Reply on RC1

Yes, this would be the general assumption and is often the best line of reasoning. However, in a stagnant, undynamic system, a scenario with hampered ventilation will lead to an OMZ but the OMZ, in my view, can only lose all the oxygenif enough respiration takes place. This can only happen if enough nutrients are transported into the surface to allow for organic matter production, which then can be respired. Therefore, the question of how much oxygen is left would boil down to physical ventilation by water mass dynamics, organic matter respiration and topography. The latter would be important because if we have a shallower water column, we could assume that even lower organic matter export rates would lead to enough respiration to cause anoxia. In contrast, if we lose a lot of organic material to the deeper water column we would possibly not see the same effect. In the end it’s question of balance, or of what has the stronger effect. Because the BoB has this strongly stratified water column and ventilation would therefore not be a strong factor, my line of reasoning would be that the biological contribution to OMZ formation is the one to look at. I clarified my standpoint as follows: ‘Assuming decreasing nitrate concentrations and concurrent decreasing biomass production, export and respiration, oxygen concentrations within the oxygen minimum zone would not be expected to further decrease. This effect could be enhanced by stronger stratification as a result of future warming, and thus possibly counteract oxygen decrease as a direct effect of stratification.’


L50-51: empirical records?
That would be the problem, the available collection of empirical data as presented in table 1 is to scattered and patchy to derive a solid pattern from it, therefore, satellite based data are the only constant record available to us.
L56: I would propose adding the time of year when the other rates were measured as a comparison to this indication of summer monsoon here. I added the time frames to the summer monsoon situation (Aug.Sept 1976), the other two records do unfortunately not have an information on the monsoon situation.  Sensing, vol. 9, no. 11, pp. 5044-5054, Nov. 2016, doi: 10.1109 L112: Is it? do we have evidence that different community compositions of Synecho/Procholoro provide comparatively quantities of primary production?
Actually you may have a point, here, we do not really know in the case of those specific clades of Synechococcus/ Prochlorococcus. We do have an idea of the overall difference between Prochlorococcus, Synechococcus and picoeukaryotes, described e.g. in Liang et al 2017 and Buitenhus et al, 2012, to be around 36, 255, and 2590 fg C cell -1 for Prochlorococcus, Synechococcus, and picoeukaryotes.
The different clades occupy different niches, they are different in several regards and they may produce different rates, this would make sense but we still may not have a good enough grasp of how the clades in the BoB contribute to primary production. I therefore changed the sentence to 'A community shift in small cyanobacteria may be somewhat speculative and with unknown impacts on bulk primary production. However, an overall increase in abundance of small cyanobacteria in concert with a decrease of eukaryotic primary producers would be expected to impact BoB biogeochemistry, especially with regard to the spatial expansion and the intensity of the OMZ through modified export production and respiration in low oxygen intermediate waters. L145: Can you elaborate, maybe earlier in the text when the different Prochloros are introduced, on their potential different contribution to primary production? Are some more productive than others? Or is their level of productivity spatiotemporally controlled by the availability/dynamics of the resources that are specifically limiting for each clade?
This is an interesting topic and issue you are raising, here. Most studies focus either on the distribution of the various ecotypes, other studies talk about bulk rates produced by Prochlorococcus. The authors of the cited study (Larkin et al) conclude that the ecotypes will distribute in response to biogeochemical factors, rates are, however, not metioned. For the specific ecotypes in the BoB it will be difficult to say if there is a difference in primary production per ecotype or if the productivity will be the same in general but vary in response to biogeochemical variables. 'While there is a body of literature describing distribution patterns of Prochlorococcus ecotype (e.g. Johnson et al., 2006, Martiny et al, 2009, Moore et al., 1998, the relative contribution of different Prochlorococcus ecotypes to primary production in the ocean is not well resolved. In addition, information on the specific contribution of Prochlorococcus ecotypes detected in the BoB to bulk primary production is not available. Thus, it is unclear if a change in Prochlorococcus ecotype composition as suggested by Larkin et al., 2019, in response to changing temperatures, nutrient concentration, or iron stress would correspond to changes in overall Prochlorococcus primary production rates.' 'This result is indeed consistent with a long term trend with decreasing productivity since the last glacial maximum (Contreras-Rosales et al. 2016;Shetye et al. 2014). With primary production leading to respiration and a concurrent oxygen loss in intermediate waters, this may provide an explanation for why the BoB is the only oxygen minimum zone region with traces of oxygen left in its core waters. It has often been suggested that the BoB is at a tipping point to developing severe anoxia (Bristow et al. 2017;Canfield et al. 2019;Rixen et al. 2020), which is a threshold with only minor changes in biogeochemistry leading to a consumption of oxygen traces in the oxygen minimum zone. This scenario is, however, challenged by decreasing primary production on long-term, as well as decadal time scales.' L157 onwards: Probably all these factors co-occur and interact.
True, I added ' or a combination of those factors' to l. 162. L168: Remove "could"?
It's been removed L169: But point out the seasonal bias in measurements available so far.
I added the following statement in l. 171: However, our understanding of the diazotroph community composition and N2 fixation rates is hampered by the low number of available datasets and their spatial and seasonal bias.
L170: But there would probably be a sort of successional equilibrium where nutrient limitation would decrease primary production creating a niche for N2 fixation, followed by DIN availability stemming from diazotrophs reviving non-diazotroph primary producers back. I guess the key question here is P availability. This is what we would classically expect but it doesn't happen, maybe it doesn't happen just yet.
L179-183: I agree that limited primary production due to stratification would lead to decreased export and less oxygen consumption in intermediate layers, but wouldn't it also impede ventilation? Agreed, however, as already discussed above, this is a question of oxygen import versus respiration. If we assume hampered ventialtion beyond what we have now, it would take less respiration to cause anoxia, but we would still need some respiration. Therefore it would boil down to which process responds faster, i.e. if we lose oxygen via stratification and lower ventilation or if we safe oxygen because less is respired because of lower nutrient fluxes due to stratification. What we see now are traces of oxygen maintained in the OMZ with an extreme stratification already, the enigma we don't understand making it cruicial to go back to the BoB and do organic matter addition experiments as well as oxygen additions in OMZ-water incubations. L201: I would maybe refer to sub/mesoscale features in general, as features other than eddies (e.g. driven by wind) can drive localized vertical mixing Correct, and changed accordingly. I, admittedly, have a weakness for eddies.
Again, thank you for your time to review this manuscript, your ideas, questions and suggestions were very helpful and appreciated.