The manuscript titled "Surface CO₂ Gradients Challenge Conventional CO₂ Emission Quantification in Lentic Water Bodies under Calm Conditions" by Patrick Aurich et al. presents an insightful study on CO₂ dynamics in the near-surface layer of the Bautzen Reservoir. I think the first author is a PhD student, and I would like to extend my congratulations to him, recognizing the complexity and difficulty of this research.

The manuscript benefits from a high temporal and spatial resolution dataset, and the analysis provides valuable insights worthy of publication. However, I think the manuscript requires further development and refinement before it is ready for publication. As I am not concerned with remaining anonymous (and I can’t remain anonymous given the nature of my comments 😊), I have attached my detailed feedback in the accompanying PDF document.

I look forward to reviewing the revised version of this manuscript.

This is a very relevant study as it points to one of the major concerns of the traditional methods to estimate air-water CO2 exchange, the assumption of constant concentration of CO2 in the water. The experiment and data are interesting and well worth publishing. I, however, think that a substantial revision with rewriting and reanalysis is required.

Below are some comments and suggestions, I will not comment much on language, but I think a thorough revision of the writing is necessary as well.

General comments of the introduction:

There exists a range of literature on the role of lakes in the carbon cycle, and this part should be updated base on more and more recent literature. Please look at the papers by Golub et al and Guseva et al for a range of suggested papers.

I think the ambition to look at 5 and 25 cm are good, but the major gradient is closer to the surface, please have a more thorough discussion on this aspect.

Line 35: There are several EC sites worldwide (see Golub and Guseva).

Line 50 to 55: The cooling induced convection and the impact on the gas exchange is very important here (in particularly when seeing the strong diurnal cycle in the result section) and should be further discussed also in the introduction. There exists a range of literature from seas and lakes (Rutgersson and Smedman, 2010, Podgrajsek et al 2015, Eugster et al, 2003, 2023; Heiskanen 2014, Andersson et al 2017)

Line 75: The study of Rudeberg is interesting and well discusses how spatial and temporal variations influences the flux. It is, however, limited to chambers. Other studies use EC-fluxes (Rutgersson et al, Dong et al).

I think the limitations of chambers in relation to EC should be further discussed (se for example Podgrajsek et al 2014).

The surrounding areas is considered unimportant, please ale note the possibility of non-local effects (eeg Esters et al).

Section 2.4: Please do not name the routines used. If this is important explain what they do to the data (if this paper is read in 10 years’ time, it might be impossible to understand as now written). The name of the routines could be in an appendix, if the authors consider it important information.

Results:

In Figure 1 you show a really nice diurnal cycle, with significant gradients during night-time, this is explained by the phytoplankton activity, but you really should consider the effect of physical processes with a strong waterside convection during night-time. This is seen during low winds, when the convection is found to dominate.

Andersson, A., E. Falck, A. Sjöblom, N. Kljun, E. Sahlée, A. M. Omar, and A. Rutgersson (2017), Air-sea gas transfer in high Arctic fjords, Geophys. Res. Lett., 44, doi:10.1002/2016GL072373.

Dong et al 2021, https://doi.org/10.5194/acp-21-8089-2021

Esters L, Rutgersson A, Nilsson E and Sahlee E 2020 Non-local impacts on eddy-covariance air-lake CO₂ fluxes Bound.-Layer Meteorol. 178 283–300

Eugster W et al 2003 CO₂ exchange between air and water in an arctic Alaskan and midlatitude Swiss lake: importance of convective mixing J. Geophys. Res. 108 4362

Eugster W, DelSontro T, Shaver G R and Kling G W 2020 Interannual, summer, and diel variability of CH₄ and CO₂ effluxes from Toolik Lake, Alaska, during the ice-free periods 2010–2015 Environ. Sci.: Process. Impacts 22 2181–98

Golub et al., 2023 Diel, seasonal, and inter-annual variation in carbon dioxide effluxes from lakes and reservoirs, Environ. Res. Lett. 18 034046, https://doi.org/10.1088/1748-9326/acb834

Heiskanen J J, Mammarella I, Haapanala S, Pumpanen J, Vesala T, Macintyre S and Ojala A 2014 Effects of cooling and internal wave motions on gas transfer coefficients in a boreal lake Tellus B 66 22827

Guseva, S., etal , 2023. Bulk Transfer Coefficients Estimated from Eddy-Covariance Measurements over Lakes and Reservoirs. J Geophys Res.-Atmospheres, 128, e2022JD037219, doi:10.1029/2022JD037219. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022JD037219 

Podgrajsek E, Sahlée E and Rutgersson A 2015 Diel cycle of lake-air CO₂ flux from a shallow lake and the impact of waterside convection on the transfer velocity J. Geophys. Res. 120 29–38

Rutgersson A. and Smedman, A. Enhancement of CO2 transfer velocity due to water-side convection, J. Marine Syst., 80, 125-134,. 2010

Rutgersson, A., M. Norman, B. Schneider, H. Pettersson, E. , Sahlée. The annual cycle of carbon-dioxide and parameters influencing the air-sea carbon exchange in the Baltic Proper. J. Mar. Syst., 74, 381-394. 2008