|L8 : « biggest” in terms of what ? Discharge ? Drainage area ?|
Please define the abbreviations (qPCR, MISA, etc…) in the abstract. The abstract should be understood on its own, which requires that abbreviations are defined. Refer to instructions “The abstract should be intelligible to the general reader without reference to the text”, which is obviously not the case if the abstract is full of abbreviations. Since there is no size limit for abstracts in BG it should be not a problem to define all of the abbreviations.
L64: “MOBs” abbreviation is not defined, as it should the first time an abbreviation is used. Same applies to DOC, TDN, etc… The authors should take the time to polish the presentation of their work. It’s not the reviewer’s job to check this, especially at the second round of review.
L74: Here, explain how the present paper adds to the previous papers by the authors in the area.
L97: Specify what was the delay between the sampling and the analysis of CH4 in the home laboratory ? How were samples stored between sampling and analysis in the home lab: room temperature ? Did the authors check if poisoning with H2SO4 is efficient to stop biological activity and for how much time ? While I can imagine that acidification can preserve a sample for a few days, I not sure this is adequate for storage for months.
L101: specify the concentration of the NaOH solution.
L185: it’s Liss not Lisa
L 195: the relationship was not “developed for coastal seas”, it was derived from an experiment in a continental shelf. In fact the Nightingale relationship converges with relationships derived in the Southern Ocean (Ho et al. 2006).
L 235: “very high” is not adequate to describe CH4 concentrations since they vary by several orders of magnitude accross aquatic systems. 400 nmol/L might be "very high" for the Laptev Sea but will be very low for a tropical reservoir. Refer to actual numbers.
L 236: use an uniform unit throughout the text, a not a mix of µM, nM, nmol/L.
L 321: Did the vertical profiles of T and S show a mixing in response to “wind increase” ?
L 327, 363: “Unfortunately” is a subjective evaluation. Scientific writing should neutral and objective.
L 333: Oxygen in surface waters of rivers/estuaries can also be lowered by sediment organic matter degradation, since sediment respiration is usually equivalent to water column respiration in estuaries (Gazeau et al. 2004). So the correlation of CH4 and O2 does not necessarily imply that CH4 is produced in the water column due to a process that consumes O2 in the water column.
L336: Is there evidence that DMSP occurs in the Lena river/estuaries ? The study of Florez-Leiva et al. 2013 shows that the increase of CH4 in response to a spike in DMS was 2 nmol/L/d. Could this production term sufficient to account for the CH4 concentrations ?
L338-345: The CH4 concentrations in the Arctic estuaries (Lena, Ob and Yenisei) are much lower than those in temperate and tropical estuaries. This is an important information from this paper that should be mentioned and discussed. This probably results from the low temperatures and possibly lower organic matter concentrations in rivers (although this needs checking and discussing). This is relevant because temperature and possible organic matter concentrations in rivers can change in future in Arctic estuaries.
L 460: what is the reproducibility and accuracy of these measurements ? Is a 0.02 ppm difference significant, given differences in calibration gases, methods, etc… ?
L 466: This not the case in the Southern part of the North Sea where there is no thermal stratification, and fluxes are much higher than in the Northern part of the North Sea where stratification occurs (Borges et al. 2016).
L 520: diverse
Figure 2: remove PSU. Salinity is measured as a ratio of two conductivities (sample:standard), hence, unitless.
In the plot of Figure 2 replace nM by nmol L-1
In plot of Figure 3 replace nM by nmol L-1 and remove @ depth (m) = first
It could be useful to add the salinity plot (appendix figure) to figure 3, so that the CH4 concentration can be compared directly to the salinity (on the same figure).
In plots of Figure 5 add the units of MOX, and remove @ depth (m) = first
Appendix Table A1: specify if this was done only for the surface data or the full data-set (all depths). Specify what do the bold numbers mean.
Borges AV, W Champenois, N Gypens, B Delille, J Harlay (2016) Massive marine methane emissions from near-shore shallow coastal areas, Scientific Reports, 6:27908, doi:10.1038/srep27908
Gazeau et al. (2004) The European coastal zone: characterization and first assessment of ecosystem metabolism, Estuarine, Coastal and Shelf Science 60, 673-694
Ho, D. T., C. S. Law, M. J. Smith, P. Schlosser, M. Harvey, and P. Hill (2006), Measurements of airsea gas exchange at high wind speeds in the Southern Ocean: Implications for global parameterizations, Geophys. Res. Lett., 33, L16611, doi:10.1029/2006GL026817.