GENERAL COMMENTS

The paper extensively describes a detailed data-set of CO2 and CH4 in surface waters of NE Baltic Sea. The figures are of good quality but the text could be improved. The terminology is in place awkward. The first part of the Introduction is a succession of unrelated statements.

MAJOR COMMENTS

Most of the paper is based on textual descriptions of changes of CO2/CH4 that are stated to relate to changes of salinity, temperature, or depth. It could useful to plot CO2/CH4 as a function of these variables to back these statements. Such plots allow to explore possible additional features in the data-set.

It’s unclear why the mixed layer depth is used to explain patterns in CO2 and CH4. I suggest that the authors compute a stratification index such potential energy anomaly (PEA) according to Simpson (1981). This is a simple computation from the density (salinity-temp) vertical profiles that allows to quantify the strength of water column stratification.

SPECIFIC COMMENTS

I do not see what is the logical link between the first, second and third paragraphs of the introduction. The content is correct, but it’s unclear how these statements connect together to introduce the paper. I suggest to remove the first two paragraphs and start the introduction by the section on the Baltic.

L44: There are recent papers showing long term changes in salinity and alkalinity that should also affect the “CO2 system of surface waters in the Baltic Sea”

L 55 : The collapse of phytoplankton blooms and delivery of fresh material to sediments poor in organic matter seem to stimulate CH4 release and a seasonal peak that does not coincide with the peak in temperature (Borges et al. 2018). Temperature seems to control seasonality in sediments rich in organic matter.

L 60 : Production of methane in aerobic conditions seem to be only relevant in the deep ocean but not the coastal ocean (Weber et al. 2019), although, concentrations and emissions of CH4 in the deep ocean are negligible compared to the coastal ocean.

I suggest that the authors use the full names of the regions instead of the abbreviations (GoF, GoR, and NBP). In the journal Biogeosciences, there is no word limit, so it is unnecessary to abbreviate. For the readers that are unfamiliar with the Baltic Sea it is already difficult to follow the reasoning with these different sub-regions. The use of abbreviations leads to further confusion (letter soup).

L200: why “rapid” ? in relation to what ?

L210: It is not necessary to use subscripts A and W for Henry’s constant (α). The same value is applied to both pCO2 in air and water. 

L216: Equation (2) is incorrect. cCH4 corresponds to dissolved concentration so Henry’s constant (α) is not necessary and in fact chemically meaningless.

L222: This is a strange result. Please briefly explain why “negligible differences in the average net CO2 flux were observed when using the different gas transfer parametrisations”. I guess this reflects that wind speed was generally low since all parameterisations converge at low wind speed. Please list the different gas transfer parametrisations that were tested.

L 320 : it’s water that is under-saturated not CO2 itself.

REFS

Borges AV, G Speeckaert, W Champenois, M.I. Scranton & N Gypens (2018) Productivity and temperature as drivers of seasonal and spatial variations of dissolved methane in the Southern Bight of the North Sea, Ecosystems, 21, 583–599, https://doi.org/10.1007/s10021-017-0171-7

Simpson, J.H., 1981. The shelf-sea fronts: Implications of their existence and behaviour. Philos. Trans. R. Soc. Lond. 302, 531-546. https://doi.org/10.1098/rsta.1981.0181

Weber, T., Wiseman, N. A., & Kock, A. (2019) Global ocean methane emissions dominated by shallow coastal waters. Nature Communications, 10 (1), 4584, https://doi.org/10.1038/s41467-019-12541-7

This is a rather lengthy manuscript for a study in a small area lasting only one year. There is little new information(besides the site-specific data), and the manuscript should be significantly shortened.

The abstract is way too descriptive and has little useful information for readers. Quantitative ones in the abstract and the conclusions should replace qualitative statements

Minor points:

1. It may be confusing to state that pCO2 and cCH4 were calculated from continuous measurements. They were indeed measured, albeit with an intermediate step of converting xCO2 and xCH4 to pCO2 and cCH4.

2. The pCO2 vertical scale should be modified to eliminate the empty space below 350 in Fig. 4 so that the signals are enlarged.