|The authors put a lot of efforts into addressing mine and the other reviewer’s concerns, and defending the original version of the MS. They did a good job in responding, and I think that the MS has benefitted from the changes made. I am still not convinced by the way they sell their data set (evidence for true AOM is weak), and I still think there are some issues, but I guess that this is part of the scientific exchange. We do not need to all agree. It is definitely a nice data set, certainly worth to be published.|
A couple of things:
I am wondering about the fact that there was not an appropriate d18O_NO2 standard used. Coming then back to R1’s question: how were the nitrite isotope values corrected?
R1 argues correctly that the observed NO2 d18O data seem too low, assuming O isotope equilibrium with water. In their response, the authors write: “relatively constant δ18O values for nitrite close to -5‰ indicate that an isotopic exchange occurred between the oxygen of the water and the oxygen of nitrite and that the latter was predominately controlled by biotic reactions”. But doesn’t the d18O of nitrite argue against this?
I agree with R1 that the nitrite d18O data may be left out of the paper, as they do not really help.
The authors tone down in their response the value of incubation experiments. I do not agree. Experiments, even if not directly reflecting the natural conditions, would have helped to calibrate the observed isotopic signatures, and to gain confidence in their interpretation. I was not referring to isotope effect experiments, and I am aware that rate measurements will deliver potential rates only, but clearly, rate measurements would have helped to indicate the potential for the use of specific substrates, and thus would have helped to support (or not) the claimed links between different biogochemical reactions (anammox and AOM).
The authors state that there is barely any mixing during summer (“no mixing, no advection”); I assume they mean no advective mixing, because turbulent diffusive mixing, even if sluggish will take place.
As for the choice of a value for Kz: In my first review; I was not suggesting a value of 0.03 m2/d. I was referring to a paper by Oswald et al. from Lake La Cruz (Oswald et al. 2016), for which this value was reported. I mixed up the different studies/lakes investigated by Oswald et al., and values from Lake Rotsee, as well as values for Lake Lugano seem to be higher, indeed. But the broad range of Kz values observed for the different meromictic lakes highlights that there is not “a typical literature value”, and that just assuming a value for Kz is difficult.
I am still confused with regards to the d18O_NOx values. Again, in a sample that contains nitrite and nitrate, O isotope fractionation during the conversion to N2O (independent of the method of conversion) must be different for nitrite and nitrate (via nitrite), simply because one versus two O atoms will be plucked off during the fractionating transformation, respectively. Hence it remains unclear to me how the 18O of the combined NO3+NO2 sample is standardized. The problem is not the yield, the problem is that the N2O from the nitrate will likely have a much higher d18O than the d18O of the N2O generated from the NO2.
As for the interpretation of the NH4 concentration versus the d15NH4 profiles: I see a more or less steady decrease from 1 to <0.1 mgN/l between 22 and 12 m, without a strong overall 15N enrichment. From these patterns, I find it quite difficult to pinpoint anaerobic ammonium oxidation in the deep water column. Within the error of the d15NH4 analyses, the isotope profile looks almost straight to me….an observation that could most plausibly be explained by aerobic ammonium explanation at the redox transition zone, which serves as an efficient NH4 sink without much N isotope fractionation.
L915: established is misspelled