See attached review report.

See attached review notes.

Summary

The technical note by Iwe and colleagues presents an analytical approach for the determination of N₂-fixation rates, which the authors envision as a tool for obtaining continuous, high-resolution measurements of N₂, Ar, and O₂. The main advantages of the proposed approach are that it improves the spatial and temporal coverage with respect to sporadic surveys (some of which use discrete sampling methods), and that it would enable the users to conduct detailed assessments of net community production (NCP) in surface waters of different oceanic regions, while accounting for small-scale variability.    

General assessment

Strengths: Given the pivotal role of N₂-fixation, the topic of this technical note is certainly relevant. Although the principles of the individual methods (gas equilibration and MIMS) have been used in other studies for similar applications, their combination and optimization for underway measurements is novel. Besides the obvious advantages of being able to derive N₂-fixation rates and NCP over large areas and with potentially unprecedented temporal coverage, this approach might enable a better understanding of carbon and nitrogen dynamics in surface waters. Overall, the manuscript is well written, the approach followed is clear and the specific aims (1. Assessing equilibration times and full equilibrium; and 2. Assessing the system’s performance in terms of precision, accuracy, limits of detection) are adequately addressed and substantiated with laboratory-based experiments.   

Weaknesses: The major drawback of this contribution is that the authors present it in a way that it has been optimized for surveys on board voluntary observing ships (VOS), without providing data/experiments derived from an at-sea deployment. As it stands, the manuscript shows an assessment that the system is, in principle, capable of conducting measurements on such a vessel just as much as it could do in any other type of application. Beyond this, perhaps semantic issue, there are practical considerations that need to be accounted for when systems are installed in an unattended manner (as I am sure it is known to some of the coauthors). These include strong temperature variability (potentially affecting both hardware and software), potential contamination, vibration, biofouling, etc.. Because of this, several parts of the text (starting with the title) can be considered misleading in the absence of direct evidence.  

Overall, it is my opinion that this is a contribution worthy of being published after some issues are addressed. I would be reluctant to ask the authors for data from an at-sea deployment at this stage, but my recommendation would be to reformulate so that it is clear that their approach paves the way for further studies that do carry out the deployments on VOS.

Specific comments:

Throughout the text: I spotted a few format inconsistencies with the usage of chemical names (e.g. sometimes “O₂”, sometimes “oxygen”, and also not all subscripts are correct).

l. 1 – 3 (Title): This approach can, in principle be applied to any survey type and in this manuscript no data from VOS is shown. I would therefore include this as a potentially useful application in the context of long-term observatories.

l. 18 – 19 (“The GE-MIMS is designed for…”): Perhaps the authors could describe this as a "proof-of-concept" in view of its future application to conduct observations in VOS.

l. 84: “provide” instead or “provides”

l. 99 (Figure 1 caption): To me most abbreviations were clear, but there might be readers not yet familiar with this kind of analytical setup. Therefore I would recommend the authors to include abbreviations also here (I noticed that they are used in the text, which is good, but some are far from the actual figure).

l. 103 – 104 (“A pressure gauge (P2) was installed”): I was wondering whether the authors could add some values (or an empirical threshold) here. This would be good both to ensure repeatability and also guide potential new users of this approach.

l. 113 – 115: Virtually unattended deployment in a VOS will require a suitable alternative. I am guessing the authors might be able to provide useful suggestions on this.

l. 150 – 154: This information could be conveyed more clearly with a graph (e.g. an Allan plot).

l. 215 – 249: The full mathematical derivation is not novel and it seems unnecessary in this part of the manuscript. I would suggest the authors to shift this to an appendix.

l. 258 – 260: It is hard to grasp how the underlying assumption of no water flow could be directly applied to operation conditions in which indeed there will be seawater flowing through the system. In my opinion this needs further explanation.

l. 382 (“(…) denitrification in deep waters.”): A citation seems to be missing here.

l. 394 (“Ignoring vertical mixing (…)”): This choice should be substantiated.

l. 434 (“(…) such that also currently used for continuous pCO₂ measurements (…)”): A citation seems to be missing here.

l. 435 (“(…) will facilitate determinations of NCP”): A further potential application of the approach presented by the authors would be to combine it with underway measurements of N₂O, since this might help further constraining uncertainties in O₂/Ar based NCP estimates (see Cassar et al., GRL, 8961–8970, 2014).