|This is a revised version of a manuscript focusing on sediment C accumulation and sediment CH4 concentration in a tropical reservoir. The study is an important addition to the literature about the net C and greenhouse gas emissions caused by hydropower reservoirs and dams. Valuable work has been done to estimate the sediment C accumulation rates and spatial patterns in this reservoir. The current work is not capable of closing the full C or GHG budget, but perhaps, that is the ultimate goal of the researchers. The work is topical as it seems that hydropower business is increasing, particularly in the tropics. I agree with the reviewers 1 and 2 that the study deserves to be published. I also agree with the criticisms raised by those reviewers.|
It looks that the authors have improved the manuscript quite well according to the comments on the earlier version by the reviewers 1 and 2. I have, however, still some reservations. I think that the presentation needs some work to be publishable and I suggest a rewrite. I suggest introducing the big questions behind and how sediment C accumulation and potential for CH4 emissions relate to that. Is the question about the GHG emissions caused by hydropower reservoirs and dams and/or how sediment C accumulation affect catchment C and GHG budgets, or something else? In addition, I found problematic how the CH4 data were dealt. Authors considered the CH4 concentration data as an indicator of potential for CH4 ebullition if concentration high enough for bubble formation. This is a bit problematic, because CH4 concentration, or CH4 saturation if high enough, may not translate to actual CH4 emission. The ‘potential’ is not considered in quantitative manner in this study and the approach needs a motivation. I suggest that authors could elaborate the analysis or the text to the direction of quantitative analysis how the sediment and C accumulation and qualitative sediment properties determine the sediment CH4 concentration/CH4 saturation. I am confident that the authors can improve the presentation. I have listed some detailed comments and suggestions below.
The title is awkward, because the emissions vs accumulation aspect was not really studied in the current study. How about: High sediment carbon accumulation and high sediment methane concentrations in an Amazonian hydroelectric reservoir. Other thing, I am more used to the term “carbon accumulation” rather than “C burial”. I.e. sediment accumulation rate (SAR), carbon ccumulation rate (CAR), but up to you.
Abstract 31-34. Difficult to read. Maybe make two sentences.
36. CUN, explain
46-47. It is not only transport from land to the Ocean, but also net sequestration by autotrophs at places.
49. But see also Kortelainen et al. 2004, GCB
61. You just gave an estimate so rewrite to express the uncertainty
75- Overall C accumulation is one component in the C and GHG budgets and you shoul acknowledge also the other parts. Examining the the sediment C accumulation is important, though.
83. There are also older references
87-88. Awkward, suggest editing. To capture the total emissions both high emission and not so high emissions sites count.
94. Note that CH4 emission may not affect the C pool much even though it has significange as a greenhouse gas. How about CO2 and CH4 emissions.
96- Shoudn’t you formulate research questions or hypotheses?
122-124. here and elsewhere, unnecessary use of parentheses.
Fig S1. Why not in the main document.
140. Delete (see…)
153-159. Specify that sampling capaigns were targeted to rising and falling water periods?
158. Quite large GPS error!
184. Unnecessary parentheses
185. Explain, how saturation concentration defined
192. Be specific, how many cores were measured for [CH4]
199. Injected unnecessary > …and analyzed for CH4 concentration within the same day using…
208. was lost
218. do you mean that ‘therefore, the mean accumulation rate cannot reveal short-term …’
226. The average C accumulation rate (xx) was calculated dividing the total C mass (g C m-2) by the accumulation time
231. We used empirical relationship between SAR and CAR (y=..) to estimate the CAR for the reamining…
235. 237. Suggestion, ..’to produce maps of SAR and CAR’.
245. was > were?
260. Sediment and C accumulation rates
262. Why supplement if the main result? To my opinnion belongs to the main document
276. The same here
278-279. to the site description
286. Isn’t this unnecessary reference to the supplement, because you have fig. 3?
289. Is the test mentioned in the M&M?
292. ‘this indicates…’ belongs to the discussion
294. S5 unnecessary?
295. So the same areas have also high CAR. Maybe emphasize.
317. What you mean with ‘margins’?
323. What the ‘muddy lake area’ actually means?
328. I think that the ref to S6 is unnecessary
352. If CUN has high CAR (& other rates), why it is important? How these findings will improve our understanding on the reservoir systems?
386. Can you refer to the Fig 1 instead of the supplement?
388- I found this section unclear
398. Can you include S6 in the main document
401- Write open the idea behind
417. Mention that the C:N in the sediment is a mixture
444. How about diffusive transport? Totally neglected
Fig5. A scatterplot instead?
448-457. I found this only marginally relevant
458. What the earlier CUN study says about the CH4 ebullition? Do you have own measurements? If [CH] stays the same, what it means? A stable reservoir with no release but no production either / continuous production and emission at same rates/
474. But there are earlier CH4 flux measurements.
475- Net effect on what? Reservoir radiative forcing, catchment C balance? What are the other components in the budgets?