Dear Authors,

I now have received two reviewer reports for your revised manuscript. Both reviewers find that the revised version is improved but both also raise a number of further comments, especially Emmanuel Boss has a few concerns that I agree with. I would like you to consider these carefully and make the appropriate revisions. With your revised manuscript, please submit a detailed, point-by-point response that contains the complete reviewer comments.

Best regards, Katja Fennel

Dear Authors,

Thank you for revising your manuscript further. There are two remaining issues which have to be addressed before your manuscript can be published.

In your additions to section 4.2, please remove the sentence:
"This assumption has been proved by Mellard et al. (2011)."
Often there is a significant amount of plankton in the surface layer even if chlorophyll is low (see e.g. Fennel and Boss 2003 for examples using BATS data).

I also don't agree with this sentence:
"If nutrient input directly to the mixed layer due to riverine inputs, surface runoff, or atmospheric deposition, Chl a concentration within the surface mixed layer will be sustained, while a SCM by itself will be not possible (Mellard et al. 2011)."
Subsurface chlorophyll maxima (SCM) do occur in coastal regions with riverine inputs, etc. I have seen numerous examples in observations. Just because your highly simplified model breaks down in these cases doesn't mean there are no SCM in reality. Please change this sentence accordingly.

Best, Katja Fennel

Dear Authors,

Thank you for revising your manuscript further. It is now accepted for publication.

Thank you for supporting Open Access publications like Biogeosciences.

Best regards, Katja Fennel