REVIEWER 1 

The article explores the possibility of using gliders equipped with oxygen and turbulence sensors to estimate vertical oxygen fluxes, which has the potential to increase spatial and temporal resolution of measurements while reducing the carbon footprint. The approach of using the CTD values combined with the oxygen/temperature sensors seems to give  good results and may be a good solution to the issue related with the slow response of oxygen sensors when using gliders in high gradient zones. I have a few comments: 

In Methodology: 

Noted from R1 and R2, we have amended the manuscript to include a discussion section and a shorter conclusions section.  

REVIEWER 2 

RC2: 'Comment on bg-2023-100', Anonymous Referee #2, 07 Aug 2023  reply  

General comments 

We have calculated the oxygen density gradient from the base of the pycnocline from the RINKO data (see S.I. Fig. 1), this can be taken as our ‘true’ oxygen gradient. 

We have then used the CTD temperature, lagged optode oxygen, and the lagged optode thermistor data at the base of the pycnocline (where 1027.5 <rho<1027.8) to calculate the first two terms of our equation 1 to compare our against the ‘true’ oxygen gradient. 

The true oxygen gradient from linear regression of S.I. Figure 4: 

m = 168 mmol/m4  

The estimated oxygen gradient taken from (dT/drho) * (dC(O2_lagged)/dT_thermistor): 

dT/drho = -4.2 deg C m^3/kg 

DC(O2_lagged)/dT_thermistor = 45 mmol m-3/deg C 

Giving dC(O2)/drho = -189 mmol/m4 

Our estimated value of the oxygen gradient using lagged oxygen values from the slow response optode gives us an oxygen gradient within 12.5% of the true gradient value. Note that the slow response optode used in our example here has also not been calibrated (our glider slow response optode was calibrated), we would expect this to slightly change our value for dC(O2)/dT and be more accurate. 

We think this represents independently that our method can be used to calculate the oxygen gradient pretty well, and if our manuscript is accepted we would add this method validation into our supplementary information (with more detail to be added on figures and calculations). 

S.I. Figure 1: CTD profile of temperature (blue dots) with optode thermistor values (red). 

S.I. Figure 2: Density profile from the CTD showing two step structure, red lines mark the area where the oxygen-density gradient has been calculated (1027.5 to 1027.75). 

S.I. Figure 3: Fast response oxygen from RINKO (blue) and lagged and uncalibrated optode (red circles), like the optode used in our study on the glider. 

S.I Figure 4: oxygen and density relationship from the RINKO (reliable oxygen measurement). 

Specific comments 

Materials and Methods 

- L.98 : What is the potential impact of the simply solution employement (= constant mixing efficiency) on the results ? 

Although there is no consensus on a mixing efficiency it is well documented that below a certain buoyancy Reynolds number (Reb) turbulence is not mixing diapycnally thus  Eq (1) in the manuscript is valid only in developed turbulence.  There are certainly limitations of this approach, especially in low energy systems as the Arctic, the Black Sea and the Baltic Sea. The low energy levels in this regions make it much more complicated to easily derive fluxes from single turbulence profiles (e.g. Holtermann et al. 2019, Scheifele et al. 2018). 

The vast majority of studies have not conclusively arrived at a suitable improvement to that proposed by Osborn (1980), so in our manuscript we have employed this simple solution as also current best practice as outlined in Gregg et al., 2018. 

- L101 to 105 : see ‘m substitution’ comments « general » section 

As above, I think we have now provided adequate evidence that our method accurately resolves the oxycline. 

- oxygen optode calibration/validation. Informations are missing in this section as it is mentioned in the author contribution.  

This point was also raised by Reviewer 1 and we agree needs to be included in the manuscript. The glider optode was calibrated against both Winkler analysed discrete samples and another calibrated glider optode. We agree this information needs to be within the manuscript and will add this both to the manuscript methodology text with further details of the calibration (calibration coefficients) within the appendix. 

Results 

- L116 Figure 2b rather than 2a 

Thank you this typo has been corrected. 

- L125 : Figure 2d does not exist  

This is a typo and should be 2c. 

- L 127 : Fig 3c ? 

Fig. 3c shows the episodic spikes so we have removed Fig. 2c from the figure referencing in L127 

>> better referencing figures 

- L.137: 10-7 ? not 10-6 

This is a typo and has been corrected to  10-6 

- L.139 : « with the highest values corresponding to the highest €pycno values » this is not visible on current figure 3. 

We think Fig 3d mirrors 3c spikes quite clearly on Fig 3, but take reviewer 2’s comment on board and are happy to mark the oxygen flux spikes out with grey boxes. 

- L.142 to 144 : add figure references to help the reading 

Noted, we have updated the manuscript to reference the figures more frequently. 

- L.145 – 151 : see ‘spike’ comments « general » section 

We have added a section here describing our rationale for chosing this value of epsilon yo describe a ‘spike’. 

Conclusion 

Conclusion includes a discussion part that is missing in the paper. Change the section title or include a section « discussion » 

Noted from R1 and R2, we have amended the manuscript to include a discussion section and a shorter conclusions section.  

Technical corrections 

- L. 57 - 61: add the time coverage of your dataset in this section compare to 17 month of data for Alter ECO for clarification. For example, <(Sep. to Nov.)> just before « autumn breakdown of stratification in the North Sea » and/or < Using this 3-month dataset > 

Thank you have clarified. 

- L70 : replace <(‘Kelvin’, unit 444)> by <(called « Kelvin », unit 444)> 

Thank you we have changed this. 

- L75 : replace <Kelvin was equipped … > by <Glider « Kelvin » was equipped … > as line 113 

- L118 : add model references. 

> This is in comparison to AMM7 temperature (results not shown) we can add this to supplementary information if needed.  

Plot shows temperatures (deg C) from AMM7 for the area where the glider was sampling, stratification shown to break down by October 2018. 

- L142 : replace <-34-4> by <34.4> 

Changed 

- L150 1.0 mmol m-2 s-1 for A and 1 mmol m-2 s-1 for B > harmonize 

Have changed thank you. 

- figure 3  caption : add <(black line)> after <within the pycnocline> 

Changed 

- Explain TKE abbreviation in the text 

Done 

- Figure improvement suggestions : add bottom depth of the shelf sea on the sections of Figure 2 and 3 to complete them 

We agree that this would improve the figures.