Nitrogen dynamics and nitrate stable isotopes indicate nitrogen loss in the Bay of Bengal

Gesa Schulz, Kirstin Dähnke, Tina Sanders, Jan Penopp, Hermann W. Bange, Rena Czeschel, and Birgit Gaye

General comments:

The manuscript looks at the nitrogen cycling in the Bay of Bengal as well as East Equatorial Indian Ocean. The methodologies are detailed, the data well presented and the discussion follows a logical pathway.

Motivation:

The authors make a good case of why the Bay of Bengal OMZ is important to study. It is not clear, however, why they chose EEIO. Please include a section about why you study this region.

Specific comments:

L21: “Below”. Do the authors mean “subsurface waters”?

L27: Please change to “unavailable for most organisms”

L31: Denitrification may also be autotrophic. Please restructure this sentence.

L41: Define “low oxygen” and “anoxic”

L43&44:  Add references for “…oxygen concentrations upto 20µM” and “…concentrations of 6µM”

L59: “dual”: please mention N and O in parentheses.

L87: “large amount of freshwater input”: a number would be helpful

Figure 2: I would recommend adding BoB and EEIO in the figures too.

L124: How was particulate nitrogen measures? Did the authors scrape material from the filter paper or pellets or any other way?

L133-134 : The international standards are reported for NO3- and not Nox. I suggest writing NO3- . Same goes for the samples: since the authors have ensured nitrite removal.

L165: Where is the figure for ammonium concentrations?

L177: “heaviest isotope signature….upto 13.9‰ “ yet Fig 3e shows δ15N>20‰

Figure 3: Please keep the labels identical for all . (a)-(j) are on the bottom right while (k)-(n) on bottom left.

L201: It is not clear what is a “strong oxygen minimum zone”?

L340: Is the calculation of δ15N of nitrite shown?

L367:  I would like to see the authors discuss a few more Indian Ocean studies. For e.g. how distinct is it to the Arabian Sea OMZ?

Technical corrections:

L98: onboard R/V Sonne

L124: Dried filters were used “to”

L127-132: The authors use the past tense in some sentences and the present tense. I believe the past tense is more appropriate.