|Throughout the text specify the flux and concentration units, as grams can stand for grams of CH4 or grams of carbon.|
L 31 : Sentence “Our findings show that accurate short- and long-term projections of lake CH4 emissions can be based on distinct weather- and climate controlled drivers.” is very vague and not very informative. Either remove sentence or modify it to specify which “weather- and climate controlled drivers”, and also clarify what is meant by “projections”. However, I’m not convinced that the authors actually demonstrate this statement by their analysis. For instance just looking at CH4 concentration, there is a general relation between CH4 and temperature in Figure 4 if all of the data is merged together, yet the CH4 concentration at a given temperature is distinctly lower in Villasjön than Mellersta Harrsjön. Yet the authors do not provide a conclusive explanation for this (depth ?). So I’m unsure if they can claim to make “accurate short- and long-term projections of lake CH4 emissions”.
L 65 : “A key control on emissions is the periodicity at which dissolved gases are brought to the air-water interface” this statement is incorrect for “dissolved gases” in general. Water bodies are not necessarily sources of gases in general and can be temporary or permanent sinks of some gases. During phytoplankton blooms water bodies can be sinks of atmospheric CO2. Conversely, in net heterotrophic systems, water bodies absorb O2 from the atmosphere. Finally, not all gases are produced in bottom waters, for instance during phytoplankton blooms O2 is produced in surface waters.
L 127: explain how atmospheric pressure was regulated inside the chamber during the deployments to avoid over-pressuring (for instance at the moment of the deployment that leads to a partial compression of the gas inside the chamber) or under-pressuring when the gas was sampled (volume of gas retrieved). Over-pressure and under-pressure will artificially decrease or increase the flux measurement, respectively.
L148: Vachon et al. (2010) also compared measurements of turbulence during deployments of chambers and on the contrary to the Ribas-Ribas work concluded that chamber deployments lead to a substantial artificial enhancement of turbulence and hence the estimate of gas transfer velocity. This seems to be an open question, and the artificial enhancement of turbulence cannot be discarded lightly. Please note that the floating chamber described by Ribas-Ribas provides gas transfer velocity values in the ocean reported by Banko-Kubis et al. (2019) that are between 2 to 10 times higher than the values predicted at the same wind speed by the conventionally accepted parameterization of Wanninkhof (2004). This would strongly suggest an artificial enhancement of gas transfer velocity measurements with floating chambers, even with the one described by Ribas-Ribas.
L173 : I’m aware that the FID has a linear response but it is still good practice to calibrate the FID with a standard that is relatively close to the expected values rather than using a CH4 standard of 2 ppm to measure values that are 10 to 100 times higher. This is because small uncertainty on the value of the standard, and the determination (integration) of the peak area of the standard (signal to noise ratio due to baseline fluctuations) will propagate into relatively large errors on the sample concentration computation if the difference between sample and standard values are very large, even if the response of the FID is absolutely perfectly linear.
L 854-858: I do not understand how CH4 oxidation can possibly influence the gas transfer velocity computation. You measured simultaneously a flux and a concentration from which you compute a corresponding gas transfer velocity. I do not see how methane oxidation can play a role in this computation and in the interpretation of the derived data. Sentence “This additional removal process invalidates the implicit assumption in Eq. 1 and 2 that all dissolved CH4 that we measure in the surface water is emitted to the atmosphere” does not make sense. The CH4 gradient drives the flux that both are instantaneously measured, independently of methane oxidation.
Banko-Kubis HM, O Wurl, NIH Mustaffa, M Ribas-Ribas (2019) Gas transfer velocities in Norwegian fjords and the adjacent North Atlantic waters, Oceanologia, 61, 460—470
Vachon D, YT Prairie and JJ. Cole (2010)The relationship between near-surface turbulence and gas transfer velocity in freshwater systems and its implications for floating chamber measurements of gas exchange, Limnol. Oceanogr., 55(4), 2010, 1723–1732.
Wanninkhof, R., 2014. Relationship between wind speed and gas exchange over the ocean revisited. Limnol. Oceanogr 12 (6), 351— 362.