rrs555]. This notation would than be consistent
with your axis label in figure 5.
Reply: Notation has been changed to make it consistent with Fig. 5.
7. pg. 9. I'm happy to see that you included the log based errors (eqn. 4) in your analysis. The
relative error, x = (S-I)/I, is highly skewed, and can be misleading. It can never be less than
-1 (i.e., the % error can never be less than -100%) as long as S is positive, whereas it can be
arbitrarily large (i.e., >> 100%). Thus, stating the error bounds as mean +/- std. makes little
sense. (e.g., what does it mean when the std is larger than the mean?). I have recommended
that the "one sigma" range for a ratio (S/I) be reported as inverse transforms of M-S and
M+S where M and S are the mean and standard deviation of the log error: log(S) - log(I).
(See RSE-06-00318).
Reply: We appreciate the comment and corresponding explanation.
8. pg. 10. change "Although the RMS errors (in natural or log-transformed forms) for the
two?" to "Although the RMS errors for the two?" The term "natural" could be confused with
natural log.
Reply: The phrase “in natural or log transformed forms” has been removed from the
statement
9. pg. 11. State whether the remote sensing reflectance data used in figure 5 is from in situ
measurements or from the satellite data. If the latter, then be sure you've applied the
algorithm to level 2 data and not to any averaged (e.g., level 3) Rrs data.
Reply: We have reworded the sentence to: “By using the spectral remote sensing reflectance
data (Rrs) derived from the satellite measurements (Fig. 5), the OC4v4 algorithm yielded
comparable results to those obtained from HPLC measurements”
10. pg. 11. Reword: "Clearly, the ratio of CaFluor/CaHPLC is not uniform, but appears to
decrease with increasing concentrations (Table 2). Indeed, for CaHPLC <0.05 mg m-3 and
CaHPLC >3.0 mg m-3, the statistical results may not be reliable ?" to "The ratio of
CaFluor/CaHPLC appears to decrease with increasing concentrations (Table 2), although
for CaHPLC <0.05 mg m-3 and CaHPLC >3.0 mg m-3, the statistical results may not be
reliable ?"
Reply: The sentence has been reworded as suggested.
11. pg. 11, last paragraph. Words like "clearly" and "insignificant" should be used cautiously.
When you state that the bias is insignificant, it implies that you've done a statistical
hypothesis test, but nothing has been said about such statistical tests in your methods section.
Thus, I am assuming that you didn't really do any hypothesis tests (and that's fine), but to
avoid this issue, it's best not to make statements about a result being "significant" or
"insignificant." Where you say that "there is clearly an overestimation in CaFluor.." .. this
assumes that the HPLC is the right answer. Here's a suggested rewording of that sentence:
"For CaHPLC between 1.5 and 3.0 mg m-3, the bias is small (15%) and the ratio of
CaFluor/CaHPLC is close to unity (1.15 +/- 0.73). Between 0.05 and 1.5 mg m-3,
however, CaFluor is much higher than CaHPLC (CaFluor/CaHPLC = 2.48 +/- 2.23, n =
647), and this difference is believed to be due to errors in the CaFluor measurements as
described above."
Reply: The sentence has been reworded as suggested.
12. Your discussion section (pp. 12-13) is largely focused on the effects of Cb and Cc, whereas
earlier I said this issue should be downplayed. It's okay to include a discussion about it here,
but don't assume that the reader will understand why Cb or Cc can interfere with the
fluorometric method. e.g, your sentence "Hence, the presence of significant amounts of Cc
appears to be the cause for the overestimation of Ca by the fluorometric method." doesn't
follow from the previous sentences in that paragraph. It would be better simply to state "The
presence of significant amounts of Cc is known to cause an overestimation of Ca by the
fluorometric method (cite references)." In other words, this "known" effect can be evolked as
the cause for the fluorometric overestimation of Ca - without assuming that the reader
understands.
Reply: We appreciate the reviewer’s suggestions. The statement in question has been left
without modifications except for the removal of “Hence”. The reader can refer to the studies
cited later in the discussion (page 13, 2nd paragraph) for details on how accessory pigments
can interfere with the fluorometric signal. [Marina: I think the reviewer has a point. Although
you removed the word “Hence”, the following sentence “…appears to be the cause…” still
has no ground because I couldn’t find in the text WHY Cc can lead to overestimation. In page
6 you said “On the other hand, high concentrations of chlorophyll c (Cc),
typically found in diatoms, dinoflagellates, prasinophytes and haptophytes,
lead to an overestimation of Ca with respect to fluorometric measurements.”
but no reference was given. In the discussion section (p13, 2nd para) I can’t
find the reason or reference either. So I suggest you follow the reviewer’s
suggestion and put the reference there]
13. pg. 13. "Cb and Cc are variable in different regions of the world's ocean.." reword "Cb
and Cc vary widely throughout the world's ocean.."
Reply: The statement has been modified as suggested.
14. pg. 13 "Lohrenz et al. (2003) reported that even if the ratio of accessory pigments (sum
of carotenoids and Cb + Cc) over Ca is ~100%, the perturbation to the pigment
absorption is <30%, corresponding to <30% error in the satellite-retrieved Ca." reword
to "Lohrenz et al. (2003) reported that even if the amount of accessory pigments (sum of
carotenoids and Cb + Cc) is equal to Ca, the perturbation to the pigment absorption is
<30%, suggesting a relatively small error in the satellite-retrieved Ca."
Reply: The statement has been modified following the reviewer’s suggestion.
15.
pg. 13-14. Your last sentence "Hence, the large differences ?" doesn't make sense to
me. Maybe something like this would be better: "Although differences between CaFluor
and CaHPLC may be the result of the effect of accessory pigments,this effect cannot
explain the large differences between CaSWF and CaFluor."
Reply: We did not change the sentence because our reasoning is for absorption rather
than for fluorescence..
16.
Conclusion. I don't think you should say that the "satellite estimates closely agree
with those determined from water samples." It is more accurate to say that the satellite
estimates agree [delete 'closely'] with those determined from water samples by HPLC
methods.
Reply: The word “closely” has been deleted from the sentence.
17. Last sentence: "However, if primary productivity models have been tuned based on
CaFluor as input, the satellite estimates of Ca used in primary productivity estimates
would need adjustment to be consistent with the models."
Reply: The word “would” has been replaced by “will”, referring to future modeling work
Reviewer #3:
I have reviewed the manuscript entitled " Are SeaWiFS chlorophyll a concentrations
underestimated in the Southern Ocean" by M. Marrari, C. Hu, and K. L. Daly. The authors
compared the in situ chlorophyll a (Ca) data in the Southern Ocean with SeaWiFS-derived Ca
concentration and found that SeaWiFS-derived Ca data from SeaDAS4.8, OC4v4 algorithm is a
good estimation of the in situ Ca if it is determined by the HPLC method. The authors spent
much effort to discuss the property of the in situ data and their work has some scientific merit; I
think it is worthwhile to be published in RSE.
Reply: We appreciate the reviewer’s positive comments.
My suggestions to improve the presentation of the paper are:
1). It is suggested to draw the 2000m (not 1000m) isobath in Fig.1, because the sampling data
were collected within the 2000m isobath (see Fig.2 and Fig.3).
Reply: We thank the reviewer for the recommendation. However, we did not modify Figure 1
to include the 2000m isobath instead of the 1000m. Although the in situ data were collected
within the 2000m line, the location of the shelf break in this area is usually considered to be
between 500 and 2000m, thus the 1000m isobath indicates its mean location, giving the
reader a good idea of the spatial extent of the continental shelf. [Marina: I don’t think this is a
good reason. Actually I agree with the reviewer – to be consistent with other figures the
isobath line in Fig. 1 needs to be 2000 m, unless this is too troublesome for you to redraw]
2). The units are necessary in Figs 4, 5, 6 and 8.
Reply: Units were added to Figures 4 and 5, but not to Figs 6 and 8 because Fig. 6 caption
already has units and Fig. 8 units are dimensionless.
Reviewer #4:
=============================
Reviewer #4:
Work is devoted to the important problem - intrecomparison of the "in-situ" and satellite
measurements of the Chl-a. Authors processed SeaWiFS and "in-situ" data for the
Southern Ocean basin. Quality of the both in_situ methods (HPLS and Fluorometric)
were analyzed. Generally problem of satellite chlorophyll measurements quality
estimation consists from the set of the sub-tasks:
- atmospheric correction
- existence of the universal relation Chl-a and WLR for different species
- quality of the in-situ measurements
- spatial and temporal difference between satellite and in-situ data
Presented work takes in to account last two sub-tasks and is aimed for regional and
seasonal study, that partially limits the taxonomic variability (sub-task 2).
Authors collected significant data set for in -situ Chl measurements and made filtration
of the data for preparation of the reference sampling to exclude influence of the errors
induced by time difference, depth distribution ( I recommend additionally make some
spatial filtration to exclude gradient areas).
Comparison of the satellite data with filtered in-situ data set demonstrates good
agreement in the range Ca 0.05-1.5 mg/m3 for investigated area and season.
Some remarks:
- p8 -
( ratio = 1.05 ?0.99) corresponds inverse ratio ~ 0.5-16 is it not significant?
Reply: After reviewing our in situ data, we found that indeed for the HPLC data,
the weighted and surface chlorophyll values are significantly different. For the
fluorometric samples, surface and weighted chl are not different (see t-test
summaries below). We have corrected this error in the Methods section (page 8):
t-Test: Surface vs. weighted Cs (fluorometric)
Mean
Variance
Observations
Pearson Correlation
df
t Stat
P(T<=t) one-tail
t Critical one-tail
P(T<=t) two-tail
Surface
1.44367
7.011669
775
0.977917
774
0.199886
0.420811
1.646825
0.841622
Weighted
1.439692202
6.577036311
775
t Critical two-tail
1.963034
t-Test: Surface vs. weighted Cs (HPLC)
Mean
Variance
Observations
Pearson Correlation
df
t Stat
P(T<=t) one-tail
t Critical one-tail
P(T<=t) two-tail
t Critical two-tail
Surface
1.245435
7.844801
189
0.820289
188
2.295788
0.011396
1.652999
0.022791
1.972663
Weighted
0.967443171
3.403379117
189
[Marina: in the text you said “190”, not “189” samples. Make sure they are consistent,
and also make sure all related numbers are also consistent throughout the text]
Fig 4-6 - scales without dimension ( mg/m3)
Reply: Units were added to the corresponding figures.
It is good methodological work focused on "true" intercomparison of the heterogeneous data
sets. Article can be published without significant changes.
Reply: We thank the reviewer for the positive suggestion.