Sediment properties and CO<sub>2</sub> efflux from intact and cleared temperate mangrove forests

Bulmer, R. H.; Lundquist, C. J.; Schwendenmann, L.

doi:https://doi.org/10.5194/bg-12-6169-2015

Articles | Volume 12, issue 20

https://doi.org/10.5194/bg-12-6169-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-12-6169-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 12, issue 20

Research article

|

28 Oct 2015

Research article |

| 28 Oct 2015

Sediment properties and CO₂ efflux from intact and cleared temperate mangrove forests

R. H. Bulmer, C. J. Lundquist, and L. Schwendenmann

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Final revised paper (published on 28 Oct 2015)
Supplement to the final revised paper
Preprint (discussion started on 25 Feb 2015)
Supplement to the preprint

Interactive discussion

Status: closed

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

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- Supplement

RC C908: 'Referee comments', Anonymous Referee #1, 30 Mar 2015
- AC C2855: 'Response to referee 1', Richard Bulmer, 16 Jun 2015
RC C950: 'Review of “Sediment CO2 efflux from cleared and intact temperate mangroves and tidal flats” by Bulmer et al.', Anonymous Referee #2, 31 Mar 2015
- AC C2856: 'Response to referee 2', Richard Bulmer, 16 Jun 2015
RC C1170: 'Review of Bulmer et al.: ”Sediment CO2 efflux from cleared and intact temperate mangroves and tidal flats”', Anonymous Referee #3, 13 Apr 2015
- AC C2857: 'Response to referee 3', Richard Bulmer, 16 Jun 2015

Peer-review completion

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

ED: Reconsider after major revisions (17 Jun 2015) by Steven Bouillon

AR by Richard Bulmer on behalf of the Authors (18 Jun 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (18 Jun 2015) by Steven Bouillon

RR by Anonymous Referee #2 (02 Jul 2015)

Suggestions for revision or reasons for rejection

There still remains a number of concerns that need to be adressed to bring this MS up to a standard suitable for publication in Biogeosciences.
L16 Check repeated text
L71-72 This sentence is out of place
Line 78 and various other place throughout manuscript - temperature and tropical mangroves? I am assuming the authors mean temperate (also line 84 but check throughout MS)
Line 108 "hydrodynamic exposure" what is meant by this? In my original review I requested information on hydrodynamics such as current velocities, inundation time etc, this seems to be the response of the authors to this request but the term is not defined
L117 The shading experiment is flawed in 2 ways - 1) The measurements were not undertaken over the same sediment (they were adjacent), and considering the large within site variability, it is no wonder there was no significant difference!
2) The experiments were undertaken in the "intact" mangroves. This is the location where the affect of MPB on measured fluxes is likely the lowest due to the lower light penetration under the mangrove canopy. This experiemnt should have been undertaken in the cleared environments were presumably benthic photosynthetic rates would have been highest, therefore the artifact associated with continuing CO2 uptake in the chambers would have been highest.
L150 30 seconds between biofilm removal and flux measurements - no wonder fluxes were significantly higher, need time to re-establish diffusion gradients
L152 As written the second term of this equation is not correct, should be (P.V)/(R.T.A)
L154 Regression of concentration vs time should give units of ppm/second
L168 "dionized"?
L168 - What temperature was the hotplate? Not too hot I hope as OM degradation will occur if temps are raised too high!
L178 How was sample stored for the 1 month prior to analysis? Chl a requires very specific storage to prevent degradation
L216 See comments above - you cannot do a paired t-test on samples that are technically not the same site
L251 - Visual analysis? Any method to determine these cuttofs (eg. cluster analysis). Otherwise this kind of seperation by non statistical methods opens up the possibility of "p-hacking" whereby the group classification is determined based on producing significant differences between the groups. I would like to see a more quantitative seperation of these groups.
L260 - Again some method other than "visual analysis" to seperate groups is required.
L305-308 If microbial communities drive the higher fluxes at the high efflux sites, why were the fluxes higher when the biofilm was removed? This argument does not fit the biofilm removal story
L316-317 True - but you do not have any light data to discuss this
L332 - What is meant by exposure?
L344-348 - Any data on nutrients? If not this is a bit speculative. Also high clay content is usually a covariate with SOC because of hydrodynamics - i.e. where there is depositional conditions (i.e. low current velocity), both clays and OM settle out. While there is some effect of surface charge on OM adsorption - generally clay content and SOC are just covariates
L386-388 See comment above regarding the problems with the shading experiment
L389-391 This is not really plausable unless the CO2 concentration in the sediments is lower than ambient air - which is not likely
L403-404 I do not believe that chemosynthetic CO2 uptake can exceed respiration in these organic rich sediments

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RR by Anonymous Referee #3 (26 Jul 2015)

Suggestions for revision or reasons for rejection

The manuscript has improved very much by the revision. I like that the authors took the time to examine the effect of pre-shading. To me it looks like there is an effect, although the statistics could not prove it – the CO2 flux for intact sediment almost doubled. In any case, the pre-darkening could not explain the reason for the large difference when the surface 2 mm sediment is removed. It might be due to the exposure of a steeper gradient driving the higher flux. Anyway, I am still not sure I understand why this artificial manipulation was done. What does it tell us? The authors argue in the discussion (p. 15) that one reason for the observed difference is the homogenization of surface sediment by the removal procedure. This is quite likely and then the results depend on the type of handling – and as such do not tell us anything about the system. They also argue that chemoautotrophs in the biofilms fix carbon and are responsible for the lower flux in intact sediment. I agree that this to some extent is true, but do not explain the results measured here – which in some cases even provided CO2 uptake without removal of the biofilm. This is impossible under steady state conditions. If the chemoautotrophs are driven by sulfide, then it will require oxidation of about 5 sulfides to fix one CO2. The reduction of sulfate to sulfide by sulfate reduction on the other hand will gain two CO2. From this calculation it is obvious that the role of chemoautotrophs for the CO2 flux is not as important as stated.

I am also somewhat concerned about the lack of information about number of pneumatophores and burrows inside the measuring chamber. The authors are aware of the problem, but it would be nice to see if there are any correlations of flux results with the presence of these biogenic structures.

I now find the manuscript acceptable for publication when revised according to my concerns above and the corrections listed below.

Specific points:

Abstract:
Line 11: Delete “temperate”
Line 16-17: There is a copy-paste error here as the same text is shown twice.

Introduction:
Line 33: Change to “These forests are subject to….”
Line 36: Change to “….temperate mangrove sediments….”
Line 47: Change to “…expansion of mangrove forests…”
Line 77-78: Change to “Lovelock (2008) found a positive correlation between leaf area index and sediment CO2 flux in temperate and tropical mangrove forests.”
Materials and methods:
Line 124 & 127: It reads better writing “removal”
Line 153-154: How can the units of the regression slope be µmol mol-1 when the regression describes change in CO2 concentration over time? It must be µmol s-1
Line 166: Why was it only a subset of samples?
Line 202: It is not clear if this equation was used for all trees in Mangere 1 and Hatea 1, or if it was only trees exceeding the maximum height (248 cm) of equation 3.
Line 210: It seems not logic than root biomass was not measured in mangrove sites. Here these structures must be expected to be most important!

Results
Line 243 and elsewhere: The unit for fluxes given per second is not standard in these types of studies. In fact at line 281, the authors make the conversion to per day for comparative purpose. This also makes Table 2 somewhat strange because the same data are given in two columns, one per day and one per second. Please convert the results to per day throughout. Then Table 2 will only consist of one data column – and readers familiar with the per day notation need not do the calculation to get rates they can compare with others.
Line 251-252: Are these clusters shown anywhere?

Discussion
Line 288-289: Of this reason it would be nice with some data on numbers of burrows and pneumatophores.
Line 313: Measurements were made at low to mid-tide. I suppose that the sediment was still air exposed?
Line 334-340: Do you find any indication of lower organic quality in older cleared sites?
Line 346: Was this accumulation of carbon due to sedimentation or what?
Line 349: Change to “….particularly in dwarf/stunted mangrove forests…”
Line 350-351: Is this correlation shown anywhere?
Line 361-370: Do these differences in organic carbon be due to sedimentation of particles from the tidal water or what? It is not clear from where the enrichment originates.
Line 386-388: True, but there seems to be a marginally significant effect.
Line 389-397: I am not fully satisfied be the explanations for the strongly increased flux after removing the upper 2 mm of the sediment. The authors could mention displaced profiles and thus strongly increased gradients.
Line 398: It is true that chemoautotrophs fix carbon, but they can never fix more that generated by heterotrophic processes in the entire sediment column.

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ED: Reconsider after major revisions (03 Aug 2015) by Steven Bouillon

AR by Richard Bulmer on behalf of the Authors (14 Sep 2015) Author's response Manuscript

ED: Publish subject to technical corrections (09 Oct 2015) by Steven Bouillon

AR by Richard Bulmer on behalf of the Authors (13 Oct 2015) Author's response Manuscript

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Short summary

This is the first study investigating the effect of clearing on sediment CO2 efflux in temperate Avicennia marina forests. We found that rates of sediment CO2 efflux from cleared and intact temperate Avicennia marina forests are comparable to rates observed in other temperate and tropical forests. Our results show that greater consideration should be made regarding the rate of carbon released from mangrove forest following clearance and the relative contribution to global carbon emissions.

Sediment properties and CO2 efflux from intact and cleared temperate mangrove forests

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Interactive discussion

Peer-review completion

Sediment properties and CO₂ efflux from intact and cleared temperate mangrove forests