|The authors revised their paper in response to many of the reviewers’ comments, but they didn’t really try to address with my main complaint. Why this study? They still have only a rather vague statement about wishing “to quantify the importance of nitrogen loss processes in overall nitrogen cycling.”|
Maybe a more precise question is, why do the mesocosms? The authors’ experiment is really unprecedented and fantastic, but why not just measure rates in the real ocean if the goal is only “to quantify the importance of nitrogen loss processes”?
A start of an answer is that with the mesocosms the authors can construct a N budget. But then, what is the purpose of a budget? I think if the authors grapple with these questions, they will have a better, more impactful paper.
L9: The argument why substrate limitation explains the difference is not clear. See below my comment on L265.
L33: I’m not sure what this means: “the net balance in terms of bioavailable nitrogen is negative.” All of the processes discussed before this sentence are about how N is lost, so of course, they are “negative.” Why is this sentence needed?
L48: The authors here could add a couple sentence about what the hope to learn from their mesocosm experiment.
L84: The N/P ratio, to be picky, isn’t given in Table 1, just N/P/Si. More obvious than the difference in N/P ratios between the two waters is the presence or absence of NO3 and NO2. That’s seen in the first two columns of Table 1. I think it’s a better way to label the two waters.
L265: If denitrification were substrate-limited in the mesocosms, wouldn’t you expect rates to be lower, not higher than in situ? The authors’ argument is not clear.
L270: The authors changed “measured/maximum” to “measured/maximum sustainable rates”, in response to my comment that it’s ambiguous. In their response to the reviewers, they say they made the change to make “clear that in cases where substrate limitation was encountered, maximum-sustainable rather than measured rates were used for in-situ N-loss estimates.” But that’s not clear in the Discussion. “measured/maximum sustainable rates” is still ambiguous by itself. Use of “/” often leads to ambiquity.
L279: The authors say inorganic N didn’t differ much between the two mesocosms, citing Table 1. But that table has fairly high NO3 and NO2 (>1 uM) for one mesocosm and zero NO3 and NO2 for the other. That seems like a big difference to me. Maybe over time, the difference disappeared?
Table 2: The author revised this table in response to most of my criticisms. I still think it would be better if they label the rows in the table, not give explanations in the caption, to indicate the expected maximum rates and the anammox values. I had to think too much to interpret the * and the italics used to indicate the anammox values. What’s the “overall mean”? It needs to be explained.
Also, I suggest putting a “-“ in front of the N-loss estimates (make them negative—they are losses) to make clear they can be compared directly with the N-budget numbers.
Table 3 and Figure 5: Although the authors have simplified Table 3, I still think it and Figure 5 aren’t needed. The authors don’t really use these to make any points. I still don’t understand Figure 5.