Temperature dependence of the relationship between <i>p</i>CO<sub>2</sub>  and dissolved organic carbon in lakes

Pinho, L.; Duarte, C. M.; Marotta, H.; Enrich-Prast, A.

doi:https://doi.org/10.5194/bg-13-865-2016

Articles | Volume 13, issue 3

https://doi.org/10.5194/bg-13-865-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-13-865-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 13, issue 3

Research article

|

15 Feb 2016

Research article |

| 15 Feb 2016

Temperature dependence of the relationship between pCO₂ and dissolved organic carbon in lakes

L. Pinho, C. M. Duarte, H. Marotta, and A. Enrich-Prast

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Final revised paper (published on 15 Feb 2016)
Supplement to the final revised paper
Preprint (discussion started on 06 Feb 2015)

Interactive discussion

Status: closed

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

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- Supplement

SC C239: 'a large bias in pCO2 calculation in freshwaters', Gwenaël Abril, 17 Feb 2015
RC C944: 'Review of "Temperature-dependence of the relationship between pCO2 and dissolved organic carbon in lakes"', Anonymous Referee #1, 31 Mar 2015
- AC C1054: 'Response to referee #1', Luana Pinho, 07 Apr 2015
RC C978: 'Review of "Temperature-dependence of the relationship between pCO2 and dissolved organic carbon in lakes"', Anonymous Referee #2, 01 Apr 2015
- AC C1061: 'Response to referee #2', Luana Pinho, 07 Apr 2015

Peer-review completion

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

AR by Luana Pinho on behalf of the Authors (20 May 2015) Manuscript

ED: Referee Nomination & Report Request started (12 Jun 2015) by Minhan Dai

RR by Anonymous Referee #1 (05 Jul 2015)

Suggestions for revision or reasons for rejection

General comments

The manuscript, "Temperature-dependence of the relationship between pCO2 and dissolved organic carbon in lakes" by Pinho et al. has been improved substantially by the authors' revisions. It is my feeling that this is a timely study that will be well-cited, as it presents data on ecosystems often underrepresented in the carbon cycling literature. Methods are clear and appropriate, and the authors have done a good job of addressing potential shortcomings of this methodology. As in prior versions of the paper, it is concise and generally well written, though many of the new revisions contain grammatical errors that need attention. Specific comments are below.

Minor specific comments

1. ln 34: This point can be supported with fewer references.
2. ln 36: the word “tactility” should be removed so that the sentence reads, “….supporting heterotrophy, …..”
3. ln 39: Again, though I appreciate the thoroughness in updating references, this point can be supported with fewer citations.
4. ln 49: Wording is unclear. I’m not sure what is meant by the “global caught”.
5. ln 50: Should read, “One of the priorities of a comparative study…” or “One priority of a comparative study is….”
6. ln 178: average, not averages
7. ln 180: Change to “….we did not find…” rather than “…we continuous observing…”
8. ln 217: This sentence makes a very important point, but has very awkward construction. I would consider shortening it to something like, “Despite limitations of our methodology, our work contributes data to the literature from tropical lakes that is frequently missing from global calculations.”
9. Figure 1: Enlarge axis labels.
10. Figure 3: Use open circles, or if making plots in R (ggplot2 package), you could use alpha blending so that your trendline is visible. As presented, it is not visible.
11. Figure 3: Edit the x-axis labels for the break between 30 and 40 so that they are farther apart. Alternately, you could just remove either the 30 or 40 from the axis labels.
12. Figure 4: Same comment as above regarding trendline. Use the same format and axis scale for figures 3 and 4. I would suggest combining figures 3 and 4 into a block of 4 figures (a-d), so that the corrected plots are easily compared to the original.

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RR by Anonymous Referee #3 (10 Aug 2015)

Suggestions for revision or reasons for rejection

This paper reports the relationship of pCO2 and DOC in Brazilian tropical lakes, indicating no or weak relationship was observed. These findings are intriguing and they are different from those derived from lower temperature temperate/boreal lakes. The authors’ work adds new data to our collective body of knowledge and is important.
General comments:
1. As a whole, “introduction” is clearly addressed. The authors summarize the previous studies of the relationship of pCO2 and DOC, indicating a lack of data in tropical lakes. Then the authors introduce the importance of temperature. However, logic transition is not smooth in some paragraphs and somewhat confusing. The specific comments are listed below:
a. Line 52, what do “low latitudes” mean? I think that the authors want to compare the differences between high latitudes and low latitudes. It might not be appropriate to use ONLY “temperate systems” and “WINTER” to represent high latitudes conditions.
b. Line 59, “heterotrophic activity…support high fluxes of CO2, leading to CO2 enrichment”. Here “fluxes” should be “production”. In contrast, CO2 outgassing (high flux) from lakes to the atmosphere leads to a decrease in CO2 content in lake waters.
c. Line 62, the authors indicated that there was a paucity of low latitude data in Sobek et al. (2005). It is not correct. Sobek et al. (2005) included 148 tropical lakes. If the authors want to divide tropical lakes into high-temp and low-temp tropical regimes and indicate Sobek et al. (2005) doesn’t have high temp data, they should address it clearly here.
d. The last paragraph in Introduction: The authors should address that pCO2 and DOC are related but pCO2 is independent of temperature at high latitudes FIRST. Otherwise, it’s confusing why temperature is related to pCO2 and DOC. But here the relationship of pCO2 and DOC might be influenced “OR NOT” by temperature in tropical lakes.

2. There is no detailed description of methods. Detailed methods need to be addressed for a comparison of different datasets, including method precision/accuracy, progress of calculation, and unbiased data processing between different datasets. ONLY Based on these work, the authors can yield confident results. Some specific comments listed below:
a. Data source of published data? In line 122.
b. Line 126, pH is measured on which pH scale and is calibrated against what standards? If the standard has a precision of 0.1, how could we get pH results with a precision of 0.01 pH unit?
c. Line 136-137, pCO2 calculated from pH and TA should use dissociation constants of carbonic acid. Weiss (1974) provides the equation of CO2 solubility. The authors should address which set of constants is used, and if these constants are valid for low salinity lake waters?
d. Line 138, the authors might cite correction methods as G. W. Kling, G. W. Kipphut, M. C. Miller, Hydrobiologia, 240, 23 (1992).
e. Line 139-141. “aware of the difficulties in determining the pCO2…”. It should be noted that there has no difficulty in determining (such as direct measurement) of pCO2. The problem in pCO2 calculation is raised from the existence of Organic-TA. As well, there is no problem in TA and pH methods. In addition, sentence construction in the following sentences in the paragraph should be revised to make it clear. Avoid using “TA” and “Alkalinity” interchangeably.
f. The authors should address clearly how they remove the influence of TA in method section, or at least describe it concisely here and put details in the supplementary.

3. In the results part, the authors never address why they compare specific regions for temperature and DOC beyond a general description. It gives me a sense that the authors will compare their differences and the magnitude of temperature and DOC should be related to somewhat.
4. There is still no solid and strong evidence to support the primary conclusion of non-significant or weak negative relationship of pCO2 and DOC in Fig. 3a and 4a. What I see is a strong relationship for pCO2 and DOC at DOC<10 region with high pCO2. In contrast, there is no relationship for low pCO2 region. The authors should separate these regions to see if there is spatial difference which can be explained by different biomes.
5. There is no evidence of temperature related issue for the relationship of pCO2 and DOC in tropical lakes to support authors’ point. The authors do not need to remove fig. 3b in the previous manuscript. Using Ln(pCO2) vs. temperature can explain the physical control of pCO2 over temperature (see Takahashi et al. (1993) published in GBC). The authors also can check the relationship between DOC and temperature in tropical/temperature lakes. If there is no relationship between DOC and temperature, latitudinal difference is more convincible than temperature difference.
6. Suitable interpretation of the mechanisms controlling the weak relationship of pCO2 and DOC and its broad impact are recommended in discussion.
Minor comments: Gramma/sentence construction/spelling need attention.
1. Line 48, “because of” should be “because”.
2. Line 79, “conducted a survey of pCO2…” should be “of pH, TA, and DOC”. Here pCO2 is calculated from pH and TA. Same as in line 87.
3. Line 80, “0 to 33 °” should add “south”, same as in Line 83.
4. Line 86, link ftp://geoftp.ibge.gov.br/mapas_tematicos/mapas_murais/biomas is invalid, should be “biomas.pdf”.
5. Line 133, GF/F filter is 0.7 um, not mm.
6. Line 175, “(pCO2 = 45,70 ± 1,84 x DOC + 623,7 ± 18,83, R2= 0,12, p <0,0001, n = 4433),” I have no idea if it’s “pCO2 = (45,70 ± 1,84) x DOC + (623,7 ± 18,83)???
7. The authors wrongly replace decimal point with comma in a lot of places. Such as line 175…
8. Fig. 1, font of coordinates is too small. Link is invalid.
9. pCO2 cannot be written as pCO2.
10. Figure captions in support material are unclear.

Referee Report: PDF

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ED: Reconsider after major revisions (26 Aug 2015) by Minhan Dai

AR by Luana Pinho on behalf of the Authors (08 Oct 2015)

ED: Referee Nomination & Report Request started (25 Oct 2015) by Minhan Dai

RR by Anonymous Referee #4 (31 Oct 2015)

Suggestions for revision or reasons for rejection

Review of Biogeosciences manuscript (MS No.: bg-2014-593)
Title: Temperature-dependence of the relationship between pCO2 and dissolved organic carbon in lakes
Authors: Pinho et al.

The manuscript by Pinho et al set out to examine the effect of temperature on the relationship between pCO2 and dissolved organic carbon (DOC) in lake waters. pCO2 data were collected from 194 Brazilian lakes in low-latitude and tropical regions in South American. The authors found no significant correlation between pCO2 and DOC in low-latitude lakes, which is different from those observed in temperate regions. They then concluded the temperature dependence of the pCO2 and DOC relation. However, this work is solely based on the comparison with those reported by Sobeck et al. (2005). No further discussion was made in the manuscript.

First of all, DOC is not the only parameter regulating the pCO2 abundance in the water column. Depending on the sources of DOC, pCO2 could be positively or negatively correlated with DOC when DOC is mostly derived from terrestrial input or mostly produced in situ. Secondly, temperature could be a master parameter affecting pCO2 abundance only when all other environmental settings, biological, chemical and physical parameters, remain the same. Now, the question is then “so what” and why pCO2 has to be correlated with DOC or why temperature has to be a factor affecting this correlation?

Most importantly, in stead of using DIC, their pCO2 data were derived from total alkalinity, which includes additional contributions from dissolved organic ligands (DOC), but was not even mentioned or corrected. Overestimation of TA values cannot be ignored when DOC concentration is high especially in lake waters (up to >100 ppm in some samples, see Fig. 2). Therefore, pCO2 reported in this manuscript could be questionable.

Overall, I suggest that the authors exploit their data set further, teasing out different factors that could regulate pCO2, and produce additional meaningful conclusions.

Specific comments:
1. Methods: The authors should add more detailed information on the methods, such as QA&QC for DOC and total alkalinity measurements, including the use of certified reference materials and the diluted HCl for titration.
2. Line 152-153: Delete the repeated equation.
3. Line 152-153: Is this equation applicable to all DOC concentrations or for a specific range of DOC concentrations?
4. Line 220-224: High temperature and solar radiation may increase primary production and the uptake of CO2, resulting in a negative correlation between pCO2 and DOC assuming DOC is positively related to biomass or Chl-a. However, if the degradation of terrestrial DOC prevails in lake waters under high temperature/solar radiation, could pCO2 be positively correlated to terrestrial DOC ?.

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RR by Anonymous Referee #1 (09 Nov 2015)

ED: Reconsider after major revisions (30 Nov 2015) by Minhan Dai

AR by Luana Pinho on behalf of the Authors (11 Jan 2016) Author's response Manuscript

ED: Publish as is (24 Jan 2016) by Minhan Dai

AR by Luana Pinho on behalf of the Authors (25 Jan 2016) Author's response Manuscript

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Short summary

Unlike the positive relationship reported before between partial pression of carbon dioxide and dissolved organic carbon for lake waters, we found no significant relationship in our low-latitude lakes, despite very broad ranges in both variables. The strength of this relationship declines with increasing water temperature, suggesting substantial differences in carbon cycling in warm lakes, which must be considered when upscaling limnetic carbon cycling to global scales.

Temperature dependence of the relationship between pCO2 and dissolved organic carbon in lakes

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Temperature dependence of the relationship between pCO₂ and dissolved organic carbon in lakes