Seasonal dynamics of carbon and nutrients from two contrasting tropical floodplain systems in the Zambezi River basin

Zuijdgeest, A. L.; Zurbrügg, R.; Blank, N.; Fulcri, R.; Senn, D. B.; Wehrli, B.

doi:https://doi.org/10.5194/bg-12-7535-2015

Articles | Volume 12, issue 24

https://doi.org/10.5194/bg-12-7535-2015

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

https://doi.org/10.5194/bg-12-7535-2015

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

Articles | Volume 12, issue 24

Research article

|

21 Dec 2015

Research article |

| 21 Dec 2015

Seasonal dynamics of carbon and nutrients from two contrasting tropical floodplain systems in the Zambezi River basin

A. L. Zuijdgeest, R. Zurbrügg, N. Blank, R. Fulcri, D. B. Senn, and B. Wehrli

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Final revised paper (published on 21 Dec 2015)
Supplement to the final revised paper
Preprint (discussion started on 09 Jul 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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RC C3694: 'Seasonal dynamics of carbon and nutrients from two contrasting tropical floodplain systems in the Zambezi river basin', Lex Bouwman, 20 Jul 2015
- AC C6084: 'Author response to C3694 - Lex Bouwman', Alissa Zuijdgeest, 02 Oct 2015
RC C4240: 'Review of bgd-12-10545-2015 "Seasonal dynamics of carbon and nutrients from two contrasting tropical floodplain systems in the Zambezi River Basin" by Zuijdgeest et al.', Tim Jennerjahn, 13 Aug 2015
- AC C6090: 'Author response to C4240 - Tim Jennerjahn', Alissa Zuijdgeest, 02 Oct 2015

Peer-review completion

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

ED: Reconsider after major revisions (09 Oct 2015) by Steven Bouillon

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

ED: Referee Nomination & Report Request started (14 Oct 2015) by Steven Bouillon

RR by Tim Jennerjahn (05 Nov 2015)

Suggestions for revision or reasons for rejection

Review of revised version of bgd-12-10545-2015 "Seasonal dynamics of carbon and nutrients from two contrasting tropical floodplain systems in the Zambezi River Basin" by Zuijdgeest et al.

General comments:

After having reviewed the first version of this manuscript I have now read the revised version. The authors are to commend for improving the paper a lot. In most instances it became much clearer and the figures look much better. However, there are still some major issues in the discussion that need to be solved before the paper can be published. The source/sink calculations are partly contradictory and the organic matter discussion is partly unclear and therefore possibly wrong. Also, I am not sure if the major finding of this study is really pointed out. Isn't it the fact that the flooding of floodplains during the wet season releases large amounts of dissolved organic matter that then is exported by the river? Once these few (but important) issues are solved, the paper will make a valuable contribution to the literature.

Detail comments:

Abstract:
L. 42-51: There still some weaknesses in the discussion of C/N ratios and stable isotopes which is reflected in this part of the abstract (see comments on discussion). The final statement is not really clear and satisfying. It is not necessary to mention the need for more monitoring.

Study sites:
L. 144-145: 6,000 km2 is not "slightly" smaller than 7,700 km2; the difference is almost one third of the smaller area.
L. 146-148: Here you should add: ", but the overall annual discharge did not change.". This is what I read in the response letter, right?

Methods:
Still, the data on accuracy/precision of the methods are partly missing. Though many of the analyses are termed a "standard analysis" by many people does not mean that data quality must not be documented. Please add these numbers.

Discussion:
L. 354-355: Shouldn't it read "…primary production AND potentially a minor contribution…"?
L. 360-371: This is where I have a major problem to follow the reasoning. I think, there is a calculation error.
Table 1 and figure 5 provide daily loads representative of the whole season. As both seasons are equally long (6 months each) the sum of both seasonal terms subdivided by two provides the annual load. So, if I believe in table 1, on an annual basis the Barotse Plains are a sink for POC and PN and a source for DOC, DIC and DON (DIN=0); the Kafue Flats are a source for all parameters except DIN. Table 1 looks nice and is very clear in its result now, but I do absolutely not understand how numbers in table 2 were produced and how the Kafue Flats that were a source in table 1 now become a sink. Simply normalizing the load to unit area cannot change the algebraic sign, i.e. a source cannot become a sink as a result of this simple mathematical operation; it is simply impossible. The table 2 caption mentions "see methods section for how inundation areas were estimated", but in the methods section there is nothing on it. In "Study sites" the size of both of these areas was mentioned, but not how it was used in the calculations. In the end, the numbers shown in these tables and the statements made in the text are to some extent contradictory. The problem may lie in the way in which differences in flooded areas between seasons are accounted for. The total load given in table 1 should be independent of it, it should simply be the total amount that leaves the floodplain. So, these calculations have to be carefully inspected again and they have to be better documented in the methods section.
What is also striking in this context is the one order of magnitude higher POM yield in the Barotse Plains as compared to the Beusen et al. numbers for the whole Zambezi. If the numbers in table 2 are correct, which I doubt, then this is an extraordinary finding which, however, is then not discussed at all. So, if it is true, what could be the reason for it?
L. 361: There is a typo, it is "PN" not "PON".
L. 380-382: This statement appears to be partly wrong, at least contradicting the below statement (l. 384). I don't think that "…organic matter inputs from the floodplain…may have decreased the DOC concentrations" in the Barotse Plains. I would expect that it is similar to what you wrote about the Kafue Flats, that uptake by primary producers lowers DOC and DON concentrations.
L. 398-406: I am a bit puzzled by this explanation, because you only mention that your result "corresponds to the general observation that DOC export increases with runoff, caused by shallowing of the flow paths through organic-rich upper soils". Doesn't this touch the major point of your study, the floodplains as an important biogeochemical reactor? Shouldn't the larger area flooded during the wet season and hence the much more intensive leaching and transformation of organic matter from the plants and soils in the flooded areas be the major factor for the high DOC? To my opinion, this is a major finding of the study and needs to be highlighted. It deserves more discussion. Just mentioning other examples is not sufficient. What are the reasons for the high DOC and DON in Hawaii and in the Congo? That is important.
L. 417-429: The discussion of C/N ratios is still not very satisfying. The use of terms "higher" and "elevated" (elevated over what?) is given without any basis, hence suggests something which is not there. Again, the statements made here are unjustified assertions.
L. 422-425: No, this statement has to be toned down. The small difference in the C/N ratio of suspended matter and reservoir sediment sediment must not necessarily be the result of source differences. The C/N ratio also increases during degradation. So, the sediment in the reservoir may represent quite a long time span during which the organic matter has probably undergone degradation and the C/N ratio has possibly increased. The argument made might be correct, but the discussion is missing. You may have a look on soil C/N ratios in the following papers: Aitkenhead and McDowell, 2000, Global Biogeochemical Cycles 14, 127-138; Kirkby et al., 2011, Geoderma 163, 197-208.
L. 437-443: Here is a conclusion drawn and then explained by mentioning other examples. This needs to be done the opposite way: mention other examples and explain (!) the processes/controls/factors responsible and then draw a conclusion on your own findings.
L. 451-455: This is not really clear to me. Do you mean that you have a higher contribution of C4 plants during the dry season? Then say so. And what does the last sentence tell the reader?
L. 460-462: This is more or less a commonplace and not really true. The second half-sentence is rather speculation. Isn't his large amount of dissolved organic matter extracted/leached from the large areas of inundated flood plains during the wet season your major finding? I would highlight that.

Conclusions:
L. 474-477: This needs to be revised, see previous comments.
L. 480-481: This statement is unclear, see previous comment.
L. 483-485: This stamen may be true, but as is is rather speculation. Moreover, the second part of the sentence can hardly be understood.
L. 486-488: Second part of sentence hard to understand, see previous comment.
L. 488-490: If you want to stress the importance of dams, you need to expand this a little. Do you mean the altered hydrology has changed floodplain vegetation over time and accordingly also the contributions of OM from various sources? It is not yet coming out that clear (if meant that way).

Tables:
Table 2 is not really clear. The caption mentions parts of the methods section which are not existing and the way of calculation is not clear.

Figures:
Figure 7 is not absolutely clear. What does the direction of the arrow mean? If I look at a large arrow pointing into the floodplain I would assume that it indicates input into the floodplain. However, if I am right, it means the opposite, e.g. high DOM export from the Barotse Plains during the wet season. That's a bit counterintuitive. You better clarify this in the figure caption. This is a visual summary, the message should be unequivocal and easily extractable for the reader.

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ED: Publish subject to minor revisions (Editor review) (20 Nov 2015) by Steven Bouillon

AR by Alissa Zuijdgeest on behalf of the Authors (03 Dec 2015) Author's response

ED: Publish as is (11 Dec 2015) by Steven Bouillon

AR by Alissa Zuijdgeest on behalf of the Authors (14 Dec 2015)

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Short summary

Two large floodplains hold the peak flows in the Zambezi River system – the dam-impacted Kafue Flats and the largely pristine Barotse Plains. Here we show that the inputs of organic matter and nutrients from such floodplains to the river are strongly affected by the presence of hydropower dams and highlight how floodplains act as large biogeochemical reactors that can behave distinctly differently from the entire catchment.